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Full text of "On the structure and distribution of coral reefs; also, Geological observations on the volcanic islands and parts of South America visited during the voyage of H.M.S. Beagle"

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GersteiiT Sderrce Infarmation Centre 



DARWIN. 



CORAL REEFS, VOLCANIC ISLANDS, 
SOUTH AMERICAN GEOLOGY. 



The Minerva Library 

OF FAMOUS BOOKS. 
Edited by G. T. BETTANY, M.A. 



/. CHARLES DARWIN'S JOURNAL during a 
V'oyage in the ' Beagle.' 

2. THE INGOLDSBY LEGENDS. 

3. BORROWS BIBLE IN SPAIN. 

4. EMERSON'S PROSE IVORKS. 

5. GALTONS TROPICAL SOUTH AFRICA. 

6. MANZONI'S THE BETROTHED LOVERS. 

7. GOETHE'S FAUST {Complete). Bayard Taylor. 

8. WALLACE'S TRAVELS ON THE AMAZON, 
g. DEAN STANLEY'S LIFE OF DR. ARNOLD. 

10. POE'S TALES. 

11. COMEDIES BY MOLIERE. 

12. FORSTER'S LIFE OF GOLDSMITH. 

13. LANE'S MODERN EGYPTIANS. 
li. TORRENS' LIFE OF MELBOURNE. 

15. THACKERAY'S VANITY FAIR. 

16. EARTH'S TRAVELS IN AFRICA. 

17. VICTOR HUGO: SELECT POEMS, &>€. 
iS. DARWINS CORAL REEFS, &-c. 



London : Ward, Lock & Co. 



j®c3 ^:=^""ri 




THE MINERVA LIBRARY OF FAMOUS BOOKS. 

Edited by G. T. BETTANY, M.A., B.Sc. 



ON THE STRUCTURE AND DISTRIBUTION OF 

CORAL' REEFS; 

ALSO 

GEOLOGICAL OBSERVATIONS 

ON THE 

VOLCANIC ISLANDS 



AND PARTS OF J ^>» 

• |£3 5P CD 

SOUTH AMERICir - - 

Visited during the Voyage of HM.S. Beagle. \^ ^>-i 



Jij 



BY i^U^^ 



CHARLES DARWIN 






W^/r// MAPS, PLATES, AND NUMEROUS ILLUSTRATIONS. 



AND A CRITICAL INTRODUCTION TO EACH WORK BY 
Prof. JOHN W. JUDD, F.R.S., 

Professor of Geology in the Normal College qf Science, South Kensington. 



WARD, LOCK, AND CO., 

LONDON, NEW YORK, AND MELBOURNE. 
1890. 



LA LIBRARY C) 




5j4 



EDITORIAL NOTE. 



Although in some respects more technical in their subjects and 
style than Darwin's " Journal," the books here reprinted will never 
lose their value and interest for the originality of the observa- 
tions they contain. Many parts of them are admirably adapted 
for giving an insight into problems regarding the structure 
and changes of the earth's surface, and in fact they form a 
charming introduction to physical geology and physiography 
in their application to special domains. The books themselves 
cannot be obtained for many times the price of the present 
volume, and both the general reader, who desires to know more of 
Darwin's work, and the student of geology, who naturally wishes 
to know how a master mind reasoned on most important 
geological subjects, will be glad of the opportunity of possessing 
them in a convenient and cheap form. 

The three introductions, which my friend Professor Judd has 
kindly furnished, give critical and historical information which 
makes this edition of special value. 

G. T. B. 



CONTE NTS. 



CORAL REEFS. 

PAGE 

Critical Intjioduction 3 

Introduction .,,,,,. . , , . ii 

CHAPTER I. 

ATOLLS OR LAGOON-ISLANDS. 

Section I. — Description of keeling atoll. 

Corals on the outer margin. — Zone of Nulliporae. — Exterior reef. — 
Islets. — Coral-conglomerate, — Lagoon. — Calcareous sediment. 
— Scari and Holuthuriae subsisting on corals. — Changes in the 
condition of the reefs and islets. — Probable subsidence of the 
atoll. — Future state of the lagoon . . . . , .15 

Section II. — General description of atolls. 

General form and size of atolls, their reefs and islets. — P'xternal 
slope. — Zone of Nulliporae. — Conglomerate. — Depth of lagoons. 
— Sediment. — Reefs submerged wholly or in part. — Breaches 
in the reef. — Ledge-formed shores round certain lagoons. — 
Conversion of lagoons into land ...... 24 

Section III. — Atolls of the maldiva archipelago — great 

CHAGOS BANK. 

Maldiva Archipelago. — Ring-formed reefs, marginal and central, — 
Great depth in the lagoons of the southern atolls. — Reefs in 
the lagoons all rising to the surface. — Position of islets and 
breaches in the reefs, with respect to the prevalent winds and 



CONTENTS. 

PAGE 

action of the waves. — Destruction of islets. — Connection in the 
position and submarine foundation of distinct atolls. — The 
apparent disseverment of large atolls. — The Great Chagos 
Bank. — Its submerged condition and extraordinary structure . 33 



CHAPTER II. 

BARRIER-REEFS. 

Closely resemble in general form and structure atoll-reefs. — Width 
and depth of the lagoon-channels. — Breaches through the reef 
in front of valleys, and generally on the leeward side. — Checks 
to the filling up of the lagoon-channels. — Size and constitution 
of the encircled islands. — Number of islands within the same 
reef. — Barrier-reefs of New Caledonia and Australia. — Position 
of the reef relative to the slope of the adjoining land. — Probable 
great thickness of barrier-reefs 38 

CHAPTER. III., 

FRINGING OR SHORE-REEFS. 

Reefs of Mauritius. — Shallow channel within the reef. — Its slow 
filling up. — Currents of water formed within it. — Upraised 
reefs. — Narrow fringing-reefs in deep seas. — Reefs on the 
coast of E. Africa and of Brazil — Fringing-reefs in very shallow 
seas, round banks of sediment and on worn-down islands. — 
Fringing-reefs affected by currents of the sea. — Coral coating 
the bottom of the sea, but not forming reefs . , . -45 

CHAPTER IV. 

ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS. 

Section I. — On the distribution of coral-reefs, and on 

THE conditions FAVOURABLE TO THEIR INCREASE . . 50 

Section II. — On the rate of growth of coral-reefs . . 57 

Section III. — On . the. depths at which reef-building 
polypifers can live 63 

CHAPTER V. 

THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF 
CORAL-REEFS. 

The atolls of the larger archipelagoes are not formed on submerged 
craters, or on banks of sediment.— Immense areas interspersed 



CONTENTS. ix 

PAGE 

with atolls. — Recent changes in their state. — The origin of 
barrier-reefs and of atolls. — Their relative forms. — The step- 
formed ledges and walls round the shores of some lagoons. — 
The ring-formed reefs of the Maldiva atolls. — The submerged 
condition of parts or of the whole of some annular reefs. — The 
disseverment of large atolls. — The union of atolls by linear 
reefs. — The Great Chagos Bank. — Objections, from the area 
and amount of subsidence required by the theory, considered. 
— The probable composition of the lower parts of atolls . . 69 



CHAPTER VI. 

ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO 
THE THEORY OF THEIR FORMATION. 

Description of the coloured map. — Proximity of atolls and barrier- 
reefs. — Relation in form and position of atolls with ordinary 
islands. — Direct evidence of subsidence difficult to be detected. 
— Proofs of recent elevation where fringing-reefs occur. — 
Oscillations of level. — Absence of active volcanoes in the areas 
of subsidence. — Immensity of the areas which have been 
elevated and have subsided. — Their relation to the present 
distribution of the land. — Areas of subsidence elongated, their 
intersection and alternation with those of elevation. — Amount 
and slow rate of the subsidence. — Recapitulation . 



APPENDIX. 

Containing a detailed description of the reefs and islands in 

Plate III Ill 



VOLCANIC ISLANDS. 

PAGE 

Critical Introduction 157 

CHAPTER I. 

ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO. 

Rocks of the lowest series. — A calcareous sedimentary deposit, 
with recent shells, altered by the contact of superincumbent 
lava, its horizontality and extent. — Subsequent volcanic erup- 
tions, associated with calcareous matter in an earthy and 
fibrous form, and often enclosed within the separate cells of 
the scoriae. — Ancient and obliterated orifices of eruption of 
small size. — Difficulty of tracing over a bare plain recent 
streams of lava. — Inland hills of more ancient volcanic rock. — 
Decomposed olivine in large masses. — Feldspathic rocks be- 
neath the upper crystalline basaltic strata. — Uniform structure 
and form of the more ancient volcanic hills. — Form of the 
valleys near the coast. — Conglomerate now forming on the 
sea beach 167 

CHAPTER] : II. 

FERNANDO NORONHA ; TERCEIRA ; TAHITI, ETC. 

Fernando Noronha. — Precipitous hill of phonolite. Terceira. — 
Trachytic rocks : their singular decomposition by steam of 
high temperature. Tahiti. — Passage from wacko into trap ; 
singular volcanic rock with the vesicles half-filled with mcso- 
type. Mauritius. — Proofs of its recent elevation. — Structure 
of its more ancient mountains ; similarity with St. Jago. St. 
Paul's Rocks. — Not of volcanic origin. — Their singular minera- 
logical composition 181 



i 



CONTENTS. 
CHAPTER III. 

ASCENSION. 

] 

Basaltic lavas. — Numerous craters truncated on the same side. — 
Singular structure of volcanic bombs. — Aeriform explosions. — 
Ejected granite fragments. — Trachytic rocks. — Singular veins. 
— Jasper, its manner of formation, — Concretions in pumiceous 
tuff. — Calcareous deposits and frondescent incrustations on the 
coast. — Remarkable laminated beds, alternating with, and pass- 
ing into obsidian. — Origin of obsidian. — Lamination of volcanic 
rocks ........... 



CHAPTER IV. 

ST. HELENA. 

Lavas of the feldspathic, basaltic, and submarine series. — Section 
of Flagstaff Hill and of the Barn. — Dikes. — Turk's Cap and 
Prosperous Bays. — Basaltic ring. — Central crateriform ridge, 
with an internal ledge and a parapet. — Cones of phonolite. — 
Superficial beds of calcareous sandstone. — Extinct land-shells. 
— Beds of detritus. — Elevation of the land. — Denudation. — 
Craters of elevation 214 

CHAPTER V. 

GALAPAGOS ARCHIPELAGO. 

Chatham Island. — Craters composed of a peculiar kind of tuff. — 
Small basaltic craters, with hollows at their bases. — Albemarle 
Island ; fluid lavas, their composition. — Craters of tuff ; incli- 
nation of their exterior diverging strata, and structure of their 
interior converging strata. — James Island, segment of a small 
basaltic crater ; fluidity and composition of its lava-streams, 
and of its ejected fragments. — Concluding remarks on the 
craters of tuff, and on the breached condition of their southern 
sides. — Mineralogical composition of the rocks of the archi- 
pelago. — Elevation of the land. — Direction of the fissures of 
eruption 229 

CHAPTER VI. 

TRACHYTE AND BASALT. — DISTRIBUTION OF VOLCANIC ISLES. 

The sinking of crystals in fluid lava. — Specific gravity of the con- 
stituent parts of trachyte and of basalt, and their consequent 



xii CONTENTS. 

PACK 

separation. — Obsidian. — Apparent non-separation of the ele- 
ments of plutonic rocks. — Origin of trap-dikes in the plutonic 
series. — Distribution of volcanic islands ; their prevalence in 
the great oceans. — They are generally arranged in lines. — The 
central volcanoes of Von Buch doubtful. — Volcanic islands 
bordering continents. — Antiquity of volcanic islands, and their 
elevation in mass. — Eruptions on parallel lines of fissure 
within the same geological period 243 

CHAPTER VIL 

AUSTRALIA ; NEW ZEALAND ; CAPE OF GOOD HOPE. 

New South Wales. — Sandstone formation. — Embedded pseudo- 
fragments of shale. — Stratification. — Current-cleavage. — Great 
valleys. — Van Diemen's Land. — Palaeozoic formation. — Newer 
formation with volcanic rocks. — Travertin with leaves of extinct 
plants. — Elevation of the land. — New Zealand. — King George's 
Sound. — Superficial ferruginous beds. — Superficial calcareous 
deposits, with casts of branches ; its origin from drifted particles 
of shells and corals. — Their extent. — Cape of Good Hope. — 
Junction of the granite and clay-slate. — Sandstone formation . 251 



GEOLOGICAL OBSERVATIONS ON SOUTH 
AMERICA. 

PAGE 

Critical Introduction . 269 



CHAPTER I. 

ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. 

Upraised shells of La Plata. — Bahia Blanca, Sand-dunes and 
Pumice-pebbles. — Step-formed plains of Patagonia, with up- 
raised shells. — Terrace-bounded valley of Santa Cruz, formerly 
a sea-strait. — Upraised shells of Tierra del Fuego. — Length 
and breadth of the elevated area. — Equability of the move- 
ments, as shown by the similar heights of the plains. — Slow- 
ness of the elevatory process. — Mode of formation of the step- 
formed plains. — Summary. — Great shingle formation of Pata- 
gonia ; its extent, origin, and distribution. — Formation of 
sea-cliffs 277 



CHAPTER n. 

ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. 

Chonos Archipelago. — Chiloe, recent and gradual elevation of, tra- 
ditions of the inhabitants on this subject. — Concepcion, earth- 
quake and elevation of. Valparaiso, great elevation of, 
upraised shells, earth or marine origin, gradual rise of the land 
within the historical period. Coquimbo, elevation of, in recent 
times ; terraces of marine origin, their inclination, their escarp- 
ments not horizontal. — Guasco, gravel terraces of. — Copiapo. 
Peru. — Upraised shells of Cobija, Iquique, and Arica. — Lima, 
shell-beds and sea-beach on San Lorenzo. — Human remains, 
fossil earthenware, earthquake debacle, recent subsidence. — On 
the decay of upraised shells, — General summary . • . 30J, 



CONTENTS. 



CHAPTER III. 

ON THE PLAINS AND VALLEYS OF CHILE: — SALIFEROUS SUPERFICIAL 

DEPOSITS. 

PAGE 

Basin-like plains of Chile ; their drainage, their marine origin. — 
Marks of sea-action on the eastern flanks of the Cordillera. — 
Sloping terrace-like fringes of stratified shingle within the 
valleys of the Cordillera ; their marine origin. — Boulders in the 
valley of the Cachapual. — Horizontal elevation of the Cordillera. 
• — Formation of valleys.— Boulders moved by earthquake- 
waves. — Saline superficial deposits. — Bed of nitrate of soda at 
Iquique. — Saline incrustations. — Salt-lakes of La Plata and 
Patagonia; purity of the salt ; its origin 333 



CHAPTER IV. 

ON THE FORMATIONS OF THE PAMPAS. 

Mineralogical constitution. — Microscopical structure. — Buenos 
Ayres, shells embedded in tosca-rock. — Buenos Ayres to the 
Colorado.— S. Ventana. — Bahia Blanca ; M. Hermoso, bones 
and infusoria of; P. Alta, shells, bones, and infusoria of; co- 
existence of the recent shells and extinct mammifers. — Buenos 
Ayres to St. Fe. — Skeletons of M^todon. — Infusoria. — Inferior 
marine tertiary strata, their age. — Horse's tooth, Banda 
Oriental. — Superficial Pampcan formation. — Inferior tertiary 
strata, variation of, connected with volcanic action ; Macrau- 
chenia Patachonica at S. Julian in Patagonia, age of, subse- 
quent to living mollusca and to the erratic block period. 
Summary. — Area of Pampean formation. — Theories of origin. 
— Source of sediment. — Estuary origin. — Contemporaneous 
with existing mollusca. — Relations to underlying tertiary strata. 
Ancient deposit of estuary origin. — Elevation and successive 
deposition of the Pampean formation. — Number and state of 
the remains of mammifers ; their habitation, food, extinction, 
and range. — Conclusion. — Supplement on the thickness of the 
Pampean formation. — Localities in Pampas at which mammi- 
ferous remains have been found 350 

CHAPTER V. 

ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE. 

Rio Negro. — S. Josef. — Port Desire, white pumiceous mudstone 
with infusoria. — Port S. Julian. — Santa Cruz, basaltic lava of. 
— P. Gallegos. — Eastern Tierra del Fuego ; leaves of extinct 



CONTENTS. XV 

PAGE 

beech-trees. — Summary on the Patagonian tertiary formations. 
— Tertiary formations of the Western Coast.— Chonos and 
Chiloe groups, volcanic rocks of. — Concepcion. — Navidad. — 
Coquimbo. — Summary. — Age of the tertiary formations. — Lines 
of elevation. — Silicified wood. — Comparative ranges of the 
extinct and living mollusca on the West Coast of S. America. 
— Climate of liie tertiary period. — On the causes of the absence 
of recent conchiferous deposits on the coasts of S. America. — 
On the contemporaneous deposition and preservation of sedi- 
mentary formations 381 



CHAPTER VI. 

PLUTONIC AND METAMORPHIC ROCKS : — CLEAVAGE AND FOLIATION. 

Brazil, Bahia, gneiss with disjointed metamorphosed dikes. — Strike 
of foliation. — ^Rio de Janeiro, gneiss-granite, embedded frag- 
ment in, decomposition of. — La Plata, metamorphic and old 
volcanic rocks of. — S. Ventana. — Claystone porphyry formation 
of Patagonia ; singular metamorphic rocks ; pseudo-dikes. — 
Falkland Islands, palaeozoic fossils of. — Tierra del Fuego, clay- 
slate formation, cretaceous fossils of ; cleavage and fqliation ; 
form of land. — Chonos Archipelago, mica-schists, foliation dis- 
turbed by granitic axis ; dikes. — Chiloe. — Concepcion, dikes, 
successive formation of. — Centml and Northern Chile. — Con- 
cluding remarks on cleavage anafoliation. — Their close analogy 
and similar origin. — Stratification of metamorphic schists. — 
Foliation of intrusive rocks. — Relation of cleavage and folia- 
tion to the lines of tension during metamorphosis . . .416 



CHAPTER VII. 

CENTRAL CHILE : -STRUCTURE OF THE CORDILLERA. 

Central Chile. — Basal formations of the Cordillera. — Origin of 
the porphyritic claystone conglomerate. — Andesite. — Volcanic 
rocks. — Section of the Cordillera by the Peuquenes or Portillo 
Pass. — Great gypseous formation. — Peuquenes line ; thickness 
of strata, fossils of. — Portillo line. — Conglomerate, orthitic 
granite, mica-schist, volcanic rocks of. — Concluding remarks on 
the denudation and elevation of the Portillo line. — Section by 
the Cumbre, or Uspallata Pass. — Porphyries. — Gypseous strata. 
— Section near the Puente del Inca ; fossils of. — Great subsi- 
dence. — Intrusive porphyries. — Plain of Uspallata. — Section of 
the Uspallata chain. — Structure and nature of the strata. — 
Sihcified vertical trees. — Great subsidence. — Granitic rocks of 



CONTENTS. 

PAGE 

axis.— Concluding remarks on the Uspallata range ; origin 
subsequent to that of the main Cordillera ; two periods of 
subsidence ; comparison with the Portillo chain . , .441 



CHAPTER VIII. 

NORTHERN CHILE. — CONCLUSION. 

Section from Illapel to Combarbala ; gypseous formation with 
silicified wood. — Panuncillo. — Coquimbo ; mines of Arqueros ; 
section up valley ; fossils. — Guasco, fossils of — Copiapo, section 
up valley ; Las Amolanas, silicified wood. — Conglomerates, 
nature of former land, fossils, thickness of strata, great subsi- 
dence. — Valley of Despoblado, tufaceous fossils, deposit, com- 
plicated dislocations of — Relations between ancient orifices of 
eruption and subsequent axes of injection. — Iquique, Peru, 
fossils of, salt-deposits. — Metalliferous veins. — Summary on 
the porphyritic conglomerate and gypseous formations. — 
Great subsidence with partial elevations during the cretaceo- 
oolitic period. — On the elevation and structure of the Cordillera. 
— Recapitulation on the tertiary series. — Relations between 
movements of subsidence and volcanic action. — Pampean for- 
mation. — Recent elevatory movements. — Long-continued vol- 
canic action in the Cordillera. — Conclusion .... I479 



Index to " Coral-Reefs " 519 

Index to " Volcanic Islands " 528 

Index to " South American Observations " 535 



DESCRIPTION OF THE PLATES. 



PLATE I. 

Map showing the resemblance in form between barrier coral-reefs 
surrounding mountainous islands, and atolls or lagoon-islands. 

{Frontispiece?) 

In the several original surveys, from which the small plans on this 
plate have been reduced, the coral-reefs are engraved in very 
different styles. For the sake of uniformity, I have adopted the 
style used in the charts of the Chagos Archipelago, published by 
the East Indian Company, from the survey by Captain Moresby 
and Lieutenant Powell. The surface of the reef, which dries at 
low water, is represented by a surface with small crosses : the 
coral-islets on the reef are marked by small linear spaces, on 
which a few cocoa-nut trees, out of all proportion too large, have 
been introduced for the sake of clearness. The entire aitftular 
reef, which when surrounding an open expanse of water, forms an 
" atoll," and when surrounding one or more high islands, forms 
an encircling " barrier-reef," has a nearly uniform structure. The 
reefs in some of the original surveys are represented merely by a 
single line with crosses, so that their breadth is not given ; I have 
had such reefs engraved of the width usually attained by coral- 
reefs. I have not thought it worth while to introduce all those 
small and very numerous reefs, which occur within the lagoons of 
most atolls and within the lagoon-channels of most barrier-reefs, 
and which stand either isolated, or are attached to the shores of 
the reef or land. At Peros Banhos none of the lagoon-reefs rise 
to the surface of the water ; a few of them have been introduced, 
and are marked by plain dotted circles. A few of the deepest 
soundings are laid down within each reef ; they are in fathoms, of 
six English feet. 

Fig. I. — Vanikoro, situated in the western part of the South Pacific ; 
taken from the survey by Captain D'Urville in the Astrolabe ; the 
soundings on the southern side of the island, namely, from thirty 
to forty fathoms, are given from the voyage of the Chev. Dillon ; 
the other soundings are laid down from the survey by D'Urville ; 

b 



xviii DESCRIPTION OF THE PLATES. 

height of the summit of the island is 3,032 feet. The principal 
small detached reefs within the lagoon-channel have in this instance 
been represented. The southern shore of the island is narrowly 
fringed by a reef: if the engraver had carried this reef entirely 
round both islands, this figure would have served (by leaving out 
in imagination the barrier-reef) as a good specimen of an abruptly- 
sided island, surrounded by a reef of the fringing class. 

Fig. 2. — HoGOLEU, or Roug, in the Caroline Archipelago ; taken from 
the " Atlas of the Voyage of the Astrolabe" compiled from the sur- 
veys of Captains Duperrey and D'Urville ; the depth of the immense 
lagoon-like space within the reef is not known. 

Fig. 3. — Raiatea, in the Society Archipelago ; from the map given in 
the quarto edition of " Cook's First Voyage ; " it is probably not 
accurate. 

Fig. 4. — Bow, or Heyou atoll (or lagoon-island), in the Low Archi- 
pelago, from the survey by Captain Beechey, R.N. ; the lagoon is 
choked up with reefs, but the average greatest depth of about 
twenty fathoms, is given from the published account of the voyage. 

Fig. 5. — BoLABOLA, in the Society Archipelago, from the survey of 
Captain Duperrey in the Coquille : the soundings in this and the 
following figures have been altered from French feet to English 
fathoms; height of highest point of the island 4,026 feet. 

Fig. 6. — Maurua, in the Society Archipelago ; from the survey by 
Captain Duperrey in the Coquille: height of land ^bout eight 
hundred feet. 

Fig. 7. — PouYNiPETE, or Seniavine, in the Caroline Archipelago; from 
the survey by Admiral Lutke. 

Fig. 8, — ^Gambier Islands, in the southern part of the Low Archi- 
pelago ; from the survey by Captain Beechey ; height of highest 
island, 1,246 feet ; the islands are surrounded by extensive and 
irregular reefs ; the reef on the southern side is submerged. 

Fig. 9. — Peros Banhos atoll (or lagoon-island), in the Chagos group 
in the Indian Ocean ; from the survey by Captain Moresby and 
Lieutenant Powell ; not nearly all the small submerged reefs in 
the lagoon are represented ; the annular reef on the southern side 
is submerged. 

Fig. 10. — Keeling, or Cocos atoll (or lagoon-island), in the Indian 
Ocean ; from the survey by Captain Fitzroy ; the lagoon south of 
the dotted line is very shallow, and is left almost bare at low 
water; the i)art north of the line is choked up with irregular reefs. 
The aiuiular reef on the north-west side is broken, and blends into 
a shoal sandbank, on which the sea breaks. 



DESCRIPTION OF THE PLATES, xix 

PLATE II. 

Great Chagos Bank, New Caledonia, Menchikoff atoll, etc. ippp.p. 36.) 

Fig. I. — Great Chagos Bank, in the Indian Ocean; taken from the 
survey by Captain Moresby and Lieutenant Powell ; the parts 
which are shaded, with the exception of two or three islets on the 
western and northern sides, do not rise to the surface, but are sub- 
merged from four to ten fathoms ; the banks bounded by the 
dotted lines lie from fifteen to twenty fathoms beneath the surface, 
and are formed of sand ; the central space is of mud, and from 
thirty to fifty fathoms deep. 

Fig. 2. — A vertical section, on the same scale, in an eastern and western 
line across the Great Chagos Bank, given for the sake of exhibiting 
more clearly its structure. 

Fig. 3. — Menchikoff atoll (or lagoon-island), in the Marshall Archi- 
pelago, Northern Pacific Ocean; from Krusenstern's "Atlas of the 
Pacific ; " originally surveyed by Captain Hagemeister ; the depth 
within the lagoons is unknown. 

Fig. 4. — Mahlos Mahdoo atoll, together with Horsburgh atoll, in the 
Maldiva Archipelago ; from the survey by Captain Moresby and 
Lieutenant Powell ; the white spaces in the middle of the separate 
small reefs, both on the margin and in the middle part, are meant 
to represent little lagoons ; but it was found not possible to dis- 
tinguish them clearly from the small islets, which have been formed 
on these same small reefs ; many of the smaller reefs could not be 
introduced ; the nautical mark (-i-) over the figures 250 and 200, 
between Mahlos Mahdoo and Horsburgh atoll and Powell's island, 
signifies that soundings were not obtained at these depths. 

Fig. 5. — New Caledonia, in the western part of the Pacific; from 
Krusenstern's "Atlas," compiled from several surveys ; I have slightly 
altered the northern point of the reef, in accordance with the " Atlas 
of the Voyage of the Astrolabe." In Krusenstern's " Atlas," the reef 
is represented by a single line with crosses ; I have for the sake of 
uniformity added an interior line. 

Fig, 6.— Maldiva Archipelago, in the Indian Ocean ; from the survey 
by Captain Moresby and Lieutenant Powell. 

PLATE III. 

Map showing the distribution of coral-reefs and active volcanoes. 

((9//./. 89.) 

The principles, on which tliis map was coloured, are explained in the 
beginning of Chapter VI. ; and the authorities for each particular 
spot are detailed in the Appendix to Coral Reefs. The names 
printed in italics in the Index refer to the Appendix. 



XX DESCRIPTION OF PLATES. 

PLATE IV. 

The Island of Ascension. {0pp. p. i88.) 

PLATE V. 

Geological sections through the Cordilleras. {0pp. p. 441.) 

PLATE VI. 

Map of southern portion of South America. {End of Volume^ 



THE STRUCTURE AND DISTRIBUTION 
OF CORAL-REEFS. 



/ 



DEPARTMENT OF GEOLOGY 

LIBRARY 

PRESENTED BY 

Professor A. P. COLEMAN 
UNIVERSITY OF TORONTO 



CRITICAL INTRODUCTION. 



A SCIENTIFIC discovery is the outccme of an interesting 
process of evolution in the mind of its author. When we 
are able to detect the germs of thought in which such a discovery 
has originated, and to trace the successive stages of the reasoning 
by which the crude idea has developed into an epoch-making 
book, wc have the materials for reconstructing an important 
chapter of scientific history. Such a contribution to the story of 
the "making of science" may be furnished in respect to Darwin's 
famous theory of coral-reefs, and the clearly reasoned treatise in 
which it was first fully set forth. 

The subject of corals and coral-reefs is one concerning which 
much popular misconception has always prevailed. The mislead- 
ing comparison of coral-rock with the combs of bees and the 
nests of wasps is perhaps responsible for much of this misunder- 
standing ; one writer has indeed described a coral-reef as being 
" built by fishes by means of their teeth." Scarcely less mislead- 
ing, however, are the references we so frequently meet with, both 
in prose and verse, to the "skill," "industry," and "persever- 
ance " of the " coral-insect " in " building " his " home." As well 
might we praise men for their cleverness in making their own 
skeletons, and laud their assiduity in filling churchyards with the 
same. The polyps and other organisms, whose remains accumu- 
late to form a coral-reef, simply live and perform their natural 
functions, and then die, leaving behind them, in the natural course 
of events, the hard calcareous portions of their structures to add 
to the growing reef. 

While the forms of coral-reefs and coral-islands are sometimes 
very remarkable and worthy of attentive study, there is no ground, 
it need scarcely be added, for the suggestion that they afford 
proofs of design on the part of the living builders, or that, in the 



4 CRITICAL INTRODUCTION. 

words of Flinders, they constitute breastworks, defending the 
workshops from whence " infant colonies might be safely sent 
forth." 

It was not till the beginning of the present century that 
travellers like Beechey, Chamisso, Quoy and Gaimard, Moresby, 
Nelson, and others, began to collect accurate details concerning 
the forms and structure of coral-masses, and to make such obser- 
vations on the habits of reef-forming polyps, as might serve as a 
basis for safe reasoning concerning the origin of coral-reefs and 
islands. In the second volume of Lyell's "Principles of Geology," 
published in 1832, the final chapter gives an admirable summary 
of all that was then known on the subject. At that time, the 
ring-form of the atolls was almost universally regarded as a proof 
that they had grown up on submerged volcanic craters; and Lyell 
gave his powerful support to that theory. 

Charles Darwin was never tired of acknowledging his indebted- 
ness to Lyell. In dedicating to his friend the second edition of 
his "Naturalist's Voyage Round the World," Darwin writes that he 
does so " with grateful pleasure, as an acknowledgment that the 
chief part of whatever scientific merit this journal and the other 
works of the author may possess, has been derived from studying 
the v/ell-known and admirable ' Principles of Geology.' " 

The second volume of Lyell's " Principles " appeared after 
Darwin had left England ; but it was doubtless sent on to him 
without delay by his faithful friend and correspondent, Professor 
Henslow. It appears to have reached Darwin at a most oppor- 
tune moment, while, in fact, he was studying the striking evidences 
of slow and long-continued, but often interrupted movement on 
the west coast of South America. Darwin's acute mind could 
not fail to detect the weakness of the then prevalent theory con- 
cerning the origin of the ring-shaped atolls — and the difficulty 
which he found in accepting the volcanic theory, as an explanation 
of the phenomena of coral-reefs, is well set forth in his book. 

In an interesting fragment of autobiography, Darwin has given 
us a very clear account of the way in which the leading idea of 
the theory of coral-reefs originated in his mind ; he writes, " No 
other work of mine was begun in so deductive a spirit as this, for 
the whole theory was thought out on the west coast of South 
America, before I had seen a true coral-reef. I had therefore 
only to verify and extend my views by a careful examination of 
living reefs. But it should be observed that I had during the two 
previous years been incessantly attending to the effects on the 



CRITICAL INTRODUCTION. 5 

shores of South America of the in'-ermittent elevation of the land, 
together with the denudation and deposition of sediment. This 
necessarily led me to reflect much on the effects of subsidence, 
and it was easy to replace in imagination the continued deposition 
of sediment by the upward growth of corals. To do this was to 
form my theory of the formation of barrier-reefs and atolls." 

On her homeward voyage, the Beagle visited Tahiti, Australia, 
and some of the coral-islands in the Indian Ocean, and Darwin 
had an opportunity of testing and verifying the conclusion at 
which he had arrived by studying the statements of other 
observers. 

I well recollect a remarkable conversation I had with Darwin, 
shortly after the death of Lyell. With characteristic modesty, he 
told me that he never fully realised the importance of his theory 
of coral-reefs till he had an opportunity of discussing it with Lyell, 
shortly after the return of the Beagle. Lyell, on receiving from 
the lips of its author a sketch of the new theory, was so overcome 
with delight that he danced about and threw himself into the 
wildest contortions, as was his manner when excessively pleased. 
He wrote shortly afterwards to Darwin as follows : — " I could 
think of nothing for days after your lesson on coral-reefs, but of 
the tops of submerged continents. It is all true, but do not flatter 
yourself that you will be believed till you are growing bald like 
me, with hard work and vexation at the incredulity of the world." 
On May 24th, 1837, Lyell wrote to Sir John Herschel as 
follows : — " I am very full of Darwin's new theory of coral-islands, 
and have urged Whewell to make him read it at our next meeting 
I must give up my volcanic crater for ever, though it cost me a 
pang at first, for it accounted for so much." Dr. Whewell was 
president of the Geological Society at the time, and on May 31st, 
1837, Darwin read a paper entitled "On Certain Areas of Eleva- 
tion and Subsidence in the Pacific and Indian Oceans, as deduced 
from the Study of Coral Formations," an abstract of which appeared 
in the second volume of the Society's proceedings. 

It was about this time that Darwin, having settled himself in 
lodgings at Great Marlborough Street, commenced tlie writing ol 
his book on " Coral-Reefs." Many delays from ill-health and the 
interruption of other work, caused the progress to be slow, and 
his journal speaks of "recommencing" the subject in February 
1839, shortly after his marriage, and again in October of the same 
year. In July 1841, he states that he began once more "after 
more than thirteen months' interval," and the last proof-sheet of 



6 CRITICAL INTRODUCTION. 

the book was not corrected till May 6th, 1842. Darwin writes in 
his autobiography, " This book, though a small one, cost me 
twenty months of hard work, as I had to read every work on the 
islands of the Pacific, and to consult many charts." The task of 
elaborating and writing out his books was, with Darwin, always 
a very slow and laborious one ; but it is clear that in accomplish- 
ing the work now under consideration, there was a long and 
constant struggle with the lethargy and weakness resulting from 
the sad condition of his health at that time. 

Lyell's anticipation that the theory of coral-reefs would be slow 
in meeting with general acceptance was certainly not justified by 
the actual facts. On the contrary the new book was at once 
received with general assent among both geologists and zoologists, 
and even attracted a considerable amount of attention from the 
general public. 

It was not long before the coral-reef theory of Darwin found an 
able exponent and sturdy champion in the person of the great 
American naturalist, Professor James D. Dana. Two years after 
the return of the Beagle to England, the ships of the United States 
Exploring Expedition set sail upon their four years' cruise, under 
the com.mand of Captain Wilkes, and Dana was a member of the 
scientific staff. When, in 1839, the expedition arrived at Sydney, 
a newspaper paragraph was found which gave the American 
naturalist the first intimation of Darwin's new theory of the origin 
of atolls and barrier-reefs. Writing in 1872, Dana describes the 
effect produced on his mind by reading this passage: — "The 
paragraph threw a flood of light over the subject, and called forth 
feelings of peculiar satisfaction, and of gratefulness to Mr. Darwin, 
which still come up afresh whenever the subject of coral islands 
is mentioned. The Gambler Islands in the Paumotus, which 
gave him the key to the theory, I had not seen ; but on reaching 
the Feejees, six months later, in 1840, I found there similar facts 
on a still grander scale and of a more diversified character, so 
that I was afterward enabled to speak of his theory as established 
with more positivcness than he himself, in his philosophic caution, 
had been ready to adopt. His work on coral-reefs appeared in 
1842, when my report on the subject was already in manuscript. 
It showed that the conclusions on other points, which we had 
independently reached, were for the most part the same. The 
principal points of difference relate to the reason for the absence 
of corals from some coasts, and the evidence therefrom as to 
changes of level, and the distribution of the oceanic regions of 



CRITICAL INTRODUCTION. 7 

elevation and subsidence — topics which a wide range of travel 
over the Pacific brought directly and constantly to my attention." 

Among the Reports of the United States Exploring Expedition, 
two important works from the pen of Professor Dana made their 
appearance; — one on "Zoophytes," which treats at length on 
" Corals and Coral- Animals," and the other on " Coral-Reefs and 
Islands." In 1872, Dana prepared a work of a more popular cha- 
racter in which some of the chief results of his studies are described ; 
it bore the title of " Corals and Coral-Islands." Of this work, new 
and enlarged editions appeared in 1874 and 1890 in America, 
while two editions were published in this country in 1872 and 
1875. I^ ^11 these works their author, while maintaining an 
independent judgment on certain matters of detail, warmly defends 
the views of Darwin on all points essential to the theory. 

Another able exponent and illustrator of the theory of coral- 
reefs was found in Professor J. B. Jukes, who accompanied 
H.M.S. Fly, as naturalist, during the survey of the Great Barrier- 
Reef — in the years 1842 to 1846. Jukes, who was a man of great 
acuteness as well as independence of mind, concludes his account 
of the great Australian reefs with the following words : — " After 
seeing much of the Great Barrier-Reefs, and reflecting much 
upon them, and trying if it were possible by any means to evade 
the conclusions to which Mr. Darwin has come, I cannot help 
adding that his hypothesis is perfectly satisfactory to my mind, 
and rises beyond a mere hypothesis into the true theory of coral- 
reefs." 

As the result of the clear exposition of the subject by Darwin, 
Lyell, Dana, and Jukes, the theory of coral-reefs had, by the middle 
of the present century, commanded the almost universal assent of 
both biologists and geologists. In 1859 Baron von Richthofen 
brought forward new facts in its support, by showing that the 
existence of the thick masses of dolomitic limestone in the Tyrol 
could be best accounted for if they were regarded as of coralline 
origin and as being formed during a period of long continued 
subsidence. The same views were maintained by Professor 
Mojsisovics in his " Dolomut-riffe von Siidtirol und Venetien," 
which appeared in 1879. 

The first serious note of dissent to the generally accepted theory 
was heard in 1863, when a distinguished German naturalist, 
Dr. Karl Semper, declared that his study of the Pelew Islands 
showed that uninterrupted subsidence could not have been going 
on in that region. Dr. Scmper's objections were very carefully 



8 CRITICAL INTRODUCTION, 

considered by Mr. Darwin, and a reply to them appeared in the 
second and revised edition of his " Coral-Reefs," which was pub- 
lished in 1874. With characteristic frankness and freedom from 
prejudice, Darwin admitted that the facts brought forward by 
Dr. Semper proved that in certain specified cases, subsidence 
could not have played the chief part in originating the peculiar 
forms of the coral-islands. But while making this admission, he 
firmly maintained that exceptional cases, like those described in 
the Pelew Islands, were not sufficient to invalidate the theory of 
subsidence as applied to the widely spread atolls, encircling reefs, 
and barrier-reefs of the Pacific and Indian Oceans. It is worthy 
of note that to the end of his life Darwin maintained a friendly 
correspondence with Semper concerning the points on which they 
were at issue. 

After the appearance of Semper's work, Dr. J. J. Rein published 
an account of the Bermudas, in which he opposed the interpre- 
tation of the structure of the islands given by Nelson and other 
authors, and maintained that the facts observed in them are 
opposed to the views of Darwin. Although, so far as I am aware, 
Darwin had no opportunity of studying and considering these 
particular objections, it may be mentioned that two American 
geologists have since carefully re-examined the district — Professor 
W. N. Rice in 1884 and Professor A. Heilprin in 1889 — and they 
have independently arrived at the conclusion that Dr. Rein's 
objections cannot be maintained. 

The most serious opposition to Darwin's coral-reef theory, 
however, was that which developed itself after the return of 
H.M.S. Challenger from her famous voyage. Mr. John Murray, 
one of the staff of naturalists on board that vessel, propounded 
a new theory of coral-reefs, and maintained that the view that 
they were formed by subsidence was one that was no longer 
tenable ; these objections have been supported by Professor 
Alexander Agassiz in the United States, and by Dr. A. Geikie, 
and Dr. PI. B. Guppy in this country. 

Although Mr. Darwin did not live to bring out a third edition 
of his " Coral-Reefs," I know from several conversations with 
him that he had given the most patient and thoughtful considera- 
tion to Mr. Murray's paper on the subject. He admitted to me 
that had he known, when he wrote his work, of the abundant de- 
position of the remains of calcareous organisms on the sea floor, 
he might have regarded this cause as sufficient in a few cases-to 
raise tlv '^mniniis ()( mi]ni-tfT"^<vl volfnufv-s or other mountains 



CRITICAL INTRODUCTION, 9 

to a level at which reef-forming corals can commence to flourish. 
But he did not think that the admission that under certain 
favourable conditions, atolls might be thus formed without sub- 
sidence, necessitated an abandonment of his theory in the case 
of the innumerable examples of the kind which stud the Indian 
and Pacific Oceans. 

A letter written by Darwin to Professor Alexander Agassiz in 
May 1 88 1 shows exactly the attitude which careful consideration 
of the subject led him to maintain towards the theory propounded 
by Mr. Murray : — " You will have seen," he writes, " Mr. Murray's 
views on the formation of atolls and barrier-reefs. Before pub- 
lishing my book, I thought long over the same view, but only as 
far as ordinary marine organisms are concerned, for at that time 
little was known of the multitude of minute oceanic organisms. 
I rejected this view, as from the few dredgings made in the 
Beagle^ in the south temperate regions, I concluded that shells, 
the smaller corals, etc., decayed and were dissolved when not 
protected by the deposition of sediment, and sediment could not 
accumulate in the open ocean. Certainly, shells, etc., were in 
several cases completely rotten, and crumbled into mud between 
my fingers; but you will know whether this is in any degree 
common. I have expressly said that a bank at the proper depth 
would give rise to an atoll, which could not be distinguished from 
one formed during subsidence. I can, however, hardly believe in 
the existence of as many banks (there having been no subsidence) 
as there are atolls in the great oceans, within a reasonable depth, 
on which minute oceanic organisms could have accumulated to 
the depth of many hundred feet." 

Darwin's concluding words in the same letter written within 
a year of his death, are a striking proof of the candour and 
openness of mind which he preserved so well to the end, in this 
as in other controversies. 

" If I am wrong, the sooner I am knocked on the head and 
annihilated so much the better. It still seems to me a marvellous 
thing that there should not have been much, and long-continued, 
subsidence in the beds of the great oceans. I wish some doubly 
rich millionaire would take it into his head to have borings made 
in some of the Pacific and Indian atolls, and bring home cores 
for slicing from a depth of 500 or 600 feet." 

It is noteworthy that the objections to Darwin's theory have for 
the most part proceeded from zoologists, while those who have 
fully appreciated the geological aspect of the question, have been 



10 CRITICAL INTRODUCTION. 

the staunchest supporters of the theory of subsidence. The 
desirabiUty of such boring operations in atolls has been insisted 
upon by several geologists, and it may be hoped that before many 
years have passed away, Darwin's hopes may be realised, either 
with or without the intervention of the " doubly rich millionaire." 

Three years after the death of Darwin, the veteran Professor 
Dana re-entered the lists and contributed a powerful defence of 
the theory of subsidence in the form of a reply to an essay written 
by the ablest exponent of the anti-Darwinian views on this subject, 
Dr. A. Geikie. While pointing out that the Darwinian position 
had been to a great extent misunderstood by its opponents, he 
showed that the rival theory presented even greater difficulties 
than those which it professed to remove. 

During the last five years, the whole question of the origin of 
coral-reefs and islands has been re-opened, and a controversy 
has arisen, into which, unfortunately, acrimonious elements have 
been very unnecessarily introduced. Those who desire it, will find 
clear and impartial statements of the varied and often mutually 
destructive views put forward by different authors, in three works 
which have made their appearance within the last year, — " The 
Bermuda Islands," by Professor Angelo Heilprin ; '• Corals and 
Coral-islands," new edition by Professor J. D. Dana ; and the 
third edition of Darwin's " Coral-Reefs," with Notes and Appendix 
by Professor T. G. Bonney. 

Most readers will, I think, rise from the perusal of these works 
with the conviction that, while on certain points of detail it is 
clear that, through the want of knowledge concerning the action 
of marine organisms in the open ocean, Darwin was betrayed into 
some grave errors, yet the main foundations of his argument 
have not been seriously impaired by the new facts observed in 
the deep-sea researches, or by the severe criticism to which his 
theory has been subjected during the last ten years. On the 
other hand, I think it will appear that much misapprehension has 
been exhibited by some of Darwin's critics, as to what his views 
and arguments really were ; so that the reprint and wide circula- 
tion of the book in its original form is greatly to be desired, and 
cannot but be attended with advantage to all those who will have 
the fairness to acquaint themselves with Darwin's views at firit 
hand, before attempting to reply to them. 

John W. Judd. 



CORAL-REEFS. 



INTRODUCTION. 

THE object of this volume is to describe from my own observation 
and the works of others, the principal kinds of coral-reefs, more 
especially those occurring in the open ocean, and to explain the origin 
of their peculiar forms. I do not here treat of the polypifers, which 
construct these vast works, except so far as relates to their distribution, 
and to the conditions favourable to their vigorous growth. Without 
any distinct intention to classify coral-reefs, most voyagers have spoken 
of them under the following heads : " lagoon-islands," or " atolls," 
"barrier " or " encircling reefs," and " fringing " or " shore-reefs." The 
lagoon-islands have received much the most attention ; and it is not 
surprising, for every one must be struck with astonishment, when he 
first beholds one of these vast rings of coral-rock, often many leagues 
in diameter, here and there surmounted by a low verdant island with 
dazzling white shores, bathed on the outside by the foaming breakers 
of the ocean, and on the inside surrounding a calm expanse of water, 
which from reflection, is of a bright but pale green colour. The 
naturalist will feel this astonishment more deeply after having examined 
the soft and almost gelatinous bodies of these apparently insignificant 
creatures, and when he knows that the solid reef increases only on the 
outer edge, which day and night is lashed by the breakers of an ocean 
never at rest. Well did Francois Pyrard de Laval, in the year 1605, 
exclaim, " C'est une merueille de voirchacun de ces atollons, enuironn6 
d'un grand banc de pierre tout autour, n'y ayant point d'artifice humain." 
The accompanying sketch of Whitsunday Island, in the South Pacific, 
taken from Captain Beechey's admirable "Voyage," although excellent 
of its kind, gives but a faint idea of the singular aspect of one of these 
lagoon-islands. 

Whitsunday Island is of small size, and the whole circle has been 
converted into land, which is a comparatively rare circumstance. As 
the reef of a lagoon-island generally supports many separate small 
islands, the word "island," applied to the whole, is often the cause of 
confusion; hence I have invariably used in this volume the term "atoll," 
which is tjic name given to these cimilnr rrroups of coral-islets by their 



12 CORAL-REEFS, 

inhabitants in the Indian Ocean, and is synonymous with "lagoon- 
island." 

Barrier-reefs, when encircling small islands, have been comparatively 
little noticed by voyagers ; but they well deserve attention. In their 
structure they are little less marvellous than atolls, and they give a 
singular and most picturesque character to the scenery of the islands 
they surround. In the accompanying sketch, taken from the " Voyage of 




the Coquille," the reef is seen from within, from one of the high peaks 
of the island of Bolabola.* Here, as in Whitsunday Island, the whole 
of that part of the reef which is visible is converted into land. This is 
a circumstance of rare occurrence ; more usually a snow-white line of 
great breakers, with here and there an islet crowned by cocoa-nut 
trees, separates the smooth waters of the lagoon-like channel from the 
waves of the open sea. The barrier-reefs of Austraha and of New 




Caledonia, owing to their enormous dimensions, have excited much 
attention : in structure and form they resemble those encircling many 
of the smaller islands in the Pacific Ocean. 

With respect to fringing, or siiore-reefs, there is little in their struc- 
ture which needs explanation ; and their name expresses their compara- 

* I have taken the liberty of simplifying the foreground, and leaving out a 
mountainous island in the far distance. 



INTRODUCTION. 13 

lively small extension. They differ from barrier-reefs in not lying so 
far from the shore, and in not having within a broad channel of deep 
water. Keefs also occur around submerged banks of sediment and ot 
worn-down rock ; and others are scattered quite irregularly where the 
sea is very shallow ; these in most respects are allied to those of the 
fringing class, but they are of comparatively little interest 

I have given a separate chapter to each of the above classes, and 
have desci ibed some one reef or island, on which I possessed most in- 
formation, as typical ; and have afterwards compared it with others 01 
a like kind. Although this classification is useful from being obvious, 
and from including most of the coral-reefs existing in the op'^'n sea, it 
admits of a more fundamental division into barrier and atoll-formed 
reefs on the one hand, where there is a great apparent difficulty with 
respect to the foundation on which they must hrst have grown ; and 
into fringing-reefs on the other, where, owing to the nature of the slope 
of the adjoining land, there is no such difficulty. The two blue tints 
and the red colour * on the map (Plate III.), represent this main divi- 
sion, as explained in the beginning of the last chapter. In the Appendix, 
every existing coral-reef, except some on the coast of Brazil not included 
in the map, is briefly described in geographical order, as far as I 
possessed information ; and any particular spot may be found by con- 
sulting the Index. 

Several theories have been advanced to explain the origin of atolls 
or lagoon-islands, but scarcely one to account for barrier-reefs. From 
the limited depths at which reef-building polypifers can flourish, taken 
into consideration with certain other circumstances, we are compelled 
to conclude, as it will be seen, that both in atolls and barrier-reefs, the 
foundation on which the coral was primarily attached, has subsided ; 
and that during this downward movement, the reefs have grown 
upwards. This conclusion, it will be further seen, explains most satis- 
factorily the outline and general form of atolls and barrier-reefs, and 
likewise certain peculiarities in their structure. The distribution, also, 
of the different kinds of coral-reefs, and their position with relation to 
the areas of recent elevation, and to the points subject to volcanic 
eruptions, fully accord with this theory of their origin, f 

* Replaced by numbers in this edition. 

f A brief, account of my views on coral formations, now published in my 
Journal of Researches, was read May 31st, 1837, before the Geological 
Society, and an abstract has appeared in the Proceedings. 



CHAPTER I. 

ATOLLS OR LAGOON-ISLANDS. 

Sectiojt I. — Keeling Atoll. 

Corals on the outer margin. — Zone of Nulliporae exterior reef. — Islets — 
Coral-conglomerate.— Lagoon calcareous sediment. — Scari and Holo- 
thuriae subsisting on corals. — Changes in the condition of the reefs and 
islets. — Probable subsidence of the atoll. — Future state of the lagoon. 

Keeling or Cocos atoll is situated in the Indian Ocean, in 12° 5' S., 
and longitude 90° 5 5' E. : a reduced chart of it from the survey of Captain 
Fitzroy and the Officers of H.M.S. Beagle, is given in Plate L, Fig. 10. 
The greatest width of this atoll is nine miles and a half. Its structure 
is in most respects characteristic of the class to which it belongs, with 
the exception of the shallowness of the lagoon. The accompanying 
woodcut represents a vertical section, supposed to be drawn at low 
water from the outer coast across one of the low islets (one being taken 
of average dimensions) to within the lagoon. 




A.— Level of the sea at low water : where the letter A is placed, the depth is twenty- 
five fathoms, and the distance rather more than one hundred and filty yards from the 
edge of the reef. 

B.— Outer edge of that flat part of the reef, which dries at low water : the edge 
either consists of a convex mound, as represented, or of rugged points, like those a 
little farther seaward, beneath the water. 

C. — A flat of coral-rock, covered at high water. 

D.— A low projecting ledge of brecciated coral-rock washed by the waves at high 
water. 

E.— A slope of loose fragments, reached by the sea only during gales : the upper 
part, which is from six to twelve feet high, is clothed With vegetation. The surface of 
the islet gently slopes to the lagoon. 

F.— Level ot the lagoon at low water. jr 

The section is true to the scale in a horizontal line, but it could not 
be made so in a vertical one, as the average greatest height of the 
land is only between six and twelve feet above high-water mark. 



1 6 KEELING ATOLL. 

I will describe the section, commencing with the outer margin. I must 
first observe that the reef-building polypifers, not being tidal animals, 
require to be constantly submerged or washed by the breakers. I was 
assured by Mr. Liesk, a very intelligent resident on th°se islands, as 
well as by some chiefs at Tahiti (Otaheite), that an exposure to the 
rays of the sun for a very short time invariably causes their destruction. 
Hence it is possible only under the most favourable circumstances, 
afforded by an unusually low tide and smooth water, to reach the outer 
margin, where the coral is alive. I succeeded only twice in gaining 
this part, and found it almost entirely composed of a living Porites, 
which forms great irregularly rounded masses (like those of an Astraea, 
but larger) from four to eight feet broad, and little less in thickness. 
These mounds are separated from each other by narrow crooked 
channels, about six feet deep, most of which intersect the line of reef at 
right angles. On the furthest mound, which I was able to reach by 
the aid of a leaping-pole, and over which the sea broke with some 
violence, although the day was quite calm and the tide low, the polypi- 
fers in the uppermost cells were all dead, but between three and four 
inches lower down on its side they were living, and formed a projecting 
border round the upper and dead surface. The coral being thus 
checked in its upward growth, extends laterally, and hence most of the 
masses, especially those a little further inwards, had broad flat dead 
summits. On the other hand I could see, during the recoil of the 
breakers, that a few yards further seaward, the whole convex surface of 
the Porites was alive ; so that the point where we were standing was 
almost on the exact upward and shoreward limit of existence of those 
corals which form the outer margin of the reef. We shall presently 
see that there are other organic productions, fitted to bear a somewhat 
longer, exposure to the air and sun 

Next, but much inferior in importance to the Porites, is the Millepora 
complanata* 

It grows in thick vertical plates, intersecting each other at various 
angles, and forms an exceedingly strong honeycombed mass, which 
generally affects n circular form, the marginal plates alone being alive. 
Between these plates and in the protected crevices on the reef, a multitude 
of branching zoophytes and other productions flourish, but the Porites 
and Millepora alone seem able to resist the fury of the breakers 
on its upper and outer edge : at the depth of a lew fathoms other 
kinds of stony corals live. Mr. Liesk, who was intimately acquainted 
with every part of this reef, and likewise with that of North Keeling 
atoll, assured me that these corals invariably compose the outer margin. 
The lagoon is inhabited by quite a distinct set of corals, generally 
brittle and thinly branched; but a Porites, apparently of the same 
species with that on the outside, is found there, although it does not 
seem to thrive, and certainly does not attain the thousandth part in 
bulk of the masses opposed to the breakers. 

* This Millepora (Palmipora of Blainville), as well as the M. alcicomis, 
possesses the singular property of stinging the skin where it is delicate, as on 
the face and arm. 



CORALS ON THE OUTER MARGIN. 17 

The woodcut shows the form of the bottom off the reef : the water 
deepens for a space between one and two hundred yards wide, very 
gradually to twenty-five fathoms (A in section), beyond which the sides 
plunge into the unfathomable ocean at an angle of 45°.* To the 
depth of ten or twelve fathoms the bottom is exceedingly rugged, and 
seems formed of great masses of living coral, similar to those on the 
margin. The arming of the lead here invariably came up quite clean, 
but deeply indented, and chains and anchors which were lowered, in 
the hopes of tearing up the coral, were broken. Many small fragments, 
however, of Millepora alcicornis were brought up ; and on the arming 
from an eight-fathom cast, there was a perfect impression of an Astraea, 
apparently alive. I examined the rolled fragments cast on the beach 
during gales, in order further to ascertain what corals grew outside the 
reef. The fragments consisted of many kinds, of which the Porites 
already mentioned and a Madrepora, apparently the M. corymbosa, were 
the most abundant. As I searched in vain in the hollows on the reef 
and in the lagoon, for a living specimen of this Madrepore, I conclude 
that it is confined to a zone outside, and beneath the surface, where it 
must be very abundant. Fragments of the Alillepora alcicornis and of 
an Astraea were also numerous ; the former is found, but not in propor- 
tionate numbers, in the hollows on the reef; but the Astraea I did not 
see living. Hence we liiay infer, that these are the kinds of coral which 
form the rugged sloping surface (represented in the woodcut by an un- 
even line), round and beneath the external margin. Between twelve and 
twenty fathoms the arming came up an equal number of times smoothed 
with sand, and indented with coral : an anchor and lead were lost at 
tiie respective depths of thirteen and sixteen fathoms. Out of twenty- 
five soundings taken at a greater depth than twenty fathoms, every one 
showed that the bottom was covered w^ith sand ; whereas, at a less depth 
than twelve fathoms, every sounding showed that it was exceedingly 
rugged, and free from all extraneous particles. Two isoundings were 
obtained at the depth of 360 fathoms, and several between two hundred 
and three hundred fathoms. The sand brought up from these depths 
consisted of finely triturated fragments of stony zoophytes, but not, as 
far as I could distinguish, of a particle of any lamelliform genus: 
fragments of shells were rare. 

At a distance of 2,200 yards from the breakers. Captain Fitzroy 
found no bottom with a line of 7,200 feet in length ; hence the sub- 
marine slope of this coral formation is steeper than that of any volcanic 
cone. Off the mouth of the lagoon, and likewise off the northern point 
of the atoll, where the currents act violently, the inclination, owing to 
the accumulation of sediment, is less. As the arming of the lead from 

* The soundings from which this section is laid down were taken with 
great care by Captain Fitzroy himself. He used a bell-shaped lead, having 
a diameter of four inches, and the armings each time were cut off and 
brought on board for me to examine. The arming is a preparation of tallow, 
placed in the concavity at the bottom of the lead. Sand, and even small 
fragments of rock, will adhere to it ; and if the bottom be of rock it brings up 
an exact impression of its surface. 

2 



i8 KEELING ATOLL. 

all the greater depths showed a smooth sandy bottom, I at first con- 
cluded that the whole consisted of a vast conical pile of calcareous 
sand, but the sudden increase of depth at some points, and the 
circumstance of the line having been cut, as if rubbed, when between 
five hundred and six hundred fathoms were out, indicate the probable 
existence of submarine cliffs. 

On the margin of the reef, close within the line where the upper 
surface of the Porites and of the Millepora is dead, three species of 
Nullipora flourish. One grows in thin sheets, like a lichen on old 
trees ; the second in stony knobs, as thick as a man's finger, radiating 
from a common centre ; and the third, which is less common, in a 
moss-like reticulation of thin, but perfectly rigid branches.* The three 
species occur either separately or mingled together ; and they form by 
their successive growth a layer two or three feet in thickness, which in 
some cases is hard, but where formed of the lichen-like kind, readily 
yields an impression to the hammer : the surface is of a reddish colour. 
These Nulliporae, although able to exist above the limit of true corals, 
seem to require to be bathed during the greater part of each tide by 
breaking water, for they are not found in any abundance in the pro- 
tected hollows on the back part of the reef, where they might be 
immersed either during the whole or an equal proportional time of each 
tide. It is remarkable that organic productions of such extreme 
simplicity, for the Nulliporae undoubtedly belong to one of the lowest 
classes of the vegetable kingdom, should be limited to a zone so 
peculiarly circumstanced. Hence the layer composed by their growth 
merely fringes the reef for a space of about twenty yards in width, 
either under the form of separate mammillated projections, where the 
outer masses of coral are separate, or, more commonly, where the 
corals are united into a solid margin, as a continuous smooth convex 
mound (B in woodcut), like an artificial breakwater. Both the mound 
and mammillated projections stand about three feet higher than any 
other part of the reef, by which term I do not include the islets, formed 
by the accumulation of rolled fragments. We shall hereafter see that 
other coral reefs are protected by a similar thick growth of Nulliporae 
on the outer margin, the part most exposed to the breakers, and this 
must effectually aid in preserving it from being worn down. 

The woodcut represents a section across one of the islets on the 
reef, but if all that part which is above the level of C were removed, 
the section would be that of the simple reef, as it occurs where no islet 
has been formed. It is this reef which essentially forms the atoll. 
It is a ring, enclosing the lagoon on all sides except at the northern 
end, where there are two open spaces, through one of which ships can 
enter. The reef varies in width from two hundred and fifty to five 

* This last species is of a beautiful bright peach-blossom colour. Its 
branches are about as thick as crow-quills ; they are slightly flattened and 
knobbed at the extremities. The extremities only are alive and brightly 
coloured. The two other species are of a dirty purplish-white. The second 
species is extremely hard ; its short knob-like branches are cylindrical, and 
do not grow thicker at their extremities. 



ISLETS ON THE KEEP. 19 

hundred yards ; its surface is level, or very slightly inclined towards 
the lagoon, and at high tide the sea breaks entirely over it : the water 
at low tide thrown by the breakers on the reef, is carried by the many 
narrow and shoal gullies or channels on its surface, into the lagoon : 
a return stream sets out of the lagoon through the main entrance. 
The most frequent coral in the hollows on the reef is Pocillopora ver- 
rucosa, which grows in short sinuous plates, or branches, and when 
alive is of a beautiful pale lake-red : a Madrepora, closely allied or 
identical with M. pocillifera, is also common. As soon as an islet is 
formed, and the waves are prevented breaking entirely over the reef, 
the channels and hollows in it become filled up with cemented frag- 
ments, and its surface is converted into a hard smooth floor (C of 
woodcut), like an artificial one of freestone. This flat surface varies 
in width from one hundred to two hundred, or even three hundred 
yards, and is strewed with a few large fragments of coral torn up 
during gales : it is uncovered only at low water. I could with difficulty, 
and only by the aid of a chisel, procure chips of rock from its surface, 
and therefore could not ascertain how much of it is formed by the 
aggregation of detritus, and how much by the outward growth of 
mounds of corals, similar to those now living on the margin. Nothing 
can be more singular than the appearance at low tide of this "flat" of 
naked stone, especially where it is externally bounded by the smooth 
convex mound of Nulliporae, appearing like a breakwater built to resist 
the waves, which are constantly throwing over it sheets of foaming 
water. The characteristic appearance of this "flat" is shown in the 
foregoing woodcut of Whitsunday atoll. 

The islets on the reef are first formed between two hundred and 
three hundred yards from its outer edge, through the accumulation of 
a pile of fragments, thrown together by some unusually strong gale. 
Their ordinary width is under a quarter of a mile, and their length 
varies from a few yards to several miles. Those on the south-east and 
windward side of the atoll, increase solely by the addition of fragments 
on their outer side ; hence the loose blocks of coral, of which their 
surface is composed, as well as the shells mingled with them, almost 
exclusively consist of those kinds which live on the outer coast. The 
highest part of the islets (excepting hillocks of blown sand, some of 
which are thirty feet high), is close to the outer beach (E of the wood- 
cut), and averages from six to ten feet above ordinary high-water mark. 
From the outer beach the surface slopes gently to the shores of the 
lagoon, which no doubt has been caused by the breakers the further 
they have rolled over the reef, having had less power to throw up 
fragments. The little waves of the lagoon heap up sand and fragments 
of thinly-branched corals on the inner side of the islets on the leeward 
side of the atoll ; and these islets are broader than those to windward, 
some being even eight hundred yards in width ; but the land thus 
added is very low. The fragments beneath the surface are cemented 
into a solid mass, which is exposed as a ledge (D of the woodcut), 
projecting some yards in front of the outer shore and from two to four 
feet high. This ledge is just reached by the waves at ordinary high- 



20 KEELING ATOLL 

water: it extends in front of all the islets, and everywhere has a 
water-worn and scooped appearance. The fragments of coral which 
are occasionally cast on the " flat " are during gales of unusual violence 
swept together on the beach, where the waves each day at high-water 
tend to remove and gradually wear them down ; but the lower fragments 
having become firmly cemented together by the percolation of calcareous 
matter, resist the daily tides longer, and hence project as a ledge. The 
cemented mass is generally of a white colour, but in some few parts 
reddish from ferruginous matter ; it is very hard, and is sonorous 
under the hammer ; it is obscurely divided by seams, dipping at a small 
angle seaward ; it consists of fragments of the corals which grow on 
the outer margin, some quite and others partially rounded, some small 
. and others between two and three feet across ; and of masses of 
previously formed conglomerate, torn up, rounded, and re-cemented ; 
or it consists of a calcareous sandstone, entirely composed of rounded 
particles, generally almost blended together, of shells, corals, the spines 
of echini, and other such organic bodies ; rocks, of this latter kind, 
occur on many shores, where there are no coral reefs. The structure 
of the coral in the conglomerate has generally been much obscured by 
the infiltration of spathose calcareous matter ; and I collected a very 
interesting series, beginning with fragments of unaltered coral, and 
ending with others, where it was impossible to discover with the naked 
eye any trace of organic structure. In some specimens I was unable, 
even with the aid of a lens, and by wetting them, to distinguish the 
boundaries of the altered coral and spathose limestone. Many even of 
the blocks of coral lying loose on the beach, had their central parts 
altered and infiltrated. 

The lagoon alone remains to be described ; it is much shallower than 
that of most atolls of considerable size. The southern part is almost 
filled up with banks of mud and fields of coral, both dead and alive ; 
but there are considerable spaces, between three and four fathoms, 
and smaller basins, from eight to ten fathoms deep. Probably about 
half its area consists of sediment, and half of coral-reefs. The corals 
composing these reefs have a very different aspect from those on 
the outside ; they are very numerous in kind, and most of them are 
thinly branched. Meandrina, however, lives in the lagoon, and great 
rounded masses of this coral are numerous, lying quite or almost 
loose on the bottom. The other commonest kinds consist of three 
closely allied species of true Madrepora in thin branches ; of Seriata- 
pora subulata ; two species of Porites* with cylindrical branches, 
one of which forms circular clumps, with the exterior branches only 
alive ; and lastly, a coral something like an Explanaria, but with stars 
on both surfaces, growing in thin, brittle, stony, foliaceous expansions, 
especially in the deeper basins of the lagoon. The reefs on which 

* This Porites has somewhat the habit of P. clavaria, but the branches are 
not knobbed at their ends. "When alive it is of a yellow colour, but after 
having been washed in fresh water and placed to dry, a jet-black sHmy 
substance exuded from the entire surface, so that the specimen now appears 
»s if it had been dipped in ink. 



LAGOON-REEFS, 21 

these corals grow are very irregular in form, are full of cavities, and 
have not a solid flat surface of dead rock, like that surrounding the 
lagoon ; nor can they be nearly -so hard, for the inhabitants made with 
crowbars a channel of considerable length through these reefs, in whicli 
a schooner, built on the S.E. islet, was floated out. It is a very inter- 
esting circumstance, pointed out to us by Mr. Liesk, that this channel, 
although made less than ten years before our visit, was then, as we saw, 
almost choked up with living coral, so that fresh excavations would be 
absolutely necessary to allow another vessel to pass through it. 

The sediment from the deepest parts in the lagoon, when wet, ap- 
peared chalky, but when dry, like very fine sand. Large soft banks of 
similar, but even finer grained mud, occur on the S.E. shore of the 
lagoon, afi'ording a thick growth of a Fucus, on which turtle feed : this 
mud, although discoloured by vegetable matter, appears from its entire 
solution in acids to be purely calcareous. I have seen in the Museum 
of the Geological Society, a similar but more remarkable substance, 
brought by Lieutenant Nelson from the reefs of Bermuda, which, when 
shown to several experienced geologists, was mistaken by them for true 
chalk. On the outside of the reef much sediment must be formed by 
the action of the surf on the rolled fragments of coral ; but in the calm 
waters of the lagoon, this can take place only in a small degree. There 
are, however, other and unexpected agents at work here : large shoals 
of two species of Scarus, one inhabiting the surf outside the reef and 
the other the lagoon, subsist entirely, as I was assured by Mr. Liesk, 
the intelligent resident before referred to, by browsing on the living 
polypifers. I opened several of these fish, which are very numerous 
and of considerable size, and I found their intestines distended by small 
pieces of coral, and finely ground calcareous matter. This must daily 
pass from them as the finest sediment ; much also must be produced 
by the infinitely numerous vermiform and molluscous animals, which 
make cavities in almost every block of coral. Dr. J. Allan, of Forres, 
who has enjoyed the best means of observation, informs me in a letter 
that the Holothuriae (a family of Radiata) subsist on living coral ; and 
the singular structure of bone within the anterior extremity of their 
bodies, certainly appears well adapted for this purpose. The number 
of the species of Holothuria, and of the individuals which swarm on 
every part of these coral-reefs, is extraordinarily great ; and many ship- 
loads are annually freighted, as is well known, for China with the 
trepang, which is a species of this genus. The amount of coral yearly 
consumed, and ground down into the finest mud, by these several 
creatures, and probably by many other kinds, must be immense. These 
facts are, however, of more importance in another point of view, as 
showing us that there are living checks to the growth of coral-reefs, and 
that the almost universal law of "consumed and be consumed," holds 
good even with the polypifers forming those massive bulwarks, whicli 
are able to withstand the force of the open ocean. 

Considering that Keeling atoll, like other coral formations, has been 
entirely formed by the growth of organic beings, and the accumulation 
of their detritus, one is naturally led to inquire how long it has 



22 KEELING ATOLL. 

continued, and how long it is likely to continue, in its present state. 
Mr. Liesk informed me that he had seen an old chart in which the 
present long island on the S.E. side was divided by several channels 
into as many islets ; and he assures me that the channels can still be 
distinguished by the smaller size of the trees on them. On several 
islets, also, I observed that only young cocoa-nut trees were growing on 
the extremities ; and that older and taller trees rose in regular succes- 
sion behind them ; which shows that these islets have very lately 
increased in length. In the upper and south-eastern part of the lagoon, 
I was much surprised by finding an irregular field of at least a mile 
square of branching corals, still upright, but entirely dead. They 
consisted of the species already mentioned ; they were of a brown 
colour, and so rotten, that in trying to stand on them I sank halfway 
up the leg, as if through decayed brushwood. The tops of the branches 
were barely covered by water at the time of lowest tide. Several facts 
having led me to disbelieve in any elevation of the whole atoll, I was at 
first unable to imagine what cause could have killed so large a field of 
coral. Upon reflection, however, it appeared to me that the closing up 
of the above-mentioned channels would be a sufficient cause ; for before 
this, a strong breeze by forcing water through them into the head of the 
lagoon, would tend to raise its level. But now this cannot happen, and 
the inhabitants observe that the tide rises to a less height, during a high 
S.E. wind, at the head than at the mouth of the lagoon. The corals, 
which, under the former condition of things, had attained the utmost 
possible limit of upward growth, would thus occasionally be exposed 
for a short time to the sun, and be killed. 

Besides the increase of dry land, indicated by the foregoing facts, the 
exterior solid reef appears to have grown outwards. On the western 
side of the atoll, the "flat" lying between the margin of the reef and 
the beach, is very wide ; and in front of the regular beach with its 
conglomerate basis, there is, in most parts, a bed of sand and loose 
fragments with trees growing out of it, which apparently is not reached 
even by the spray at high water. It is evident some change has taken 
place since the waves formed the inner bsach ; that they formerly beat 
against it with violence was evident, from a remarkably thick and water- 
worn point of conglomerate at one spot, now protected by vegetation 
and a bank of sand ; that they beat against it in the same peculiar 
manner in which the swell from windward now obliquely curls round 
the margin of the reef, was evident from the conglomerate having been 
worn into a point projecting from the beach in a similarly oblique 
manner. This retreat in the line of action of the breakers might result, 
cither from the surface of the reef in front of the islets having been 
submerged at one time, and afterward having grown upwards, or from 
the mounds of coral on the margin having continued to grow outwards. 
That an outward growth of this part is in process, can hardly be doubted 
from the fact already mentioned of the mounds of Porites with their 
summits apparently lately killed, and their sides only three or four 
inches lower down thickened by a fresh layer of living coral. But there 
is a difficulty on this supposition which I must not pass over. If thq 



DECREASE OF THE ISLETS. 23 

whole, or a large part of the "flat," had been formed by the outward 
growth of the margin, each successive margin would naturally have 
been coated by the Nulliporae, and so much of the surface would have 
been of equal height with the existing zone of living Nulliporae : this is 
not the case, as may be seen in the woodcut. It is, however, evident 
from the abraded state of the "flat," with its original inequalities filled 
up, that its surface has been much modified ; and it is possible that the 
hinder portions of the zone of Nulliporae, perishing as the reef grows 
outwards, might be worn down by the surf. If this has not taken place, 
the reef can in no part have increased outwards in breadth since its 
formation, or at least since the Nulliporae formed the convex mound on 
its margin ; for the zone thus formed, and which stands between two 
and three feet above the other parts of the reef, is nowhere much above 
twenty yards in width. 

Thus far we have considered facts, which indicate, with more or less 
probability, the increase of the atoll in its different parts : there are 
others having an opposite tendency. On the south-east side, Lieutenant 
Sulivan, to whose kindness I am indebted for many interesting observa- 
tions, found the conglomerate projecting on the reef nearly fifty yards 
in front of the beach : we may infer from what we see in all other parts 
of the atoll, that the conglomerate was not originally so much exposed, 
but formed the base of an islet, the front and upper part of which has 
since been swept away. The degree to which the conglomerate, round 
nearly the whole atoll, has been scooped, broken up, and the fragments 
cast on the beach, is certainly very surprising, even on the view that it 
is the office of occasional gales to pile up fragments, and of the daily 
tides to wear them away. On the western side, also, of the atoll, where 
I have described a bed of sand and fragments with trees growing out 
of it, in front of an old beach, it struck both Lieutenant Sulivan and 
myself, from the manner in which the trees were being washed down, 
that the surf had lately recommenced an attack on this fine, of coast. 
Appearances indicating a slight encroachment of the water on the land, 
are plainer within the lagoon : I noticed in several places, both on its 
windward and leeward shores, old cocoa-nut trees falling with their 
roots undermined, and the rotten stumps of others on the beach, where 
the inhabitants assured us the cocoa-nut could not now grow. Captain 
Fitzroy pointed out to me, near the settlement, the fovmdation posts of 
a shed, now washed by every tide, but which the inhabitants stated, 
had seven years before stood above high watermark. In the calm 
waters of the lagoon, directly connected with a great, and therefore 
stable ocean, it seems very improbable that a change in the currents, 
sufficiently great to cause the water to eat into the land on all sides, 
should have taken place within a limited period. From these con- 
siderations I inferred, that probably the atoll had lately subsided to a 
small amount ; and this inference was strengthened by the circumstance, 
that in 1834, two years before our visit, the island had been shaken by 
a severe earthquake, and by two slighter ones during the ton previous 
years. If, during these subterranean disturbances, the atoll did subside, 
the downward movement must have been very small, as we must cou- 



24 ATOLLS, 

elude from the fields of dead coral still lipping the surface of the lagoon, 
and from the breakers on the western shore not having yet regained the 
line of their former action. The subsidence must, also, have been 
preceded by a long period of rest, during which the islets extended to 
their present size, and the living margin of the reef grew either upwards, 
or as I believe outwards, to its present distance from the beach. 

Whether this view be correct or not, the above facts are worthy of 
attention, as showing how severe a struggle is in progress on these low 
coral formations between the two nicely balanced powers of land and 
water. "With respect to the future state of Keeling atoll, if left undis- 
turbed, we can see that the islets may still extend in length ; but as 
they cannot resist the surf until broken by rolling over a wide space, 
their increase in breadth must depend on the increasing breadth of the 
reef; and this must be limited by the steepness of the submarine flanks, 
which can be added to only by sediment derived from the wear and 
tear of the coral. From the rapid growth of the coral in the channel 
cut for the schooner, and from the several agents at work in producins^ 
fine sediment, it might be thought that the lagoon would necessarily 
become quickly filled up. Some of this sediment, however, is trans- 
ported into the open sea, as appears from the soundings off the mouth 
of the lagoon, instead of being deposited within it. The deposition, 
moreover, of sediment, checks the growth of coral-reefs, so that these 
two agencies cannot act together with full effect in filling it up. We 
know so little of the habits of the many different species of corals, 
which form the lagoon-reefs, that we have no more reasons for sup- 
posing that their whole surface would grow up as quickly as the coral 
did in the schooner-channel, than for supposing that the whole surface 
of a peat-moss would increase as quickly as parts are known to do 
in holes, where the peat has been cut away. These agencies, never- 
theless, tend to fill up the lagoon ; but in proportion as it becomes 
shallower, so must the polypifers be subject to many injurious agencies, 
such as impure water and loss of food. For instance, Mr. Liesk 
informed me, that some years before our visit unusually heavy rain 
killed nearly all the fish in the lagoon, and probably the same cause 
would likewise injure the corals. The reefs also, it must be remem- 
bered, cannot possibly rise above the level of the lowest spring-tide, 
so that the final conversion of the lagoon into land must be due to the 
accumulation of sediment ; and in the midst of the clear water of the 
ocean, and with no surrounding high land, this process must be exceed- 
ingly slow. 

Section II. 

General form and size of atolls, their reefs and islets. — External slope. — Zone 
of Nulliporae. — Conglomerate. — Depth of lagoons. — Sediment. — Reefs 
submerged wholly or in part. — Breaches in the reef. — Ledge-formed 
shores round certain lagoons. — Conversion of lagoons into land. 

I will here give a sketch of the general form and structure of the 
many atolls and atoll-formed reefs which occur in the Pacific and 
Indian Oceans, comparing them with Keeling atoll. The Maldiva atolls 



ISLETS ON THE REEF 25 

and the Great Chagos Bank differ in so many respects, that I shall 
devote to them, besides occasional references, a third section of this 
chapter. Keeling atoll may be considered as of moderate dimensions 
and of regular form. Of the thirty-two islands surveyed by Captain 
Beechey in the Low Archipelago, the longest was found to be thirty 
miles, and the shortest less than a mile ; but yiiegen atoll, situated in 
another part of the same group, appears to be sixty miles long and 
twenty broad. Most of the atolls in this group are of an elongated 
form ; thus Bow Island is thirty miles in length, and on an average 
only six in width (See Fig. 4, Plate I.), and Clermont Tonnere has 
nearly the same proportions. In the Marshall Archipelago (the Raiick 
and Radack group of Kotzebue) several of the atolls are more than 
thirty miles in length, and Rimsky Korsacoff is fifty-four long, and 
twenty wide, at the broadest part of its irregular outline. Most of the 
atolls in the Maldiva Archipelago are of great size, one of them (which, 
however, bears a double name) measured in a medial and slightly 
curved line, is no less than eighty-eight geographical miles long, its 
greatest width being under twenty, and its least only nine and a half miles. 
Some atolls have spurs projecting from them ; and in the Marshall 
group there are atolls united together by linear reefs, for instance 
Menchioff Island (See Fig. 3, Plate II.), which is sixty miles in length, 
and consists of three loops tied together. In far the greater number 
of cases an atoll consists of a simple elongated ring, with its outline 
moderately regular. 

The average width of the annular wreath may be taken as about a 
quarter of a mile. Captain Beechey * says that in the atolls of the Low 
Archipelago it exceeded in no instance half a mile. The description 
given of the structure and proportional dimensions of the reef and islets 
of Keeling atoll, appears to apply perfectly to nearly all the atolls in 
the Pacific and Indian Oceans. The islets are first formed some way 
back either on the projecting points of the reef, especially if its form be 
angular, or on the sides of the main entrances into the lagoon — that is 
in both cases, on points where the breakers can act during gales of 
wind in somewhat different directions, so that the matter thrown up 
from one side may accumulate against that before thrown up from 
another. In Lutke's chart of the Caroline atolls, we see many instances 
of the former case ; and the occurrence of islets, as if placed for beacons, 
on the points where there is a gateway or breach through the reef, has 
been noticed by several authors. There are some atoll-formed reefs, 
rising to the surface of the sea and partly dry at low water, on which 
from some cause islets have never been formed ; and there are others 
on which they have been formed, but have subsequently been worn 
av.'ay. In atolls of small dimensions the islets frequently become united " 
into a single horse-shoe or ring-formed strip ; but Diego Garcia, although 
an atoll of considerable size, being thirteen miles and a half in length, 
has its lagoon entirely surrounded, except at the northern end, by a belt 
of land, on an average a third of a mile in width. To show how small 
the total area of the annular reef and the land is in islands of this class, 

* Beechey's " Voyage to the Pacific and Beering's Straits," chap, viii. 



26 ATOLLS. 

I may quote a remark from the voyage of Lutk6, namely, that if the 
forty-three rings, or atolls, in the Caroline Archipelago, were put one 
within another, and over a steeple in the centre of St. Petersburg, the 
whole world would not cover that city and its suburbs. 

The form of the bottom off Keeling atoll, which gradually slopes to 
about twenty fathoms at the distance of between one and two hundred 
yards from the edge of the reef, and then plunges at an angle of 45° 
into unfathomable depths, is exactly the same * with that of the 
sections of the atolls in the Low Archipelago given by Captain Beechey. 
The nature, however, of the bottom seems to differ, for this officer t 
informs me that all the soundings, even the deepest, were on coral, but 
he does not know whether dead or alive. The slope round Christmas 
atoll (Lat. 1° 4' N., 157° 45' W.), described by Cook, J is considerably 
less ; at about half a mile from the edge of the reef, the average depth 
was about fourteen fathoms on a fine sandy bottom, and at a mile, only 
between twenty and forty fathoms. It has no doubt been owing to 
this gentle slope, that the strip of land surrounding its lagoon, has 
increased in one part to the extraordinary width of three miles ; it is 
formed of successive ridges of broken shells and corals, like those on 
the beach. I know of no other instance of such width in the reef of an 
atoll ; but Mr. F. D. Bennett informs me that the inclination of the 
bottom round Caroline atoll in the Pacific, is like that off Christmas 
Island, very gentle. Off the Maldiva and Chagos atolls, the inclination is 
much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell § found 
fifty and sixty fathoms close to the edge of the reef, and at 300 yards 
distance there was no bottom with a 300-yard line. Captain Moresby 
informs me, that at 100 fathoms from the mouth of the lagoon of Diego 
Garcia, he found no bottom with 150 fathoms ; this is the more remark- 
able, as the slope is generally less abrupt in front of channels through 
a reef, owing to the accumulation of sediment. At Egmont Island, 
also, at 150 fathoms from the reef, soundings were struck with 150 
fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef, no 
bottom was obtained, as I am informed by Captain Moresby, with a 
line of 200 fathoms ! The currents run with great force round these 
atolls, and where they are strongest, the inclination appears to be 
most abrupt. I am informed by the same authority, that wherever 
soundings were obtained off these islands, the bottom was invariably 

* The form of the bottom round the Marshall atolls in the Northern Pacific 
is probably similar : Kotzebue (" First Voyage," vol. ii., p. 16) says: "We had 
at a small distance from the reef, forty fathoms depth, which increased a 
little further so much that we could find no bottom." 

■j- I must be permitted to express my obligation to Captain Beechey, for 
the very kind manner in which he has given me information on several 
points, and to own the great assistance I have derived from his excellent 
published work. 

J Cook's "Third Voyage," vol. ii., chap. 10. 

§ This fact is taken from a MS. account of these groups lent me by Captain 
Moresby. See also Captain Moresby's paper on the Maldiva atolls in the 
Ccographi'cnl Journal, vol. v., p. 40 1, 



OUTER SLOPE OF THE REEF. 27 

sandy : nor was there any reason to suspect the existence of submarine 
cliffs, as there was at KeeHng Island.* Here then occurs a difficulty ; 
can sand accumulate on a slope, which, in some cases, appears to 
exceed fifty-five degrees ? It must be observed, that I speak of slopes 
where soundings were obtained, and not of such cases, as that of 
Cardoo, where the nature of the bottom is unknown, and where its in- 
clination must be nearly vertical. M. Elie de Beaumont f has argued, 
and there is no higher authority on this subject, from the inclination at 
which snow slides down in avalanches, that a bed of sand or mud can- 
not be formed at a greater angle than thirty degrees. Considering the 
number of soundings on sand, obtained round the Maldiva and Chagos 
atolls, which appears to indicate a greater angle, and the extreme 
abruptness of the sand-banks in the West Indies, as will be mentioned 
in the Appendix, I must conclude that the adhesive property of wet 
sand counteracts its gravity, in a much greater ratio than has been 
allowed for by M. Elie de Beaumont. From the facility with which 
calcareous sand becomes agglutinated, it is not necessary to suppose 
that the bed of loose sand is thick. 

Captain Beechey has observed, that the submarine slope is much less 
at the extremities of the more elongated atolls in the Low Archipelago, 
than at their sides ; in speaking of Ducie's Island he says X the buttress, 
as it may be called, which " has the most powerful enemy (the S.W. 
swell) to oppose, is carried out much further, and with less abruptness 
than the other." In some cases, the less inclination of a certain part 
of the external slope, for instance of the northern extremities of the two 
Keeling atolls, is caused by a prevailing current which there accumulates 
a bed of sand. Where the water is perfectly tranquil, as within a 
lagoon, the reefs generally grow up perpendicularly, and sometimes 
even overhang their bases ; on the other hand, on the leeward side of 
Mauritius, where the water is generally tranquil, although not invariably 
so, the reef is very gently inclined. Hence it appears that the exterior 
angle varies much ; nevertheless in the close similarity in form between 
the sections of Keeling atoll and of the atolls in the Low Archipelago, 
in the general steepness of the reefs of the Maldiva and Chagos atolls, 
and in the perpendicularity of those rising out of water always tranquil, 
we may discern the effects of uniform laws ; but from the complex 
action of the surf and currents, on the growing powers of the coral and 
on the deposition of sediment, we can by no means follow out all the 
results. 

* Off some of the islands in the Low Archipelago the bottom appears to 
descend by ledges. Off Elizabeth Island, which, however, consists of raised 
coral, Captain Beechey (p. 45, 4to ed.) describes three ledges : the first had 
an easy slope from the beach to a distance of about fifty yards : the second 
extended two hundred yards with twenty-five fathoms on it, and then ended 
abruptly, like the first ; and immediately beyond this there was no bottom 
with two hundred fathoms. 

t " Memoires pour scrvir a una description Geolog. dc France," tome iv., 
p. 216. 

I Eccchcy's "Voyage," 4to ed., p. 44, 



28 ATOLLS. 

Where islets have been formed on the reef, that part which I have 
sometimes called the "flat" and which is partly dry at low water, 
appears similar in every atoll. In the Marshall group in the North 
Pacific, it may be inferred from Chamisso's description, that the reef, 
where islets have not been formed on it, slopes gently from the external 
margin to the shores of the lagoon ; Flinders states that the Australian 
barrier has a similar inclination inwards, and I have no doubt it is of 
general occurrence, although, according to Ehrenberg, the reefs of the 
Red Sea offer an exception. Chamisso observes that "the red colour 
of the reef (at the Marshall atolls) under the breakers is caused by a 
Nullipora, which covers the stone wherever the waves beat ; and, under 
favourable circumstances, assumes a stalactical form," — a description 
perfectly applicable to the margin of Keeling atoll.* Although Chamisso 
does not state that the masses of Nulliporae form points or a mound, 
higher than the flat, yet I believe that this is the case ; for Kotzebue,t 
in another part, speaks of the rocks on the edge of the reef "as visible 
for about two feet at low water," and these rocks we may feel quite 
certain are not formed of true coral. | Whether a smooth convex mound 
of Nulliporae, like that which appears as if artificially constructed to 
protect the margin of Keeling Island, is of frequent occurrence round 
atolls, I know not ; but we shall presently meet with it, under precisely 
the same form, on the outer edge of the "barrier-reefs" which encircle 
the Society Islands. 

There appears to be scarcely a feature in the structure of Keeling 
reef, which is not of common, if not of universal occurrence, in other 
atolls. Thus Chamisso describes § a layer of coarse conglomerate, 
outside the islets round the Marshall atolls which "appears on its upper 
surface uneven and eaten away." From drawings, with appended 
remarks, of Diego Garcia in the Chagos group and of several of the 

* Kotzebue's "First Voyage," vol. iii., p. 142. Near Porto Praya, in the 
Cape de Verde Islands, some basaltic rocks, lashed]by no inconsiderable surf, 
were completely enveloped with a layer of Nulliporae. The entire surface 
over many square inches, was coloured of a peach-blossomed red ; the layer, 
however, was of no greater thickness than paper. Another kind, in the 
form of projecting knobs, grew in the same situation. These Nulliporae are 
closely related to those described on the coral-reefs, but I believe are of 
different species. 

f Kotzebue's "First Voyage," vol. ii., p. 16. Lieutenant Nelson, in his 
excellent memoir in the Geological Transactions (vol. ii., p. 105), alludes to 
the rocky points mentioned by Kotzebue, and infers that they consist of 
Serpulae, which compose incrusting masses on the reefs of Bermudas, as 
they likewise do on a sandstone bar off the coast of Brazil (which I have 
described in London Phil. Journal^ Oct. 1841). These masses of Serpulae 
hold the same position, relatively to the action of the sea, with the Nulliporae 
on the coral-reefs in the Indian and Pacific Oceans. 

\ Captain Moresby, in his valuable paper "on the Northern atolls of 
Maldivas" {^Geographical Journal, vol. v.), says that the edges of the reefs 
there stand above water at low spring-tides. 

§ Kotzebue's "First Voyage," vol. iii., p. 144. 



SUBMERGED REEFS. 29 

Maldiva atolls, shown me by Captain Moresby,* it is evident that their 
outer coasts are subject to the same round of decay and renovation as 
those of Keeling atoll. From the description of the atolls in the Low 
Archipelago, given in Captain Beechey's "Voyage," it is not apparent 
that any conglomerate coral-rock was there observed. 

The lagoon in Keeling atoll is shallow ; in the atolls of the Low 
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall 
Group, according to Chamisso, from 30 to 35 ; in the Caroline atolls it 
is only a little less. Within the Maldiva atolls there are large spaces 
with 45 fathoms, and some soundings are laid down of 49 fathoms. The 
greater part of the bottom in most lagoons, is formed of sediment ; large 
spaces have exactly the same depth, or the depth varies so insensibly, 
that it is evident that no other means, excepting aqueous deposition, 
could have levelled the surface so equally. In the Maldiva atolls this 
is very conspicuous, and likewise in some of the Caroline and Marshall 
Islands. In the former large spaces consist of sand and soft clay ; and 
Kotzebue speaks of clay having been found wdthin one of the Marshall 
atolls. No doubt this clay is calcareous mud, similar to that at Keeling 
Island, and to that at Bermuda already referred to, as undistinguishable 
from disintegrated chalk, and which Lieutenant Nelson says is called 
there pipe-clay.f 

Where the waves act with unequal force on the two sides of an atoll, 
the islets appear to be first formed, and are generally of greater con- 
tinuity on the more exposed shore. The islets, also, which are placed 
to leeward, are in most parts of the Pacific liable to be occasionally 
swept entirely away by gales, equalling hurricanes in violence, which 
blow in an opposite direction to the ordinary trade-wind. The absence 
of the islets on the leeward side of atolls, or when present their lesser 
dimensions compared with those to windward, is a comparatively un- 
important fact ; but in several instances the reef itself on the leeward 
side, retaining its usual defined outline, does not rise to the surface by 
several fathoms. This is the case with the southern side of Peros 
Banhos (Plate I., Fig. 9) in the Chagos group, with Mourileu atoll, | in 
the Caroline Archipelago, and with the barrier-reef (Plate I., Fig. 8) of 
the Gambler Islands. I allude to the latter reef, although belonging to 

* See also Moresby on the Northern atolls of the Maldivas, Geographical 
Journal, vol. v., p. 400. 

f I may here observe that on the coast of Brazil, where there is much 
coral, the soundings near the land are described by Admiral Roussin, in the 
Ptlote du Bre'sil, as siliceous sand, mingled with much finely comminuted 
particles of shells and coral. Further in the oflfing, for a space of 1,300 
miles along the coast, from the Abrolhos Islands to Maranham, the bottom in 
many places is composed of " tuf blanc, mel6 ou form6 de madrdpores 
broyes," This white substance, probably, is analogous to that which occurs 
within the above-mentioned lagoons ; it is sometimes, according to Roussin, 
firm, and he compares it to mortar, 

X Frederick Lutke's " Voyage autour du Monde," vol. ii., p. 291. See also 
his account of Namonoui'to, at pp. 97 and 105, and the chart of Oulleay in 
the Atlas. 



30 ATOLLS. 

another class, because Captain Bcechey was first led by it to observe 
the peculiarity in the question. At Peros Banhos the submerged part 
is nine miles in length, and lies at an average depth of about five 
fathoms ; its surface is nearly level, and consists of hard stone, with a 
thin covering of loose sand. There is scarcely any living coral on it, 
even on the outer margin, as I have been particularly assured by Captain 
Moresby ; it is, in fact, a wall of dead coral-rock, having the same width 
and transverse section with the reef in its ordinary state, of which it is 
a continuous portion. The living and perfect parts terminate abruptly, 
and abut on the submerged portions, in the same manner as on the 
sides of an ordinary passage through the reef The reef to leeward in 
other cases is nearly or quite obHterated, and one side of the lagoon is 
left open ; for instance, at Oulleay (Caroline Archipelago), where a 
crescent-formed reef is fronted by an irregular bank, on which the other 
half of the annular reef probably once stood. At Namonouito, in the 
same Archipelago, both these modilications of the reef concur ; it consists 
of a great flat bank, with from twenty to twenty-five fathoms water on it ; 
for a length of more than forty miles on its southern side it is open and 
without any reef, whilst on the other sides it is bounded by a reef, in 
parts rising to the surface and perfectly characterised, in parts lying 
some fathoms submerged. In the Chagos group there are annular 
reefs, entirely submerged, which have the same structure as the sub- 
merged and defined portions just described. The Speaker's Bank offers 
an excellent example of this structure ; its central expanse, which is 
about twenty-two fathoms deep, is twenty-four miles across ; the 
external rim is of the usual width of annular reefs, and is well-defined ; 
it lies between six and eight fathoms beneath the surface, and at the 
same depth there are scattered knolls in the lagoon. Captain Moresby 
believes the rim consists of dead rock, thinly covered with sand, and he 
is certain this is the case with the external rim of the Great Chagos 
Bank, which is also essentially a submerged atoll. In both these cases, 
as in the submerged portion of the reef at Peros Banhos, Captain 
Moresby feels sure that the quantity of living coral, even on the outer 
edge overhanging the deep-sea water, is quite insignificant. Lastly, in 
several parts of the Pacific and Indian Oceans there are banks, lying at 
greater depths than in the cases just mentioned, of the same form and 
size with the neighbouring atolls, but with their atoll-like structure 
wholly obliterated. It appears from the survey of Freycinet, that there 
are banks of this kind in the Caroline Archipelago, and, as is reported, 
in the Low Archipelago. When we discuss the origin of the different 
classes of coral formations, we shall see that the subrnerged state of 
the whole of some atoll-formed reefs, and of portions of others, gene- 
rally but not invariably on the leeward side, and the existence of more 
deeply submerged banks now possessing little or no signs of their 
original atoll-like structure, are probably the effects of a uniform cause, — 
namely, the death of the coral, during the subsidence of the area, in 
which the atolls or banks are situated. 

There is seldom, with the exception of the Maldiva atolls, more than 
two or three channels, and generally only one leading into the lagoon, 



BREACHES THROUGH THE R£EP. 31 

of sufficient depth for a ship to enter. In small atolls, there is usually 
not even one. Where there is deep water, for instance above twenty- 
fathoms, in the middle of the lagoon, the channels through the reef are 
seldom as deep as the centre, — it may be said that the rim only of the 
saucer-shaped hollow forming the lagoon is notched. Mr. Lyell* has 
observed that the growth of the coral would tend to obstruct all the 
channels through a reef, except those kept open by discharging the 
water, which during high tide and the greater part of each ebb is thrown 
over its circumference. Several facts indicate that a considerable 
quantity of sediment is likewise discharged through these channels ; 
and Captain Moresby informs me that he has observed, during the 
change of the monsoon, the sea discoloured to a distance off the 
entrances into the Maldiva and Chagos atolls. This, probably, would 
check the growth of the coral in them, far more effectually than a mere 
current of water. In the many small atolls without any channel, these 
causes have not prevented the entire ring attaining the "surface. The 
channels, like the submerged and effaced parts of the reef, very generally 
though not invariably occur on the leeward side of the atoll, or on that 
side, according to Beechey.f which, from running in the same direction 
with the prevalent wind, is not fully exposed to it. Passages between 
the islets on the reef, through which boats can pass at high water, must 
not be confounded with ship-channels, by which the annular reef itself 
is breached. The passages between the islets occur, of course, on the 
windward as well as on the leeward side ; but they are more frequent 
and broader to leeward, owing to the lesser dimensions of the islets on 
that side. 

At Keeling atoll the shores of the lagoon shelve gradually, where 
the bottom is of sedfment, and irregularly or abruptly where there are 
coral-reefs; but this is by no means the universal structure in other 
atolls. Chamisso,J speaking in general terms of the lagoons in the 
Marshall atolls, says the lead generally sinks "from a depth of two or 
three fathoms to twenty or twenty-four, and you may pursue a line in 
which on one side of the boat you may see the bottom, and on the 
other the azure-blue deep water," The shores of the lagoon-like 
channel within the barrier-reef at Vanikoro have a similar structure. 
Captain Beechey has described a modification of this structure (and he 
believes it is not uncommon) in two atolls in the Low Archipelago, in 
which the shores of the lagoon descend by a few, broad, slightly 
inclined ledges or steps : thus at Matilda atoll, § the great exterior 
reef, the surface of which is gently inclined towards and beneath the 
surface of the lagoon, ends abruptly in a little cliff three fathoms deep ; 
at its foot, a ledge forty yards wide extends, shelving gently inwards 

* " Principles of Geology," vol. iii., p. 289. 

t Beechey's "Voyage," 4to ed., vol. i., p. 189. 

X Kotzebue's " First Voyage," vol. iii., p. 142. 

§ Beechey's "Voyage," 4to ed., vol. i., p. 160. At Whitsunday Island the 
bottom of the lagoon slopes gradually towards the centre, and then deepens 
suddenly, the edge of the bank being nearly perpendicular. This bank is 
formed of coral and dead shells. 



32 A TOLLS. 

like the siirl'ace-reef, and terminated by a second little cliff five fathoms 
deep ; beyond this, the bottom of the lagoon slopes to twenty fathoms, 
which is the average depth of its centre. These ledges seem to be 
formed of coral-rock ; and Captain Beechey says that the lead often 
descended several fathoms through holes in them. In some atolls, all 
the coral reefs or knolls in the lagoon come to the surface at low 
water ; in other cases of rarer occurrence, all lie at nearly the same 
depth beneath it, but most frequently they are quite irregular, — some 
with perpendicular, some with sloping sides, — some rising to the surface, 
and others lying at all intermediate depths from the bottom upwards. 
I cannot, therefore, suppose that the union of such reefs could produce 
even one uniformly sloping ledge, and much less two or three, one 
beneath the other, and each terminated by an abrupt wall. At Matilda 
Island, which offers the best example of the step-like structure. Captain 
Beechey observes that the coral-knolls within the lagoon are quite 
irregular in their height. We shall hereafter see that the theory 
which accounts for the ordinary form of atolls, apparently includes this 
occasional peculiarity in their structure. 

In the midst of a group of atolls, there sometimes occur small, flat, 
very low islands of coral formation, which probably once included a 
lagoon, since filled up with sediment and coral-reefs. Captain Beechey 
entertains no doubt that this has been the case with the two small 
islands, which alone of thirty-one surveyed by him in the Low Archi- 
pelago, did not contain lagoons. Romanzoff Island (in lat. 15° S.) 
is described by Chamisso * as formed by a dam of madreporitic rock 
inclosing a flat space, thinly covered with trees, into which the sea on 
the leeward side occasionally breaks. North Keeling atoll appears to 
be in a rather less forward stage of conversion into land ; it consists of 
a horse-shoe shaped strip of land surrounding a muddy flat, one mile 
in its longest axis, which is covered by the sea only at high water. 
When describing South Keeling atoll, I endeavoured to show how slow 
the final process of filling up a lagoon must be ; nevertheless, as all 
causes do tend to produce this effect, it is very remarkable that not one 
instance, as I believe, is known of a moderately sized lagoon being 
filled up even to the low water-line at spring-tides, much less of such a 
one being converted into land. It is, likewise, in some degree remark- 
able, how few atolls, except small ones, are surrounded by a single 
linear strip of land, formed by the union of separate islets. We cannot 
suppose that the many atolls in the Pacific and Indian Oceans all have 
had a late origin, and yet should they remain at their present level, 
subjected only to the action of the sea and to the growing powers of 
the coral, during as many centuries as must have elapsed since any of 
the earlier tertiary epochs, it cannot, I think, be doubted that their 
lagoons and the islets on their reef, would present a totally different 
appearance from what they now do. This consideration leads to the 
suspicion that some renovating agency (namely subsidence) comes 
into play at intervals, and perpetuates their original structure. 

* Kotzebue's "First Voyage," vol. iii., p. 221. 



RING-FORMED REEFS. 33 

Section III. 

Maldiva archipelago. — Ring-formed reefs marginal and central. — Great 
depths in the lagoons of the south atolls. — Reefs in the lagoons all 
rising to the surface. — Position of islets, and breaches in the reefs with 
respect to the prevalent winds and action of the waves. — Destruction of 
islets. — Connection in the position and submarine foundation of distinct 
atolls. — The apparent disseverment of large atolls. — The Great Chagos 
Bank. — Its submerged condition and extraordinary structure. 

Although occasional references have been made to the Maldiva atolls, 
and to the banks in the Chagos group, some points of their structure 
deserve further consideration. My description is derived from an 
examination of the admirable charts lately published from the survey 
of Captain Moresby and Lieutenant Powell, and more especially from 
information which Captain Moresby has communicated to me in the 
kindest manner. 

The Maldiva Archipelago is 470 miles in length, with an average 
breadth of about 50 miles. The form and dimensions of the atolls, 
and their singular position in a double line, may be seen, but not well, 
in the greatly reduced chart (Fig. 6) in Plate II. The dimensions of 
the longest atoll in the group (called by the double name of Milla-dou- 
Madou and Tilla-dou-Matte) have already been given ; it is 88 miles in a 
medial and slightly curved line, and is less than 20 miles in its broadest 
part. Suadiva, also, is a noble atoll, being 44 miles across in one direction, 
and ;; |. in another, and the great included expanse of water has a 
depth of between 250 and 300 feet. The smaller atolls in this group 
differ in no respect from ordinary ones ; but the larger ones are remark- 
able from being breached by numerous deep-water channels leading into 
the lagoon ; for instance, there are 42 channels, through which a 
ship could enter the lagoon ot Suadiva. In the three southern large 
atolls, the separate portions of reef between these channels have the 
ordinary structure, and are linear; but in the other atolls, especially 
the more northern ones, these portions are ring-formed, like miniature 
atolls. Other ring-formed reefs rise out of tlie lagoons, in the place 
of those irregular ones which ordinarily occur there. In the reduction 
of the chart of Mahlos Mahdoo (Plate II., Fig. 4), it was not found easy 
to define the islets and the little lagoons within each reef, so that the 
ring-formed structure is very imperfectly shown ; in the large published 
charts of Tilla-dou-Matte, the appearance of these rings, from standing 
further apart from each other, is very remarkable. The rings on the 
margin are generally elongated ; many of them are three, and some 
even five miles, in diameter ; those within the lagoon are usually 
smaller, few being more than two miles across, and the greater number 
rather less than one. The depth of the little lagoon within these small 
annular reefs is generally from five to seven fathoms, but occasionally 
more ; and in Ari atoll many of the central ones are twelve, and some 
even more than twelve fathoms deep. These rings rise abruptly from 
the platform or bank, on which they are placed ; thoir outer margin is 

3 



34 MALDIVA ATOLLS; 

invariably bordered by living coral* within which there is a flat surface 
of coral rock; on this flat, sand and fragments have in many cases 
accumulated and been converted into islets, clothed with vegetation. 
I can, in fact, point out no essential difference between these little 
ring-formed reefs (which, however, are larger, and contain deeper 
lagoons than many true atolls that stand in the open sea), and the most 
perfectly characterised atolls, excepting that the ring-formed reefs are 
based on a shallow foundation, instead of on the floor of the open sea, 
and that instead of being scattered irregularly, they are grouped closely 
togetlier on one large platform, with the marginal rings arranged in a 
rudely formed circle. 

The perfect series which can be traced from portions of simple 
linear reef, to others including long linear lagoons, and from these 
again to oval or almost circular rings, renders it probable that the latter 
are merely modifications of the linear or normal state. It is conform- 
able with this view, that the ring-formed reefs on the margin, even 
where most perfect and standing furthest apart, generally have their 
longest axes directed in the line wliich the reef would have held, if the 
atoll had been bounded by an ordinary wall. We may also infer that 
the central ring-formed reefs are modifications of those irregular ones, 
which are found in the lagoons of all common atolls. It appears from 
the charts on a large scale, that the ring-like structure is contingent 
on the marginal channels or breaches being wide ; and, consequently, 
on the whole interior of the atoll being freely exposed to the waters of 
the open sea. When the channels are narrow or few in nu nber, 
although the lagoon be of great size and depth (as in Suadiva), there 
are no ring-formed reefs ; where the channels are somiCwhat broader, 
the marginal portions of reef, and especially those close to the larger 
channels, are ring-formed, but the central ones are not so ; where they 
are broadest, almost every reef throughout the atoll is more or less 
perfectly ring-formed. Although their presence is thus contingent on 
the openness of the marginal channels, the theory of their formation, as 
we shall hereafter see, is included in that of the parent atolls, of which 
they form the separate portions. 

The lagoons of all the atolls in the southern part of the Archipelago 
are from ten to twenty fathoms deeper than those in the northern part. 
This is well exemplified in the case of Addoo, the southermost atoll in 
the group, for altliough only nine miles in its longest diameter, it has a 
depth of thirty-nine fathoms, whereas all the other small atolls have 
comparatively shallow lagoons ; I can assign no adequate cause for this 
difference in depth. In the central and deepest part of the lagoons, 
the bottom consists, as I am informed by Captain Moresby, of stiff clay 
(probably a calcareous mud) ; nearer the border it consists of sand, and 
in the channels through the reef, of hard sand-banks, sandstone, con- 
glomerate rubble, and a little live coral. Close outside the reef and the 
line joining its detached portions (where intersected by many channels), 
tlie bottom is sandy, ancl it slopes abruptly into unfathomable depths. 

* Captain Moresby informs me that Millepora complanata is one of the 
commonest kinds on the outer margin, as it is at KceHng atoll. 



THEIR LAGOONS. 35 

In most lagoons the depth is considerably greater in the centre than in 
the channels ; but in Tilla-dou-Matte, where the marginal ring-formed 
reefs stand far apart, the same depth is carried across the entire atoll, 
from the deep-water line on one side to that on the other. I cannot 
refrain from once again remarking on the singularity of these atolls, — a 
great sandy and generally concave disc rises abruptly from the un- 
fathomable ocean, with its central expanse studded and its border 
symmetrically fringed with oval basins of coral-rock, just lipping the 
surface of the sea, sometimes clothed with vegetation, and each contain- 
ing a little lake of clear water ! 

In the southern Maldiva atolls, of which there are nine large ones, 
all the small reefs within the lagoons come to the surface, and are dry 
at low w-ater spring-tides ; hence in navigating them, there is no danger 
from submarine banks. This circumstance is very remarkable, as 
within some atolls, for instance those of the neighbouring Chagos group, 
not a single reef comes to the surface, and in most other cases a few 
only do, and the rest lie at all intermediate depths from the bottom 
upwards. When treating of the growth of coral I shall again refer to 
this subject. 

Although in the neighbourhood of the Maldiva Archipelago the winds, 
during the monsoons, blow during nearly an equal time from opposite 
quarters, and although, as I am informed by Captain Moresby, the 
westerly winds are the strongest, yet the islets are almost all placed 
on the eastern side of the northern atolls, and on the south-eastern side 
of the southern atolls. That the formation of the islets is due to detritus 
thrown up from the outside, as in the ordinary manner, and not from 
the interior of the lagoons, may, I think, be safely inferred from several 
considerations, which it is hardly worth while to detail. As the easterly 
winds are not the strongest, their action probably is aided by some 
prevaihng swell or current. 

In groups of atolls, exposed to a trade-wind, the ship-channels into 
the lagoons are almost invariably situated on the leeward or less 
exposed side of the reef, and the reef itself is sometimes either wanting 
there, or is submerged. A strictly analogous, but different fact, may be 
observed at the Maldiva atolls — namely, that where two atolls stand in 
front of each other, the breaches in the reef are the most numerous on 
their near, and therefore less exposed, sides. Thus on the near sides of 
Ari and the two Nillandoo atolls, which face S. Male, Phaleedoo, and 
Moloque atolls, there are seventy-three deep-water channels, and only 
twenty-five on their outer sides ; on the near side of the three latter 
named atolls there are fifty-six openings, and only thirty-seven on their 
outsides. It is scarcely possible to attribute this difference to any other 
cause than the somewhat different action of the sea on the two sides, 
which would ensue from the protection afforded by the two rows of 
atolls to each other. I may here remark that in most cases, the condi- 
tions favourable to the greater accumulation of fragments on the reef 
and to its more perfect continuity on one side of the atoll than on the 
other, have concurred, but this has not been the case with the Maldivas ; 
for we have seen that the islets are placed on the eastern or south- 



36 MALDIVA ATOLLS. 

eastern sides, whilst the breaches in the reef occur indifferently on any 
side, where protected by an opposite atoll. The reef being more con- 
tinuous on the outer and more exposed sides of those atolls which stand 
near each other, accords with the fact, that the reef of the southern 
atolls is more continuous than that of the northern ones ; for the former, 
as I am informed by Captain Moresby, are more constantly exposed 
than the northern atolls to a heavy surf. 

The date of the first formation of some of the islets in this Archipelago 
is known to the inhabitants ; on the other hand, several islets, and even 
some of those which are believed to be very old, are now fast wearing 
away. The work of destruction has, in some instances, been completed 
in ten years. Captain Moresby found on one water-washed reef the 
marks of wells and graves, wliich were excavated when it supported an 
islet. In South Nillandoo atoll, the natives say that three of the islets 
were formerly larger: in North Nillandoo there is one now being 
washed away ; and in this latter atoll Lieutenant Prentice found a reef, 
about six himdred yards in diameter, which the natives positively 
affirmed was lately an island covered with cocoa-nut trees. It is now 
only partially dry at low water spring-tides, and is (in Lieutenant 
Prentice's words) " entirely covered v\ ith live coral and madrepore." In 
the northern part, also, of the Maldiva Archipelago and in the Chagos 
group, it is known that some of the islets are disappearing. The natives 
attribute these effects to variations in the currents of the sea. For my 
own part I cannot avoid suspecting that there must be some further 
cause, which gives rise to such a cycle of change in the action of the 
currents of the great and open ocean. 

Several of the atolls in this Archipelago are so related to each other 
in form and position, that at the first glance one is led to suspect that 
they have originated in the disseverment of a single one. Male consists 
of three perfectly characterised atolls, of which the shape and relative 
position are such, that a line drawn closely round all three, gives a 
symmetrical figure ; to see this clearly, a larger chart is required than 
that of the Archipelago in Plate II. ; the channel separating the two 
northern Male atolls is only little more than a mile wide, and no bottom 
was found in it with lOO fathoms. Powell's Island is situated at 
the distance of two miles and a half off the northern end of Mahlos 
Mahdoo (see Fig. 4, Plate II.), at the exact point where the two sides of 
the latter, if prolonged, would meet ; no bottom, however, was found 
in the channel with 200 fathoms ; in the wider channel between 
Horsburgh atoll and the southern end of Mahlos Mahdoo, no bottom 
was found with 250 fathoms. In these and similar cases, the relation 
consists only in the form and position of the atolls. But in the channel 
between the two Nillandoo atolls, although three miles and a quarter 
wide, soundings were struck at the depth of 200 fathoms ; the 
channel between Ross and Ari atolls is four miles wide, and only 1 50 
fathoms deep. Here then we have, besides the relation of form, a 
submarine connection. The fact of soundings having been obtained 
between two separate and perfectly characterised atolls is in itself 
interesting, as it has never, I beheve, been effected in any of the many 



Jl 






A 



S4' 



r.;^ 



^^^i^&'Qf'^-i:.:^^ ^._ 'SJ 



.'iw.->:-»s-i' 






3,,, 







^'^-... 












31 



%5 




GREAT CHAGOS BANK. 37 

other groups of atolls in the Pacific and Indian seas. In continuing to 
trace the connection of adjoining atolls, if a hasty glance be taken at the 
chart (Fig. 4., Plate II.) of Mahlos Mahdoo, and the line of unfathomable 
water be followed, no one will hesitate to consider it as one atoll. But 
a second look will show that it is divided by a bifurcating channel, of 
which the northern arm is about one mile and three-quarters in width, 
with an average depth of 125 fathoms, and the southern one three- 
quarters of a mile wide, and rather less deep. These channels resemble 
in the slope of their sides and general form, those which separate atolls 
in every respect distinct ; and the northern arm is wider than that 
dividing two of the Male atolls. The ring-formed reefs on the sides of 
tliis bifurcating channel are elongated, so that the northern and southern 
portions of Mahlos Mahdoo may claim, as far as their external outline 
is concerned, to be considered as distinct and perfect atolls. But the 
intermediate portion, lying in the fork of the channel, is bordered by 
reefs less perfect than those which surround any other atoll in the 
group of equally small dimensions. Mahlos Malidoo, therefore, is in 
every respect in so intermediate a condition, that it may be considered 
either as a single atoll nearly dissevered into three portions, or as three 
atolls almost perfect and intimately connected. This is an instance of 
a very early stage of the apparent disseverment of an atoll, but a still 
earlier one in many respects is exhibited at Tilla-dou-Matte. In one 
] art of this atoll, the ring-formed reefs stand so far apart from each 
other, that the inhabitants have given different names to the northern 
and southern halves ; nearly all the rings, moreover, are so perfect and 
stand so separate, and the space from which they rise is so level and 
unlike a true lagoon, that we can easily imagine the conversion of this 
one great atoll, not into two or three portions, but into a whole group 
of miniature atolls. A perfect series such as we have here traced, 
impresses the mind with an idea of actual change ; and it will hereafter 
be seen, that the theory of subsidence, with the upward growth of the 
coral, modified by accidents of probable occurrence, will account for 
the occasional disseverment of large atolls. 

The Great Chagos Bank alone remains to be described. In the 
Chagos group there are some ordinary atolls, some annular reefs rising 
to the surface but without any islets on them, and some atoll-formed 
banks, either quite submerged, or nearly so. Of the latter, the Great 
Chagos Bank is much the largest, and differs in its structure from the 
others : a plan of it is given in Plate II., Fig. i, in which, for the sake of 
clearness, I have had the parts under ten fathoms deep finely shaded : 
an east and west vertical section is given in Fig. 2, in which the vertical 
scale has been necessarily exaggerated. Its longest axis is ninety 
nautical miles, and another fine drawn at right angles to the first, across 
the broadest part, is seventy. The central part consists of a level muddy 
flat, between forty and fifty fathoms deep, which is surrounded on all 
sides, with the exception of some breaches, by the steep edges of a set 
of banks, rudely arranged in a circle. These banks consist of sand, 
with a very little live coral ; they vary in breadth from five to twelve 
miles, and on an average lie about sixteen fathoms beneath the surface ; 



38 BARRIER-REEFS; 

they are bordered by the steep edges of a third narrow and upper bank, 
which forms the rim to the whole. This rim is about a mile in width, 
and with the exception of two or three spots where islets have been 
formed, is submerged between five and ten fathoms. It consists of 
smooth hard rock, covered with a thin layer of sand, but with scarcely 
any live coral ; it is steep on both sides, and outwards slopes abruptly 
into unfathomable depths. At the distance of less than half a mile 
from one part, no bottom was found with 190 fathoms; and off another 
point, at a somewhat greater distance, there was none with 210 fathoms. 
Small steep-sided banks or knolls, covered with luxuriantly growing 
coral, rise from the interior expanse to the same level with tlie external 
rim, which, as we have seen, is formed only of dead rock. It is 
impossible to look at the plan (Fig. i, Plate II.), although reduced to so 
small a scale, without at once perceiving that the Great Chagos Bank is, 
in the words of Captain Moresby,* "nothing more than a half-drowned 
atoll." But of what great dimensions, and of how extraordinary an 
internal structure ? We shall hereafter have to consider both the cause 
of its submerged condition, a state common to other banks in the group, 
and the origin of the singular submarine terraces, which bound the 
central expanse : these, I think, it can be shown, have resulted from a 
cause analogous to that which has produced the bifurcating channel 
across Mahlos Mahdoo. 



CHAPTER II. ' 

BARRIER-REEFS. 

Closely resemble in general form and structure atoll-reefs. — Width and 
depth of the lagoon-channels. — Breaches through the reef in front of 
valleys, and generally on the leeward side. — Checks to the filling up of 
the lagoon-channels. — Size and constitution of the encircled islands. — 
Number of islands within the same reef. — Barrier-reefs of New Cale- 
donia and Australia. — Position of the reef relative to the slope of the 
adjoining land. — Probable great thickness of barrier-reefs. 

The term " barrier " has been generally applied to that vast reef which 
fronts the N.E. shore of Aiistralia, and by most voyagers likewise to 
that on the western coast of New Caledonia. At one time I thought it 
convenient thus to restrict the term, but as these reefs are similar in 
structure, and in position relatively to the land, to those, which, like 
a wall with a deep moat within, encircle many smaller islands, I 
have classed them together. The reef, also, on the west coast of 
New Caledonia, circling round the extremities of the island, is an 

* This officer has had the kindness to lend me an excellent MS. account 
of the Chagos Islands ; from this paper, from the published charts, and from 
verbal information communicated to me by Captain Moresby, the above 
account of the Great Chagos Bank is taken. 



THEIR RESEMBLANCE TO ATOLLS. 39 

intermediate form between a small encircling reef and the Australian 
barrier, which stretches for a thousand miles in nearly a straight line. 

The geographer Balbi has in effect described those barrier-reefs, 
which encircle moderately sized islands, by calling them atolls with 
high land rising from within their central expanse. The general re- 
semblance between the reefs of the barrier and atoll classes may be 
seen in the small, but accurately reduced charts on Plate I.,* and this 
resemblance can be furthe-r shown to extend to every part of the struc- 
ture. Beginning with the outside of the reef ; many scattered soundings 
off Gambler, Oualan, and some other encircled islands, show that close 
to the breakers there exists a narrow shelving margin, beyond which the 
ocean becomes suddenly unfathomable ; but off the west coast of New 
Caledonia, Captain Kentf found no bottom with 150 fathoms, at two 
ships' length from the reef; so that the slope here must be nearly as 
precipitous as off the Maldiva atolls. 

I can give little information regarding the kinds of corals which live on 
the outer margin. When I visited the reef at Tahiti, although it was 
low water, the surf was too violent for me to see the living masses ; 
but, according to what I heard from some intelligent native chiefs, they 
resemble in their rounded and branchless forms, those on the margin of 
Keeling atoll. The extrem^e verge of the reef, vv^hich was visible between 
the breaking waves at low water, consisted of a rounded, convex, 
artificial-like breakwater, entirely coated with Nulliporse, and absolutely 
similar to that which I have described at Keeling atoll. From what I 
heard when at Tahiti, and from the writings of the Revs. W. Ellis and 
J. Williams, I conclude that this peculiar structure is common to most 
of the encircled islands of the Society Archipelago. The reef within 
this mound or breakwater, has an extremely irregular surlace, even 
more so than between the islets on the reef of Keeling atoll, with which 
alone (as there are no islets on the reef of Tahiti) it can properly be 
compared. At Tahiti, the reef is very irregular in width ; but round 
many other encircled islands, for instance, Vanikoro or Gambler Islands 
(Figs. I and 8, Plate I.), it is quite as regular, and of the same average 
width, as in true atolls. Most barrier-reefs on the inner side slope 
irregularly into the lagoon-channel (as the space of deep water separat- 
ing the reef from the included land may be called), but at Vanikoro the 
reef slopes only for a short distance, and then terminates abruptly in 
a submarine wall, forty feet high, — a structure absolutely similar to that 
described by Chamisso in the Marshall atolls. 

In the Society Archipelago, Ellis % states, that the reefs generally lie 
at the distance of from one to one and a half miles, and, occasionally, 
even at more than three miles, from the shore. The central mountains 
are generally bordered by a fringe of flat, and often marshy, alluvial 

* The authorities from which these charts have been reduced, together 
with some remarks on them, are given in a separately appended page, 
descriptive of the Plates. 

f Dalrymple, "Hydrog. Mem.," vol. iii. 

X Consult, on this and other points, the "Polynesian Researches," by the 
Rev. W. Ellis, an admirable work, full of curious information. 



4C BARRIER-REEFS; 

land, from one to four miles in width. This fringe consists of coral- 
sand and detritus thrown up from the lagoon-channel, and of soil 
washed down from the hills; it is an encroachment on the channel, 
analogous to that low and inner part of the islets in many atolls which 
is formed by the accumulation of matter from the lagoon. At Hogoleu 
(Fig. 2, Plate I.), in the Caroline Archipelago,* the reef on the south side 
is no less than twenty miles ; on the east side, five ; and on the north 
side, fourteen miles from the encircled high islands. 

The lagoon channels may be compared in every respect with true 
lagoons. In some cases they are open, with a level bottom of fine sand ; 
in others they are choked up with reefs of delicately branched corals, 
which have the same general character as those within the Keeling atoll. 
These internal reefs either stand separately, or more commonly skirt 
the shores of the included high islands. The depth of the lagoon- 
channel round the Society Islands varies from two or three to thirty 
fathoms ; in Cook's t chart of Uheta, however, there is one sounding laid 
down of forty-eight fathoms ; at Vanikoro there are several of fifty-four 
and one of fifty-six and a half fathoms (English), a depth which even 
exceeds by a little that of the interior of the great Maldiva atolls. Some 
barrier-reefs have very few islets on them ; whilst others are surmounted 
by numerous ones; and those round part of Bolabola (Plate I., Fig. 5) 
form a single linear strip. The islets first appear either on the angles 
of the reef, or on the sides of the breaches through it, and are generally 
most numerous on the windward side. The reef to leeward retaining 
its usual width, sometimes lies submerged several fathoms beneath the 
surface ; I have already mentioned Gambier Island as an instance of 
this structure. Submerged reefs, having a less defined outline, dead, 
and covered with sand, have been observed (see Appendix) off some 
parts of Huaheine and Tahiti. The reef is more frequently breached to 
leeward than to windward ; thus I find in Krusentern's " Memoir on the 
Pacific," that there are passages through the encircling reef on the lee- 
ward side of each of the seven Society Islands, which possess ship- 
harbours ; but that there are openings to windward through the reef of 
only three of them. The breaches in the reef are seldom as deep as the 
interior lagoon-like channel ; they generally occur in front of the main 
valleys, a circumstance which can be accounted for, as will be seen in 
the fourth chapter, without much difficulty. The breaches being situated 
in. front of the valleys, which descend indifferently on all sides, explains 
their more frequent occurrence through the windward side of barrier- 
reefs than through the windward side of atolls, — for in atolls there is 
no included land to influence the position of the breaches. 

It is remarkable, that the lagoon-channels round mountainous islands 
have not in every instance been long ago filled up with coral and sedi- 
ment ; but it is more easily accounted for than appears at first sight. 
In cases like that of Hogoleu and the Gambier Islands, where a few 

* See " Hydrographical Mem." and the "Atlas of the Voyage of the 
Astrolabe" by Captain Dumont D'Urville, p. 428. 

f See the chart in vol. i. of Hawkesworth's 4to ed. of "Cook's First 
Voyage." 



THEIR RESEMBLANCE TO ATOLLS. 41 

small peaks rise out of a great lagoon, the conditions scarcely differ 
from those of an atoll, and I have already shown, at some length, that 
the filling up of a true lagoon must be an extremely slow process. 
Where the channel is narrow, the agency, which on unprotected coasts 
is most productive of sediment, namely the force of the breakers, is 
here entirely excluded, and the reef being breached in the front of the 
main valleys, much of the finer mud from the rivers must be transported 
into the open sea. As a current is formed by the water thrown over 
the edge of atoll-formed reefs, which carries sediment with it through 
the deep-water breaches, the same thing probably takes place in barrier- 
reefs, and this w^ould greatly aid in preventing the lagoon-channel from 
being filled up. The low alluvial border, however, at the foot of the 
encircled mountains, shows that the work of filling up is in progress ; 
and at Maura (Plate I., Fig. 6), in the Society group, it has been almost 
effected, so that there remains only one harbour for small craft. 

If we look at a set of charts of barrier-reefs, and leave out in imagina- 
tion the encircled land, we shall find that, besides the many points 
already noticed of resemblance, or rather of identity in structure with 
atolls, there is a close general agreement in form, average dimensions, 
and grouping. Encircling barrier-reefs, like atolls, are generally 
elongated, with an irregularly rounded, though sometimes angular out- 
line. There are atolls of all sizes, from less than two miles in diameter 
to sixty miles (excluding Tilla-dou-Matte, as it consists of a number of 
almost independent atoll-formed reefs) ; and there are encircling 
barrier-reefs from three miles and a half to forty-six miles in diameter, 
— Turtle Island being an instance of the former, and Hogoleu of the 
latter. At Tahiti the encircled island is thirty-six miles in its longest 
axis, whilst at Maurua it is only a little more than two miles. It will be 
shown, in the last chaper in this volume, that there is the strictest 
resemblance in the grouping of atolls and of common islands, and con- 
sequently there must be the same resemblance in the grouping of atolls 
and of encircling barrier-reefs. 

The islands lying within reefs of this class, are of very various heights. 
Tahiti* is 7,000 feet ; Maurua about 800 ; Aitutaki 360, and Manouai 
only 50. The geological nature of the included land varies : in most 
cases it is of ancient volcanic origin, owing apparently to the fact that 
islands of this nature are most frequent within all great seas ; some, 
liowever, are of madreporitic limestone, and others of primary forma- 
tion, of which latter kind New Caledonia offers the best example. The 
central land consists either of one island, or of several : thus, in the 
Society group, Eimeo stands by itself ; while Taha and Raiatea (Fig. 3, 
Plate I.), both moderately large islands of nearly equal size, are 
included in one reef. Within the reef of the Gambier group there 
are four large and some smaller islands (Fig. 8, Plate I.) ; within that of 

* The height of Tahiti is given from Captain Beechey ; Maurua from Mr. 
F. D. Bennett {^Geograph. Journ., vol. viii., p. 220); Aitutaki from measure- 
ments made on board the Beagle ; and Manouai or Harvey Island, from an 
estimate by the Rev. J. Williams. The two latter islands, however, are not 
in some respects well characterised examples of the encircled class. 



42 BA RRIER-REEFS ; 

Hogoleu (Fig. 2, Plate I.) nearly a dozen small islands are scattered 
over the expanse of one vast lagoon. 

After the details now given, it may be asserted that there is not one 
point of essential difference between encircling barrier-reefs and atolls : 
the latter enclose a simple sheet ot water, the former encircle an 
expanse with one or more islands rising from it. I was much struck 
with this fact, when viewing, from the heights of Tahiti, the distant 
island of Eimeo standing within smooth water, and encircled by a ring 
of snow-white breakers. Remove the central land, and an annular 
reef like that of an atoll in an early stage of its formation is left ; remove 
it from Bolabola, and there remains a circle of linear coral-islets, 
crowned with tall cocoa-nut trees, like one of the many atolls scattered 
over the Pacific and Indian Oceans. 

The barrier-reefs of Australia and of New Caledonia deserve a sepa- 
rate notice from their great dimensions. The reef on the west coast of 
New Caledonia (Fig. 5, Plate II.) is 400 miles in length ; and for a 
length of many leagues it seldom approaches within eight miles of the 
shore ; and near the southern end of the island, the space between the 
reef and the land is sixteen miles in width. The Australian barrier 
extends, with a few interruptions, for nearly a thousand miles ; its 
average distance from the land is between twenty and thirty miles, and 
in some parts from fifty to seventy. The great arm of the sea thus 
included, is from ten to twenty-five fathoms deep, with a sandy bottom ; 
but towards the southern end, where the reef is further from the shore, 
the depth gradually increases to forty, and in some parts to more than 
sixty fathoms. Flinders * has described the surface of this reef as con- 
sisting of a hard white agglomerate of different kinds of coral, with 
rough projecting points. The outer edge is the highest part ; it is 
traversed by narrow gullies, and at rare intervals is breached by ship- 
channels. The sea close outside is profoundly deep ; but, in front of 
the main breaches, soundings can sometimes be obtained. Some low 
islets have been formed on the reef. 

There is one important point in the structure of barrier-reefs whicli 
must here be considered. The accompanying diagrams represent nortli 
and south vertical sections, taken through the highest points of 
Vanikoro, Gambier, and Maurua Islands, and through their encircling 
reefs. The scale both in the horizontal and vertical direction is the 
same, namely, a quarter of an inch to a nautical mile. The height and 
width of these islands is known ; and I have attempted to represent the 
form of the land from the shading of the hills in the large published 
charts. It has long been remarked, even from the time of Dampicr, 
that considerable degree of relation subsists between the inclination 
of that part of the land which is beneath water and that above it ; hence 
the dotted line in the three sections, probably, does not widely differ 
in inclination from the actual submarine prolongation of the land. If we 
now look at the outer edge of the reef fAA), and bear in mind that the 
plummet on the right hand represents a depth of 1,200 feet, we must con- 
clude that the vertical thickness of these barrier coral-reefs is very great. 
* Flinders' " Vo3-age to Terra Australis," vol. ii., p. 88. 



THEIR GREAT VERTICAL THICKNESS. 

3032^ 



43 




Souta 



Rori*- 



1. Vanikoro, irom the " Atlas of the Voyage of the Astrolahe," by D, D'Ui-ville. 

2. Gambler Island, from Eeechey. 

3. Maurua, from the " Atlas of the Voj^age of the Cc<jnUle" by Duperrey, 

The horizontal line is the level of the sea, from which on the right hand a plummet 
descends, representing a depth of 200 fathoms, or 1,200 feet. The vertical shading 
shows the section of the land, and the horizontal shading that of the encircling barrier- 
reef: from the smallness of the scale, the lagoon-channel could not be represented. 

AA. — Outer edge of the coral-reefs, where the sea breaks 

BB. — The shore of the encircled islands. 

I must observe that if the sections had been taken in any other 
direction across these islands, or across other encircled islands,* the 
result would have been the same. In the succeeding chapter it will 
be shown that reef-building polypifers cannot flourish at great depths, — 
for instance, it is highly improbable that they could exist at a quarter 
of the depth represented by the plummet on the right hand of the 
woodcut. Here there is a great apparent difficulty — how were the 
basal parts of these barrier-reefs formed ? It will, perhaps, occur to 
some, that the actual reefs formed of coral are not of great thickness, 
but that before their first growth, the coasts of these encircled islands 
were deeply eaten into, and a broad but shallow submarine ledge thus 
left, on the edge of which the coral grew ; but if this had been the case, 
the shore would have been invariably bounded by lofty cliffs, and not 
have sloped down to the lagoon-channel, as it does in many instances. 
On this view,t moreover, the cause of the reef springing up at such a 
great distance from the land, leaving a deep and broad moat within, 
remains altogether unexplained. A supposition of the same nature, 

* In the fifth chapter an east and west section across the Island of 
Bolabola and its barrier-reefs is given, for the sake of illustrating another 
point. The unbroken line in it (woodcut No. 5) is the section referred to. 
The scale is -57 of an inch to a mile; it is taken from the "Atlas of the 
Voyage of the Coquille^'^ by Duperrey. The depth of the lagoon-channel 
is exaggerated, 

t The Rev. D. Tyerman and Mr. Bennett (" Journal of Voyage and Travels," 
vol. i., p. 215) have briefly suggested this explanation of the origin of the 
encircling reefs of the Society Islands. 



44 BARRIER-REEFS. 

and appearing at first more probable is, that the reefs sprung up irom 
banks of sediment, which had accumulated round the shore previously 
to the growth of the coral ; but the extension of a bank to the same 
distance round an unbroken coast, and in front of those deep arms of 
the sea (as in Raiatea, see Plate II., Fig. 3) which penetrate nearly to 
the heart of some encircled islands, is exceedingly improbable. And 
why, again, should the reef spring up, in some cases steep on both 
sides like a wall, at a distance of two, three or more miles from the 
shore, leaving a channel often between two hundred and three hundred 
feet deep, and rising from a depth which we have reason to believe is 
destructive to the growth of coral ? An admission of this nature cannot 
possibly be made. The existence, also, of the deep channel, utterly 
precludes the idea of the reef having grown outwards, on a foundation 
slowly formed on its outside, by the accumulation of sediment and 
coral detritus. Nor, again, can it be asserted, that the reef-building 
corals will not grow, excepting at a great distance from the land ; for, 
as we shall soon see, there is a whole class of reefs, which take their 
name from growing closely attached (especially where the sea is deep) 
to the beach. At New Caledonia (see Plate II., Fig. 5) the reefs which 
run in front of the west coast are prolonged in the same line 150 
miles beyond the northern extremity of the island, and this shows that 
some explanation, quite different from any of those just suggested, is 
required. The continuation of the reefs on each side of the submarine 
prolongation of New Caledonia, is an exceedingly interesting fact, if 
this part formerly existed as the northern extremity of the island, and 
before the attachment of the coral had been worn down by the action of 
the sea, or if it originally existed at its present height, with or without 
beds of sediment on each flank, how can we possibly account for the 
reefs, not growing on the crest of this submarine portion, but fronting 
its sides, in the same line with the reefs which front the shores of the 
lofty island ? We shall hereafter see, that there is one, and I believe 
only one, solution of this difficulty. 

One other supposition to account for the position 01 encircling barrier- 
reefs remains, but it is almost too preposterous to be mentioned ; namely, 
that they rest on enormous submarine craters, surrounding the included 
islands. When the size, height, and form of the islands in the Society 
group are considered, together with the fact that all are thus encircled, 
such a notion will be rejected by almost every one. New Caledonia, 
moreover, besides its size, is composed of primitive formations, as are 
some of the Comoro Islands ;* and Aitutaki consists of calcareous rock. 
We must, therefore, reject these several explanations, and conclude 
that the vertical thickness of barrier-reefs, from their outer edges to the 
foundation on which they rest (from AA in the section to the dotted 
lines) is really great; but in this, there is no difficulty, for it is not 
necessary to suppose that the coral has sprung up from an immense 
depth, as will be evident when the theory of the upward growth of 
coral-reefs, during the slow subsidence of their foundation, is discussed. 

* I have been informed tliat this is the case by Dr. Allan of Forres, who 
has visited this group. 



REEFS OF MAURITIUS. 45 

CHAPTER III. 

FRINGING OR SHORE-REEFS. 

Reefs of Mauritius. — Shallow channel within the reef. — Its slow filling up. — 
Currents of water formed within it. — Upraised reefs. — Narrow fringing 
reefs in deep seas. — Reefs on the coast of East Africa and of Brazil, — 
Fringing-reefs in very shallow seas, round banks of sediment and on 
worn-down islands. — Fringing-reefs affected by currents of the sea. — 
Coral coating bottom of the sea, but not forming reefs. 

Fringing-reefs, or, as they have been called by some voyagers, shore- 
reefs, whether skirting an island or part of a continent, might at first be 
thought to differ little, except in generally being of less breadth, from 
barrier-reefs. As far as the superficies of the actual reef is concerned 
this is the case ; but the absence of an interior deep-water channel, 
and the close relation in their horizontal extension with the probable 
slope beneath the sea of the adjoining land, present essential points of 
difference. 

The reefs which fringe the island of Mauritius offer a good example 
of this class. They extend round its whole circumference, with the 
exception of two or three parts,* where the coast is almost precipitous, 
and where, if as is probable the bottom of the sea has a similar 
inclination, the coral would have no foundation on which to become 
attached. A similar fact may sometimes be observed even in reefs of 
the barrier class, which follow much less closely the outline of the 
adjoining land ; as, for instance, on the south-east and precipitous side 
of Tahiti, where the encircling reef is interrupted. On the western 
side of the Mauritius, which was the only part I visited, the reef 
generally lies at the distance of about half a mile from the shore ; but 
in some parts it is distant from one to two, and even three miles. But 
even in this last case, as the coast-land is gently inclined from the foot 
of the mountains to the sea-beach, and as the soundings outside the 
reef indicate an equally gentle slope beneath the water, there is no 
reason for supposing that the basis of the reef, formed by the prolonga- 
tion of the strata of the island, lies at a greater depth than that at 
which the polypifers could begin constructing the reef. Some allowance, 
however, must be made for the outward extension of the corals on a 
foundation of sand and detritus, formed from their own wear, which 
would give to the reef a somewhat greater vertical thickness, than 
would otherwise be possible. 

The outer edge of the reef on the western or leeward side of the 
island is tolerably well defined, and is a little higher than any other 
part. It chiefly consists of large strongly branched corals, of the genus 
Madrepora, which also form a sloping bed some way out to sea : the 

* This fact is stated on the authority of the Officier du Roi, in his extremely 
interesting "Voyage a I'lsle de France," undertaken in 1768. According to 
Captain Carmichael (Hooker's Bot. Misc., vol. ii., p. 316) on one part of the 
coast there is a space for sixteen miles without a reef. 



46 FRINGING-REEFS, 

kinds of coral growing in this part will be described in the ensuing 
chapter. Between the outer margin and the beach, there is a flat 
space with a sandy bottom and a few tufts of living coral ; in some 
parts it is so shallow, that people, by avoiding the deeper holes and 
gullies, can wade across it at low water ; in other parts it is deeper, 
seldom however exceeding ten or twelve feet, so that it offers a safe 
coasting channel for boats. On the eastern and windward side of the 
island, which is exposed to a heavy surf, the reef was described to me 
as having a hard smooth surface, very sliglitly inclined inwards, just 
covered at low-water, and traversed by gullies ; it appears to be quite 
similar in structure to the reefs of the barrier and atoll classes. 

The reef of Mauritius, in front of every river and streamlet, is 
breached by a straiglit passage : at Grand Port, however, there is a 
channel like that within a barrier-reef ; it extends parallel to the shore 
for four miles, and has an average depth of ten or twelve fathoms ; its 
presence may probably be accounted for by two rivers which enter at 
each end of the channel, and bend towards each other. The fact of reefs 
of the fringing class being always breached in front of streams, even of 
those which are dry during the greater part of the year, will be explained, 
when the conditions unfavourable to the growth of coral are considered. 
Low coral-islets, like those on barrier-reefs and atolls, are seldom 
formed on reefs of this class, owing apparently in some cases to their 
narrowness, and in others to tlie gentle slope of the reef outside not 
yielding many fragments to the breakers. On the windward side, 
however, of the Mauritius, two or three small islets have been 
formed. 

It appears, as will be shown in the ensuing chapter, tliat the action 
of the surf is favourable to the vigorous growth of the stronger corals, 
and that sand or sediment, if agitated by thfe waves, is injurious to them. 
Hence it is probable that a reef on a shelving shore, like that of 
Mauritius, would at first grow up, not attached to the actual beach, but 
at some little distance from it ; and the corals on the outer margin 
would be the most vigorous. A shallow channel would thus be formed 
within the reef, and as the breakers are prevented acting on tlie shores 
of the island, and as they do not ordinarily tear up many fragments 
from the outside, and as ev-ery streamlet has its bed prolonged in a 
straight line through the reef, this channel could be filled up only very 
slowly with sediment. But a beach of sand and of fragments of the 
smaller kinds of coral seems, in the case of Mauritius, to be slowdy 
encroaching on the shallow channel. On many shelving and sandy 
coasts, the breakers tend to form a bar of sand a little way from the 
beach, with a slight increase of depth within it ; for instance. Captain 
Grey * states that the west coast of Australia, in lat. 24°, is fronted by 
a sand bar about two hundred yards in width, on which there is only 
two feet of water ; but within it the depth increases to two fathoms. 
Similar bars, more or less perfect, occur on other coasts. In these 
cases I suspect that the shallow channel (which no doubt during storms 
is occasionally obliterated) is scooped out by the flowing away of the 
* Captain Grey's "Journal of Two Expeditions," vol. i., p. 369. 



CLOSELY ATTACHED TO THE BEACH, 47 

water thrown beyond the line, on which the waves break with the 
greatest force. At Pernambuco a bar of hard sandstone,* which has 
the same external form and height as a coral-reef, extends nearly 
parallel to the coast ; within this bar currents, apparently caused by the 
water thrown over it during the greater part of each tide, run strongly, 
and are wearing away its inner wall. From these facts it can hardly be 
doubted, that within most fringing-reefs, especially within those lying 
some distance from the land, a return stream must carry away the 
water thrown over the outer edge ; and the current thus produced, 
would tend to prevent the channel being filled up with sediment, and 
might even deepen it under certain circumstances. To this latter JDelief 
I am led, by finding that channels are almost universally present within 
the fringing-reefs of those islands which have undergone recent eleva- 
tory movements; and this could hardly have been the case, if the 
conversion of the very shallow channel into land had not been counter- 
acted to a certain extent. 

A fringing-reef, if elevated in a periect condition above the level of 
the sea, ought to present the singular appearance of a broad dry moat 
within a low mound. The author f of an interesting pedestrian tour 
round the Mauritius, seems to have met with a structure of this kind : 
he says, "J' observai que la, ou la mer etale, independamment des res- 
cifs du large, il y a terre ime espece d' effoncement ou chcmin convert 
naturel. On y pourrait mettre du canon," etc. In another place he 
adds, "Avant de passer le Cap, on remarque un gros banc de corail 
eleve de plus de quinze pieds : c'est une espece de rescif, que la mer 
abandonne, il regne au pied une longue flaque d'eau, dont on pourrait 
faire un bassin pour de petits vaisseaux." But the margin of the reef, 
although the highest and most perfect part, from being most exposed 
to the surf, would generally during a slow rise of the land be either 
partially or entirely worn down to that level, at which corals could 
renew their growth on its upper edge. On some parts of the coast-land 
of Mauritius there are little hillocks of coral-rock, which are either the 
last remnants of a continuous reef, or of low islets formed on it. I 
observed two such hillocks between Tamarin Bay and the Great Black 
River ; they were nearly twenty feet high, about two hundred yards from 
the present beach, and about thirty feet above its level. They rose 
abruptly from a smooth surface, strewed with worn fragments of coral. 
They consisted in their lower part of hard calcareous sandstone, and in 
their upper of great blocks of several species of Astraea and Madrepora, 
loosely aggregated ; they were divided into irregular beds, dipping sea- 
ward, in one hillock at an angle of 8°, and in the other at 18°. I suspect 
that the superficial parts of the reefs, which have been upraised together 
with the islands they fringe, have generally been much more modified 
by the wearing action of the sea, than those of Mauritius. 

* I have described this singular slruclurc in the Lonl. and Editt. Phil. 
Mag., October 1841. 

f "Voyage a I'lslc do France, par un Officicr du Roi," part i., pp. 192, 
200. 



48 FRINGING-REEFS. 

Many islands * are fringed by reefs quite similar to those of Mauritius ; 
but on coasts where the sea deepens very suddenly the reefs are much 
narrower, and their limited extension seems evidently to depend on the 
high incHnation of the subm.arine slope ; a relation, which, as we have 
seen, does not exist in reefs of the barrier class. The fringing-reefs on 
steep coasts are frequently not more than from fifty to one hundred 
yards in width; they have a nearly smooth, hard surface, scarcely 
uncovered at low water, and without any interior shoal channel, like 
that within those fringing-reefs, which lie at a greater distance from the 
land. Tlie fragments torn up during gales from the outer margin are 
thrown over the reef on the shores of the island. I may give as 
instances, Wateeo, where the reef is described by Cook as being a 
hundred yards wide; and Mauti and Elizabeth f Islands, where it is 
only fifty yards in width : the sea round these islands is very deep. 

Fringing-reefs, like barrier-reefs, both surround islands, and front 
the shores of continents. In the charts of the eastern coast of Africa, 
by Captain Owen, many extensive fringing-reefs are laid down ; thus, for 
a space of nearly forty miles, from lat. i" 15' to 1° 45' S., a reef fringes 
the shore at an average distance of rather more than one mile, and 
therefore at a greater distance than is usual in reefs of this class ; but 
as the coast-land is not lofty, and as the bottom shoals very gradually 
(the depth being only from eight to fourteen fathoms at a mile and a 
half outside the reef), its extension thus far from the land offers no 
difficulty. The external margin of this reef is described, as formed of 
projecting points, within which there is a space, from six to twelve feet 
deep, with patches of living coral on it. At Mukdeesha (lat. 2° i' N.) 
" the port is formed," it is said,t " by a long reef extending eastward, 
four or five miles, within which there is a narrow channel, with ten to 
twelve feet of water at low spring-tides ;" it lies at the distance of a quarter 
of a mile from the shore. Again, in the plan of Mombas (lat. 4° S.), 
a reef extends for thirty-six miles, at the distance of from half a mile to 
one mile and a quarter from the shore ; within it, there is a channel 
navigable "for canoes and small craft," between six and fifteen feet 
deep : outside the reef the depth is about thirty fathoms at the distance 
of nearly "half a mile. Part of this reef is very symmetrical, and has a 
uniform breadth of two hundred yards. 

The coast of Brazil is in many parts fringed by reefs. Of these, some 
are not of coral formation ; for instance, those near Bahia and in front 

* I may give Cuba, as another instance ; Mr. Taylor (^Loudon's Mag. of 
Nat. Hist, vol. ix., p. 449) has described a reef several miles in length be- 
tween Gibara and Vjaro, which extends parallel to the shore at the distance 
of between half and the third part of a mile, and encloses a space of shallow 
water, with a sandy bottom and tufts of coral. Outside the edge of the reef, 
which is formed of great branching corals, the depth is six and seven fathoms. 
This coast has been upheaved at no very distant geological period." 

•j- Mauti ie described by Lord Byron in the voyage of H.M.S. Blonde, and 
Elizabeth Island by Captain Beechc}'. 

I Owen's "Africa," vol. i., p. 357, from which work the foregoing facts arc 
likewise taken. 



FRINGING-REEFS. 49 

of Pernambuco ; but a ifew miles south of this latter city, the reef 
follows * so closely every turn of the shore, that I can hardly doubt it 
is of coral ; it runs at the distance of three-quarters of a mile from the 
land, and within it the depth is from ten to fifteen feet. I was assured 
by an intelligent pilot that at Ports Frances and Maceio, the outer part 
of the reef consists of living coral, and the inner of a white stone, full 
of large irregular cavities, communicating with the sea. The bottom of 
the sea off the coast of Brazil shoals gradually to between thirty and 
forty fathoms, at the distance of between nine and ten leagues from 
the land. 

From the description now given, we must conclude that the dimensions 
and structure of fringing-reefs depend entirely on the greater or less 
inclination of the submarine slope, conjoined with the fact that reef- 
building polypifers can exist only at limited depths. It follows from 
this, that where the sea is very shallow, as in the Persian Gulf and in 
parts of the East Indian Archipelago, the reefs lose their fringing 
character, and appear as separate and irregularly scattered patches, 
often of considerable area. From the more vigorous growth of the coral 
on the outside, and from the conditions being less favourable in several 
respects within, such reefs are generally higher and more perfect in their 
marginal than in their central parts ; hence these reefs sometimes assume 
(and this circumstance ought not to be overlooked) the appearance of 
atolls ; but they differ from atolls in their central expanse being much 
less deep, in their form being less defined, and in being based on a 
shallow foundation. But when in a deep sea reefs fringe banks of 
sediment, which have accumulated beneath the surface, round either 
islands or submerged rocks, they are distinguished with difficulty on the 
one hand from encircling barrier-reefs, and on the other from atolls. In 
the West Indies there are reefs, which I should probably have arranged 
under both these classes, had not the existence of large and level banks, 
lying a little beneath the surface, ready to serve as the basis for the 
attachment of coral, been occasionally brought into view by the entire 
or partial absence of reefs on them, and had not the formation of such 
banks, through the accumulation of sediment now in progress, been 
sufficiently evident. Fringing-reefs sometimes coat, and thus protect 
the foundations of islands, which have been worn down by the surf to 
tlie level of the sea. According to Ehrenberg, this has been extensively 
the case with the islands in the Red Sea, which formerly ranged parallel 
to the shores of the mainland, with deep water within them : hence the 
reefs now coating their bases are situated relatively to the land like 
barrier-reefs, although not belonging to that class ; but there are, as 1 
believe, in the Red Sea some true bai rier-reefs. The reefs of this sea 
and of the West Indies will be described in the Appendix. In some 
cases, fringing-reefs appear to be considerably modified in outline by 
the course of the prevailing currents. Dr. J. Allan informs me that on 
the east coast of Madagascar almost every headland and low point of 
sand has a coral-reef extending from it in a S.W. and N.E. line, parallel 

* See Baron Roussin's " Pilote du Br6sil," and accompanying hydro- 
graphical memoir. 

4 



50 THE GROWTH OF CORAL-REEFS. 

to the currents on that shore. I should think the influence of the 
currents chiefly consisted in causing an extension, in a certain direction, 
of a proper foundation for the attachment of the coral. Round many 
intertropical islands, for instance the Abrolhos on the coast of Brazil 
surveyed by Captain Fitzroy, and, as I am informed by Mr. Cuming, 
round the Philippines, the bottom of the sea is entirely coated by irre- 
gular masses of coral, which although often of large size, do not reach 
the surface and form proper reefs. This must be owing, either to 
insufficient growth, or to the absence of those kinds of corals which can 
withstand the breaking of the waves. 

The three classes, atoll-formed, barrier, and fringing-reefs, together 
with the modifications just described of the latter, include all the most 
remarkable coral formations anywhere existing. At the commencement 
of the last chapter in the volume, where I detail the principles on which 
the map (Plate III.) is coloured, the exceptional cases will be enumerated. 



CHAPTER IV. 

ON THE GROWTH OF CORAL-REEFS. 

In this chapter I will give all the facts which I have collected, relating 
to the distribution of coral-reefs, — to the conditions favourable to 
their increase, — to the rate of their growth, — and to the depth at which 
they are formed. 

These subjects have an important bearing on the theory of the origin 
of the different classes of coral-reefs. 



Section T. 

On the distribution of coral-reefs, and on the conditions favourable to their 
increase. 

With regard to the limits of latitude, over which coral-reefs extend, 1 
have nothing new to add. The Bermuda Islands, in 32° 15' N., is the 
point furthest removed from the equator, in which they appear to exist ; 
and it has been suggested that their extension so far northward in this 
instance is owing to the warmth of the Gulf Stream. In the Pacific, the 
Loo Choo Islands, in lat. 27° N., have reefs on their shores, and there is 
an atoll in 28° 30', situated N.W. of the Sandwich Archipelago. In the 
Red Sea there are coral-reefs in lat. 30°. In the southern hemisphere 
coral-reefs do not extend so far from the equatorial sea. In the 
Southern Pacific there are only a few reefs beyond the line of the 
tropics, but Houtmans Abrolhos, on the western shores of Australia in 
lat. 29° S., are of coral formation. 

The proximity oi" volcanic land, owing to the lime generally evolved 
from it, has been thought to be favourable to the increase of coral-reefs. 



ABSENCE OF REEFS IN CERTAIN LARGE AREAS. 51 

There is, however, not much foundation for this view ; for nowhere are 
coral-reefs more extensive than on the shores of New Caledonia, and of 
north-eastern Australia, which consist of primary formations ; and in 
the largest groups of atolls, namely the Maldiva, Chagos, Marshall, 
Gilbert, and Low Archipelagoes, there is no volcanic or other kind of 
rock, excepting that formed of coral. 

The entire absence of coral-reefs in certain large areas within the 
tropical seas, is a remarkable fact. Thus no coral-reefs were observed, 
during the surveying voyages of the Beagle and her tender on the 
west coast of South America south of the equator, or round the 
Galapagos Islands. It appears, also, that there are none * north of the 
equator; Mr. Lloyd, who surveyed the Isthmus of Panama, remarked 
to me, that although he had seen corals living in the Bay of Panama, 
yet he had never observed any reefs formed by them. I at first attri- 
buted this absence of reefs on the coasts of Peru and of the Galapagos 
Islands,t to the coldness of the currents from the south, but the Gulf of 
Panama is one of the hottest pelagic districts in the world. t In the 
central parts of the Pacific there are islands entirely free from reefs ; 
in some few of these cases I have thought that this was owing to recent 
volcanic action ; but the existence of reefs round the greater part of 
Hawaii, one of the Sandwich Islands, shows that recent volcanic action 
docs not necessarily prevent their growth. 

In the last chapter I stated that the bottom of the sea round some 
islands is thickly coated with living corals, which nevertheless do not 
form reefs, either from insufficient growth, or from the species not 
being adapted to contend with the breaking waves. 

I have been assured by several people, that there are no coral-reefs 
on the west coast of Africa, § or round the islands in the Gulf of Guinea. 
This perhaps may be attributed, in part, to the sediment brought down 
by the many rivers debouching on that coast, and to the extensive mud- 

* I have been informed that this is the case, by Lieutenant Ryder, R.N., 
and others who have had ample opportunities for observation. 

f The mean temperature of the surface sea from observations made by 
the direction of Captain Fitzroy on the shores of the Galapagos Islands, 
between the 1 6th of September and the 20th of October, 1835, ^^'^^ 68^Fahr. 
The lowest temperature observed was 585° at the south-west end of Albe- 
marle Island ; and on the west coast of this island, it was several times 62'^ 
and 63*^. The mean temperature of the sea in the Low Archipelago of atolls, 
and near Tahiti, from similar observations made on board the ISeagle, was 
(although further from the equator) 77"5°, the lowest any day being 7^'5°- 
Therefore we have here a difference of 9*5° in mean temperature, and 18° 
in extremes ; a difference doubtless quite sufBcient to affect the distribution 
of organic beings in the two areas. 

X Humboldt's "Personal Narrative," vol. vii,, p. 434. 

§ It might be concluded, from a paper by Captain Owen {Geograph. 
Journ., vol. ii., p. 89), that the reefs off Cape St. Anne and the Sherboro' 
Islands were of coral, although the author states that they are not purely 
coralline. But I have been assured by Lieutenant Holland, R.N., that these 
reefs arc not of coral, or at least that they do not at all resemble those in the 
West Indies. 



52 BREAKERS FAVOUR THE GROWTH OF CORALS. 

banks, which line great part of it. But the islands of St. Helena, 
Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are, 
also, entirely without reefs, although they lie far out at sea, are com- 
posed of the same ancient volcanic rocks, and have the same general 
form, with those islands in the Pacific, the shores of which are sur- 
rounded by gigantic walls of coral-rock. With the exception of 
Bermuda, there is not a single coral-reef in the central expanse of the 
Atlantic Ocean. It will, perhaps, be suggested that the quantity of 
carbonate of lime in different parts of the sea, may regulate the presence 
of reefs. But this cannot be the case, for at Ascension, the waves 
charged to excess precipitate a thick layer of calcareous matter on the 
tidal rocks ; and at St. Jago, in the Cape Verdes, carbonate of lime not 
only is abundant on the shores, but it forms the chief part of some 
upraised post-tertiary strata. The apparently capricious distribution, 
therefore, of coral-reefs, cannot be explained by any of these obvious 
causes ; but as the study of the terrestrial and better known half of the 
world must convince every one that no station capable of supporting 
life is lost, — nay more, that there is a struggle for each station, between 
the different orders of nature, — we may conclude that in those parts oi 
the intertropical sea, in which there are no coral-reefs, there are other 
organic bodies supplying the place of the reef-building polypifers. It 
has been shown in the chapter on Keeling atoll that there are some 
species of large fish, and the whole tribe of Holothuriae which prey on 
the tenderer parts of the corals. On the other hand, the polypifers in 
their turn must prey on some other organic beings ; the decrease of 
which from any cause would cause a proportionate destruction of the 
living coral. The relations, therefore, which determine the formation 
of reefs on any shore, by the vigorous growth of the efficient kinds of 
coral, must be very complex, and with our imperfect knowledge quite 
inexplicable. From these considerations, we may infer that changes in 
the condition of the sea, not obvious to our senses, might destroy all 
the coral-reefs in one area, and cause them to appear in another : thus, 
the Pacific or Indian Ocean might become as barren of coral-reefs as the 
Atlantic now is, without our being able to assign any adequate cause 
for such a change. 

It has been a question with some naturalists, which part of a reef is 
most favourable to the growth of coral. The great mounds of living 
Porites and of Millepora round Keeling atoll occur exclusively on the 
extreme verge of the reef, which is washed by a constant succession of 
breakers ; and living coral nowhere else forms solid masses. At the 
Marshall islands the larger kinds of coral (chiefly species of Astraea. 
a genus closely allied to Porites) "which form rocks measuring several 
fathoms in thickness," prefer, according to Chamisso,* the most violent 
sui f. I have stated that the outer margin of the Maldiva atolls consists 
of living corals (some of which, if not all, are of the same species with 
those at Keeling atoll), and here the surf is so tremendous, that even 
large ships have been thrown, by a single heave of the sea, high and dry 
on the reef, all on board thus escaping with their lives. 

* Kotzcbue's "First Voyage" (Eng. Trans.), vol. iii., pp. 142, 143, 331. 



BREAKERS FAVOUR THE GROWTH OF CORALS. 53 

Ehrenberg * remarks, that in the Red Sea the strongest corals live on 
the outer reefs, and appear to love the surf; he adds, that the more 
branched kinds abound a little way within, but that even these in still 
more protected places, become smaller. Many other facts having a 
similar tendency might be adduced. t It has, however, been doubted 
by MM. Quoy and Gaimard, whether any kind of coral can even with- 
stand, much less flourish in, the breakers of an open sea : X they affirm 
that the saxigenous lithophytes flourish only where the water is tranquil, 
and the heat intense. This statement has passed from one geological 
work to another; nevertheless, the protection of the whole reef 
undoubtedly is due to those kinds of coral, which cannot exist in the 
situations thought by these naturalists to be most favourable to them. 
For should the outer and living margin perish, of any one of the many 
low coral-islands, round which a line of great breakers is incessantly 
foaming, the whole, it is scarcely possible to doubt, would be washed 
away and destroyed, in less than half a century. But the vital energies 
of the corals conquer the mechanical power of the waves ; and the large 
fragments of reef torn up by every storm, are replaced by the slow but 
steady growth of the innumerable polypifers, which form the living zone 
on its outer edge. 

From these facts, it is certain, that the strongest and most massive 
corals flourish, where most exposed. The less perfect state of the reef 
of most atolls on the leeward and less exposed side, compared vvith its 
state to windward ; and the analogous case of the greater number of 
breaches on the near sides of those atolls in the Maldiva Archipelago, 
which afford some protection to each other, are obviously explained by 
this circumstance. If the question had been, under what conditions the 
greater number of species of coral, not regarding their bulk and strength, 
were developed, I should answer, — probably in the situations described 
by MM. Quoy and Gaimard, where the water is tranquil and the heat 
intense. The total number of species of coral in the circumtropical 
seas must be very great : in the Red Sea aloite, 120 kinds, according to 
Ehrenberg, § have been observed. 

The same author has observed that the recoil of the sea from a steep 
shore is injurious to the growth of coral, although waves breaking over 
a bank are not so. Ehrenberg also states, that where there is much 
sediment, placed so as to be liable to be moved by the waves there is 
little or no coral ; and a collection of living specimens placed by him 
on a sandy shore died in the course of a few days.|| An experiment, 

* Ehrenberg, "tjber die Natur und Bildung der Corallen Banke im rothen 
Meere," p. 49. 

t In the West Indies, as I am informed by Captain Bird Allen, R.N., it is 
the common belief of those, who are best acquainted with the reefs, that the 
coral flourishes most, where freely exposed to the swell of the open sea. 

\ "Annales des Sciences Naturelles," tome vi., pp. 276, 278, — "La oil les 
ondes sont agitees, les Lytophytes ne peuvent travailler, parce qu'elles 
detruiraient leurs fragiles edifices," etc. 

§ Ehrenberg, "tjber die Natur," etc., etc., p. 46. 

U Ibid., p. 49. 



54 BREACHES IN REEFS CAUSED BY SEDIMENT. 

however, will presently be related in which some large masses of living 
coral increased rapidly in size, after having been secured by stakes on a 
sandbank. That loose sediment should be injurious to the living poly- 
pifers, appears, at first sight, probable ; and accordingly, in sounding off 
Keeling atoll, and (as will hereafter be shown) off Mauritius, the arming 
of the lead invariably came up clean, where the coral was growing 
vigorously. This same circumstance has probably given rise to a 
strange belief, which, according to Captain Owen,* is general amongst 
the inhabitants of the Maldiva atolls, namely that corals have roots, and 
therefore that if merely broken down to the surface, they grow up again ; 
but, if rooted out, they are permanently destroyed. By this means the 
inhabitants keep their harbours clear ; and thus the French Governor of 
St. Mary's in Madagascar, "cleared out and made a beautiful little port 
at that place," For it is probable that sand would accumulate in the 
hollows formed by tearing out the corals, but not on the broken and 
projecting stumps, and therefore, in the former case, the fresh growth 
of the coral might be thus prevented. 

In the last chapter I remarked that fringing-reefs are almost univer- 
sally breached, where streams enter the sea.f Most authors have 
attributed this fact to the injurious effects of the fresh water, even where 
it enters the sea only in small quantity, and during a part of the year. 
No doubt brackish water would prevent or retard the growth of coral ; 
but I believe that the mud and sand which is deposited, even by rivulets 
when flooded, is a much more efficient check. The reef on each side of 
the channel leading into Port Louis at Mauritius, ends abruptly in a 
wall, at the foot of which I sounded and found a bed of thick mud. 
This steepness of the sides appears to be a general character in such 
breaches. Cook, J speaking of one at Raiatea, says, "like all the rest, 
it is very steep on both sides." Now, if it were the fresh water mingling 
with the salt which prevented the growth of coral, the reef certainly 
would not terminate abruptly, but as the polypifers nearest the impure 
stream would grow less vigorously than those farther off, so would the 
reef gradually thin away. On the other hand, the sediment brought 
down from the land would only prevent the growth of the coral in the 
hue of its deposition, but would not check it on the side, so that the 
reefs might increase till they overhung the bed of the channel. The 
breaches are much fewer in number, and front only the larger valleys 
in reefs of the encircling barrier class. They probably are kept open m 
the same manner as those into the lagoon of an atoll, namely, by the 

* Captain Owen on the Geography of the Maldiva Islands, Geograph. 
Journal, vol. ii., p. 88. 

f Lieutenant Wellstead and others have remarked that this is the case in 
the Red Sea; Dr. Riippell (" Reise in Abyss.," Band, i., p. 142) says that there 
are pear-shaped harbours in the upraised coral-coast, into which periodical 
streams enter. Frogi this circumstance, I presume, we must infer that before 
the upheaval of the strata now forming the coast-land, fresh water and 
sediment entered the sea at these points; and the coral being thus prevented 
growing, the pear-shaped harbours were produced. 

X Ccok's "First Voyage," vol. ii., p. 271 (Ha\vkesworth's edit.). 



CORALS ADAPTED TO DIFFERENT STATIONS. 55 

force of the currents and the drifting outwards of fine sediment. Their 
position in front of valleys, although often separated from the land by 
deep water lagoon-channels, which it might be thought would entirely 
remove the injurious effects both of the fresh water and the sediment, 
will receive a simple explanation when we discuss the origin of 
barricr-rccfs. 

In the vegetable kingdom every different station has its peculiar 
group of plants, and similar relations appear to prevail with corals. 
We have already described the great difference between the corals 
within the lagoon of an atoll and those on its outer margin. The corals, 
also, on the margin of Keeling Island occurred in zones ; thus the 
Porites and Millepora co7nplanata grow to a large size only where they 
are washed by a heavy sea, and are killed by a short exposure to the 
air ; whereas, three species of Nullipora also live amidst the breakers, 
but are able to survive uncovered for a part of each tide ; at greater 
depths, a strong Madrepora and Millepora alcicornis are the commonest 
kinds, the former appearing to be confined to this part, beneath the zone 
of massive corals, minute encrusting corallines and other organic bodies 
live. If we compare the external margin of the reef at Keeling atoll 
with that on the leeward side of Mauritius, which are very differently 
circumstanced, we shall find a corresponding difference in the appear- 
ance of the corals. At the latter place, the genus Madrepora is pre- 
ponderant over every other kind, and beneath the zone of massive 
corals there are large beds of Seriatopora. There is also a marked 
difference, according to Captain Moresby,* between the great branching 
corals of the Red Sea, and those on the reefs of the Maldiva atolls. 

These facts, which in themselves are deserving of notice, bear, per- 
haps, not very remotely, on a remarkable circumstance which has been 
pointed out to me by Captain Moresby, namely, that with very few excep- 
tions, none of the coral-knolls within the lagoons of Peros Banhos, Diego 
Garcia, and the Great Chagos Bank (all situated in the Chagos group),risc 
to the surface of the water ; whereas all those, with equally few excep- 
tions, within Solomon and Egmont atolls in the same group, and likewise 
within the large southern Maldiva atolls, reach the surface. I make 
these statements, after having examined the charts of each atoll. In the 
lagoon of Peros Banhos, which is nearly twenty miles across, there is 
only one single reef which rises to the surface ; in Diego Garcia there 
are seven, but several of these lie close to the margin of the lagoon, and 
need scarcely have been reckoned ; in the Great Chagos Bank there is 
not one. On the other hand, in the lagoons of some of the great 
southern Maldiva atolls, although thickly studded with reefs, every one 
without exception rises to the surface ; and en an average there are less 
than two submerged reefs in each atoll ; in the northern atolls, however, 
the submerged lagoon-reefs are not quite so rare. The submerged reefs 
in the Chagos atolls generally have from one to seven fathoms water on 
them, but some have from seven to ten. Most of them are small with 

* Captain Moresby on the Northern Maldiva atolls, Gcograph. Joiini.^ 
vol. v., p. 401. 



56 SUBMERGED LAGOON-REEFS, 

very steep sides ; * at Peros Banhos they rise from a depth of about 
thirty fathoms, and some of them in the Great Chagos Bank from above 
forty fathoms ; they are covered, Captain Moresby informs me, with 
living and healthy coral, two and three feet high, consisting of several 
species. Why then have not these lagoon-reefs reached the surface, 
like the innumerable ones in tlie atolls abov^e named ? If we attempt 
to assign any difference in their external conditiorvs, as t?re cause of this 
diversity, we are at once baffled. Tlie lagoon of Diego Garcia is not 
deep, and is almost wholly surrounded by its reef; Peros Banhos is 
very deep, much larger, with many wide passages communicating with 
the open sea. On the other hand, of those atolls, in which all or nearly 
all the lagoon-reefs have reached the surface, some are small, others 
large, some shallow, others deep, some well-enclosed, and others open. 

Captain Moresby informs me that he has seen a French chart of 
Diego Garcia made eighty years before his survey, and apparently 
very accurate ; and from it he infers, that during this interval there has 
not been the smallest change in the depth on any of the knolls within 
the lagoon. It is also known that during the last fifty-one years, the 
eastern channel into the lagoon has neither become narrower, nor 
decreased in depth ; and as there are numerous small knolls of living 
coral within it, some change might have been anticipated. Moreover, 
as the whole reef round the lagoon of this atoll has been converted 
into land — an unparalleled case, 1 believe, in an atoll of such large 
size, — and as the strip of land is for considerable spaces more than half 
a mile wide — also a very unusual circumstance, — we have the best 
possible evidence, that Diego Garcia has remained at its present level 
for a very long period. With this fact, and with the knowledge that 
no sensible change has taken place during eighty years in the coral- 
knolls, and considering that every single reef has reached the surface 
in other atolls, which do not present the smallest appearance of being 
older than Diego Garcia and Peros Banhos, and which are placed under 
the same external conditions with them, one is led to conclude that 
these submerged reefs, although covered with luxuriant coral, have no 
tendency to grow upwards, and that they would remain at their present 
levels for an almost indefinite period. 

From the number of these knolls, from their position, size, and form, 
many of them being only one or two hundred yards across, with a 
rounded outline, and precipitous sides, — it is indisputable that they 
have been formed by the growth of coral ; and this makes the case 
much more remarkable. In Peros Banhos and in the Great Chagos 
Bank, some of these almost columnar masses are 200 feet high, and 
their summits lie only from two to eight fathoms beneath the sur- 
face ; therefore, a small proportional amount more of growth would 
cause them to attain the surface, like those numerous knolls, which 
rise from an equally great depth within the Maldiva atolls. We can 
hardly suppose that time has been wanting for the upward growth of 

* Some of these statements were not communicated to me verbally by 
Captain Moresby, but are taken from the MS. account before alluded to, of 
the Chagos Group. 



RATE OF GROWTH. 57 

the coral, whilst in Diego Garcia, tlie broad annular strip of land, formed 
by the continued accumulation of detritus, shows how long this atoll 
has remained at its present level. We must look to some other cause 
than the rate of growth ; and I suspect it will be found in the reefs 
being formed of different species of corals, adapted to live at different 
depths. 

The Great Chagos Bank is situated in the centre of the Chagos 
Group, and the Pitt and Speaker Banks at its two extreme points. These 
banks resemble atolls, except in their external rim being about eight 
fathoms submerged, and in being formed of dead rock, with very little 
living coral on it : a portion nine miles long of the annular reef of Peros 
Banhos atoll is in the same condition. These facts, as will hereafter 
be shown, render it very probable that the whole group at some former 
period subsided seven or eight fathoms ; and that the coral perished 
on the outer margin of those atolls which are now submerged, but that 
it continued alive, and grew up to the surface on those which are now 
perfect. If these atolls did subside, and if from the suddenness of the 
movement or from any other cause, those corals which are better adapted 
to live at a certain depth than at the surface, once got possession of the 
knolls, supplanting the former occupants, they would exert little or no 
tendency to grow upwards. To illustrate this, I may observe, that if 
the corals of the upper zone on the outer edge of Keeling atoll were to 
perish, it is improbable that those of the lower zone would grow to the 
surface, and thus become exposed to conditions for which they do not 
appear to be adapted. The conjecture, that the corals on the sub- 
merged knolls within the Chagos atolls have analogous habits with 
those of the lower zone outside Keeling atoll, receives some support 
from a remark by Captain Moresby, namely, that they have a different 
appearance from those on the reefs in the Maldiva atolls, which, as we 
have seen, all rise to the surface : he compares the kind of difference 
to that of the vegetation under different climates. I have entered at 
considerable length into this case, although unable to throw much light 
on it, in order to show that an equal tendency to upward growth ought 
not to be attributed to all coral-reefs, — to those situated at different 
depths, — to those forming the ring of an atoll or those on the knolls 
within a lagoon, — to those in one area and those in another. The in- 
ference, therefore, that one reef could not grow up to the surface 
within a given time, because another, not known to be covered with the 
same species of corals, and not known to be placed under conditions 
exactly the same, has not within the same time reached the surface, 
is unsound. 

Sectio7t II. 
On the rate of growth'of coral-reefs. 

The remark made at the close of the last section, naturally leads to 
this division of our subject, which has not, I think, hitherto been con- 
sidered under a right point of view. Ehrenbcrg * has stated, that in 
* Ehrenberg, as before cited, pp. 39, 46, and 50. 



58 THICKNESS OF MASSES OF ROCK 

the Red Sea, the corals only coat other rocks in a layer from one to 
two feet in thickness, or at most to a fathom and a half ; and he dis- 
believes that, in any case, they form, by their own proper growth, great 
masses, stratum over stratum. A nearly similar observation has been 
made by MM. Quoy and Gaimard,* with respect to the thickness of 
some upraised beds of coral, which they examined at Timor and some 
other places. Ehrenberg t saw certain large massive corals m the Red 
Sea, which he imagines to be of such vast antiquity, that they might 
have been beheld by Pharaoh ; and according to Mr. Lyell | there are 
certain corals at Bermuda, which are known by tradition, to have been 
living for centuries. To show how slowly coral-reefs grow upwards, 
Captain Beechey § has adduced the case of the Dolphin Reef oft" Tahiti, 
which has remained at the same depth beneath the surface, namely 
about two fathoms and a half, for a period of sixty-seven years. There 
are reefs in the Red Sea, which certainly do not appear || to have 
increased in dimensions during the last half-century, and from the 
comparison of old charts with recent surveys, probably not during the 
last two hundred years. These, and other similar facts, have so strongly 
impressed many with the belief of the extreme slowness of the growth 
of corals, that they have even doubted the possibility of islands in the 
great oceans having been formed by their agency. Others, again, who 
have not been overwhelmed by this difficulty, have admitted that it 
would require thousands, and tens of thousands of years, to form a 
mass, even of inconsiderable thickness ; but the subject has not, I 
believe, been viewed in the proper light. 

That masses of considerable thickness have been formed by the 
growth of coral, may be inferred with certainty from the following facts. 
In the deep lagoons of Peros Banhos and of tlie Great Chagos Bank, 
there are, as already described, small steep-sided knolls covered with 
living coral. There are similar knolls in the southern Maldiva atolls, 
some of which, as Captain Moresby assures me, are less than a hundred 
yards in diameter, and rise to the surface from a depth of between two 
hundred and fifty and three hundred feet. Considering their number, 
form, and position, it would be preposterous to suppose that they are 
based on pinnacles of any rock, not of coral formation ; or that sediment 
could have been heaped up into such small and steep isolated cones. 
As no kind of living coral grows above the height of a few feet, we are 
compelled to suppose that these knolls have been formed by the succes- 
sive growth and death of many individuals, — first one being broken off 
or killed by some accident, and then another, and one set of species 
being replaced by another set with different habits, as the reef rose 
nearer the surface, or as other changes supervened. The spaces between 
the corals would become filled up with fragments and sand, and 
such matter would probably soon be consolidated, for we learn from 

* " Annales des Sciences Nat.," torn, vi., p. 28. 

f Ehrenberg, ntsup., p. 42. 

X Lyell's "Principles of Geology," book iii., ch. xviii. 

§ Beechey's " Vo3'age to the Pacific," ch. viii. 

11 Ehrenberg, tit sup., p. 43. 



FORMED BY THE GROlVTH OP CORAL. 59 

Lieutenant Nelson,* that at Bermuda a process of this kind takes place be- 
neath water, without the aid of evaporation. In reefs, also, of the barrier 
class, we may feel sure, as I have shown, that masses of great thickness 
have been formed by the growth of the coral ; in the case of Vanikoro, 
judging only from the depth of the moat between the land and the reef, 
the wall of coral-rock must be at least three hundred feet in vertical 
thickness. 

It is unfortunate that the upraised coral-islands in the Pacific have 
not been examined by a geologist. The cliffs of Elizabeth Island, in 
the Low Archipelago, are eighty feet high, and appear, from Captain 
Beechey's description, to consist of a homogeneous coral-rock. From 
the isolated position of this island, we may safely infer that it is an 
upraised atoll, and therefore that it has been formed by masses of 
coral, grown together. Savage Island seems, from the description of 
the younger Forsler,t to have a similar structure, and its shores are 
about forty feet high : some of the Cook Islands also appear % to be 
similarly composed. Captain Belcher, R.N., in a letter which Captain 
Beaufort showed me at the admiralty, speaking of Bow atoll, says, 
" I have succeeded in boring forty-five feet through coral-sand, when 
the auger became jammed by the falling in of the surrounding crea??ty 
matter." On one of the Maldiva atolls, Captain Moresby bored to a 
depth of twenty-six feet, when his auger also broke : he has had the 
kindness to give me the matter brought up ; it is perfectly white, and 
like finely triturated coral-rock. 

In my description of Keeling atoll, I have given some facts, which 
show that the reef probably has grown outwards ; and I have found, 
just within the outer margin, the great mounds of Porites and of Mille- 
pora, with their summits lately killed, and their sides subsequently 
thickened by the growth of the coral : a layer, also, of Nullipora had 
already coated the dead surface. As the external slope of the reef is 
the same round the whole of this atoll, and round many other atolls, 
the angle of inclination must result from an adaption between the 
growing powers of the coral, and the force of the breakers, and their 
action on the loose sediment. The reef, therefore, could not increase 
outwards, without a nearly equal addition to every part of the slope, so 
that the original inchnation might be preserved, and this would require 
a large amount of sediment, all derived from the wear of corals and 
shells, to be added to the lower part. Moreover, at Keeling atoll, and 
probably in many other cases, the different kinds of corals would have 
to encroach on each other; thus the Nulliporae cannot increase outwards 
without encroaching on the Porites and Millepora compla7iata^ as is 
now taking place ; nor these latter without encroaching on the strongly 
branclied Madreporet, the Millepora aldcor7iis, and some Astroeas ; nor 
these again without a foundation being formed for them within the 
requisite depth, by the accumulation of sediment. How slow, then, 
must be the ordinary lateral or outward growth of such reefs. But off 

* "Geological Transactions," vol. v., p. 1 13. 

+ Forster's "Voyage round the World with Cook," vol. ii., pp. 163, 167. 

\ Williams's "Narrative of Missionary Enterprise," p. 30. 



6o OUTIVARD GROWTH OF REEFS. 

Christmas atoll, where the sea is much more shallow than is usual, we 
have good reason to believe that, within a period not very remote, the 
reef has increased considerably in width. The land has the extra- 
ordinary breadth of three miles ; it consists of parallel ridges of shells 
and broken corals, which furnish "an incontestable proof," as observed 
by Cook,* "that the island has been produced by accessions from the 
sea, and is in a state of increase." The land is fronted by a coral-reef, 
and from the manner in which islets are known to be formed, we may 
feel confident that the reef was not three miles wide, when the first, or 
most backward ridge, was thrown up ; and, therefore, we must conclude 
that the reef has grown outwards during the accumulation of the suc- 
cessive ridges. Here then, a wall of coral-rock of very considerable 
breadth has been formed by the outward growth of the living margin, 
within a period during which ridges of shells and corals, lying on the 
bare surface, have not decayed. There can be little doubt, from the 
account given by Captain Beechey, that Matilda atoll, in the Low Archi- 
pelago, has been converted in the space of thirty-four years, from being, 
as described by the crew of a wrecked whaling vessel, a " reef of rocks " 
into a lagoon-island, fourteen miles in length, with " one of its sides 
covered nearly the whole way with high trees." f The islets, also, on 
Keeling atoll, it has been shown, have increased in length, and since 
the construction of an old chart, several of them have become united 
into one long islet ; but in this case, and in that of Matilda atoll, 
we have no proof, and can only infer as probable, that the reef, that is 
the foundation of the islets, has increased as well as the islets them- 
selves. 

After these considerations, I attach little importance, as indicating the 
ordinary and still less the possible rate of outward growth of coral- 
reefs, to the fact that certain reefs in the Red Sea have not increased 
during a long interval of time ; or to other such cases, as that of Ouluthy 
atoll in the Caroline group, where every islet, described a hundred 
years before by Cantova was found in the same state by Lutke,;}: — 
without it could be shown that, in these cases, the conditions were 
favourable to the vigorous and unopposed growth of the corals living 
in the different zones of depth, and that a proper basis for the ex- 
tension of the reef was present. The former conditions must depend 
on many contingencies, and in the deep oceans where coral forma- 
tions most abound, a basis within the requisite depth can rarely be 
present. 

Nor do I attach any importance to the fact of certain submerged reefs, 
as those off Tahiti, or those within Diego Garcia not now being nearer 
the surface than they were many years ago, as an indication of the rate 

* Cook's " Third Voyage," book III., eh. x. 

f Beechey's " Voyage to the Pacific," ch. vii. and viii. 

\ F. Lutke's "Voyage autour du Monde." In the group Elato, however, 
it appears that what is now the islet Falipi, is called in Cantova's Chart, the 
Banc de Falipi. It is not stated whether this has been caused by the growth 
of coral, or by the accumulation of sand. 



UPWARD GROWTH OF REEFS, 6i 

under favourable circumstances of the upward growth of reefs ; after it 
has been shown, that all the reefs have grown to the surface in some 
of the Chagos atolls, but that in neighbouring atolls which appear to be 
of equal antiquity and to be exposed to the same external conditions, 
every reef remains submerged ; for we are almost driven to attribute 
this to a difference, not in the rate of growth, but in the habits of the 
corals in the two cases. 

In an old-standing reef, the corals, which are so different in kind on 
different parts of it, are probably all adapted to the stations they occupy, 
and hold their places, like other organic beings, by a struggle one with 
another, and with external nature ; hence we may infer that their growth 
would generally be slow, except under peculiarly favourable circum- 
stances. Almost the only natural condition, allowing a quick upward 
growth of the whole surface of a reef, would be a slow subsidence of 
the area in which it stood ; if, for instance, Keeling atoll were to subside 
two or three feet, can we doubt that the projecting njargin of live coral, 
about half an inch in thickness, which surrounds the dead upper sur- 
faces of the mounds of Porites, would in this case form a concentric 
layer over them, and the reef thus increase upwards, instead of, as at 
present, outwards ? The Nulliporae are now encroaching on the Porites 
and Millepora, but in this case might we not confidently expect that the 
latter would, in their turn, encroach on the Nulliporae ? After a sub- 
sidence of this kind, the sea would gain on the islets, and the great 
fields of dead but upright corals in the lagoon, would be covered by a 
sheet of clear water ; and might we not then expect that these reefs 
would rise to the surface, as they anciently did when the lagoon was 
less confined by islets, and as they did within a period of ten years in 
the schooner-channel, cut by the inhabitants ? In one of the Maldiva 
atolls, a reef, which within a very few years existed as an islet bearing 
cocoa-nut trees, was found by Lieutenant Prentice " entirely covered with 
live coral and Madrepore." The natives believe that the islet was 
washed away by a change in the currents, but if, instead of this, it had 
quietly subsided, surely every part of the island which offered a solid 
foundation, would in a like manner have become coated with living 
coral. 

Through steps such as these, any thickness of rock, composed of a 
singular intermixture of various kinds of corals, shells, and calcareous 
sediment, might be formed ; but without subsidence, the thickness 
would necessarily be determined by the depth at which the reef- 
bwilding polypifers can exist. If it be asked, at what rate in years I 
suppose a reef of coral favourably circumstanced could grow up from 
a given depth ; I should answer, that we have no precise evidence on 
this point, and comparatively little concern with it. We see, in 
innumerable points over wide areas, that the rate has been sufficient, 
either to bring up the reefs from various depths to the surface, or, as is 
more probable, to keep them at the surface, during progressive sub- 
sidences ; and this is a much more important standard of comparison 
than any cycle of years. 

It may, however, be inferred from the following facts, that the rate 



62 QUICK GROWTH OF CORAL. 

in years under favourable circumstances would be very far from slow. 
Dr. Allan, of Forres, has, in his MS. Thesis deposited in the 
library of the Edinburgh University (extracts from which I owe to the 
kindness of Dr. Malcolmson), the following account of some experi- 
ments, which he tried during his travels in the years 1830 to 1832 on 
the cast coast of Madagascar. " To ascertain the rise and progress of 
the coral-family, and fix the number of species met with at Foul Point 
(lat. 17° 40') twenty species of coral were taken off the reef and 
planted apart on a sand-bank three feet deep at lozv water. Each 
portion weighed ten pounds, and was kept in its place by stakes. 
Similar quantities were placed in a clump and secured as the rest. 
This was done in December 1830, In July following, each detached 
mass was nearly level with the sea at low water, quite immovable, 
and several feet long, stretching as the parent reef, with the coast 
current from north to south. The masses accumulated in a clump 
were found equally increased, but some of the species in such unequal 
ratios, as to be growing over each other." The loss of Dr. Allan's 
magnificent collection by shipwreck, unfortunately prevents its being 
known to what genera these corals belonged ; but from the numbers 
experimented on, it is certain that all the more conspicuous kinds must 
have been included. Dr. Allan informs me, in a letter, that he believes 
it was a Madrepora, which grew most vigorously. One may be per- 
mitted to suspect that the level of the sea might possibly have been 
somewhat different at the two stated periods ; nevertheless, it is quite 
evident that the growth of the ten-pound masses, during the six or 
seven months, at the end of which they were found immovably fixed * 
and several feet in length, must have been very great. The fact of 
the different kinds of coral, when placed in one clump, having increased 
in extremely unequal ratios, is very interesting, as it shows the manner 
in which a reef, supporting many species of coral, would probably be 
affected by a change in the external conditions favouring one kind more 
than another. The growth of the masses of coral in N. and S. lines 
parallel to the prevailing currents, whether due to the drifting of sedi- 
ment or to the simple movement of the water, is, also, a very interesting 
circumstance. 

A fact, communicated to me by Lieutenant Wellstead, I.N., in some 
degree corroborates the result of Dr. Allan's experiments : it is, that in 
the Persian Gulf a ship had her copper bottom encrusted in the course 
of twenty months with a layer of coral, tzvo feet in thickness, which 
it required great force to remove, when the vessel was docked : 
it was not ascertained to what order this coral belonged. The case 
of the schooner-channel choked up with coral in an interval of less 
than ten years, in the lagoon of Keeling atoll, should be here borne 

* It is stated by De la Beche ("Geological Manual," p. 143), on the 
authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that some 
specimens of Folypifers, placed by him in a sheltered pool of water, were 
found in the course of a few days firmly fixed by the secretion of a stony 
matter, to the bottom. 



THE DEPTH AT WHICH CORALS LIVE. 63 

in mind. We may also infer, . from the trouble which the inhabi- 
tants of the Maldiva atolls take to root out, as they express it, the 
coral-knolls from their harbours, that their growth can hardly be very 
slow,* 

From the facts given in this section, it may bo concluded, first, that 
considerable thicknesses of rock have certainly been formed within the 
present geological area by the growth of coral and the accumulation of 
its detritus ; and, secondly, that the increase of individual corals and of 
reefs, botli outwards or horizontally and upwards or vertically, under 
the peculiar conditions favourable to such increase, is not slow, when 
referred either to the standard of the average oscillations of level in 
the earth's crust, or to the more precise but less important one of a 
cycle of years. 

Section III. 

Oil the depths at which reef-building polypifers live. 

I have already described in detail, which might have appeared trivial, 
the nature of the bottom of the sea immediately surrounding Keeling 
atoll ; and I will now describe with almost equal care the soundings 
oft^ the fringing-reefs of Mauritius. I have preferred this arrangement, 
for the sake of grouping together facts of a similar nature. I sounded 
with the wide bell-shaped lead which Captain Fitzroy used at Keeling 
Island, but my examination of the bottom was confined to a few miles 
of coast (between Port Louis and Tomb Bay) on the leeward side of 
the island. The edge of the reef is formed of great shapeless masses 

* Mr. Stutchbury {West of England Journal, No. I., p. 50) has described a 
specimen of Agaricia, "weighing 2 lbs, 9 oz., which surrounds a species of 
oyster, whose age could not be more than two years, and yet is completely 
enveloped by this dense coral." I presume that the oyster was living when 
the specimen was procured ; otherwise the fact tells nothing. Mr. Stutchbury 
also mentions an £nchor, which had become entirely encrusted with coral in 
fifty years ; other cases, however, are recorded of anchors which have long 
remained amidst coral-reefs without having become coated. The anchor of 
the Beagle, in 1832, after having been down exactly one month at Rio de 
Janeiro, was so thickly coated by two species of Tubularia, that large spaces 
of the iron were entirely concealed ; the tufts of this horny zoophyte were 
between two and three inches in lengLh, It has been attempted to com- 
pute, but I believe erroneously, the rate of growth of a reef, from the fact 
mentioned by Captain Beechey, of the Chama gigas being embedded in 
coral-rock. But it should be remembered, that some species of this g^nus 
invariably live, both whilst young and old, in cavities, which the animal has 
the power of enlarging with its growth. I saw many of these shells thus 
embedded in the outer "flat" of Keeling atoll, which is composed of dead 
rock ; and therefore the cavities in this case had no relation whatever with 
the growth of coral. M. Lesson, also, speaking of this shell (Partie Zoolog, 
" Voyage de la Coqiiille "), has remarked, "que constamment scs valves etaient 
engages completcmcnt dans la masse dcs Madrepores," 



64 THE DEPTH AT WHICH CORALS LIVE, 

ot branching Madrepores, which chiefly consist of two species, — 
apparently M. corymbosa and pocillifera, — mingled with a few other 
kinds ot coral. These masses are separated from each other by the 
most irregular gullies and cavities, into which the lead sinks many feet. 
Outside this irregular border of Madrepores, the water deepens gradu- 
ally to twenty fathoms, which depth generally is found at the distance 
of from half to three-quarters of a mile from the reef. A little further 
out the depth is thirty fathoms, and thence the bank slopes rapidly 
into the depths of the ocean. This inclination is very gentle compared 
with that outside Keeling and other atolls, but compared with most 
coasts it is steep. The water was so clear outside the reef, that I could 
distinguish every object forming the nigged bottom. In this part, and 
to a depth of eight fathoms, 1 sounded repeatedly, and at each cast 
pounded the bottom with the broad lead, nevertheless the arming 
invariably came up perfectly clean, but deeply indented. From eight 
to fifteen fathoms a little calcareous sand w^as occasionally brought up, 
but more frequently the arming was simply indented. In all this 
space the two Madrepores above mentioned, and two species of Astraea, 
with rather large * stars, seemed the commonest kinds ; and it must 
be noticed that twice at the depth of fifteen fathoms, the arming was 
marked with a clean impression of an Astraea. Besides these litho- 
phytes, some fragments of the Millepora alcicornis, which occurs in the 
same relative position at Keeling Island, were brought up ; and in the 
deeper parts there were large beds of a Seriatopora, different from 
»S. subtilata, but closely allied to it. On the beach within the reef, the 
rolled fragments consisted chiefly of the corals just mentioned, and of a 
massive Porites, like that at Keeling atoll, of a Meandrina, Podllopora 
verrucosa^ and of numerous fragments of NuUipora. From fifteen to 
twenty fathoms the bottom was, with few exceptions, either formed of 
sand, or thickly covered with Seriatopora : this delicate coral seems 
to form at these depths extensive beds unmingled with any other kind. 
At twenty fathoms, one sounding brought up a fragment of Madrepora 
apparently M. pocillifera, and I believe it is the same species (for I 
neglected to bring specimens from both stations) which mainly forms 
the upper margin of the reef; if so, it grows in depths varying from 

* Since the preceding pages were printed off, I have received from Mr. 
Lyell a very interesting pamphlet, entitled " Remarks upon Coral Forma- 
tions," etc., by J. Couthouy, Boston, United States, 1842. There is a state- 
ment (p. 6), on the authority of the Rev. J. Williams, corroborating the 
remarks made by Ehrenberg and Lyell (p. 71 of this volume), on the 
antiquity of certain individual corals in the Red Sea and at Bermuda ; 
namely, that at Upolu, one of the Navigator Islands, " particular clumps of 
coral are known to the fishermen by name, derived from either some 
particular configuration or tradition attached to them, and handed down 
from time immemorial." With respect to the thickness of masses of coral- 
rock, it clearly appears, from the descriptions given by Mr. Couthouy 
(pp. 34, 58) that Mangaia and Aurora Islands are upraised atolls, composed 
of coral rock : the level summit of the former is about three hundred feet, 
and that of Aurora Island is two hundred feet above the sea-leveL 



OFF THE REEFS AT MAURITIUS. 65 

o to 20 fathoms. Between 20 and 23 fathoms I obtained several sound- 
ings, and they all showed a sandy bottom, with one exception at 30 
fathoms, when the arming came up scooped out, as if by the margin of 
a large Catyophyllia. Beyond 33 fathoms I sounded only once ; and 
from 86 fathoms, at the distance of one mile and a third from the edge 
of the reef, the arming brought up calcareous sand with a pebble of 
volcanic rock. The circumstance of the arming having invariably come 
up quite clean, when sounding within a certain number of fathoms off 
the reefs of Mauritius and Keeling atoll (eight fathoms in the former 
case, and twelve in the latter) and of its having always come up (with 
one exception) smoothed and covered with sand, when the depth ex- 
ceeded twenty fathoms, probably indicates a criterion, by which the 
limits of the vigorous growth of coral might in all cases be readily 
ascertained. I do not, however, suppose that if a vast number of 
soundings were obtained round these islands, the limit above assigned 
would be found never to vary, but I conceive the facts are sufficient 
to show, that the exceptions would be few. The circumstance of a 
gradual change, in the two cases, from a field of clean coral to a smooth 
sandy bottom, is far more important in indicating the depth at which 
the larger kinds of coral flourish than almost any number of separate 
observations on the depth, at which certain species have been dredged 
up. For we can understand the gradation, only as a prolonged struggle 
against unfavourable conditions. If a person were to find the soil 
clothed with turf on the banks of a stream of water, but on going to 
some distance on one side of it, he observed the blades of grass 
growing thinner and thinner, with intervening patches of sand, until he 
entered a desert of sand, he would safely conclude, especially if changes 
of the same kind were noticed in other places, that the presence of the 
water was absolutely necessary to the formation of a thick bed of turf: 
so may we conclude, with the same feeling of certainty, that thick 
beds of coral are formed only at small depths beneath the surface of 
the sea. 

I have endeavoured to collect every fact, which might either invalidate 
or corroborate this conclusion. Captain Moresby, whose opportunities 
for observation during his survey of the Maldiva and Chagos Archi- 
pelagoes have been unrivalled, informs me, that the upper part or zone 
of the steep-sided reefs, on the inner and outer coasts of the atolls in 
both groups, invariably consists of coral, and the lower parts of sand. 
At seven or eight fathoms depth, the bottom is formed, as could be seen 
through the clear water, of great living masses of coral, which at about 
ten fathoms generally stand some way apart from each other, with 
patches of white sand between them, and at a little greater depth these 
patches become united into a smooth steep slope, without any coral. 
Captain Moresby, also, informs me in support of his statement, that he 
found only decayed coral on the Padua Bank (northern part of the 
Laccadive group) which has an average depth between twenty-five and 
thirty-five fathoms, but that on some other banks in the same group 
with only ten or twelve fathoms water on them (for instance, the 
Tillacapcni bank), the coral was living. 



66 THE DEPTHS AT WHICH 

With regard to the coral-reefs in the Red Sea, Ehrenberg has the 
following passage : — " The living corals do not descend there into great 
depths. On the edges of islets and near reefs, where the depth was 
small, very many lived; but we found no more even at six fathoms. 
The pearl-fishers at Yemen and Massaua asserted that there was no 
coral near the pearl-banks at nine fathoms depth, but only sand. We 
were not able to institute any more special researches." * I am, how- 
ever, assured both by Captain Moresby and Lieutenant Wellstead, that 
in the more northern parts of the Red Sea, there are extensive beds of 
living coral at a depth of twenty-five fathoms, in which the anchors of 
their vessels were frequently entangled. Captain Moresby attributes 
the less depth, at which the corals are able to live in the places men 
tioned by Ehrenberg, to the greater quantity of sediment there ; and the 
situations, where they were flourishing at the depth of twenty-five fathoms, 
were protected, and the water was extraordinarily limpid. On the 
leeward side of Mauritius where I found the coral growing at a some- 
what greater depth than at Keeling atoll, the sea, owing apparently tc 
its tranquil state, was likewise very clear. Within the lagoons of some 
of the Marshall atolls, where the water can be but little agitated, there 
are, according to Kotzebue, living beds of coral in twenty-five fathoms. 
From these facts, and considering the manner in which the beds ot 
clean coral off Mauritius, Keeling Island, the Maldiva and Chagos 
atolls, graduated into a sandy slope, it appears very probable that the 
depth, at which reef-building polypifers can exist, is partly determined 
by the extent of inclined surface, which the currents of the sea and the 
recoiling waves have the power to keep free from sediment. 

MM. Quoy and Gaimardf believe that the growth of coral is confined 
within very limited depths ; and they state that they never found any 
fragment of an Astrsea (the genus they consider most efficient in 
forming reefs) at a depth above twenty-five or thirty feet. But we have 
seen that in several places the bottom of the sea is paved with massive 
corals at more than twice this depth ; and at fifteen fathoms (or twice 
this depth) off" the reefs of Mauritius, the arming was marked with the 
distinct impression of a living Astrsea. Millepora alcicornis lives in 
from o to 1 2 fathoms, and the genera Madrepora and Seriatopor from 
o to 20 fathoms. Captain Moresby has given me a specimen of 
Sideropora scabra (Porites of Lamarck) brought up alive from 17 
fathoms. Mr. Couthouy % states that he has dredged up on the Bahama 
banks considerable masses of Meandrina from 16 fathoms, and he has 
seen this coral growing in 20 fathoms. A Caryophyllia, half an inch 
in diameter, was dredged up alive from 80 fathoms otf Juan Fernandez 
(lat. 33° S.) by Captain P. P. King : § this is the most remarkable fact 
with which I am acquainted, showing the depth at which a genus of 

* Ehrenberg, " Ober die Niitur," etc., p. 50. 
f " Annales des Sci. Nat.," torn. vi. 
% •'Remarks on Coral Formations," p. 12. 

§ I am indebted to Mr. Stokes for having Jcindly communicated this fact to 
me, together with much other valuable information. 



CORALS CAN EXIST. 67 

corals often found on reefs, can exist.* We ought, however, to feel less 

* I will record in the form of a note all the facts that I have been able to 
collect on the depths, both within and without the tropics, at which those 
corals and corallines can live, which there is no reason to suppose ever 
materially aid in the construction of a reef. 



Name of Zoophyte. 


Depth in 

Fathoms. 


Country and S. 
Latitude. 


Authority. 


Sertularia 

Cellaria 

,, A minute scarlet encrust- 
ing species, found living . 
„ An allied, small stony 
sub-generic form 
A coral alHed to Vincularia, 

with eight rows of cells 
Tubulipora, near to T. patima . 

Do. Do. 
Cellepora, several species, and 
allied sub-generic forms 

Ditto 

Ditto 

Eschara 

Ditto 

Retepcra 

Ditto 

Millepora, a strong coral with. 
cylindrical branches, of a j 
pink colour, about two f 
inches high, resembling in I 
the form of its oritices M. \ 
aspera of Lamarck . / 

Coralium 

Antipathes .... 
Gorgonia (or an allied form) . 


40 
Ditto 

190 

48 

40 

Ditto 

94 

40 
40 and 57 
48 
30 
48 
40 

ICO 

94 and 30 

120 
16 
160 


Cape Horn 66° 
Ditto 

Keeling At. 12° 

S. Cruz Riv. 50° 

Cape Horn 

Ditto 

East Chiloe 43° 

Cape Horn 

Chonos Arch. 45° 

S. Cruz 50° 

Tierra del Fuego 53° 

S. Cruz R. 50"" 

Cape Horn 

C. Good Hope 34° 

E. Chiloe 43° Tierra 
del Fuego 53° 

Barbary 33° N. 

Chonos 45° 

f Abrolhos on the 
\ coast of Brazil 18° 


(Where none is 
given, the obser- 
vation is my own.) 

rQuoy and Gai- 
J mard, Ann. 
'S Scten. Nat., t. 
Cvi., p. 284. 

( Peyssonel in 
; paper read to 
j Royal Society 
^May 1752. 

rCapt. Beechey 
J informed me of 
) this fact in a 
( letter. 



Ellis ("Nat. Hist, of Coralline," p. 96) states that Ombellularia was 
procured in lat. 79° N. sti(kmg to a line from the depth of 236 fathoms; 
hence this coral either must have been floating loose, or was entangled 
in stray line at the bottom. Off Keeling atoll a compound Ascidia (Sigillina) 
was brought up from 39 fathoms, and a piece of sponge, apparently living, 
from 70, and a fragment of Nullipora also apparently living from 92 fathoms. 
At a greater depth than 90 fathoms off this coral island, the bottom was 
thickly strewed with joints of Halimeda and small fragments of other 
Nulliporae, but all dead. Captain B. Allen, R.N., informs me that in the 



68 THE DEPTHS AT WHICH CORALS CAN EXIST. 

surprise at this fact, as Car}^oph3'llia alone of the lamelliform genera, 
ranges far beyond the tropics ; it is found in Zetland * in Lat. 60° N. 
in deep water, and I procured a small species from Tierra del Fuego in 
Lat. 53° S. Captain Beechey informs me, that branches of pink and 
yellow coral were frequently brought up from between twenty and 
twenty-five fathoms off the Low atolls ; and Lieutenant Stokes, writing 
to me from the N.W. coast of Australia, says that a strongly branched 
coral was procured there from thirty fathoms ; unfortunately it is not 
known to what genera these corals belong. 

Although the limit of depth, at which each particular kind of coral 
ceases to exist, is far from being accurately known ; yet when we bear 
in mind the manner in whicli the clumps of coral gradually became 
infrequent at about the same depth, and wholly disappeared at a greater 
depth than twenty fathoms, on the slope round Keeling atoll, on the 
leeward side of the Mauritius, and at rather less depth, both without 
and within the atolls of the Maldiva and Chagos Archipelagoes ; and 
when we know that the reefs round these islands do not differ from 
other coral formations in their form and structure, we may, I think, 
conclude that in ordinary cases, reef-building polypifers do not flourish 
at greater depths than between twenty and thirty fathoms. 

It has been argued t that reefs may possibly rise from very great 
depths through the means of small corals, first making a platform for 
the growth of the stronger kinds. This, however, is an arbitrary sup- 
position: it is not always remembered, that in such cases there is 
an antagonist power in action, namely, the decay of organic bodies, 
when not protected by a covering of sediment, or by their own 
rapid growth. We have, moreover, no right to calculate on unlimited 
time for the accumulation of small organic bodies into great masses. 
Every fact in geology proclaims that neither the land, nor the bed of 
the sea retain for indefinite periods the same level. As well might it 
be imagined that the British Seas would in time become choked up 
with beds of oysters, or that the numerous small corallines off the 
inhospitable shores of Tierra del Fuego would in time form a solid and 
extensive coral-reef. 

survey of the West Indies it was noticed that between the depth of 10 and 
2CO fathoms, the sounding lead very generally came up coated with the dead 
joints of a Halimeda, of which he showed me specimens. Off Pernambuco, 
in Brazil, in about twelve fathoms, the bottom was covered with fragments 
dead and alive of a dull red Nullipora, and I infer from Roussin's chart, that 
a bottom of this kind extends over a wide area. On the beach, within the 
coral-reefs of Mauritius, vast quantities of fragments of Nulliporae were 
piled up. From these facts it appears, that these simply organized bodies 
are amongst the most abundant productions of the sea. 

* Fleming's "British Animals," genus Caryophyllia. 

•j" Journal of the Royal Geographical Society, 183 1, p. 218. 



ATOLLS NOT BASED ON CRATERS. 69 



CHAPTER V. 

Theory of the formation of the different classes of 
coral-reefs. 

The atolls of the larger archipelagoes are not formed on submerged craters, 
or on banks of sediment. — Immense areas interspersed with atolls. — 
Their subsidence. — The effects of storms and earthquakes on atolls. — 
Recent changes in their state. — The origin of barrier-reefs and of atolls. 
— Their relative forms. — The step-formed ledges and walls round the 
shores of some lagoons. — The ring-formed reefs of the Maldiva atolls. — 
The submerged condition of parts or of the whole of some annular reefs. 
— The disseverment of large atolls. — The union of atolls by linear reefs. 
— The Great Chagos Bank. — Objections from the area and amount of 
subsidence required by the theory, considered. — The probable com- 
position of the lower parts of atolls. 

The naturalists who have visited the Pacific, seem to have had their 
attention riveted by the lagoon-islands, or atolls, — those singular rings 
of coral-land which rise abruptly out of the unfathomable ocean — and 
have passed over, almost unnoticed, the scarcely less wonderful en- 
circling barrier-reefs. The theory most generally received on the 
formation of atolls, is that they are based on submarine craters ; but 
where can we find a crater of the shape of Bow atoll, which is five 
times as long as it is broad (Plate I., Fig. 4) ; or like that of Menchicoff 
Island (Plate II., Fig. 3), with its three loops, together sixty miles in 
length ; or like Rimsky Korsacoff, narrow, crooked, and fifty-four miles 
long ; or like the northern Maldiva atolls, made up of numerous ring- 
formed reefs, placed on the margin of a disc, — one of which discs is 
eighty-eight miles in length, and only from ten to twenty in breadth ? It 
is, also, not a little improbable, that there should have existed as many 
craters of immense size crowded together beneath the sea, as there are 
now in some parts atolls. But this theory hes under a greater difficulty, 
as will be evident, when we consider on what foundations the atolls 
of the larger archipelagoes rest : nevertheless, if the rim of a crater 
afforded a basis at the proper depth, I am far from denying that a reef 
like a perfectly characterised atoll might not be formed ; some such, 
perhaps, now exist ; but I cannot believe in the possibility of the greater 
number having thus originated. 

An earlier and better theory was proposed by Chamisso ; * he sup- 
poses that as the more massive kinds of corals prefer the surf, the outer 
portions, in a reef rising from a submarine basis, would first reach the 
surface and consequently form a ring. But on this view it must be 
assumed, that in every case the basis consists of a flat bank ; for if it 
were conically formed, like a mountainous mass, we can see no reason 
why the coral should spring up from the flanks, instead of from the 
central and highest parts : considering the number of the atolls in the 
Pacific and Indian Oceans, this assumption is very improbable. As the 

♦ Kotzebuc's "First Voyage," vol. iii., p. 331. 



70 FASTNESS OF THE AREAS 

lagoons of atolls are sometimes even more than forty fathoms deep, it 
must, also, be assumed on this view, that at a depth at which the 
waves do not break, the coral grows more vigorously on the edges of 
a bank than on its central part ; and this is an assumption without any 
evidence in support of it. I remarked, in the third chapter, that a 
reef, growing on a detached bank, would tend to assume an atoll- 
like structure ; if, therefore, corals were to grow up from a bank, 
with a level surface some fathoms submerged, having steep sides and 
being situated in a deep sea, a reef not to be distinguished from an 
atoll, might be formed : I believe some such exist in the West Indies. 
But a difficulty of the same kind with that affecting the crater theory, 
renders, as we shall presently see, this view inapplicable to the greater 
number of atolls. 

No theory worthy of notice has been advanced to account for those 
barrier-reefs, which encircle islands of moderate dimensions. The 
great reef which fronts the coast of Australia has been supposed, but 
without any special facts, to rest on the edge of a submarine precipice, 
extending parallel to the shore. The origin of the third class or of 
fringing-reefs presents, I believe, scarcely any difficulty, and is simply 
consequent on the polypifers not growing up from great depths, and 
their not flourishing close to gently shelving beaches where the water is 
often turbid. 

What cause, then, has given to atolls and barrier-reefs their charac- 
teristic forms ? Let us see whether an important deduction will not 
follow from the consideration of these two circumstances, first, the 
reef-building corals ilourishing only at limited depths ; and secondl}', the 
vastncss of the areas interspersed with coral-reefs and coral-islets, none 
of which rise to a greater height above the level of the sea, than that 
attained by matter thrown up by the waves and winds. I do not make 
this latter statement vaguely ; I have carefully sought for descriptions 
of every island in the intertropical seas ; and my task has been in 
some degree abridged by a map of the Pacific, corrected in 1834 by 
MM. D'Urville and Lottin, in which the low islands are distinguished 
from the high ones (even from those much less than a hundred feet in 
height) by being written without a capital letter ; I have detected a few 
errors in this map, respecting the height of some of the islands, which 
will be noticed in the Appendix, where 1 treat of coral formations in 
geographical order. To the Appendix, also, I must refer for a more 
particular account of the data on which the statements on the next page 
are grounded. I have ascertained, and chiefly from the writings of 
Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and Lutke, regarding 
the Pacific ; and from Moresby * with respect to the Indian Ocean, that 
in the following cases the term " low island " strictly means land of the 
height commonly attained by matter thrown up by the winds and the 

* See also Captain Owen's and Lieutenant Wood's papers in the Geogra- 
phical Journal, on the Maldiva and Laccadive Archipelagoes. These officers 
particularly refer to the lowness of the islets; but I chiefly ground my asser- 
tion respecting these two groups, and the Chagos group, from information 
communicated to me by Captain Moresby. 



STUDDED WITH LOW CORAL-ISLANDS. 71 

waves of an open sea. If we draw a line (the plan I have always 
adopted) joining the external atolls of that part of the Low Archipelago 
in which the islands are numerous, the figure will be a pointed elHpse 
(reaching from Hood to Lazaref Island), of which the longer axis is 
840 geographical miles, and the shorter 420 miles ; in this space * none 
of the innumerable islets united into great rings rise above the stated 
level. The Gilbert group is very narrow, and 300 miles in length. In 
a prolonged line from this group, at the distance of 240 miles, is the 
Marshall Archipelago, the figure of which is an irregular square, one end 
being broader than the other ; its length is 520 miles, with an average 
width of 240 ; these two groups together are 1,040 miles in length, and 
all their islets are low. Between the southern end of the Gilbert and 
the northern end of Low Archipelago, the ocean is thinly strewed with 
islands, all of which, as far as I have been able to ascertain, are low ; 
so that from nearly the southern end of the Low Archipelago, to the 
northern end of the Marshall Archipelago, there is a narrow band of 
ocean, more than 4,000 miles in length, containing a great number of 
islands, all of which are low. In the western part of the Caroline 
Archipelago, there is a space of 480 miles in length, and about 100 
broad, thinly interspersed with low islands. Lastly, in the Indian 
Ocean, the archipelago of the Maldivas is 470 miles in length, and 60 in 
breadth ; that of the Laccadives is 150 by .100 miles ; as there is a low 
island between these two groups, they may be considered as one group 
of 1,000 miles in length. To this may be added the Chagos group of 
low islands, situated 280 miles distant, in a line prolonged from the 
southern extremity of the Maldivas. This group, including the sub- 
merged banks, is 170 miles in length and 80 in breadth. So striking is 
the uniformity in direction of these three archipelagoes, all the islands 
of which are low, that Captain Moresby, in one of his papers, speaks of 
them as parts of one great chain, nearly 1,500 miles long. I am, then, 
fully justified in repeating, that enormous spaces, both in the Pacific and 
Indian Oceans, are interspersed with islands, of which not one rises 
above that height, to which the waves and winds in an open sea can 
heap up matter. 

On what foundations, then, have these reefs and islets of coral been 
constructed ? A foundation must originally have been present beneath 
each atoll at that limited depth, which is indispensable for the first 
growth of the reef-building polypifers. A conjecture will perhaps be 
hazarded, that the requisite bases might have been afforded by the 
accumulation of great banks of sediment, which owing to the action of 
superficial currents (aided possibly by the undulatory movement of the 

* I find from Mr. Couthouy's pamphlet (p. 58) that Aurora Island is about 
two hundred feet in height ; it consists of coral-rock, and seems to have been 
formed by the elevation of an atoll. It lies north-east of Tahiti, close without 
the line bounding the space coloured dark blue in the map appended to this 
volume. Honden Island, which is situated in the extreme north-west part of 
the Low Archipelago, according to measurements made on board the Beafi^le, 
whilst sailing by, is 114 feet from the summit of the trees to the water's edge. 
This island appeared to resemble the other atolls of the group. 



72 SUBSIDENCE OF THE AREAS 

sea) did not quite reach the surface, — as actually appears to have been 
the case in some parts of the West Indian Sea. But in the form and 
disposition of the groups of atolls, there is nothing to countenance this 
notion ; and the assumption without any proof, that a number of 
immense piles of sediment have been heaped on the floor of the great 
Pacific and Indian Oceans, in their central parts far remote from land, 
and where the dark blue colour of the limpid water bespeaks its purity, 
cannot for one moment be admitted. 

The many widely-scattered atolls must, therefore, rest on rocky 
bases. But we cannot believe that the broad summit of a mountain 
lies buried at the depth of a few fathoms beneath every atoll, and 
nevertheless throughout the immense areas above-named, with not one 
point of rock projecting above the level of the sea ; for we may judge 
with some accuracy of mountains beneath the sea, by those on the 
land ; and where can we find a single chain several hundred miles in 
length and of considerable breadth, much less several such chains, with 
their many broad summits attaining the same height, within from 120 
to 180 feet? If the data be thought insufficient, on which I have 
grounded my belief, respecting the depth at which the reef-building 
polj'pifers can exist, and it be assumed that they can flourish at a 
depth of even one hundred fathoms, yet the weight of the above argu- 
ment is but little diminished, for it is almost equally improbable, that 
as many submarine mountains, as there are low islands in the several 
great and widely separated areas above specified, should all rise within 
six hundred feet of the surface of the sea and not one above it, as that 
they should be of the same height within the smaller limit of one or 
two hundred feet. So highly improbable is this supposition, that we 
are compelled to believe, that the bases of the many atolls did never at 
any one period all lie submerged within the depth of a few fathoms 
beneath the surface, but that they were brought into the requisite 
position or level, some at one period and some at another, through 
movements in the earth's crust. But this could not have been effected 
by elevation, for the belief that points so numerous and so widely 
separated were successively uplifted to a certain level, but that not one 
point was raised above that level, is quite as improbable as the former 
supposition, and indeed differs little from it. It will probably occur to 
those who have read Ehrenburg's account of the Reefs of the Red Sea, 
that many points in these great areas may have been elevated, but that 
as soon as raised, the protuberant parts were cut off by the destroying 
action of the waves : a moment's reflection, however, on the basin-like 
form of the atolls, will show that this is impossible ; for the upheaval 
and subsequent abrasion of an island would leave a flat disc, which 
might become coated with coral, but not a deeply concave surface ; 
moreover, we should expect to see, in some parts at least, the rock of 
the foundation brought to the surface. If, then, the foundations of the 
many atolls were not uplifted into the requisite position, they must of 
necessity have subsided into it ; and this at once solves every difficulty,* 

* The additional difficulty on the crater hypothesis before alluded to, will 
now be evident ; for on this view the volcanic action must be supposed to 



STUDDED IVlTH LOW CORAL-ISLANDS. 73 

for we may safely infer, from the facts given in the last chapter, that 
during a gradual subsidence the corals would be favourably circum- 
stanced for building up their solid frame works and reaching the surface, 
as island after island slowly disappeared. Thus areas of immense 
extent in the central and most profound parts of the great oceans, might 
become interspersed with coral-islets, none of which would rise to a 
greater height than that attained by detritus heaped up by the sea, 
and nevertheless they might all have been formed by corals, which 
absolutely required for their growth a solid foundation within a few 
fathoms of the surface. 

It would be out of place here to do more than allude to the many 
facts, showing that the supposition of a gradual subsidence over large 
areas is by no means improbable. We have the clearest proof that a 
movement of this kind is possible, in the upright trees buried under 
the strata many thousand feet in thickness ; we have also every reason 
for believing that there are now large areas gradually sinking, in the 
same manner as others are rising. And when we consider how many 
parts of the surface of the globe have been elevated within recent 
geological periods, we must admit that there have been subsidences 
on a corresponding scale, lor otherwise the whole globe would have 
swollen. It is very remarkable that Mr. Lyell,* even in the first edition 
of his " Principles of Geology," inferred that the amount of subsidence 
in the Pacific must have exceeded that of elevation, from the area of 
land being very small relatively to the agents there tending to form it, 
namely, the growth of coral and volcanic action. But it will be asked, 
are there any direct proofs of a subsiding movement in those areas, in 
which subsidence will explain a phenomenon otherwise inexplicable ? 
This, however, can hardly be expected, for it must ever be most difficult, 
excepting in countries long civilised, to detect a movement, the tendency 
of which is to conceal the part affected. In baibarous and semi- 
have formed within the areas specified a vast number of craters, all rising 
v/ithin a few fathoms of the surface, and not one above it. The supposition 
that the craters were at different times upraised above the surface, and were 
there abraded by the surf and subsequently coated by corals, is subject to 
nearly the same objections with those given at the bottom of the last page ; 
but I consider it superfluous to detail all the arguments opposed to such a 
notion. Chamisso's theory, from assuming the existence of so many banks, 
all lying at the proper depth beneath the water, is also vitally defective. 
The same observation applies to an hypothesis of Lieutenant Nelson's 
("Geolog.Trans.,"vol. v., p. 122), who supposes that the ring- formed structure 
is caused by a greater number of germs of corals becoming attached to ihe 
declivity, than to the central plateau of a submarine bank : it likewise applies 
to the notion iormerly entertained (Forster's "Observ.," p. 151), that lagoon- 
islands owe their peculiar form to the instinctive tendencies of the polypifers. 
According to this latter view, the corals on the outer margin of the reef 
instinctively expose themselves to the surf in order to afford protection to 
corals living in the lagoon, which belong to other genera, and to other 
families ! 

* "Principles of Geology," sixth edition, vol. iii., p. 386. 



74 SUBSIDENCE OF THE AREAS 

civilised nations how long might not a slow movement, even of eleva*- 
tion such as that now affecting Scandinavia, have escaped attention ! 

Mr. Williams* insists strongly that the traditions of the natives, 
which he has taken much pains in collecting, do not indicate the appear- 
ance of any new islands : but on the theory of a gradual subsidence, all 
that would be apparent would be, the water sometimes encroaching 
slowly on the land, and the land again recovering by the accumulation 
of detritus its former extent, and perhaps sometimes the conversion of 
an atoll with coral islets on it, into a bare or into a sunken annular 
reef. Such changes would naturally take place at the periods when 
the sea rose above its usual limits, during a gale of more than ordinary 
strength ; and the effects of the two causes would be hardly distinguish- 
able. In Kotzebue's " Voyage " there are accounts of islands, both in 
the Caroline and Marshall Archipelagoes, which have been partly 
washed away during hurricanes; and Kadu, the native who was on 
board one of the Russian vessels, said " he saw the sea at Radack rise 
to the feet of the cocoa-nut trees ; but it was conjured in time."t A 
storm lately entirely swept away two of the Caroline islands, and con- 
verted them into shoals ; it partly, also, destroyed two other islands.^ 
According to a tradition which was communicated to Captain Fitzroy, 
it is believed in the Low Archipelago, that the arrival of the first ship 
caused a great inundation, which destroyed many lives. Mr. Stutchbury 
relates, that in 1825, the western side of Chain Atoll, in the same 
group, was completely devastated by a hurricane, and not less than 
300 lives lost : " in this instance it was evident, even to the natives, 
that the hurricane alone was not sufficient to account for the violent 
agitation of the ocean."§ That considerable changes have taken 
place recently in some of the atolls in the Low Archipelago, appears 
certain from the case already given of Matilda Island : with re- 
spect to Whitsunday and Gloucester Islands in this same group, 
we must either attribute great inaccuracy to their discoverer, the 
famous circumnavigator WalHs, or believe that they have undergone 
a considerable change in the period of fifty-nine years, between his 
voyage and that of Captain Beechey's. Whitsunday Island is de- 
scribed by Wallis as "about four miles long, and three wide," now it 
is only one mile and a half long. The appearance of Gloucester Island, 
in Captain Beechey's words, || "has been accurately described by its 
discoverer, but its present form and extent differ materially." Blenheim 
reef, in the Chagos group, consists of a water-washed annular reef, 
thirteen miles in circumference, surrounding a lagoon ten fathoms deep : 
on its surface there were a few worn patches of conglomerate coral- 
rock, of about the size of hovels ; and these Captain Moresby con- 

* Williams's "Narrative of Missionary Enterprise," p. 31. 
t Kotzebue's " First Voyage," vol. iii., p. 168. 
% M. Desmoulins in " Comptes Rendus," 1840, p. 837. 
§ West of England Journal, No. I., p. 35. 

II Beechey's " Voyage to the Pacific," chap, vii., and Wallis's " Voyage in 
the Dolphin" chap. iv. 



STUDDED WITH LOW CORAL-ISLANDS. 75 

sidered as being, without doubt, the last remnants of islets ; so that 
here an atoll has been converted into an atoll-formed reef. The 
inhabitants of the Maldiva Archipelago, as long ago as 1605, declared, 
•• that the high tides and violent currents were diminishing the number 
of the islands : " * and I have already shown, on the authority of 
Captain Moresby, that the work of destruction is still in progress ; but 
that on the other hand the first formation of some islets is known to the 
present inhabitants. In such cases, it would be exceedingly difficult to 
detect a gradual subsidence of the foundation, on which these mutable 
structures rest. 

Some of the archipelagoes of low coral-islands are subject to earth- 
quakes : Captain Moresby informs me that they are frequent, though 
not very strong, in the Chagos group, which occupies a very central 
position in the Indian Ocean, and is far from any land not of coral 
formation. One of the islands in this group was formerly covered by 
a bed of mould, which, after an earthquake, disappeared, and was 
believed by the residents to have been washed by the rain through the 
broken masses of underlying rock ; the island was thus rendered un- 
productive. Chamissof states, that earthquakes are felt in the Marshall 
atolls, which are far from any high land, and likewise in the islands of 
the Caroline Archipelago. On one of the latter, namely Oulleay atoll. 
Admiral Lutke, as he had the kindness to inform me, observed several 
straight fissures about a foot in width, running for some hundred yards 
obliquely across the whole width of the reef. Fissures indicate a 
stretching of the earth's crtist, and, therefore, probably changes in its 
level ; but these coral-islands, which have been shaken and fissured, 
certainly have not been elevated, and, therefore, probably they have 
subsided. In the chapter on Keeling atoll, I attempted to show by 
direct evidence, that the island underwent a movement of subsidence, 
during the earthquakes lately felt there. 

The facts stand thus ; — there are many large tracts of ocean, without 
any high land, interspersed with reefs and islets, formed by the growth 
of those kinds of corals, which cannot live at great depths ; and the 
existence of these reefs and low islets, in such numbers and at such 
distant points, is quite inexplicable, excepting on the theory, that the 
bases on which the reefs first became attached, slowly and successively 
sank beneath the level of the sea, whilst the corals continued to grow 
upwards. No positive facts are opposed to this view, and some general 
considerations render it probable. There is evidence of change in form, 
whether or not from subsidence, on some of these coral-islands ; and 
there is evidence of subterranean disturbances beneath them. Will 
then the theory, to which we have thus been led, solve the curious 
problem, — what has given to each class of reef its peculiar form ? 

Let us in imagination place within one of the subsiding areas, an 
island surrounded by a "fringing-reef," — that kind, which alone offers 
no difficulty in the explanation of its origin. Let the imbroken lines 

* See an extract from Pyrard's Voyage in Captain Owen's paper on the 
Maldiva Archipelago, in the Geographical Journal, vol. ii., p. 84. 

•f See Chamisso, in Kotzebue's "First Voyage," vol. iii , pp. 182 and 136. 



76 



THEORY OF THE FORMATION 



and the oblique shading in the woodcut (No. 4) represent a vertical 
section through such an island ; and the horizontal shading will 
represent the section of the reef. Now, as the island sinks down, either 
a lew feet at a time or quite insensibly, we may safely infer from what 
we know of the conditions favourable to the growth of coral, that the 
living masses bathed by the surf on the margin of the reef, will soon 
regain the surface. The water, however, will encroach, little by little, 
on the shore, the island becoming lower and smaller, and the space 
betweei; the edge of the reef and the beach proportionately broader. 
A section of the reef and island in this state, after a subsidence of 
several hundred feet, is given by the dotted lines : coral-islets are 
supposed to have been formed on the new reef, and a ship is anchored 
in the lagoon-channel. This section is in every respect that of an 
encircling barrier-reef; it is, in fact, a section taken* east and west 
through the highest point 0/ the encircled island of Bolabola ; of which 
a plan is given in Plate I., Fig. 5. The same section is more clearly 
shown in the following woodcut (No. 5) by the unbroken lines. The 
width of the reef, and its slope, both on the outer and inner side, will 
have been determined by the growing powers of the coral, under the 




[No. 4.] 

A A— Outer edpe of the reef at the level of the sea. 

BB— Shores of the island, , /. . ., 

A' A'— Outer edge of the reef, after its upward growth during: a period of subsidence. 

CC— The lagoon-channel between the reef and the shores of the now encirc.ed 
land. 

B'B'— The shores of the encircled island. 

N.B.— In this, and the following woodcut, the subsidence of the land could only be 
represented by an apparent rise in the level of the sea. 

conditions (for instance the force of the breakers and of the currents) to 
which it has been exposed ; and the lagoon-channel will be deeper or 
shallower, in proportion to the growth of the delicately branched corals 
within the reef, and to the accumulation of sediment, relatively, also, 
to the rate of subsidence and the length of the intervening stationary 
periods. 

It is evident in this section, that a line drawn perpendicularly down 
from the outer edge of the new reef to the foundation of solid rock, 

* The section has been made from the chart given in the " Atlas of the 
Voyage of the Coquille" The scale is -57 of an inch to a mile. The height of 
the island, according to M. Lesson, is 4,026 feet. The deepest part of the 
lagoon-channel is 162 feet; its depth is exaggerated in the woodcut for the 
sake of clearness. 



OF BARRIER-REEFS AND ATOLLS. 



77 



exceeds by as many feet as there have been feet of subsidence, that 
small limit of depth at which the effective polypifers can live — the corals 
having grown up, as the whole sank down, from a basis formed of other 
corals and their consolidated fragments. Thus the difficulty on this 
head, which before seemed so great, disappears. 

As the space between the reef and the subsiding shore continued to 
increase in breadth and depth, and as the injurious effects of the 
sediment and fresh water borne down from the land were consequently 
lessened, the greater number of the channels, with which the reef in its 
fringing state must have been breached, especially those which fronted 
the smaller streams, will have become choked up with the growth of 
coral : on the windward side of the reef, where the coral grows most 
vigorously, the breaches will probably have first been closed. In 
barrier-reefs, therefore, the breaches kept open by draining the tidal 
waters of the lagooii-channel, will generally be placed on the leeward 
side, and they will still face the mouths of the larger streams, although 
removed beyond the influence of their sediment and fresh water ; — and 
this, it has been shown, is commonly the case. 




[No. 5- 



A' A'— Outer edg^es of the barrier-reet at the level of the sea. The cocoa-nut trees 
represent coral-islets formed on the reef. 

CC — The lagoon-channel. 

B'B'— The shores of the isknd, generally formed of low alluvial land and of coral 
detritus from the lagoon-channel. 

A" A". — The outer edges of the reef now forming an atoll. 

C— The lagoon of the newly formed atoll. According to the scale, the depth of the 
lagoon and of the lagoon-channel is exaggerated. 

Referring to the diagram shown above, in which the newly formed 
barrier-reef is represented b}'' unbroken lines, instead of by dots as in 
the former woodcut, let the work of subsidence go on, and the doubly 
pointed hill will form two small islands (or more, according to the 
number of the hills) included within one annular reef. Let the island 
continue subsiding, and the coral-reef will continue growing up on its 
own foundation, whilst the water gains inch by inch on the land, until the 
last and highest pinnacle is covered, and there remains a perfect atoll. 
A vertical section of this atoll is shown in the woodcut by the dotted 
lines ;— a ship is anchored in its lagoon, but islets are not supposed yet 
to have been formed on the reef. The depth of the lagoon and the 



78 THEORY OF THE FORMATION 

width and slope of the reef, will depend on the circumstances just 
referred to under barrier-reefs. Any further subsidence will produce 
no change in the atoll, except perhaps a diminution in its size, from the 
reef not growing vertically upwards ; but should the currents of the sea 
act violently upon it, and should the corals perish on part or on the 
whole of its margin, changes would result during subsidence which will 
be presently noticed. I may here observe, that a bank either of rock or 
of hardened sediment, level with the surface of the sea, and fringed 
with living coral, would (if not so small as to allow the central space to 
be quickly filled up with detritus) by subsidence be converted immedi- 
ately into an atoll, v\ ithout passing, as in the case of a reef fringing the 
shore of an island, through the intermediate form of a barrier-reef. If 
such a bank lay a few fathoms submerged, the simple growth of the 
coral (as remarked in the third chapter) without the aid of subsidence, 
would produce a structure scarcely to be distinguished from a true 
atoll ; for in all cases the corals on the outer margin of a reef, from 
having space and being freely exposed to the open sea, will grow 
vigorously and tend to form a continuous ring whilst the growth of 
the less massive kinds on the central expanse, will be checked by the 
sediment formed there, and by that washed inwards by the breakers ; 
and as the space becomes shallower, their growth will, also, be checked 
by the impurities of the water, and probably by the small amount of 
food brought by the enfeebled currents, in proportion to the surface of 
living reefs studded with innumerable craving mouths : the subsidence 
of a reef based on a bank of this kind, would give depth to its central 
expanse or lagoon, steepness to its flanks, and through the free growth 
of the coral, symmetry to its outline : — I may here repeat that the larger 
groups of atolls in the Pacific and Indian Oceans cannot be supposed 
to be founded on banks of this nature. 

If, instead of the island in the diagram, the shore of a continent 
fringed by a reef had subsided, a great barrier-reef, like that on the north- 
east coast of Australia, would have necessarily resulted ; and it would 
have been separated from the main land by a deep-water channel, broad 
in proportion to the amount of subsidence, and to the less or greater 
inclination of the neighbouring coast-line. The effect of the continued 
subsidence of a great barrier-reef of this kind, and its probable conver- 
sion into a chain of separate atolls, will be noticed, when we discuss 
the apparent progressive disseverment of the larger Maldiva atolls. 

We now are able to perceive that the close similarity in form, dimen- 
sions, structure, and relative position (which latter point will hereafter 
be more fully noticed) between fringing and encircling barrier-reefs, 
and between these latter and atolls, is the necessary result of the trans- 
formation, during subsidence of the one class into the other. On this 
view, the three classes of reefs ought to graduate into each other. 
Reefs having intermediate character between those of the fringing and 
barrier classes do exist ; for instance, on the south-west coast of 
Madagascar, a reef extends for several miles, within which there is a 
broad channel from seven to eight fathoms deep, but the sea does not 
deepen abruptly outside the reef. Such cases, however, arc open to 



OF BARRIER-REEFS AND ATOLLS, 79 

some doubts, for an old fringing-reef, which had extended itself a little 
on a basis of its own formation, would hardly be distinguishable from 
a barrier-reef, produced by a small amount of subsidence, and with its 
lagoon-channel nearly filled up with sediment during a long stationary 
period. Between barrier-reefs, encircling either one lofty island or 
several small low ones, and atolls including a mere expanse of water, 
a striking series can be shown : in proof of this, I need only refer 
to the first plate in this volume, which speaks more plainly to the 
eye, than any description could to the ear. The authorities from which 
the charts have been engraved, together with some remarks on them, 
are given on a separate page descriptive of the plates. At New 
Caledonia (Plate II., Fig. 5.) the barrier-reefs extend for 150 miles on 
each side of the submarine prolongation of the island ; and at their 
northern extremity they appear broken up and converted into a vast 
atoll-formed reef, supporting a few low coral-islets: we may imagine 
that we here see the effects of subsidence actually in progress, the 
water always encroaching on the northern end of the island, towards 
which the mountains slope down, and the reefs steadily building up 
their massive fabrics in the lines of their ancient growth. 

We have as yet only considered the origin of barrier-reefs and atolls 
in their simplest form ; but there remain some peculiarities in structure 
and some special cases, described in the two first chapters, to be 
accounted for by our theory. These consist — in the inclined ledge 
terminated by a wall, and sometimes succeeded by a second ledge with 
a wall, round the shores of certain lagoons and lagoon-channels ; a 
structure which cannot, as I endeavoured to show, be explained by the 
simple growing powers of the corals, — in the ring or basin-like forms 
of the central reefs, as well as of the separate marginal portions of the 
northern Maldiva atolls, — in the submerged condition of the whole, or 
of parts of certain barrier and atoll-formed reefs ; where only a part is 
submerged, this being generally to leeward, — in the apparent progres- 
sive disseverment of some of the Maldiva atolls, — in the existence of 
irregularly formed atolls, some being tied together by linear reefs, and 
others with spurs projecting from them, — and, lastly, in the structure 
and origin of the Great Chagos Bank. 

Step-for?ned ledges round certain lagoons. — If we suppose an atoll 
to subside at an extremely slow rate, it is difficult to follow out the 
complex results. The living corals would grow up on the outer margin ; 
and likewise probably in the gullies and deeper parts of the bare surface 
of the annular reef; the water would encroach on the islets, but the 
accumulation of fresh detritus might possibly prevent their entire sub- 
mergence. After a subsidence of this very slow nature., the surface 
of the annular reef sloping gently into the lagoon, would probably 
become united with the irregular reefs and banks of sand, which line 
the shores of most lagoons. Should, however, the atoll be carried 
down by a more rapid movement, the whole surface of the annular reef, 
where there was a foundation of solid matter, would be favourably 
circumstanced for the fresh growth of coral ; but as the corals grew 
upwards on its exterior margin, and the waves broke heavily on tliis 



8o STEP-FORMED REEFS, 

part, the increase of the massive polypifers on the inner side would be 
checked from the want of water. Consequently, the exterior parts 
would first reach the surface, and the new annular reef thus formed on 
the old one, would have its summit inclined inwards, and be terminated 
by a subaqueous wall, formed by the upward growth of the coral 
(before being much checked), from the inner edge of the solid parts 
of the old reef. The inner portion of the new reef, from not having 
grown to the surface, would be covered by the waters of the lagoon. 
Should a subsidence of the same kind be repeated, the corals would 
again grow up in a wall, from all the solid parts of the resunken reef, 
and, therefore, not from within the sandy shores of the lagoon ; and the 
inner part of the new annular reef would, from being as before checked 
in its upward growth, be of less height than the exterior parts, and 
therefore would not reach the surface of the lagoon. In this case the 
shores of the lagoon would be surrounded by two inclined ledges, one 
beneath the other, and both abruptly terminated by subaqueous cliffs.* 
T/ie ring or basm-forfucd reefs of the northern Maldiva atolls. — I 
may first observe, that the reefs within the lagoons of atolls and within 
lagoon-channels, would, if favourably circumstanced, grow upwards 
during subsidence in the same manner as the annular rim ; and, there- 
fore, we might expect that such lagoon-recfs, when not surrounded and 
buried by an accumulation of sediment more rapid than the rate of 
subsidence, would rise abruptly from a greater depth than that at which 
the efficient polypifers can flourish : we see this well exemplified in the 
small abruptly-sided reefs, with which the deep lagoons of the Chagos 
and Southern Maldiva atolls are studded. With respect to the ring or 
basin-formed reefs of the Northern Maldiva atolls, it is evident, from 
the perfectly continuous series which exists that the marginal rings, 
although wider than the exterior or bounding reef of ordinary atolls, are 
only modified portions of such a reef; it is also evident that the central 
rings, although wider than the knolls or reefs which commonly occur 
in lagoons, occupy their place. The ring-like structure has been shown 
to be contingent on the breaches into the lagoon being broad and 
numerous, so that all the reefs which are bathed by the waters of the 
lagoon are placed under nearly the same conditions with the outer 
coast of an atoll standing in the open sea. Hence the exterior and 
living margins of these reefs must have been favourably circumstanced 
for growing outwards, and increasing beyond the usual breadth ; and 
they must likewise have been favourably circumstanced for growing 

* According to Mr. Couthouy (p. 26) the external reef round many atolls 
descends by a succession of ledges or terraces. He attempts, I doubt 
whether successfully, to explain this structure somewhat in the same manner 
as I have attempted, with respect to the internal ledges round the lagoons of 
some atolls. More facts are wanted regarding the nature both of the interior 
and exterior step-like ledges : are all the ledges, or only the upper ones, 
covered with living coral ? If they are all covered, are the kinds different on 
the ledges according to the depth? Do the interior and exterior ledges 
occur together in the same atolls; if so, what is their total width, and is the 
intervening surface-reef narrow, etc.? 



Submerged and dead reefs. Si 

vigorously upwards, during the subsiding movements, to which by our 
theory the whole archipelago has been subjected ; and subsidence with 
this upward growth of the margins would convert the central space of 
each little reef into a small lagoon. This, however, could only take 
place with those reefs, which had increased to a breadth sufficient to 
prevent their central spaces from being almost immediately filled up 
with the sand and detritus driven inwards from all sides : hence it is that 
few reefs, which are less than half a mile in diameter, even in the atolls 
where the basin-like structure is most strikingly exhibited, include 
lagoons. This remark, I may add, applies to all coral-reefs wherever 
found. The basin-formed reefs of the Maldiva Archipelago may, in 
fact, be briefly described, as small atolls formed during subsidence over 
the separate portions of large and broken atolls, in the same manner as 
these latter were formed over the barrier-reefs, which encircled the 
islands of a large archipelago now wholly submerged. 

Subuierged and dead reefs. — In the second section of the first 
chapter, 1 have shown that there are in the neighbourhood of atolls, 
some deeply submerged banks, with level surfaces ; that there are 
others, less deeply but yet wholly submerged, having all the characters 
of perfect atolls, but consisting merely of dead coral-rock ; that there 
are barrier-reefs and atolls with merely a portion of their reef, generally 
on the leeward side, submerged ; and that such portions either retain 
their perfect outline, or they appear to be quite effaced, their former 
place being marked only by a bank, conforming in outline wilh that 
part of the reef which remains perfect. These several cases are, 
I believe, intimately related together, and can be explained by the 
same means. There, perhaps, exist some submerged reefs, covered 
with living coral and growing upwards, but to these I do not here 
refer. 

As we see that in those parts of the ocean, where coral-reefs are 
most abundant, one island is fringed and another neighbouring one is 
not fringed ; as we see in the same archipelago, that all the reefs are 
more perfect in one part of it than in another, for instance, in the 
southern half compared with the northern half of the Maldiva Archi- 
pelago, and likewise on the outer coasts compared with the inner coasts 
of the atolls in this same group, which are placed in a double row; as 
we know that the existence of the innumerable polypifers forming a 
reef, depends on their sustenance, and that they are preyed on by other 
organic beings ; and, lastl}^, as we know that some inorganic causes are 
highly injurious to the growth of coral, it cannot be expected that during 
the round of change to which earth, air, and water are exposed, the 
reef-building polypifers should keep alive for perpetuity in any one 
place ; and still less can this be expected, during the progressive sub- 
.sidences, perhaps at some periods more rapid than at others, to which 
by our theory these reefs and islands have been subjected and are 
liable. It is, then, not improbable that the corals should sometimes 
perish either on the whole or on part of a reef; if on part, the dead 
portion, after a small amount of subsidence, would still retain its proper 
outline and position beneath the water. After a more prolonged sub- 

6 



82 DISSEVERMENT OF THE 

sidence, it would probably form, owing to the accumulation of seaiment, 
only the margin of a flat bank, marking the limits of the former lagoon. 
Such dead portions of reef would generally lie on the leeward side,* 
for the impure water and fine sediment would more easily flow out 
from the lagoon over this side of the reef, where the force of the 
breakers is less than to windward ; and therefore the corals would be 
less vigorous on this side, and be less able to resist any destroying 
agent. It is likewise owing to this same cause, that reefs are more 
frequently breached to leeward by narrow channels, serving as by ship- 
channels, than to windward. If the corals perished entirely, or on the 
greater part of the circumference of an atoll, an atoll-shaped bank of 
(lead rock, more or less entirely submerged, would be produced ; and 
further subsidence, together with the accumulation of sediment, would 
often obliterate its atoll-like structure, and leave only a bank with a 
level surface. 

In the Chagos group of atolls, within an area of i6o miles by 60, 
there are two atoll-formed banks of dead rock (besides another very 
imperfect one), entirely submerged ; a third, with merely two or three 
very small pieces of living reef rising to the surface ; and a fourth, 
namely, Peros Banhos (Plate I., Fig. 9), with a portion nine miles in 
length dead and submerged. As by our theory this area has subsided, 
and as there is nothing improbable in the death, either from changes in 
the state of the surrounding sea or from the subsidence being great or 
sudden, of the corals on the whole, or on portions of some of the atolls, 
the case of the Chagos group presents no difficulty. So far indeed are 
any of the above-mentioned cases of submerged reefs from being inex- 
plicable, that their occurrence might have been anticipated on our theory, 
and as fresh atolls are supposed to be in progressive formation by the 
subsidence of encircling barrier-reefs, a weighty objection, namely that 
the number of atolls must be increasing infinitely, might even have 
been raised, if proofs of the occasional destruction and loss of atolls 
could not have been adduced. 

l^he disseverme?it of the larger Maldiva atolls. — The apparent pro- 
gressive disseverment in the Maldiva Archipelago of large atolls into 
smaller ones, is, in many respects, an important consideration, and 
requires an explanation. The graduated series which marks, as I 

* Mr. Lyell, in the first edition of his " Principles of Geology," offered a 
somewhat different explanation of this structure. He supposes that there 
has been subsidence ; but he was not aware that the submerged portions of 
reef were in most cases, if not in all, dead ; and he attributes the difference 
in height in the two sides of most atolls, chiefly to the greater accumulation 
of detritus to windward than to leeward. But as matter is accumulated only 
on the backward part of the reef, the front part would remain of the same 
height on both sides. I may here observe that in most cases (for instance, 
at Peros Banhos, the Gambier group and the Great Chagos Bank), and I 
suspect in all cases, the dead and submerged portions do not blend or slope 
into the living and perfect parts, but are separated from them by an abrupt 
line. In some instances small patches of living reef rise to the surface from 
the middle of the submerged and dead parts. 



LARGER MALDIVA ATOLLS. 83 

believe, this process, can be observed only in the northern half of the 
group, where the atolls have exceedingly imperfect margins, consisting 
of detached basin-formed reefs. The currents of the sea flow across 
these atolls, as I am informed by Captain Moresby, with considerable 
force, and drift the sediment from side to side during the monsoons, 
transporting much of it seaward ; yet the currents sweep with greater 
force round their flanks. It is historically known that these atolls have 
long existed in their present state ; and we can believe, that even during 
a very slow subsidence they might thus remain, the central expanse 
being kept at nearly its original depth by the accumulation of sediment. 
But in the action of such nicely balanced forces during a progressive 
subsidence (like that, to which by our theory this archipelago has been 
subjected), it would be strange if the currents of the sea should never 
make a direct passage across some one of the atolls, through the many 
wide breaches in their margins. If this were once effected, a deep- 
water channel would soon be formed by the removal of the finer sedi- 
ment, and the check to its further accumulation ; and the sides of the 
channel would be worn into aslope like that on the outer coasts, which 
are exposed to the same force of the currents. In fact, a channel pre- 
cisely like that bifurcating one which divides Mahlos Mahdoo (Plate II., 
Fig. 4.), would almost necessarily be formed. The scattered reefs 
situated near the borders of the new ocean-channel, from being favour- 
ably placed for the growth of coral, would, by their extension, tend to 
produce fresh margins to the dissevered portions ; such a tendency is 
very evident (as may be seen in the large published chart) in the 
elongated reefs on the borders of the two channels intersecting Mahlos 
Mahdoo. Such channels would become deeper with continued subsi- 
dence, and probably from the reefs not growing up perpendicularly, 
somewhat broader. In this case, and more especially if the channels 
had been formed originally of considerable breadth, the dissevered 
portions would become perfect and distinct atolls, like Ari and Ross 
atolls (Plate II., Fig. 6), or like the two Nillandoo atolls, which must be 
considered as distinct, although related in form and position, and 
separated from each other by channels, which though deep have been 
sounded. Further subsidence would render such channels unfathomable, 
and the dissevered portions would then resemble Phaleedoo and Moluque 
atolls, or Mahlos Mahdoo and Horsburgh atolls (Plate II., Fig. 4), which 
are related to each other in no respect except in proximity and position. 
Hence, on the theory of subsidence, the disseverment of large atolls, 
which have imperfect margins (for otherwise their disseverment would 
be scarcely possible), and which are exposed to strong currents, is far 
from being an improbable event ; and the several stages, from close 
relation to entire isolation in the atolls of the Maldiva Archipelago, are 
readily explicable. 

We might go even further, and assert as not improbable, that the 
first formation of the Maldiva Archipelago was due to a barrier-reef, of 
nearly the same dimensions with that of New Caledonia (Plate II., 
Fig. 5), for if, in imagination, we complete the subsidence of that great 
island, we might anticipate from the present broken condition of the 



84 IRREGULAR-FORMED ATOLLS. 

northern portion of the reef, and from the almost entire absence of reefs 
on the eastern coast, that the barrier-reef after repeated subsidences, 
would become during its upward growth separated into distinct portions ; 
and these portions would tend to assume an atoll-like structure, from 
the coral growing with vigour round their entire circumferences, when 
freely exposed to an open sea. As we have some large islands partly 
submerged with barrier-reefs marking their former limits, such as New 
Caledonia, so our theory makes it probable that there should be other 
large islands wholly submerged ; and these, we may now infer, would 
be surmounted, not by one enormous atoll, but by several large elongated 
ones, like the atolls in the Maldiva group ; and these again, during long 
periods of subsidence, would sometimes become dissevered into smaller 
atolls. I may add, that both in the Marshall and Caroline Archipelagoes, 
there are atolls standing close together, which have an evident relation- 
ship in form : we may suppose, in such cases, either that two or more 
encircled islands originally stood close together, and afforded bases for 
two or more atolls, or that one atoll has been dissevered. From the 
position, as well as form, of three atolls in the Caroline Archipelago 
(the Namourrek and Elato group), which are placed in an irregular 
circle, I am strongly tempted to believe that they have originated by 
the process of disseverment.* 

Irrcgulaj^ly formed atolls. — In the Marshall group, Musquillo atoll 
consists of two loops united in one point ; and Menchicoff atoll is formed 
of three loops, two of which (as may be seen in Fig. 3, Plate II.) are 
connected by a mere ribbon-shaped reef, and the three together are 
sixty miles in length. In the Gilbert group some of the atolls have 
narrow strips of reef, like spurs, projecting from them. There occur 
also in parts of the open sea, a few linear and straight reefs, standing 
by themselves ; and likewise some few reefs in the form of crescents, 
with their extremities more or less curled inwards. Now, the upward 
growth of a barrier-reef which fronted only one side of an island, or one 
side of an elongated island with its extremities (of which cases exist), 
would produce after the complete subsidence of the land, mere strips 
or crescent or hook-formed reefs : if the island thus partially fronted 
became divided during subsidence into two or more islands, these 
islands would be united together by linear reefs ; and from the further 
growth of the coral along their shores together with subsidence, reefs 
of various forms might ultimately be produced, either atolls united 
together by linear reefs, or atolls with spurs projecting from them. 
Some, however, of the more simple forms above specified, might, as 
we have seen, be equally well produced by the coral perishing during 

* The same remark is, perhaps, applicable to the islands of OUap, Fanadik, 
and Tamatam in the Caroline Archipelago, of which charts are given in the 
atlas of Duperrey's voyage : a line drawn through the linear reefs and lagoons 
of these three islands forms a semicircle. Consult also, the atlas of Lutke's 
voyage ; and for the Marshall group that of Kotzebue ; for the Gilbert group 
consult the atlas of Duperrey's voyage. Most of the points here referred 
to may, however, be seen in Krusenstern's general Atlas of the Pacific. 



THE GREAT CHAGOS BANK. 85. 

subsidence on part of the circumference of an atoll, whilst on the other 
parts it continued to grow up till it reached the surface. 

The Great Chagos Bank. — I have already shown that the submerged 
condition of the Great Chagos Bank (Plate II., Fig. i, with its section 
Fig. 2), and of some other banks in the Chagos group, may in all proba- 
bility be attributed to the coral having perished before or during the 
movements of subsidence, to which this whole area by our theory has 
been subjected. The external rim or upper ledge (shaded in the chart), 
consists of dead coral-rock thinly covered with sand ; it lies at an 
average depth of between five and eight fathoms, and perfectly resembles 
in form the annular reef of an atoll. The banks of the second level, the 
boundaries of which are marked by dotted lines in the chart, lie from 
about fifteen to twenty fathoms beneath the surface ; they are several 
miles broad, and terminate in a very steep slope round the central 
expanse. This central expanse I have already described, as consisting 
of a level muddy flat between thirty and forty fathoms deep. The banks 
of the second level, might at first sight be thought analogous to the 
internal step-like ledge of coral-rock which borders the lagoons of some 
atolls, but their much greater width, and their being formed of sand, are 
points of essential difference. On the eastern side of the atoll some of 
the banks are linear and parallel, resembling islets in a great river, and 
pointed directly towards a great breach on the opposite side of the 
atoll ; these are best seen in the large published chart. I inferred from 
this circumstance, that strong currents sometimes set directly across 
this vast bank ; and I have since heard from Captain Moresby that this 
is the case. I observed, also, that the channels or breaches through the 
rim, were all of the same depth as the central lagoon-like space into 
which they lead ; whereas the channels into the other atolls of the 
Chagos group, and as I believe into most other large atolls, are not 
nearly as deep as their lagoons : for instance at Peros Banhos, the 
channels are only of the same depth, namely between ten and twenty 
fathoms, as the bottom of the lagoon for a space about a mile and a half 
in width round its shores, whilst the central expanse of the lagoon is 
from thirty-five to forty fathoms deep. Now, if an atoll during a gradual 
subsidence once became entirely submerged, like the Great Chagos 
Bank, and therefore no longer exposed to the surf, very little sediment 
could be formed from it ; and consequently the channels leading into 
the lagoon from not being filled up with drifted sand and coral detritus, 
would continue increasing in depth, as the whole sank down. In this 
case, we might expect that the currents of the open sea, instead of any 
longer sweeping round the submarine flanks, would flow directly 
through the breaches across the lagoon, removing in their course the 
finer sediment, and preventing its further accumulation. We should 
then have the submerged reef forming an external and upper rim of 
rock, and beneath this portion of the sandy bottom of the old lagoon, 
intersected by deep-water channels or breaches, and thus formed into 
separate marginal banks ; and these would be cut off by steep slopes, 
overhanging the central space, worn ^io\vn by the passage gf the 
oceanic current^, 



S6 OBJECTIONS TO THE 

By these means, I have scarcely any doubt that the Great Chagos 
Bank has originated, — a structure which at first appeared to me far 
more anomalous than any I had met with. The process of formation is 
nearly the same with that, by which Mahlos Mahdoo had been trisected ; 
but in the Chagos Bank the channels of the oceanic currents entering 
at several different quarters, have united in a central space. 

This great atoll-formed bank appears to be in an early stage of dis- 
severment ; should the work of subsidence go on, from the submerged 
and dead condition of the whole reef, and the imperfection of the south- 
east quarter a mere wreck would probably be left. The Pitt's Bank, 
situated not far southward, appears to be precisely in this state ; it 
consists of a moderately level, oblong bank of sand, lying from lo to 
20 fathoms beneath the surface, with two sides protected by a narrow 
ledge of rock which is submerged between 5 and 8 fathoms. A little 
further south, at about the same distance as the southern rim of the 
Great Chagos Bank is from the northern rim, there are two other small 
banks with from 10 to 20 fathoms on them ; and not far eastward 
soundings were struck on a sandy bottom, with between no and 145 
fathoms. The northern portion with its ledge-Hke margin, closely 
resembles any one segment of the Great Chagos Bank, between two 
of the deep-water channels, and the scattered banks, southward appear 
to be the last wrecks of less perfect portions. 

I have examined with care the charts of the Indian and Pacific 
Oceans, and have now brought before the reader all the examples, which 
I have met with, of reefs differing from the type of the class to which 
they belong ; and I think it has been satisfactorily shown, that they are 
all included in our theory, modified by occasional accidents which might 
have been anticipated as probable. In this course we have seen, that 
in the lapse of ages encircling barrier-reefs are occasionally converted 
into atolls, the name of atoll being properly applicable, at the moment 
when the last pinnacle of encircled land sinks beneath the surface ol 
the sea. We have, also, seen that large atolls during the progressive 
subsidence of the areas in which they stand, sometimes become dis- 
severed into smaller ones ; at other times, the reef-building polypifers 
having entirely perished, atolls are converted into atoll-formed banks 
of dead rock ; and these again through further subsidence and the 
accumulation of sediment modified by the force of the oceanic currents, 
pass into level banks with scarcely any distinguishing character. Thus 
may the history of an atoll be followed from its first origin, through the 
occasional accidents of its existence, to its destruction and final 
obliteration. 

Objections to the theory of the formation of atolls and b arrief -reefs. — 
The vast amount of subsidence, both horizontally or in area, and verti- 
cally or in depth, necessary to have submerged every mountain, even 
the highest, throughout the immense spaces of ocean interspersed with 
atolls, will probably strike most people as a formidable objection to my 
theory. But as continents, as large as the spaces supposed to have 
subsided, have been raised above the level of the sea, — as whole regions 
are now rising, for instance, in Scandinavia and South America, — and as 



THEORY OF SUBSIDENCE. 87 

no reason can be assigned, why subsidences should not have occurred 
in some parts of the earth's crust on as great a scale both in extent and 
amount as those of elevation, objections of this nature strike me as of 
little force. The remarkable point is that movements to such an extent 
should have taken place within a period, during which the polypifers 
have continued adding matter on and above the same reefs. Another 
and less obvious objection to the theory will perhaps be advanced from 
the circumstance, of the lagoons within atolls and within barrier-reefs 
never having become in any one instance during prolonged subsidences 
of a greater depth than sixty fathoms, and seldom more than forty 
fathoms; but we already admit, if the theory be worth considering, that 
the rate of subsidence has not exceeded that of the upward growth of 
the coral on the exterior margin ; we are, therefore, only further required 
to admit, that the subsidence has not exceeded in rate the filling up of 
the interior spaces by the growth of the corals living there, and by the 
accumulation of sediment. As this filling up must take place very 
slowly within barrier-reefs lying far from the land, and within atolls 
which are of large dimensions and which have open lagoons with very 
few reefs, we are led to conclude that the subsidence thus counter- 
balanced, must have been slow in an extraordinary degree ; a conclusion 
which accords with our only means, namely, with what is known of the 
rate and manner of recent elevatory movements, of judging by analogy 
what is the probable rate of subsidence. 

In this chapter it has, I think, been shown, that the theory of subsi- 
dence, which we were compelled to receive from the necessity of giving 
to the corals, in certain large areas, foundations at the requisite depth, 
explains both the normal structure and the less regular forms of those 
two great classes of reefs, which have justly excited the astonishment 
of all persons who have sailed through the Pacific and Indian Oceans. 
But further to test the truth of the theory, a crowd of questions will 
occur to the reader: Do the different kinds of reefs, ^which have been 
produced by the same kind of movement, generally lie within the same 
areas ? Wiiat is their relation of form and position, — for instance, do 
adjoining groups of atolls, and the separate atolls in these groups, bear 
the same relation to each other which islands do in common archipela- 
goes ? Have we reason to believe, that where there are fringing-reefs, 
there has not lately been subsidence ; or, for it is almost our only way 
of ascertaining this point, are there frequently proofs of recent eleva- 
tion ? Can we by this means account for the presence of certain classes 
of reefs in some large areas, and their entire absence in others ? Do 
the areas which have subsided, as indicated by the presence of atolls 
and barrier-reefs, and the areas which have remained stationary or have 
been upraised, as shown by fringing-reefs, bear any determinate relation 
to each other ; and are the dimensions of these areas such as harmonise 
with the greatness of the subterranean changes, which, it must be sup- 
posed, have lately taken place beneath them ? Is there any connec- 
tion between the movements thus indicated, and recent volcanic action ? 
All these questions ought to receive answers in accordance uitli th.c 
theory ; and if this can be satisfactorily shown, not only is the theory 



88 GEOLOGICAL COMPOSITION OF CORAL FORMATIONS. 

confirmed, but as deductions, the answers are in themselves important. 
Under this latter point of view, these questions will be chiefly con- 
sidered in the following chapter.* 

* I may take this opportunity of briefly considering the appearances, which 
would probably be presented by a vertical and deep section across a coral 
formation (referring chiefly to an atoll), formed by the upward growth of 
coral during successive subsidences. This is a subject worthy of attention, 
as a means of comparison with ancient coral-strata. The circumferential 
parts would consist of massive species, in a vertical position, with their 
interstices filled up with detritus; but this would be the part most subject 
to subsequent denudation and removal. It is useless to speculate how large 
a portion of the exterior annular reef would consist of upright coral, and 
how much of fragmentary rock, for this would depend on many contingen- 
cies, — such as on the rate of subsidence, occasionally allowing a fresh growth 
of coral to cover the whole surface, and on the breakers having force suffi- 
cient to throw fragments over this same space. The conglomerate which 
composes the base of the islets, would (if not removed by denudation to- 
gether with the exterior reef on which it rests) be conspicuous from the 
size of the fragments, — the diflerent degrees in which they have been 
rounded, — the presence of fragments of conglomerate torn up, rounded, and 
recemented, — and from the oblique stratification. The corals which lived in 
the lagoon-reefs at each successive level, would be preserved upright, and 
they would consist of many kinds, generally much branched. In this part, 
however, a very large proportion of the rock (and in some cases nearly all 
of it) would be formed of sedimentary matter, either in an excessively fine, 
or in a moderately coarse state, and with the particles almost blended to- 
gether. The conglomerate which was formed of rounded pieces of the 
branched corals, on the shores of the lagoon, would differ from that formed 
on the islets and derived from the outer coast ; yet both might have accumu- 
lated very near each other. I have seen a conglomerate limestone from 
Devonshire like a conglomerate now forming on the shores of the Maldiva 
atolls. The stratification taken as a whole, would be horizontal ; but the 
conglomerate beds resting on the exterior reef, and the beds o'f sandstone on 
the shores of the lagoon (and no doubt on the external flanks) would pro- 
bably be divided (as at Keeling atoll and at Mauritius) by numerous layers 
dipping at considerable angles in different directions. The calcareous sand- 
stone and coral-rock would almost necessarily contain innumerable shells, 
echini, and the bones offish, turtle, and perhaps of birds; possibly, also, the 
bones of small saurians, as these animals find their way to the islands far 
remote from any continent. The large shells of some species of Tridacna 
would be found vertically imbedded in the solid rock, in the position in 
which they lived. We might expect also to find a mixture of the remains of 
pelagic and littoral animals in the, strata formed in the lagoon, for pumice 
and the seeds of plants are floated from distant countries into the lagoons of 
many atolls : on the outer coast of Keeling atoll, near the mouth of the 
lagoon, the case of a pelagic Pteropodous animal was brought up on the 
arming of the sounding lead. All the loose blocks of coral on Keeling atoll 
were burrowed by vermiform animals ; and as every cavity, no doubt, ultima- 
tely becomes filled with spathose limestone, slabs of the rock taken from a 
considerable depth, would, if polished, probably exhibit the excavations of 
such burrowing animals. The conglomerate and fine-grained beds of coral- 
rock would be hard, sonorous, white and composed of nearly pure calcareous 



DESCRIPTION OF THE MAP. 89 

CHAPTER VI. 

ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE 
THEORY OF THEIR FORMATION. 

Description of the coloured map. — Proximity of atolls and barrier-reefs. — ■ 
Relation in form and position of atolls with ordinary islands. — Direct 
evidence of subsidence difficult to be detected. — Proofs of recent elevation 
where fringing-reefs occur. — Oscillations of level. — Absence of active 
volcanoes in the areas of subsidence. — Immensity of the areas which 
have been elevated and have subsided. — Their relation to the present 
distribution of the land. — Areas of subsidence elongated, their inter- 
section and alternation with those of elevation. — Amount, and slow rate 
of the subsidence. — Recapitulation. 

It will be convenient to give here a short account of the appended map 
(Plate III.) : * a fuller one, with the data for colouring each spot, is reserved 

matter; in some few parts, judging from the specimens at Keeling atoll, they 
would probably contain a small quantity of iron. Floating pumice and 
scoriae, and occasionally stones transported in the root of trees (see my 
"Journal of Researches," p. 549) appear the only sources, through which foreign 
matter is brought to coral-formations standing in the open ocean. The area 
over which sediment is transported from coral-reefs must be considerable : 
Captain Moresby informs me that during the change of monsoons the sea is 
discoloured to a considerable distance off the Maldiva andChagos atolls. The 
sediment of fringing and barrier coral-reefs must be mingled with the mud, 
which is brought down from the land, and is transported seaward through 
the breaches, which occur in front of almost every valley. If the atolls of the 
larger archipelagoes were upraised, the bed of the ocean being converted into 
land, they would form flat-topped mountains, varying in diameter from a few 
miles (the smallest atolls being worn away) to sixty miles ; and from being 
horizontally stratified and of similar composition, they would, as Mr. Lyell has 
remarked, falsely appear as if they had originally been united into one vast 
continuous mass. Such great strata of coral-rock would rarely be associated 
with erupted volcanic matter, for this could only take place, as may be inferred 
from what follows in the next chapter, when the area, in which they were 
situated, commenced to rise, or at least ceased to subside. During the 
enormous period necessary to effect an elevation of the kind just alluded to, 
the surface vvculd necessarily be denuded to a great thickness; hence it is 
highly improbable that any fringing-reef, or even any barrier-reef, at least 
of those encircling small islands, would be preserved. From this same 
cause, the strata which were formed within the lagoons of atolls and lagoon- 
channels of barrier-reefs, and which must consist in a large part of sedi- 
mentary matter, would more often be preserved to future ages, than the 
exterior solid reef, composed of massive corals in an upright position ; al- 
though it is on this exterior part that the present existence and further 
growth of atolls and barrier-reefs entirely depend. 

* Inasmuch as the coloured map would have proved too costly to be given 
in this series, the indications of colour have been replaced by numbers 
referring to the dotted groups of reefs, etc. The author's original wording, 
however, is retained in full, as it will be easy to refer to the map by the 
numbers, and thus the flow of the narrative is undisturbed. 



90 DESCRIPTION OF THE MAP 

for the Appendix ; and every place there referred to may be found in the 
Index. A larger chart would have been desirable ; but, small as the 
adjoined one is, it is the result of many months' labour. I have con- 
sulted, as far as I was able, every original voyage and map ; and the 
colours were first laid down on charts on a larger scale. The same 
blue colour, with merely a difference in the depth of tint, is used for 
atolls or lagoon-islands, and barrier-reefs, for we have seen, that as far 
as the actual coral-formation is concerned, they have no distinguishing 
character. Fringing-reefs have been coloured red, for between them on 
the one hand, and barrier-reefs and atolls on the other, there is an 
important distinction with respect to the depth beneath the surface, at 
which we are compelled to believe their foundations lie. The two 
distinct colours, therefore, mark two great types of structure. 

The dark blue <:<9/(?;/r [represented by (3) in our plate] represents atolls 
and submerged annular reefs, with deep water in their centres. I have 
coloured as atolls, a few low and small coral-islands, without lagoons ; 
but this has been done only when it clearly appeared that they originally 
contained lagoons, since filled up with sediment : when there were not 
good grounds for this belief, they have been left uncoloured. 

The pale blue colour [represented by (2)] represents barrier-reefs. 
The most obvious character of reefs of this class is the broad and deep- 
water moat within the reef : but this, like the lagoons of small atolls, 
is liable to become filled up with detritus and with reefs of delicately 
branched corals : when, therefore, a reef round the entire circumference 
of an island extends very far into a profoundly deep sea, so that it can 
hardly be confounded with a fringing-reef which must rest on a founda- 
tion of rock within a small depth, it has been coloured pale blue, 
although it does not include a deep-water moat : but this has only been 
done rarely, and each case is distinctly mentioned in the Appendix. 

The red colour (4) represents reefs fringing the land quite closely 
where the sea is deep, and where the bottom is gently inclined extending 
to a moderate distance from it, but not having a deep-water moat 
or lagoon-like space parallel to the shore. It must be remembered that 
fringing-reefs are frequently breached in front of rivers and valleys by 
deepish channels, where mud has been deposited. A space of thirty 
miles in width has been coloured round or in front of the reefs of each 
class, in order that the colours might be conspicuous on the appended 
map, which is reduced to so small a scale. 

The verinillion spots, and streaks (i) represent volcanoes now in action, 
or historically known to have been so. They are chiefly laid down from 
Von Buch's work on the Canary Islands ; and my reasons for making 
a few alterations are given in the note below.* 

* I have also made considerable use of the geological part of Berghaus' 
" Physical Atlas." Beginning at the eastern side of the Pacific, I have added 
to the number of the volcanoes in the southern part of the Cordillera, and 
have coloured Juan Fernandez according to observations collected during 
the voyage of the Beagle ("Geol. Trans.," vol. v., p. 601). I have added a 
volcano to Albemarle Island, one of the Galapagos Archipelago (the author's 
" Journal of Researches," p. 457). In the Sandwich group there are no active 



OF DISTRIBUTION OF CORAL REEFS. 91 

The uncoloured coasts consist, first and chiefly, of those, where there 
are no coral-reefs, or such small portions as to be quite insignificant. 
Secondly, of those coasts where there are reefs, but where the sea is 
very shallow, for in this case the reefs generally lie far from the land, 
and become very irregular, in their forms : where they have not become 
irregular, they have been coloured. Thirdly, if I had the means of as- 
certaining the fact, I should not colour a reef merely coating the edges 
of a submarine crater, or of a level submerged bank ; for such superficial 

volcanoes, except at Hawaii ; but the Rev. W. Ellis informs me, there are 
streams of lava apparently modern on Maui, having a very recent appearance, 
which can be traced to the craters whence they flowed. The same gentle- 
man informs me, that there is no reason to believe that any active volcano 
exists in the Society Archipelago ; nor are there any known in the Samoa 
or Navigator group, although some of the streams of lava and craters there 
appear recent. In the Friendly group, the Rev. J. Williams says (" Narrative 
of Missionary Enterprise," p. 29) that Toofoa and Proby Islands are active 
volcanoes. I infer from Hamilton's "Voyage in the Pandora" (p. 95), that 
Proby Island is synonymous with Onouafou, but I have not ventured to 
colour it. There can be no doubt respecting Toofoa, and Captain Edwards 
(Von Buch, p. 386) found the lava of recent eruption at Amargura still 
smoking. Berghaus marks four active volcanoes actually within the Friendly 
group ; but I do not know on what authority ; I may mention that Maurelle 
describes Latte as having a burnt-up appearance: I have marked only Toofoa 
and Amargura. South of the New Hebrides lies Matthews Rock, which is 
drawn and described as an active crater in the "Voyage of the Astrolabe." 
Between it and the volcano on the eastern side of New Zealand, lies Brim- 
stone Island, which from the high temperature of the water in the crater, 
may be ranked as active (Berghaus "Vorbemerk," ii Lief. S. 56). Make 
Brun, vol. xii., p. 231, says that there is a volcano near port St. Vincent in 
New Caledonia. I believe this to be an eiTor, arising from a smoke seen on 
the opposite coast by Cook (" Second Voyage," vol. ii., p. 23) which smoke went 
out at night. The Mariana Islands, especially the northern ones, contain 
many craters (see Freycinet's " Hydrog. Descript.") which are not active. Von 
Buch, however, states (p. 462) on the authority of La Peyrouse, that there 
are no less that seven volcanoes between these islands and Japan. Gemelli 
Careri (Churchill's " Collect.," vol. iv., p. 458), says there are two active 
volcanoes in lat. 23° 30', and in lat. 24° : but I have not coloured them. From 
the statements in Beechey's " Voyage" (p. 518, 4to edit.) I have coloured one 
in the northern part of the Bonin group. M. S. Julien has clearly made out 
from Chinese manuscripts not very ancient ("Comptes Rendus," 1840, p. 
832), that there are two active volcanoes on the eastern side of Formosa. In 
Torres Straits, on Cap Island (9° 48' S., 142° 39' E.) a volcano was seen 
burning with great violence in 1793 by Captain Bampton (see Introduction to 
Flinders' " Voyage," p. 41). Mr. M'Clelland (Report of Committee for investi- 
gating Coal in India, p. 39) has shown that the volcanic band passing through 
Barren Island must be extended northwards. It appears by an old chart, 
that Cheduba was once an active volcano (see also Silliinans North 
American Journal, vol. xxxviii,, p. 385). In Berghaus' " Phys. Atlas," 1840, 
No. 7 of Geological Part, a volcano on the coast of Pondicherry is said to have 
burst forth in 1757. Ordinaire ("Hist. Nat, des Volcans," p. 218) says that 
there is one at the mouth of the Persian Gulf, but I have not coloured it, 



92 DESCRIPTION OF THE MAP. 

formations differ essentiall)-, even when not in external appearance, 
from reefs whose foundations as well as superficies have been wholly 
formed by the growth of coral. Fourthly, in the Red Sea, and within 
some parts of the East Indian Archipelago (if the imperfect charts of 
the latter can be trusted), there are many scattered reefs, of small size, 
represented in the chart by mere dots, which rise out of deep water : 
these cannot be arranged under either of the three classes : in the Red 
Sea, however, some of these little reefs, from their position, seem once 
to have formed parts of a continuous barrier. There exist, also, 
scattered in the open ocean, some linear and irregularly formed strips 
of coral-reef, which, as shown in the last chapter, are probably allied in 
their origin to atolls ; but as they do not belong to that class, they have 
not been coloured ; they are very few in number and of insignificant 
dimensions. Lastly, some reefs are left uncoloured from the want of 
information respecting them, and some because they are of an inter- 
mediate structure between the barrier and fringing classes. The value 
of the map is lessened, in proportion to the number of reefs which I 
have been obliged to leave uncoloured, although, in a theoretical point 
of view, few of them present any great difficulty : but their number is 
not very great, as will be found by comparing the map with the state- 
ments in the Appendix. I have experienced more difficulty in colouring 
fringing-reefs than in colouring barrier-reefs, as the former, from their 
much less dimensions, have less attracted the attention of navigators. 
As 1 have had to seek my information from all kinds of sources, and 
often from indirect ones, I do not venture to hope that the map is free 
from many errors. Nevertheless, I trust it will give an approximately 
correct view of the general distribution of the coral-reefs over the 
whole world (with the exception of some fringing-reefs on the coast 
of Brazil, not included within the limits of the map), and of their arrange- 
ment into the three great classes, which, though necessarily very 
imperfect from the nature of the objects classified, have been adopted 
by most voyagers. I may further remark, that the dark blue colour 
represents land entirely composed of coral-rock ; the pale blue, land 
with a wide and thick border of coral-rock ; and the red, a mere narrow 
fringe of coral-rock. 

Looking now at the map under the theoretical point of view indicated 
in the last chapter, the two blue tints signify that the foundations of the 
reefs thus coloured have subsided to a considerable amount, at a slower 
rate than that of the upward growth of the corals, and that probably in 
many cases they are still subsiding. The red signifies that the shores 
which support fringing-reefs have not subsided (at least to any consider- 
as he gives no particulars. A volcano in Amsterdam, or St. Paul's, in the 
southern part of the Indian Ocean, has been seen {Naut. Mag., 1838, p. 
842) in action. Dr. J. Allan, of Forres, informs me in a letter, that when he 
was at Joanna, he saw at night flames apparently volcanic, issuing from the 
chief Comoro Island, and that the Arabs assured him that they were volcanic, 
adding that the volcano burned more during the wet season. 1 have marked 
this as a volcano, though with some hesitation, op account of the possibility 
of the flanie arising from gaseous sources, 



I 



GROUPING OP THE DIFFERENT CLASSES OF REEFS. 93 

able amount, for the effects of a subsidence on a small scale would in 
no case be distinguishable) ; but that they have remained neatly 
stationary since the period when they first became fringed by reefs ; or 
that they are now rising or have been upraised, with new Hues of reefs 
successively formed on them : these latter alternatives are obviously 
implied, as newly formed lines of shore, after elevations of the land, 
would be in the same state with respect to the growth of fringing-reefs, 
as stationary coasts. If during the prolonged subsidence of a shore, 
coral-reefs grew for the first time on it, or if an old barrier-reef were 
destroyed and submerged, and new reefs became attached to the land, 
these would necessarily at first belong to the fringing class, and, there- 
fore, be coloured red, although the coast was sinking : but I have no 
reason to believe, that from this source of error, any coast has been 
coloured wrongly with respect to movement indicated. Well charac- 
terised atolls and encircling barrier-reefs, where several occur in a group, 
or a single barrier-reef if of large dimensions, leave scarcely any doubt 
on the mind respecting the movement by which they have been pro- 
duced ; and even a small amount of subsequent elevation is soon 
betrayed. The evidence from a single atoll or a single encircling barrier- 
reef, must be received with some caution, for the former may possibly 
be based upon a submerged crater or bank, and the latter on a submerged 
margin of sediment, or of worn-down rock. From these remarks we 
may with greater certainty infer that the spaces, especially the larger 
ones, tinted blue in the map, have subsided, than that the red spaces 
have remained stationary, or have been upraised. 

Oti the grotipi7tg of the different classes of reefs. — Having made these 
preliminary remarks, I will consider first how far the grouping of the 
different kinds of coral-islands and reefs is corroborative of the truth of 
the theory. A glance at the map shows that the reefs, coloured blue 
and red, produced under widely different conditions, are not indis- 
criminately mixed together. Atolls and barrier-reefs, on the other hand, 
as may be seen by the two blue tints, generally lie near each other ; 
and this would be the natural result of both having been produced 
during the subsidence of the areas in which they stand. Thus, the 
largest group of encircled islands is that of the Society Archipelago ; 
and these islands are surrounded by atolls, and only separated by a 
narrow space from the large group of Low atolls. In the midst of the 
Caroline atolls, there are three fine encircled islands. The northern 
point of the barrier-reef of New Caledonia seems itself, as before 
remarked, to form a complete large atoll. The great Australian barrier 
is described as including both atolls and small encircled islands. 
Captain King * mentions many atoll-formed and encircling coral-reefs, 
some of which lie within the barrier, and others may be said (for instance 
between lat. 16° and 13°) to form part of it. Flinders f has described 
an atoll-formed reef in lat. 10°, seven miles long and from one to three 
broad, resembling a boot in shape, with apparently very deep water 

* Sailing Directions, appended to vol. ii. of his " Surveying Voyage to 
Australia." 

\ "Voyage to Terra Australis," vol. ii., p. ^d. 



94 THE GROUPING OF ATOLLS. 

within. Eight miles westward of this, and forming part of the barrier, 
He the Murray Islands, which are high and are encircled. In the 
Corallian Sea, between the two great barriers of Australia and New 
Caledonia, there are many low islets and coral-reefs, some of which are 
annular, or horse-shoe shaped. Observing the smallness of the scale 
of the map, the parallels of latitude being nine hundred miles apart, we 
see that none of the large groups of reefs and islands supposed to have 
been produced by long-continued subsidence, lie near extensive lines 
of coast coloured red, which are supposed to have remained stationary 
tince the growth of their reefs, or to have been upraised and new lines 
of reefs formed on them. Where the red and blue circles do occur 
near each other, I am able, in several instances, to show that there 
have been oscillations of level, subsidence having preceded the eleva- 
tion of the red spots ; and elevation having preceded the subsidence of 
the blue spots: and in this case the juxtaposition of reefs belonging to 
the two great types of structure is little surprising. We may, therefore, 
conclude that the proximity in the same areas of the two classes of 
reefs, which owe their origin to the subsidence of the earth's crust, and 
their separation from those formed during its stationary or uprising 
condition, holds good to the full extent, which might have been 
anticipated by our theory. 

As groups of atolls have originated in the upward growth, at each 
fresh sinking of the land, of those reefs which primarily fringed the 
shores of one great island, or of several smaller ones ; so we might 
expect that these rings of coral-rock, like so many rude outline charts, 
will still retain some traces of the general form, or at least general 
range, of the land, round which they were first modelled. That this is 
the case with the atolls in the Southern Pacific as far as their range 
is concerned, seems highly probable, when we observe that the three 
principal groups are directed in north-west and south-east lines, and that 
nearly all the land in the S. Pacific ranges in this same direction ; 
namely, N. Western Australia, New Caledonia, the northern half of 
New Zealand, the New Hebrides, Saloman, Navigator, Society, Mar- 
quesas, and Austral archipelagoes : in the Northern Pacific, the Caroline 
atolls abut against the north-west line of the Marshall atolls, much in 
the same manner as the east and west line of islands from Ccram to 
New Britain do on New Ireland : in the Indian Ocean the Laccadive 
and Maldiva atolls extend nearly parallel to the western and moun- 
tainous coast of India. In most respects, there is a perfect resemblance 
with ordinary islands in the grouping of atolls and in their form: thus 
the outline of all the larger groups is elongated ; and the greater number 
of the individual atolls are elongated in the same direction with the 
group, in which they stand. The Chagos group is less elongated than 
is usual with other groups, and the individual atolls in it are likewise 
but little elongated ; this is strikingly seen by comparing them with the 
neighbouring Maldiva atolls. In the Marshall and Maldiva archipelagoes, 
the atolls are ranged in two parallel lines, like the mountains in a great 
double mountain-chain. Some of the atolls, in the larger archipelagoes, 
stand so near to each other, and have such an evident relationship in 



EVIDENCES OF SUBSIDENCE. 95 

form, that they compose little sub-groups : in the Caroline Archipelago, 
one such sub-group consists of Pouynipete, a lofty island encircled by a 
barrier-reef, and separated by a channel only four miles and a half 
wide from Andeema atoll, with a second atoll a little further off. In all 
these respects an examination of a series of charts will show how 
perfectly groups of atolls resemble groups of common islands. 

On the direct evide7tce of the blue spaces m the map having subsided 
during the upward growth of the reefs so coloured, and of the red 
spaces havi?tg remained statio7tary, or having been upraised. — With 
respect to subsidence, I have shown in the last chapter, that we cannot 
expect to obtain in countries inhabited only by semi-civilised races, 
demonstrative proofs of a movement, which invariably tends to conceal 
its own evidence. But on the coral-islands supposed to have been 
produced by subsidence, we have proofs of changes in their external 
appearance — of a round of decay and renovation — of the last vestiges 
of land on some — of its first commencement on others : we hear of 
storms desolating them to the astonishment of their inhabitants : we 
know by the great fissures with which some of them are traversed, and 
by the earthquakes felt under others, that subterranean disturbances of 
some kind are in progress. These facts, if not directly connected with 
subsidence, as I believe they are, at least show how difficult it would 
be to discover proofs 01 such movement by ordinary means. At Keel- 
ing atoll, however, I have described some appearances, which seem 
directly to show that subsidence did take place there during the late 
earthquakes. Vanikoro, according to the Chevalier Dillon,* is often 
violently shaken by earthquakes, and there, the unusual depth of the 
channel between the shore and the reef, — the almost entire absence of 
islets on the reef, — its wall-like structure on the inner side, and the 
small quantity of low alluvial land at the foot of the mountains, all 
seem to show that this island has not remained long at its present 

* See Captain Dillon's " Voyage in search of La Peyrouse." M. Cordier in 
his "Report on the Voyage of the Astrolabe '' (p. cxi., vol. i.) speaking of 
Vanikoro, says the shores are surrounded by reefs of madrepore, " qu' on 
assure 6tre de formation tout-a-fait moderne." I have in vain endeavoured 
to learn some further particulars about this remarkable passage. I may 
here add, that according to our theory, the island of Pouynipete (Plate I., Fig. 
7), in the Caroline Archipelago, being encircled by a barrier-reef, must have 
subsided. In the New S. Wales Lit. Adveii., Feb. 1 835 (which I have seen 
through the favour of Dr. Lloghtsky), there is an account of this island 
(subsequently confirmed by Mr. Campbell), in which it is said, "At the N.E. 
end, at a place called Tamen, there are ruins of a town, now only accessible 
by boats, the waves reaching to the steps of the houses." Judging from this 
passage, one would be tempted to conclude that the island must have 
subsided, since these houses were built. 1 may, also, here append a state- 
ment in Malte Brun (vol. ix., p. 775, given without any authority), that the 
sea gains in an extraordinary manner on the coast of Cochin China, which 
lies in front and near the subsiding coral-reefs in the China Sea : as the 
coast is granitic, and not alluvial, it is scarcely possible that the encroach- 
ment of the sea can be owing to the washing away of the land ; and if so, it 
must be due to subsidence. 



96 EVIDENCES OF SUBSIDENCE. 

level, with the lagoon-channel subjected to the accumulation of sedi* 
ment, and the reef to the wear and tear of the breakers. At the Society 
Archipelago, on the other hand, where a slight tremor is only rarely felt, 
the shoaliness of the lagoon-channels round some of the islands, the 
number of islets formed on the reefs of others, and the broad belt of 
low land at the foot of the mountains, indicate that, although there 
must have been great subsidence to have produced the barrier-reefs, 
there has since elapsed a long stationary period.* 

* Mr. Couthouy states ("Remarks," p. 44) that at Tahiti and Eimco the 
space between the reef and the shore has been nearly filled up by the 
extension of those coral-reefs, which within most barrier-reefs merely fringe 
the land. From this circumstance, he arrives at the same conclusion as I 
have done, that the Society Islands since their subsidence, have remained 
stationary during a long period ; but he further believes that they have 
recently commenced rising, as well as the whole area of the Low Archipelago. 
He does not give any detailed proofs regarding the elevation of the Society 
Islands, but I shall refer to this subject in another part of this chapter. 
Before making some further comments, I may observe how satisfactory it is 
to me, to find Mr. Couthouy affirming, that " having personally examined a 
large number of coral-islands, and also residing eight months among the 
volcanic class, having shore and partially encircling reefs, I maybe permitted 
to state that my own observations have impressed a conviction of the 
correctness of the theory of Mr. Darwin," 

This gentleman believes, that subsequently to the subsidence by which 
the atolls in the Low Archipelago were produced, the whole area has been 
elevated to the amount of a few feet ; this would indeed be a remarkable 
fact ; but as far as I am able to judge, the grounds of his conclusion are not 
sufficiently strong. He states that he found in almost every atoll which he 
visited, the shores of the lagoon raised from eighteen to thirty inches above 
the sea-level, and containing imbedded Tridacnae and corals standing as 
they grew ; some of the corals were dead in their upper parts, but below 
a certain line they continued to flourish. In the lagoons, also, he fre- 
quently met with clusters of Madrepore, with their extremities standing 
from one inch to a foot above the surface of the water. Now, these appear- 
ances are exactly what I should have expected, without any subsequent 
elevation having taken place ; and I think Mr. Couthouy has not borne in 
mind the indisputable fact, that corals, when constantly bathed by the surf, 
can exist at a higher level than in quite tranquil water, as in a lagoon. 
As long, therefore, as the waves continued at low water to break entirely 
over parts of the annular reef of an atoll, submerged to a small depth, the 
corals and shells attached on these parts might continue living at a level 
above the smooth surface of the lagoon, into which the waves rolled ; but as 
soon as the outer edge of the reef grew up to its utmost possible height, or 
if the reef were very broad nearly to that height, the force of the breakers 
would be checked, and the corals and shells on the inner parts near the 
lagoon would occasionally be left dry, and thus be partially or wholly 
destroyed. Even in atolls, which have not lately subsided, if the outer 
margin of the reef continued to increase in breadth seaward (each fresh 
zone of corals rising to the same vertical height as at Keeling atoll), the 
line where the waves broke most heavily would advance outwards, and 
therefore the corals, which when living near the margin, were washed by 



EVIDENCE OF ELEVATION. ' 97 

Turning now to the red colour ; as on our map, the areas which have 
sunk slowly downwards to great depths are many and large, we might 
naturally have been led to conjecture, that with such great changes of 
level in progress, the coasts which have been fringed probably for ages 
(for we have no reason to believe that coral-reefs are of short duration), 
would not have remained all this time stationary, but would frequently 

the breaking waves during the whole of each tide, would cease being so, and 
would therefore be left on the backward part of the reef standing exposed 
and dead. The case of the madrepores in the lagoons with the tops of their 
branches exposed, seems to be an analogous fact, to the great fields of dead 
but upright corals in the lagoon of Keeling atoll; a condition of things 
which I have endeavoured to show, has resulted from the lagoon having 
become more and more enclosed and choked up with reefs, so that during 
high winds, the rising of the tide (as observed by the inhabitants) is checked, 
and the corals, which had formerly grown to the greatest possible height, 
are occasionally exposed, and thus are killed : and this is a condition oi 
things, towards which almost every atoll in the intervals of its subsidence 
must be tending. Or if we look to the state of an atoll directly after a 
subsidence of some fathoms, the waves would roll heavily over the entire 
circumference of the reef, and the surface of the lagoon would, like the ocean, 
never be quite at rest, and therefore the corals in the lagoon, from being 
constantly laved by the rippling water, might extend their branches to a 
little greater height than they could, when the lagoon became enclosed and 
protected. Christmas atoll (2° N. lat.) which has a very shallow lagoon, 
and differs in several respects from most atolls, possibly may have been 
elevated recently ; but its highest part appears (Couthouy, p. 46) to be only 
ten feet above the sea-level. The facts of a second class, adduced by Mr. 
Couthouy, in support of the alleged recent elevation of the Low Archipelago, 
are not all (especially those referring to a shelf of rock) quite intelligible to 
me ; he believes that certain enormous fragments of rock on the reef, must 
have been moved into their present position, when the reef was at a lower 
level ; but here again the force of the breakers on any inner point of the 
reef being diminished by its outward growth without any change in its level, 
has not, I think, been borne in mind. We should, also, not overlook the 
occasional agency of waves caused by earthquakes and hurricanes. Mr. 
Couthouy further argues, that since these great fragments were deposited 
and fixed en the reef, they have been elevated ; he infers this from the 
greatest amount of erosion not being near their bases, where they are un- 
ceasingly washed by the reflux of the tides, but at some height on their sides, 
near the line of high-water mark, as shown in an accompanying diagram. 
My former remark again applies here, with this further observation, that as 
the waves have to roll over a wide space of reef before they reach the 
fragments, their force must be greatly increased with the increasing depth 
of water as the tide rises, and therefore I should have expected that the 
chief line of present erosion would have coincided with the line of high- 
water mark; and if the reef had grown outwards, that there would have 
been lines of erosion at greater heights. The conclusion, to which I am 
finally led by the interesting observations of Mr. Couthouy is, that the atoUg 
in the Low Archipelago have, like the Society Islands, remained at a 
stationary level for a long period : and this probably is the ordinary course 
of events, subsidence supervening after long intervals of rest. 

7 



9S PROOFS OF RECENT ELEVATION 

have undergone movements of elevation. This supposition, we shall 
immediately see, holds good to a remarkable extent ; and although a 
stationary condition of the land can hardly ever be open to proof, from 
the evidence being only negative, we are, in some degree, enabled to 
ascertain the correctness of the parts coloured red on the map, by the 
direct testimony of upraised organic remains of a modern date. Before 
going into the details on this head (printed in small type), I may 
mention, that when reading a memoir on coral formations by MM. Quoy 
and Gaimard * I was astonished to find, for I knew that they had 
crossed both the Pacific and Indian Oceans, that their descriptions were 
applicable only to reefs of the fringing class ; but my astonishment 
ended satisfactorily, when I discovered that, by a strange chance, all 
the islands which these eminent naturalists had visited, though several 
in number, namely, the Mauritius, Timor, New Guinea, the Mariana, 
and Sandwich Archipelagoes, could be shown by their own statements 
to have been elevated within a recent geological era. 

In the eastern half of the Pacific, the Sandwich Islands are all fringed, and 
almost every naturalist who has visited them, has remarked on the abund- 
ance of elevated corals and shells, apparently identical with living species. 
The Rev. W. Ellis informs me, that he has noticed round several parts of 
Hawaii, beds of coral-detritus, about twenty feet above the level of the sea, 
and where the coast is low they extend far inland. Upraised coral-rock 
forms a considerable part of the borders of Oahu ; and at Elizabeth Island f 
it composes three strata, each about ten feet thick. Nihau, which forms the 
northern, as Hawaii does the southern end of the group (350 miles 
in length), likewise seems to consist of coral and volcanic rocks. Mr. 
Couthouy j has lately described with interesting details, several upraised 
beaches, ancient reefs with their surfaces perfectly preserved, and beds of 
recent shells and corals, at the islands of Maui, Morokai, Oahu, and Tauai 
(or Kauai) in this group. Mr. Pierce, an intelligent resident at Oahu, is 
convinced, from changes which have taken place within his memory, during 
the last sixteen j'^ears, "that the elevation is at present going forward at a 
very perceptible rate." The natives at Kauai state that the land is there 
gaining rapidly on the sea, and Mr. Couthouy has no doubt, from the nature 
of the strata, that this has been effected by an elevation of the land. 

In the southern part of the Low Archipelago, Elizabeth Island is de- 
scribed by Captain Beechey,§ as being quite flat, and about eighty feet in 
height ; it is entirely composed of dead corals, forming a honeycombed, 
but compact rock. In cases like this, of an island having exactly the appear- 
ance, which the elevation of any one of the smaller surrounding atolls with a 
shallow lagoon would present, one is led to conclude (with little better reason, 
however, than the improbability of such small and low fabrics lasting, for an 
immense period, exposed to the many destroying agents of nature), that the 
elevation has taken place at an epoch not geologically remote. When 
merely the surface of an island of ordinary formation is strewed with marine 

* " Annales des Sciences Nat.," tom. vi., p. 279, etc. 

f " Zoology of Captain Beechey's Voyage," p. 176. Sea also MM. Quoy and 
Gaimard in " Annales de Scien. Nat.," tom. vi. 
X "Remarks on Coral Formations," p, 51. 
§ Beechey's " Voyage in the Pacific," p. 46, 4to ed. 



ON THE SHORES FRINGED BY CORAL-REEFS. 99 

bodies, and that continuously, or nearly so, from the beach to a certain 
height, and not above that height, it is exceedingly improbable that such 
organic remains, although they may not have been specially examined, 
should belong to any ancient period. It is necessary to bear these remarks 
in mind, in considering the evidence of the elevatory movements in the 
Pacific and Indian Oceans, as it does not often rest on specific determina- 
tions, and therefore should be received with caution. Six of the Cook and 
Austral Islands (S.W. of the Society group), are fringed ; of these, five were 
described to me by the Rev. J. Williams, as formed of coral-rock, associated 
with some basalt in Mangaia), and the sixth as lofty and basaltic. Mangaia is 
nearly three hundred feet high, with a level summit ; and according to Mr, S. 
Wilson * it is an upraised reef; " and there are in the central hollow, formerly 
the bed of the lagoon, many scattered patches of coral-rock, some of them 
raised to a height of forty feet." These knolls of coral-rock were evidently 
once separate reefs in the lagoon of an atoll. Mr. Martens, at Sydney, 
informed me that this island is surrounded by a terrace-like plain at about 
the height of a hundred feet, which probably marks a pause in its elevation. 
From these facts we may infer, perhaps, that the Cook and Austral Islands 
have been upheaved at a period probably not very remote. 

Savage Island (S.E. of the Friendly group), is about forty feet in height, 
Forster f describes the plants as already growing out of the dead, but still 
upright and spreading trees of coral ; and the younger Forster % believes 
that an ancient lagoon is now represented by a central plain ; here we can- 
not doubt that the elevatory forces have recently acted. The same con- 
clusion may be extended, though with somewhat less certainty, to the 
islands of the Friendly Group, which have been well described in the 
second and third voyages of Cook. The surface of Tongatabou is low and 
level, but with some parts a hundred feet high ; the whole consists of coral- 
rock, " which yet shows the cavities and irregularities worn into it by the 
action of the tides." § On Eoua the same appearances were noticed at an 
elevation of between two hundred and three hundred feet. Vavao, also, at 
the opposite or northern end of the group, consists, according to the Rev. J. 
Williams, of coral-rock. Tongatabou, with its northern extensive reefs, re- 
sembles either an upraised atoll with one half originally imperfect, or one 
unequally elevated ; and Anamouka, an atoll equally elevated. This latter 
island contains || in its centre a salt-water lake, about a mile-and-a-half in dia- 
meter, without any communication with the sea, and around it the land rises 
gradually like a bank ; the highest part is only between twenty and thirty 
feet ; but on this part, as well as on the rest of the land (which, as Cook ob- 
serves, rises above the height of true lagoon-islands), coral-rock, like that on 
the beach, was found. In the Navigator Archipelago, Mr. Couthouy ^ found 
on Manua many and very large fragments of coral at the height of eighty 
feet, "on a steep hill-side, rising half a mile inland from a low sandy plain 
abounding in marine remains." The fragments were embedded in a mixture 
of decomposed lava and sand. It is not stated whether they were accom- 
panied by shells, or whether the corals resembled recent species ; as these 

* Couthouy's "Remarks," p. 34. 

t "Observations made during Voyage round the World," p. 147. 

+ "Voyage," vol. ii., p. 163. 

§ Cook's "Third Voyage" (4to edition), vol. i,, p. 314. 

II Ibid., vol. i., p. 235. 

IT "Remarks on Coral-Formations," p. 5a 



100 Proofs of recent elevatioi^ 

remains were embedded they possibly may belong to a remote epoch ; biit 1 
presume this was not the opinion of Mr. Couthouy. Earthquakes are very 
frequent in this archipelago. 

Still proceeding westward we come to the Neiv Hebrides ; on these islands, 
Mr. G. Bennett (author of "Wanderings in New South Wales"), informs me 
he found much coral at a great altitude, which he considered of recent origin. 
Respecting Santa Crtiz, and the Solomon Archipelago, I have no information ; 
but at New Ireland, which forms the northern point of the latter chain, both 
Labillardiere and Lesson have described large beds of an apparently very 
modern madreporitic rock, with the form of the corals little altered. The 
latter author * slates that this formation composes a newer line of coast, 
modelled round an ancient one. There only remains to be described in the 
Pacific, that curved line of fringed islands, of which the Marianas form the 
main part. Of these Guam, Rota, Tiniam, Saypan, and some islets farther 
north, arc described by Quoy and Gaimard,f and Chamisso,J as chiefly com- 
posed of madreporitic limestone, which attains a considerable elevation, and 
is in several cases worn into successively rising cliflfs : the two former 
naturalists seem to have compared the corals and shells with the existing 
ones, and state that they are of recent species. Fais, which lies in the pro- 
longed line of the Marianas, is the only island in this part of the sea which is 
fringed ; it is ninety feet high, and consists entirely of madreporitic rock.§ 

In the East Indian Archipelago, many authors have recorded proofs of 
recent elevation. M. Lesson || states, that near Port Dory, on the north coast 
of New Guinea, the shores are flanked, to the height of 150 feet, by madre- 
poritic strata of modern date. He mentions similar formations at Waigiou, 
Amboina, Bourou, Ceram, Sonda, and Timor : at this latter place, MM. Quoy 
and Gaimard ^ have likewise described the primitive rocks, as coated to a 
considerable height with coral. Some small islets eastward of Timor are 
said in Kolff^s "Voyage" ** to resemble sm,all coral islets upraised some feet 
above the sea. Dr. Malcolmson informs me that Dr. Hardie found in Java 
an extensive formation, containing an abundance of shells, of which the 
greater part appear to be of existing species. Dr. Jack f f has described some 
upraised shells and corals, apparently recent, on Pulo Nias o^ Sumatra ; and 
Marsden relates in his history of this great island, that the names of many 
promontories, show that they were originally islands. On part of the west 

* " Voyage de la Coquille,''' Part. Zoo!og. 

•f- Freycinet's "Voyage autour du Monde." See also the " Hydrographical 
Memoir," p. 215. 

X Kotzebue's "First Vo5'age." 

§ Lutke's "Voyage," vol, ii,, p. 304. 

II Partie Zoolog., " Voyage de la Coquille.^^ 

i[ "Ann. des Scien. Nat," tom. vi., p. 281. 

** Translated by Windsor Earl, chapters vi., vii. 

ff " Geolog. Transact.," 2nd series, vol. i., p. 403. On the Peninsula of 
Malacca, in front of Pinang, 5° 30' N., Dr. Ward collected some shells, which 
Dr. Malcolmson informs me, although not compared with existing species, 
had a recent appearance. Dr. Ward describes in this neighbourhood 
("Trans. Asiat. Soc," vol. xviii., part ii., p. 166) a single water-worn rock, 
with a cenglomerate of sea-shells at its base, situated six miles inland, which, 
according to the traditions of the natives, was once surrounded by the sea. 
Ca:ptain Low has also described {Ibid., part i., p. 131) mounds of shells 
lying tv/o miles inland on this line of coast. 



ON THE SHORES FRINGED BY CORAL-REEFS. loi 

coast o[ Borneo and at the Sooloo Islands, the form of the land, the nature of 
the soil, and the water-washed rocks, present appearances * (although it is 
doubtful whether such vague evidence is worthy of mention), of having 
recently been covered by the sea ; and the inhabitants of the Sooloo Islands 
believe that this has been the case. Mr. Cuming, who has lately investigated, 
with so much success, the natural history of the Philippines, found near 
Cabagan, in Luzon, about fifty feet above the level of the R. Cagayan, and 
seventy miles from its mouth, a large bed of fossil shells : these, he informs 
me, are of the same species with those now existing on the shores of the 
neighbouring islands. Frcm the accounts given us by Captain Basil Hall 
and Captain Beecheyf of the lines of inland reefs, and walls of coral-rock 
worn into caves, above the present reach of the waves, at the Loo CJioo 
Islands, there can be little doubt that they have been upraised at no very 
remote period. 

Dr. Davy \ describes the northern province of Ceylon as being very low, 
and consisting of a limestone with shells and corals of very recent origin ; 
he adds, that it does not admit of a doubt that the sea has retired from this 
district even within the memory of man. There is also some reason for 
believing that the western shores of India, north of Ceylon, have been 
upraised within the recent period. § JSIaiiritiiis has certainly been upraised 
within the recent period, as I have stated in the chapter on fringing-reefs. 
The northern extremity of Madagascar is described by Captain Owen || as 
formed of madreporitic rock, as likewise are the shores and outlying islands 
along an immense space oi Eastern Africa, from a little north of the equator 
for nine hundred miles southward. Nothing can be more vague than the 
expression " madreporitic rock;" but at the same time it is, I think, scarcely 
possible to look at the chart of the linear islets, which rise to a greater 
height than can be accounted for by the growth of coral, in front of the coast, 
from the equator to 2° S., without feeling convinced that a line of fringing- 
retfs has been elevated at a period so recent, that no great changes have 
since taken place on the surface of this part of the globe. Some, also, of the 

* "Notices of the East Indian Arch.," Singapore, 1828, p. 6, and 
Append., p. 43. 

f Captain B.Hall, "Voyage to Loo Choo," Append., pp. xxi. and xxv. Captain 
Bccchey's " Voj^age," p. 496. 

X "Travels in Ceylon," p. 13. This madreporitic formation is mentioned 
by M. Cordier in his report to the Institute (May 4th, 1839), on the voyage 
of the Chevrelie, as one of immense extent, and belonging to the latest 
tertiary period. 

§ Dr. Bcnza, in his "Journey through the N. Circars " (the Madras Lil. 
end Scient. Jciirn., vol. v.) has described a formation with recent fresh-water 
and marine shells, occurring at the distance of three or four miles from the 
present shore. Dr. Bcnza, in conversation with me, attributed their position 
to a rise of the land. Dr. Malcolmson, however (and there cannot be a 
higher authority on the geology of India), informs me that he suspects that 
these beds may have been formed by the mere action of the waves and 
currents accumulating sediment. From analogy I should much incline to 
Dr. Benza's opinion. 

II Owen's " Africa," vol. ii., p. 37, for Madagascar ; and for S. Africa, vol. i., 
pp. 412 and 426. Lieutenant Boteler's narrative contains fuller particulars 
regarding the coral-rock, vol. i., p. 174, and vol. ii.,pp. 41 and 54. Sec also 
Rus^henbcrger's "Voyage round the World," vol, i., p. 60. 



102 PROOFS OF RECENT ELEVATION 

higher islands of madreporitic rock on this coast, for instance Pemba, have 
very singular forms, which seem to show the combined effect of the growth 
of coral round submerged banks, and their subsequent upheaval. Dr, Allan 
informs me that he never observed any elevated organic remains on the 
Seychelles, which come under our fringed class. 

The nature of the formations round the shores of the Red Sea, as described 
by several authors, shows that the whole of this large area has been elevated 
within a very recent tertiary epoch. A part of this space in the appended 
map, is coloured blue, indicating the presence of barrier-reefs : on which 
circumstance I shall presently make some remarks. Riippell * states that 
the tertiary formation, of which he has examined the organic remains, forms 
a fringe along the shores with a uniform height of from thirty and forty feet 
from the mouth of the Gulf of Suez to about lat. 26° ; but that south of 26°, the 
beds attain only the height of from twelve to fifteen feet. This, however, can 
hardly be quite accurate ; although possibly there may be a decrease in the 
elevation of the shores in the middle parts of the Red Sea, for Dr. Malcolmson 
(as he informs me) collected from the cliffs of Camaran Island (lat. 15° 30' S.) 
shells and corals, apparently recent, at a height between thirty and forty feet ; 
and Mr. Salt ("Travels in Abyssinia") describes a similar formation a little 
southward on the opposite shore at Amphila. Moreover, near the mouth of the 
Gulf of Suez, although on the coast opposite to that on which Dr. Riippell says 
that the modern beds attain a height of only thirty to forty feet, Mr. Burton f 
found a deposit replete with existing species of shells, at the height of 
200 feet. In an admirable series of drawings by Captain Moresby, I could 
see how continuously the cliff-bounded low plains of this formation extended 
with a nearly equable height, both on the eastern and western shores. The 
southern coast of Arabia seem to have been subjected to the same elevatory 
movement, for Dr. Malcolmson found at Sahar low cliffs containing shells 
and corals, apparently of recent species. 

The Persian C^Z/'abounds with coral-reefs; but as it is difficult to distin- 
guish them from sand-banks in this shallow sea, I have coloured only some 
near the mouth ; towards the head of the gulf Mr. Ainsworth+ says that the 
land is worn into terraces, and that the beds contain organic remains of 
existing forms. The West Indian Archipelagic of " fringed" islands, alone 
remains to be mentioned ; evidence of an elevation within a late tertiary 
epoch of nearly the whole of this great area, may be found in the works of 
almost all the naturalists who have visited it. I will give some of the 
principal references in a note.§ 

* Riippell, "Reise in Abyssinien," Band i., s. 141. 

t Lyell's " Principles of Geology," 5th edition, vol. iv., p. 25. 

j Ainsworth's "Assyria and Babylon," p. 217. 

§ On Florida and the north shores of the Gulf of Mexico, Rogers' " Report 
to Brit. Assoc," vol. iii,, p. 14. — On the shores of Mexico, Humboldt, "Polit. 
Essay on New Spain," vol. i., p. 62. (I have also some corroborative facts 
with respect to the shores of Mexico.) — Honduras and the Antilles, Lyell's 
" Principles," 5th ed., vol. iv., p. 22. — Santa Cruz and Barbadoes, Prof. 
Hovey, Silliman' s Journ., vol. xxxv,, p. 74, — St. Domingo, Courrojolles,y<7wr. 
de Phys., tom. liv., p. 106. — Bahamas, United Sei-vice Journal, No. Ixxi. 
pp. 218 and 224. Jamaica, De la Beche, "Geol. Man.," p. 142. — Cuba, Taylor 
in Lond. and Edin. Mag., vol. xi., p. 1 7. Dr. Daubeny also, at a meeting of 
the Geolog. Soc, orally described some very modern beds lying on the N.W. 
parts of Cuba. I might have added many other less important references. 



ON THE SHORES FRINGED BY CORAL-REEFS. 103 

It is very remarkable on reviewing these details, to observe in how 
many instances fringing-reefs round the shores, have coincided with 
the existence on the land of upraised organic remains, which seem, 
from evidence more or less satisfactory, to belong to a late tertiary 
period. It may, however, be objected, that similar proofs of elevation, 
perhaps, occur on the coasts coloured blue in our map : but this cer- 
tainly is not the case with the few following and doubtful exceptions. 

The entire area of the Red Sea appears to haye been upraised within 
a modern period ; nevertheless I have been compelled (though on 
unsatisfactory evidence, as given in the Appendix) to class the reefs 
in the middle part, as barrier-reefs ; should, however, the statements 
prove accurate of the less height of the tertiary bed in this middle part, 
compared with the northern and southern districts, we might well sus- 
pect that it had subsided subsequently to the general elevation by which 
the whole area has been upraised. Several authors * have stated that 
that they have observed shells and corals high up on the mountains of 
the Society Islands, — a group encircled by barrier-reefs, and, therefore, 
supposed to have subsided : at Tahiti Mr. Stutchbury found on the 
apex of one of the highest mountains, between 5,000 and 7,000 feet 
above the level of the sea, "a distinct and regular stratum of semi- 
fossil coral." At Tahiti, however, other naturalists, as well as myself, 
have searched in vain at a low level near the coast, for upraised shells 
or masses of coral-reef, where if present they could hardly have been 
overlooked. From this fact, I concluded that probably the organic 
remains strewed high up on the surface of the land, had originally been 
embedded in the volcanic strata, and had subsequently been washed 
out by the rain. I have since heard from the Rev. W. Ellis, that the 
remains which he met with, were (as he believes) interstratified with 
an argillaceous tuff ; this likewise was the case with the shells observed 
by the Rev. D. Tyerman at Huaheine. These remains have not been 
specifically examined ; they may, therefore, and especially the stratum 
observed by Mr, Stutchbury at an immense height, be contemporaneous 
with the first formation of the Society Islands, and be of any degree 
of antiquity ; or they may have been deposited at some subsequent, 
but probably not very recent, period of elevation ; for if the period had 
been recent, the entire surface of the coast land of these islands, where 
the reefs are so extensive, would have been coated with upraised coral 

* Ellis, in his " Polynesian Researches," was the first to call attention to 
these remains (vol. i., p. 38), and the tradition of the natives concerning 
them. See also Williams, '* Nar. of Miss. Enterprise," p. 21 ; also Tyermfin 
and G. Bennett, "Journ. of Voyage," vol. i., p. 213; also Mr. Couthouy's 
"Remarks," p. 51 ; but this principal fact, namely, that there is a mass of 
upraised coral on the narrow peninsula of Tiarubu, is frjm hearsay evidence ; 
also Mr. Stutchbury, West of England Journ., No. i., p. 54. There is a 
passage in Von Zach, "Corres. Astronom.," vol. x., p. 266, inferring an uprising 
at Tahiti, from a footpath now used, which was formerly impassable ; but I 
particularly inquired from several native chiefs, whether they knew of any 
change of this kind, and they were unanimous in giving mc an answer in 
the negative. 



104 THE PRESENCE OF ACTIVE VOLCANOES 

which certainly is not the case. Two of the Harvey, or Cook Islands, 
namely, Aitutaki and Manouai, are encircled by reefs, which extend so 
far from the land, that 1 have coloured them blue, although with much 
hesitation, as the space within the reef is shallow, and the outline of 
the land is not abrupt. These two islands consist of coral-rock ; but I 
have no evidence of their recent elevation, besides, the improbability of 
Mangaia, a fringed island in the same group (but distant 170 miles), 
having retained its nearly perfect atoll-like structure, during any im- 
mense lapse of time after its upheaval. The Red Sea, therefore, is the 
only area in which we have clear proofs of the recent elevation of 
a district, which, by our theory (although the barrier-reefs are there not 
well characterised), has lately subsided. But we have no reason to 
be surprised at oscillation, of level of this kind having occasionally 
taken place. There can be scarcely any doubt that Savage, Aurora,* and 
Mangaia Islands, and several of the islands in the Friendly group, existed 
originally as atolls, and these have undoubtedly since been upraised to 
some height above the level of the sea ; so that by our theory, there 
has here, also, been an oscillation of level, — elevation having suc- 
ceeded subsidence, instead of, as in the middle part of the Red Sea 
and at the Harvey Islands, subsidence having probably succeeded 
recent elevation. 

It is an interesting fact, that Fais, which, from its composition, 
form, height, and situation at the western end of the Caroline Archi- 
pelago, one is strongly induced to believe existed before its upheaval 
as an atoll, lies exactly in the prolongation of the curved line of the 
Mariana group, which we know to be a line of recent elevation. I may 
add, that Elizabeth Island, in the southern part of the Low Archipelago, 
which seems to have had the same kind of origin as Fais, lies near 
Pitcairn Island, the only one in this part of the ocean which is high, 
and at the same time not surrounded by an encircling barrier-reef. 

O71 the absence of active volca?toes in the areas of subsidence, and on 
their freqiie7it presence iji the areas of elevation. — Before making some 
concluding remarks on the relations of the spaces coloured blue and 
red, it will be convenient to consider the position on our map of the 
volcanoes historically known to have been in action. It is impossible 
not to be struck, first with the absence of volcanoes in the great areas of 
subsidence tinted pale and dark blue, — namely, in the central parts 
of the Indian Ocean, in the China Sea, in the sea between the barriers 

* Aurora Island is described by Mr. Couthouy ("Remarks," p. 58); it lies 
120 miles north-east of Tahiti ; it is not coloured in the appended map, 
because it does not appear to be fringed by hving reefs. Mr. Couthouy 
describes its summit as "presenting a broad table-land which declines a few 
feet towards the centre, where we may suppose the lagoon to have been 
placed." It is about two hundred feet in height, and consists of reef-rock and 
conglomerate, with existing species of coral embedded in it. The island 
has been elevated at two successive periods ; the cliffs being marked half- 
way up with a horizontal water-worn line of deep excavations. Aurora 
Island seems closely to resemble in structure pli^abeth Island, c\t the southern 
end of the Low Archipelago. 



DETERMINED BY THE MOVEMENTS IN PROGRESS. 105 

of Australia and New Caledonia, in the Caroline, Marshall, Gilbert, and 
Low Archipelagoes ; and, secondly, with the coincidence of the principal 
volcanic chains with the parts coloured red, which indicates the pre- 
sence of fringing-reefs ; and, as we have just seen, the presence in most 
cases of upraised organic remains of a modern date. I may here 
remark that the reefs were all coloured before the volcanoes were added 
to the map, or indeed before I knew of the existence of several of them. 

The volcano in Torres Strait, at the northern point of Australia, is 
that which lies nearest to a large subsiding area, although situated 
125 miles within the outer margin of the actual barrier-reef. The Great 
Comoro Island, which probably contains a volcano, is only twenty miles 
distant from the barrier-reef of Mohila; Ambil volcano, in the PhiHppines, 
is distant only a little more than sixty miles from the atoll-formed Appoo 
reef: and there are two other volcanoes in the map within ninety miles 
of circles coloured blue. These few cases, which thus offer partial 
exceptions to the rule, of volcanoes being placed remote from the areas 
of subsidence, lie either near single and isolated atolls, or near small 
groups of encircled islands ; and these by our theory can have, in few 
instances, subsided to the same amount in depth or area, as groups of 
atolls. There is not one active volcano within several hundred miles 
of an archipelago, or even a small group of atolls. It is, therefore, a 
striking fact that in the Friendly Archipelago, which owes its origin to 
the elevation of a group of atolls, two volcanoes, and, perhaps, others 
are known to be in action : on ,the other hand, on several of the 
encircled islands in the Pacific, supposed by our theory to have sub- 
sided, there are old craters and streams of lava, which show the 
effects of past and ancient eruptions. In these cases, it would appear 
as if the volcanoes had come into action, and had become extinguished 
on the same spots, according as the elevating or subsiding movements 
prevailed. 

There are some other coasts on the map, where volcanoes in a state 
of action concur with proofs of recent elevation, besides those coloured 
red from being fringed by coral-reefs. Thus I hope to show in a future 
volume, that nearly the whole line of the west coast of South America, 
which forms the greatest volcanic chain in the world, from near the 
equator for a space of between 2,000 and 3,000 miles southward, has 
undergone an upward movement during a late geological period. The 
islands on the north-western shores of the Pacific, which form the 
second greatest volcanic chain, are very imperfectly known ; but Luzon, 
in the Philippines, and the Loo Choo Islands, have been recently 
elevated ; and at Kamtschatka * there are extensive tertiary beds of 
modern date. Evidence of the same nature, but not very satisfactory, 
may be detected in Northern New Zealand where there arc two 
volcanoes. The co-existence in other parts of the world of active 
volcanoes, with upraised beds of a modern tertiary origin, will occur to 

* At Sctlanka, in lat. 58° N. (Von Buch's " Descrip. des Isles Canaries," 
p. 455)- In a forthcoming part, I shall give the evidence referred to with 
respect to tjic plev^tion of New Zealand. 



ic6 ON THE AREAS OF 

every geologist.* Nevertheless, until it could be shown that volcanoes 
were inactive, or did not exist in subsiding areas, the conclusion that 
their distribution depended on the nature of the subterranean move- 
ments in progress, would have been hazardous. But now, viewing the 
appended map, it may, I think, be considered as almost established, 
that volcanoes are often (not necessarily always) present in those areas 
where the subterranean motive power has lately forced, or is now 
forcing outwards, the crust of the earth, but that they are invariably 
absent in those, where the surface has lately subsided or is still sub- 
siding. t 

On the relations of the areas of subsidence and elevation. — The 
immense surfaces on the map, which, both by our theory and by the 
plain evidence of upraised marine remains, have undergone a change of 
level either downwards or upwards during a late period, is a most 
remarkable fact. The existence of continents shows that the areas 
have been immense which at some period have been upraised ; in South 
America we may feel sure, and on the north-western shores of the 
Indian Ocean we may suspect, that this rising is either now actually in 
progress, or has taken place quite recently. By our theory, we may 
conclude that the areas are likewise immense which have lately sub- 
sided, or, judging from the earthquakes occasionally felt and from 
other appearances, are now subsiding. The smallness of the scale of 
our map should not be overlooked : each of the squares on it contains 
(not allowing for the curvature of the earth) 810,000 square miles. 
Look at the space of ocean from near the southern end of the Low 
Archipelago to the northern end of the Marshall Archipelago, a length 
of 4,500 miles, in which, as far as is known, every island, except Aurora 
which lies just without the Low Archipelago, is atoll-formed. The 
eastern and western boundaries of our map are continents, and they 
are rising areas : the central spaces of the great Indian and Pacific 
Oceans, are mostly subsiding ; between them, north of Australia, lies 
the most broken land on the globe, and there the rising parts are 
surrounded and penetrated by areas of subsidence,! so that the pre- 
vailing movements now in progress, seem to accord with the actual 
states of surface of the great divisions of the world. 

The blue spaces on the map are nearly all elongated ; but it does 
not necessarily follow from this (a caution, for which I am indebted to 
Mr. Lyell), that the areas of subsidence were likewise elongated ; for 

* During the subterranean disturbances which took place in Chile, in 
1835, I have shown ("Geolog. Trans.," 2nd Ser., vol. v., p. 606) that at the 
same moment that a large district was upraised, volcanic matter burst forth 
at widely separated points, through both new and old vents. 

f We may infer from this rule, that in any old deposit, which contains 
interstratified beds of erupted matter, there was at the period, and in the 
area of its formation, a tendency to an upward movement in the earth's sur- 
face, and certainly no movement of subsidence. 

I I suspect that the Arru and Timor-laut Islands present an included 
small area of subsidence, like that of the China Sea but I have not ventured 
to colour them from my imperfect information, as given in the Appendix, 



SUBSIDENCE AND ELEVATION. 107 

the subsidence of a long, narrow space of the bed of the ocean, includ- 
ing in it a transverse chain of mountains, surmounted by atolls, would 
only be marked on the map by a transverse blue band. But where a 
chain of atolls and barrier-reefs lies in an elongated area, between 
spaces coloured red, which therefore have remained stationary or have 
been upraised, this must have resulted either from the area of subsidence 
having originally been elongated (owing to some tendency in the earth's 
crust thus to subside), or from the subsiding area having originally been 
of an irregular figure, or as broad as long, and having since been 
narrowed by the elevation of neighbouring districts. Thus the areas, 
which subsided during the formation of the great north and soutli lines 
of atolls in the Indian Ocean, — of the east and west line of the Caroline 
atolls, — and of the north-west and south-east line of the barrier-reefs 
of New Caledonia and Louisiade, must have originally been elongated, 
or if not so, they must have since been made elongated by elevations, 
which we know to belong to a recent period. 

I infer from Mr. Hopkins' researches,* that for the formation of a 
long chain of mountains, with few lateral spurs, an area elongated in 
the same direction with the chain, must have been subjected to an 
elevatory movement. Mountain-chains, however, when already formed, 
although running in very different directions, it seems t may be raised 
together by a widely-acting force : so, perhaps, mountain-chains may 
subside together. Hence, we cannot tell, whether the Caroline and 
Marshall Archipelagoes, two groups of atolls running in different 
directions and meeting each other, have been formed by the subsidence 
of two areas, or of one large area, including two distinct lines of 
mountains. We have, however, in the southern prolongation of the 
Mariana Islands, probable evidence of a line of recent elevation having 
intersected one of recent subsidence. A view of the map will show 
that, generally, there is a tendency to alternation in the parallel areas 
undergoing opposite kinds of movement ; as if the sinking of one area 
balanced the rising of another. 

The existence in many parts of the world of high table-land, proves 
that large surfaces have been upraised in mass to considerable heights 
above the level of the ocean ; although the highest points in almost 
every country consist of upturned strata, or erupted matter : and from 

* "Researches in Physical Geology," Transact. Cambridge Phil. Soc., vol. 
vi., part i. 

f For instance in S. America from lat. 34°, for many degrees southward 
there are upraised beds containing recent species of shells, on both the 
Atlantic and Pacific side of the continent, and from the gradual ascent of 
the land, although with very unequal slopes, on both sides towards the 
Cordillera, I think it can hardly be doubted that the entire width has been 
upraised in mass within the recent period. In this case the two W.N.W. 
and E.S.E. mountain-lines, namely the Sierra Ventana and the S. Tapalguen, 
and the great north and south line of the Cordillera have been together raised. 
In the West Indies the N. and S. line of the Eastern Antilles, and the E. and 
W. line of Jamaica, appear both to have been upraised within the latest 
geological period. 



io8 RECAPITULATION. 

the immense spaces scattered with atolls, which indicate that land 
originally existed there, although not one pinnacle now remains above 
the level of the sea, we may conclude that wide areas have subsided to 
an amount, sufficient to bury not only any formerly existing table-land, 
but even the heights formed by fractured strata, and erupted matter. 
The effects produced on the land by the later elevatory movements, 
namel)', successively rising cliffs, lines of erosion, and beds of literal 
shells and pebbles, all requiring time for their production, prove that 
these movements have been very slow ; w^e can, however, infer this 
with safety, only with respect to the few last hundred feet of rise. But 
with reference to the whole vast amount of subsidence, necessary to 
have produced the many atolls widely scattered over immense spaces, 
it has already been shown (and it is, perhaps, the most interesting 
conclusion in this volume), that the movements must either have been 
uniform and exceedingly slow, or have been effected by small steps, 
separated from each other by long intervals of time, during which the 
reef-constructing polypifers were able to bring up their solid frame- 
works to the surface. We have little means of judging whether many 
considerable oscillations of level hav'e generally occurred during the 
elevation of large tracts ; but we know, from clear geological evidence, 
that this has frequently taken place ; and we have seen on our map, 
that some of the same islands have both subsided and been upraised. I 
conclude, however, that most of the large blue spaces, have subsided 
without many and great elevatory oscillations, because only a few 
upraised atolls have been observed : the supposition that such eleva- 
tions have taken place, but that the upraised parts have been worn 
down by the surf, and thus have escaped observation, is overruled by 
the very considerable depth of the lagoons of all the larger atolls ; tor 
this could not have been the case, if they had suffered repeated eleva- 
tions and abrasion. From the comparative observations made in these 
latter pages, we may finally conclude, that the subterranean changes 
which have caused some large areas to rise, and others to subside, have 
acted in a very similar manner. 

Recapitulation. — In the three first chapters, the principal kinds of 
coral-reefs were described in detail, and they were found to differ little, 
as far as relates to the actual surface of the reef. An atoll differs from 
an encircling barrier-reef only in the absence of land vvilhin its central 
expanse ; and a barrier-reef differs from a fringing-reef, in being placed 
at a much greater distance from the land with reference to the probable 
inclination of its submarine foundation, and in the presence of a deep- 
water lagoon-like space or moat within the reef. In the fourth chapter 
the growing powers of the reef-constructing polypifers were discussed ; 
and it was shown, that they cannot flourish beneath a very limited 
depth. In accordance with this limit, there is no difficulty respecting 
the foundations on which fringing-reefs are based ; whereas, with 
barrier-reefs and atolls, there is a great apparent difficulty on this 
hesd ; in barrier-reefs from the improbability of the rock of the coast or 
of banks of sediment extending, in every instance, so far seaward 
within tlie required depth ; — and in atolls, from the immensity of the 



kECAPITULATIOK\ i6^ 

spaces Ovet which they are interspersed, and the apparent necessity 
for believing that they are all supported on mountain-summits, which 
although rising very hear to the surface-level oT the sea, in no one 
instance emerge above it. To escape this latter most improbable 
admission, which implies the existence of submarine chains of moun- 
tains of almost the same height, extending over areas of many thousand 
square miles, there is but one alternative ; namely, the prolonged 
subsidence of the foundations, on which the atolls were primarily based, 
together with the upward growth of the reef-constructing corals. On 
this view every difficulty vanishes ; fringing-reefs are thus converted 
into barrier-reefs ; and barrier-reefs, when encircling islands, are thus 
converted into atolls, the instant the last pinnacle of land sinks beneath 
the surface of the ocean. 

Thus the ordinary forms and certain peculiarities in the structure of 
atolls and barrier-reefs can be explained ; — namely, the wall-like structure 
on their inner sides, the basin or ring-like shape both of the marginal and 
central reefs in the Maldiva atolls — the union of some atolls as if by a 
ribbon — the apparent disseverment of others — and the occurrence, in 
atolls as well as in barrier-reefs, of portions of reef, and of the whole of 
some reefs, in a dead and submerged state, but retaining the outline of 
living reefs. Thus can be explained the existence of breaches through 
barrier-reefs in front of valleys, though separated from them by a wide 
space of deep water ; thus, also, the ordinary outline of groups of atolls 
and the relative forms of the separate atolls one to another; thus can 
be explained the proximity of the two kinds of reefs formed during 
subsidence, and their separation from the spaces where fringing-reefs 
abound. On searching for other evidence of the movements supposed 
by our theory, we find marks of change in atolls and in barrier-reefs, 
and of subterranean disturbances under them; but from the nature of 
things, it is scarcely possible to detect any direct proofs of subsidence, 
although some appearances are strongly in favour of it. On the fringed 
coasts, however, the presence of upraised marine bodies of a recent 
epoch, plainly show, that these coasts, instead of having remained 
stationary, which is all that can be directly inferred from our theory, 
have generally been elevated. 

Finally, when the two great types of structure, namely barrier-reefs 
and atolls on the one hand, and fringing-reefs on the other, were laid 
down in colours on our map, a magnificent and harmonious picture of 
the movements, which the crust of the earth has within a late period 
undergone, is presented to us. We there see vast areas rising, with 
volcanic matter every now and then bursting forth through the vents or 
fissures with which they are traversed. We see other wide spaces 
slowly sinking without any volcanic outburst, and we may feel sure, that 
this sinking must have been immense in amount as well as in area, 
thus to have buried over the broad face of the ocean every one of those 
mountains, above which atolls now stand like monuments, marking the 
place of their former existence. Reflecting how powerful an agent 
with respect to denudation, and consequently to the nature and thick- 
ness of the deposits in accumulation, the sea must ever be, when acting 



no RECAPITULATION. 

for prolonged periods on the land, during either its slow emergence or 
subsidence ; reflecting, also, on the final effects of these movements in 
the interchange of land and ocean-water on the climate of the earth, 
and on the distribution of organic beings, I may be permitted to hope, 
that the conclusions derived from the study of coral-formations, originally 
attempted merely to explain their peculiar forms, may be thought worthy 
of the attention of geologists. 



APPENDIX. 

CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND 
ISLANDS IN PLATE III. 



In the beginning of the last chapter I stated the principles on which 
the map is coloured. There only remains to be said, that it is an exact 
ropy of one by M. C. Gressier, published by the D6p6t General de la 
Marine, in 1835. The names have been altered into P^nglish, and the 
longitude has been reduced to that of Greenwich. The colours were 
first laid down on accurate charts, on a large scale. The data, on which 
the volcanoes historically known to have been in action, have been 
marked with vermillion, were given in a note to the last chapter. I 
will commence my description on the eastern side of the map, and will 
describe each group of islands consecutively, proceeding westward 
across the Pacific and Indian Oceans, but ending with the West Indies. 

The Western Shores of America appear to be entirely without 
coral-reefs ; south of the equator the survey of the Beagle, and north of 
it, the published charts show that this is the case. Even in the Bay of 
Panama, where corals flourish, there are no true coraf-reefs, as I have 
been informed by Mr. Lloyd. There are no coral-reefs in the Galapagos 
Archipelago, as I know from personal inspection ; and I believe there 
are none on the Cocos, Revilla-gigedo, and other neighbouring islands. 
Clipjerton rock, 10° N., 109° W., has lately been surveyed by Captain 
Belcher; in form it is like the crater of a volcano. From a drawing 
appended to the MS. plan in the Admiralty, it evidently is not an atoll. 
The eastern parts of the Pacific present an enormous area, without any 
islands, except Easter, and Sala, and Gomez Islands, which do not 
appear to be surrounded by reefs. 

The Low Archipelago. — This group consists of about eighty atolls : 
it would be quite superfluous to refer to descriptions of each. In 
D'Urville and Lottin's chart, one island {Wolchonsky) is written with a 
capital letter, signifying, as explained in a former chapter, that it is a 
high island ; but this must be a mistake, as the original chart by 
Bellinghausen shows that it is a true atoll. Captain Beechey says of 
the thirty-two groups which he examined (of the greater number of which 



112 APPENDIX. 

I have seen beautiful MS. ciiarts in the Admiralty), that twenty-nine now 
contain kigoons, and he believes the other three originall}'- did. Belling- 
hausen (see an account of his Russian voyage, in the " Biblioth. des 
Voyages," 1834, p. 443) says, that the seventeen islands which he dis- 
covered resembled each other in structure, and he has given charts on 
a large scale of all of them. Kotzebue has given plans of several; Cook 
and Bligh mention others ; a few were seen during the vo3^age of the 
Beagle; and notices of other atolls are scattered through several publi- 
cations. The Acidon group in this archipelago has lately been dis- 
covered {Geograph. Joiirn., vol. vii., p. 454) ; it consists of three 
small and low islets, one of which has a lagoon. Another lagoon- 
island has been discovered {Nant. Mag:, 1839, p. 770), in 22° 4' S., 
and 136° 20' W. Towards the S.E. part of the group, there are some 
islands of different formation : Elizabeth Island is described by Beechey 
(p. 46, 4to ed.) as fringed by reefs, at the distance of between two and 
three hundred yards ; coloured red. Pitcaini Island, in the immediate 
neighbourhood, according to the same authority, has no reefs of any 
kind, although numerous pieces of coral are thrown up on the beach ; 
the sea close to its shore is very deep (see "Zool. of Beechey's 
Voyage," p. 164); it is left uncoloured. GaiJibier Islands (see Plate 
I., Fig. 8), are encircled by a barrier-reef; the greatest depth within is 
thirty-eight fathoms ; coloured pale blue, Aurora Island, which lies 
N.E. of Tahiti close to the large space coloured dark blue in the map, 
has been already described in a note (p. 71), on the authority of Mr. 
Couthouy ; it is an upraised atoll, but as it does not appear to be 
fringed by living reefs, it is left uncoloured. 

The Society Arch, is separated by a narrow space Irom the Low 
Arch. ; and in their parallel direction they mxanifest some relation to 
each other. I have already described the general character of the reefs 
of these fine encircled islands. In the " Atlas of the Coquilles Voyage " 
there is a good general chart of the group, and separate plans of some 
of the islands. Tahiti, the largest island in the group, is almost sur- 
rounded, as seen in Cook's chart, by a reef from half a mile to a mile 
and a half from the shore, with from ten to thirty fathoms within it. 
Some considerable submerged reefs lying parallel to the shore, with a 
broad and deep space within, have lately been discovered {Natct. 
Mag,, 1836, p. 264) on the N.E. coast of the island, where none are 
laid down by Cook. At Eii?ieo the reef " which like a ring surrounds 
it, is in some places one or two miles distant from the shore, in others 
united to the beach" (Ellis, "Polynesian Researches," vob i., p. 18, 
i2mo edit.). Cook found deep water (twenty fathoms) in some of the 
harbours within the reef. Mr. Couthouy, however, states ("Remarks," 
p. 45) that both at Tahiti and Eimeo, the space between the barrier-reef 
and the shore, has been almost filled up, — " a nearly continuous fringing- 
reef surrounding the island, and varying from a few yards to rather 
more than a mile in width, the lagoons merely forming canals between 
this and the sea-reef," that is the barrier-reef. Tapanianoa is sur- 
rounded by a reef at a considerable distance from the shore ; from the 
island being small it is breached, as I am informed by the Rev. W. 



PACIFIC OCEAN. 113 

Ellis, only by a narrow and crooked boat channel. This is the lowest 
island in the group, its height probably not exceeding 500 feet. A little 
way north of Tahiti, the low coral-islets of TeUiroa are situated ; from 
the description of them given me by the Rev. J. Williams (the author 
of the •' Narrative of Missionary Enterprise "), I should have thought 
they had formed a small atoll, and likewise from the description given 
by the Rev. D. Tyerman and G. Bennett ("Journ. of Voy. and Travels," 
vol. i., p. 183), who say that ten low coral-islets "are comprehended 
within one general reef, and separated from each other by interjacent 
lagoons;" but as Mr. Stutchbury {West of England Journal, vol. i., 
p. 54) describes it as consisting of a mere narrow ridge, I have left it 
uncolourcd. Maitea, eastward of the group, is classed by Forstcr as a 
high encircled island ; but from the account given by the Rev. D. 
Tyerman and G. Bennett (vol. i., p. 57) it appears to be an exceedingly 
abrupt cone, rising from the sea without any reef; I have left it un- 
coloured. It would be superfluous to describe the northern islands in 
this group, as they may be well seen in the chart accompanying the 4to 
edition of Cook's '* Voyages," and in the "Atlas of the CoqicillesVoyvLgi:." 
Matirua is the only one of the northern islands, in which the water 
within the reef is not deep, being only four and a half fathoms ; but the 
great width of the reef, stretching three miles and a half southward of 
the lai!d (which is represented in the drawing in the "Atlas of the 
Coquilles Voyage" as descending abruptly to the water) shows, on 
the principle explained in the beginning of the last chapter, that it 
belongs to the barrier class. I may here mention, from information 
communicated to me by the Rev. V/. Ellis, that on the N.E. side of 
Huaheine there is a bank of sand, about a quarter of a mile wide, 
extending parallel to the shore, and separated from it by an extensive 
and deep lagoon ; this bank of sand rests on coral-rock, and un- 
doubtedly was originally a living reef. North of Bolabola lies the atoll 
of Toubai (Motou-iti of the " Coqtiille's Atlas ") which is coloured dark 
blue ; the other islands, surrounded by barrier-reefs, are pale blue ; 
three of them are represented in Figures 3, 4, and 5, in Plate I. There 
are three low coral-groups lying a little E. of the Society Arch., 
and almost forming part of it, namely, Bellinghat(SC7i, which is said by 
Kotzebue ("Second Voyage," vol. ii., p. 255), to be a lagoon-island; 
Mopeha, which, from Cook's description (" Second Voyage," book iii., 
chap, i.), no doubt is an atoll ; and the Scilly Islands, which are said by 
Wallis ("Voyage," chap, ix.) to form 2^ group of low islets and shoals, 
and, therefore, probably, they compose an atoll : the two former have 
been coloured blue, but not the latter. 

Mendana or Marquesas Group. — These islands are entirely without 
reefs, as may be seen in Krusenstern's Atlas, making a remarkable 
contrast with the adjacent group of the Society Islands. Mr. F. D. 
Bennett has given some account of this group, in the seventh volume of 
the Geograph. Joicrn. He informs me that all the islands have the same 
general character, and that the water is very deep close to their shores. 
He visited three of them, namely, Do7ninicana, Christiana, and 
Roapoa; their beaches are strewed with rounded masses of coral, and 

8 



114 APPENDIX. 

although no regular reefs exist, yet the shore is in many places lined 
by coral-rock, so that a boat grounds on this formation. Hence these 
islands ought probably to come within the class of fringed islands and 
be coloured red ; but as I am determined to err on the cautious side, I 
have left them uncoloured. 

Cook or Harvey and Austral Isld. — Palmeysio7i Island is minutely 
described as an atoll by Captain Cook during his voyage in 1774; 
coloured blue. Aitictaki was partially surveyed by the Beagle (see 
map accompanying " Voyages of Adventure and Beagle"^ ; the land is 
hilly, sloping gentlyto the beach ; the highest point is 360 feet ; on the 
southern side the reef projects five miles from the land : off this 
point the Beagle found no bottom with 270 fathoms : the reef is 
surmounted by many low coral-islets. Although within the reef the 
water is exceedingly shallow, not being more than a few feet deep, as 
I am informed by the Rev. J. Williams, nevertheless, from the great 
extension of this reef into a profoundly deep ocean, this island probably 
belongs, on the principle lately adverted to, to the barrier class, and I 
have coloured it pale blue ; although with much hesitation. — Manouai 
or Hajvey Isld. The highest point is about fifty feet : the Rev. J. 
Williams informs me that the reef here, although it lies far from the 
shore, is less distant than at Aitutaki, but the water within the reef is 
rather deeper : I have also coloured this pale blue, with many doubts. — 
Round Mitia7'o Isld., as I am informed by Mr. Williams, the reef is 
attached to the shore ; coloured red. — Maiiki or Maouti ; the reef round 
this isld. (under the name of Parry Isld., in the " Voyage of H.M.S. 
Blonde" p. 209) is described as a coral-flat, only fifty yards wide, and 
two feet under water. This statement has been corroborated by Mr. 
Williams, who calls the reef attached ; coloured red. — Atiu, or Wateeo ; 
a moderately elevated hilly island, like the others of this group. The 
reef is described in Cook's *' Voyage," as attached to the shore, and about 
one hundred yards wide ; coloured red. — Fenotia-iii ; Cook describes 
this isld. as very low, not more than six or seven feet high (vol. i., bk. 
ii., chap, iii., 1777) ; in the chart published in the " Coquilles Atlas," a reef 
is engraved close to the shore : this isld. is not mentioned in the list given 
by Mr. Williams (p. 16) in the f* Narrative of Missionary Enterprise ; " 
nature doubtful. As it is so near Atiu, it has been unavoidably coloured 
red. — Rarotofiga; Mr. Williams informs me that it is a lofty basaltic isld. 
with an attached reef ; coloured red. — There are three islands, Rouroicti, 
Roxburgh, and PItdl, of which I have not been able to obtain any 
account, and have left them uncoloured. Hull Isld., in the French chart, 
is written with small letters as being low. — Mangaia; height about three 
hundred feet ; " the surrounding reef joins the shore " (Williams, '* Narra- 
tive," p. 18); coloured red. — Rimetara ; Mr. WilHams informs me that 
the reef is rather close to the shore ; but, from information given me by 
Mr. Ellis, the reef does not appear to be quite so closely attached to it as 
in the foregoing cases : the island is about three hundred feet high {Naut. 
Mag., 1839, p. 738) ; coloured red. — Ruruiu; Mr. Williams and Mr. Ellis 
inform me that this island has an attached reef; coloured red. It is 
described by Cook under the name of Oheteroa : he says it is not sur- 



PACIFIC OCEAN. 1 15 

rounded, like the neighbouring islds. by a reef ; he must have meant a 
distant reef. — Tottbouat; in Cook's chart (" Second Voyage," vol. ii., p. 2) 
the reef is laid down in part one mile, and in part two miles from the 
shore. Mr. Ellis (" Polynes. Res.," vol. iii., p. 381) says the low land 
round the base of the isld. is very extensive ; and this gentleman informs 
me that the water within the reef appears deep ; coloured blue. — 
Raivaivai, or Vivitao ; Mr. Williams informs me that the reef is here 
distant : Mr. Ellis, however, says that this is certainly not the case on 
one side of the island ; and he believes that the water within the reef 
is not deep ; hence I have left it uncoloured. — Lancaster Reef, described 
in N'aut Mag., 1833 (p. 693), as an extensive crescent-formed coral-reef. 
I have not coloured it. — Rapa, or Oparree ; from the accounts given 
of it by Ellis and Vancouver, there does not appear to be any reef. — /. 
de Bass is an adjoining isld., of which I cannot find any account. — 
Kemin Isld. ; Kruscnstern seems hardly to know its position, and gives 
no further particulars. 

Islands between the Low and Gilbert Archipelagoes. 

Caroline Isld. (10° S., 150° W.) is described by Mr. F. D. Bennett 
{Geograph. Journ., vol. vii., p. 225) as containing a fine lagoon ; coloured 
blue. — Flint Isld. (11° S., 151° W.) ; Krusenstern beheves that it is the 
same with Peregrino, which is described by Quiros (Burney's " Chron. 
Hist.," vol. ii., p. 283) as " a cluster of small islands connected by a reef, 
and forming a lagoon in the middle ; " coloured blue. — Wostock is an 
isld. a little more than half a mile in diameter, and apparently quite 
flat and low, and was discovered by Bellinghausen ; it is situated a 
little west of Caroline Isld., but it is not placed on the French charts ; 
I have not coloured it, although I entertain little doubt from the chart 
of Bellinghausen, that it originally contained a small lagoon. — Penrhyn 
Isld. (9° S., 158° W.) ; a plan of it in the "Atlas of the First Voyage " of 
Kotzebuc, shows that it is an atoll ; blue.— 5/«r^?^(r/^ Isld. (5° S., 156° W.) 
is described in Byron's "Voyage in the Blonde" (p. 206) as formed 
of a flat coral-rock, with no trees ; the height not given ; not coloured. — 
Maiden Isld. (4° S., 154° W.) ; in the same voyage (p. 205) this isld. is 
said to be of coral-formation, and no part above forty feet high ; I have 
not ventured to colour it, although, from being of coral-formation, 
it is probably fringed ; in which case it should be red. — Jarvis, or 
Bimker Isld. (0° 20' S., 160'' W.) is described by Mr. F. D. Bennett 
{Geograph. Jotcrn., vol. vii., p. 227) as a narrow, low strip of coral- 
formation ; not coloured. — Brook, is a small low isld. between the two 
latter ; the position, and perhaps even the existence of it is doubt- 
ful ; not coloured. — Pescado and Humphrey Islands ; I can find out 
nothing about these islands, except that the latter appears to be small 
and low ; not coloured. — Pearson, or Grand Duke Alexander's (10 S., 
161" W.) ; an atoll, of which a plan is given by Bellinghausen ; blue. — 
Souvoroff Islands (13° S,, 163° W.); Admiral Kruscnstern, in the most 
obliging manner, obtained for me an account of these islands from 
Admiral Lazareff, who discovered them. They consist of five very low, 



ii6 APPENDIX. 

islands of coral-formation, two of which are connected by a reef, with 
deep water close to it. They do not surround a lagoon, but are so 
placed that a line drawn though them includes an oval space, part of 
which is shallow ; these islets, therefore, probably once (as is the case 
with some of the islands in the Caroline Arch.) formed a single atoll ; 
but I have not coloured them. — Danger Isld. (io° S., i66°W.) ; described 
as low by Commodore Byron, and more lately surveyed by Bellinghausen; 
it is a small atoll with three islets on it ; blue. — Clai'eiice Isld. (9° S., 
172° W.) ; discovered in the Pandora (G. Hamilton's " Voyage," p. 75) : 
it is said, "in running along the land, we saw several canoes crossing 
the lagoons ; " as this island is in the close vicinity of other low islands, 
and as it is said, that the natives make reservoirs of water in old cocoa- 
nut trees (whicli shows the nature of the land), I have no doubt it is an 
atoll, and have coloured it blue. Vor/c Isld. (8'' S., 172° W.) is described 
by Commodore Byron (chap. x. oi his " Voyage ") as an atoll ; blue. — 
Syd7tey Isld. (4° S., 172° W.) is about three miles in diameter, with its 
interior occupied by a lagoon (Captain Tromelin, "Annal. Marit.," 1829, 
p. 297); blue. — Phoeiiix Isld. (4° S., 171° W.) is nearly circular, low, 
sandy, not more than two miles in diameter, and very steep outside 
(Tromelin, " Annal. Marit.," 1829, p. 297) ; it may be inferred that this 
isld. originally contained a lagoon, but I have not coloured it. — Neiv 
Natihickct (0° 15' N., 174° W.). From the French chart it must be a low 
isld. ; I can find nothing more about it or about Mary Isld. ; both 
micoloured. — Gardner Isld. (5°S., 174° W.) from its position is certainly 
the same as Ke7nin Isld. described (Krusenstern, p. 435, Appen. to 
Mem., publ. 1827) as having a lagoon in its centre ; blue. 

Islands south of the Sandwich Archipelago, 

Christ77ias Isld. (2° N., 1 57° W.). Captain Cook, in his " Third Voyage " 
(vol. ii., chap, x.), has given a detailed account of this atoll. The breadth 
of the islets on the reef is unusually great, and the sea near it does not 
deepen so suddenly as is generally the case. It has more lately been 
visited by Mr. F. D. Bennett (Geograph. Jottrn., vol. vii., p. 226) ; and he 
assures me tliat it is low and of coral-formation : I particularly mention 
this, because it is engraved with a capital letter, signifying a high isld., in 
D'Urville and Lottin's chart. Mr. Couthouy, also, has given some ac- 
count of it ("Remarks," p. 46) from the Hawaiian ^^^^r/^/^^r/ he beheves 
it has lately undergone a small elevation, but his evidence does not 
appear to me satisfactory ; the deepest part of the lagoon is said to be 
only ten feet ; nevertheless, I have coloured it blue. — Fanning Isld. 
(4° N., 158° W.) according to Captain Tromelin ("Ann. Maritim.," 1829, 
p. 283), is an atoll : his account as observed by Krusenstern, differs 
from that given in Fanning's " Voyage " (p. 224), which, however, is far 
from clear; coloured blue. — Washington Isld. (4° N., 159 W.) is en- 
graved as a low island in D'Urville's chart, but is described by Fanning 
(p. 226) as having a much greater elevation than Fanning Isld., and 
hence I presume it is not an atoll ; not coloured. — Palniyra Isld. (6°N., 
162° W.) is an atoll divided into two parts (Krusenstern's " Mem. Suppl.," 
p. 50, also Fanning's " Voyage," p. 233) ; blue. — Smyth's or Johnston's 



PACIFIC OCEAN. tiy 

Islds. (17'' N., 170° W.). Captain Smyth, R.N., has had the kindness to 
inform me that' they consist of two very low, small islands, with a dan- 
gerous reef off the east end of them. Captain Smyth does not recollect 
whether these islets, together with the reef, surrounded a lagoon ; 
uncoloured. 

Sandwich Arch. — Hawaii; in the chart in Freycinet's " Atlas," small 
portions of the coast are fringed by reefs ; and in the accompanying 
" Hydrog. Memoir," reefs are mentioned in several places, and the coral 
is said to injure the cables. On one side of the islet of Kohaihai there 
is a bank of sand and coral with five feet water on it, running parallel 
to the shore, and leaving a channel of about fifteen feet deep within. 
I have coloured this isld. red, but it is very much less perfectly fringed 
than others of the group. — Maui ; in Freycinet's chart of the anchorage 
of Kaheina, two or three miles of coast are seen to be fringed ; and in 
the " Hydrog. Memoir," " banks of coral along shore " are spoken of. 
Mr. F. D. Bennett informs me that the reefs, on an average, extend about 
a quarter of a mile from the beach ; the land is not very steep, and 
outside the reefs the sea does not become deep very suddenly ; coloured 
red. — Morotoi, I presume, is fringed : Freycinet speaks of the breakers 
extending along the shore at a little distance from it. From the chart, 
I believe it is fringed ; coloured red. — OaJm ; Freycinet, in his " Hydrog. 
Memoir," mentions some of the reefs. Mr. F. D. Bennett informs me 
that the shore is skirted for forty or fifty miles in length. There is even 
a harbour for ships termed by the reefs, but it is at the mouth of a 
valley ; red. — Atooi, in La Peyrouse's charts, is represented as fringed 
by a reef, in the same manner as Oahu and Morotoi ; and this, as I 
have been informed by Mr. Ellis, on part at least of the shore, is of 
coral-formation : the reef does not leave a deep channel within ; red. — 
Oneehow ; Mr. Ellis believes that this island is also fringed by a coral- 
reef: considering its close proximity to the other islands, I have ven- 
tured to colour it red. I have in vain consulted the works of Cook, 
Vancouver, La Pcyrouse, and Lisiansky, for any satisfactory account 
of the small islands and reefs, which lie scattered in a N.W. line 
prolonged from the Sandwich group, and hence have left them un- 
coloured, with one exception ; for 1 am indebted to Mr. F. D. Bennett 
for informing me of an atoll-formed reef, in lat. 28° 22', long. 178° 30' W., 
on which the Glcdstanes was wrecked in 1837. It is apparently of 
large size, and extends in a N.W. and S.E. line : very few islets have 
been formed on it. The lagoon seems to be shallow; at least, the 
deepest part which was surveyed was only three fathoms. Mr. Couthouy 
(" Remarks," p. 38) describes this isld. under the name of Ocean Isld. 
Considerable doubts should be entertained regarding the nature of a 
reef of this kind, with a very shallow lagoon, and standing far from any 
other atoll, on account of the possibility of a crater or flat bank of rock 
lying at the proper depth beneath the surface of the water, thus affording 
a foundation for a ring-formed coral-reef. I have, however, thought 
myself compelled, from its large size and symmetrical outline, to colour 
it blue. 

Samoa or Navigator Group. — Kotzebuc, in his " Second Voyage," 



ii8 APPENDIX. 

contrasts the structure of these islands with many others in the Pacific, in 
not being furnished with liarbours for ships, formed by distant coral- 
reefs. The Rev. J. Williams, however, informs me, that coral-reefs do 
occur in irregular patches on the shores of these islands ; but that they 
do not form a continuous band, as round Mangaia, and other such 
perfect cases of fringed islands. From the charts accompanying La 
PejTouse's " Voyage," it appears that the north shore of Savaii, Maotma, 
0?'osenga, and Maiiita, are fringed by reefs. La Peyrouse, speaking of 
Maouna (p. 126), says that the coral-reef surrounding its shores, almost 
touches the beach ; and is breached in front of the little coves and 
streams, forming passages for canoes, and probably even for boats. 
Further on (p. 159), he extends the same observation to all the islands 
which he visited. Mr. Williams in his " Narrative," speaks of a reef 
going round a small island attached to Oyolava, and returning again to 
it : all these islands have been coloured red. — A chart of Rose Island, 
at the extreme west end of the group, is given by Freycinet, from which 
I should have thought that it had been an atoll ; but according to 
Mr. Couthouy (" Remarks," p. 43), it consists of a reef, only a league in 
circuit, surmounted by a very few low islets ; the lagoon is very shallow, 
and is strewed with numerous large boulders of volcanic rock. This 
island, therefore, probably consists of a bank of rock, a few feet sub- 
merged, with the outer margin of its upper surface fringed with reefs ; 
hence it cannot be properly classed with atolls, in which the foundations 
are always supposed to lie at a depth, greater than that at which the 
reef-constructing polypifcrs can live; not coloured. 

Beveridge Reef, 20° S., 167° W., is described in the Naut. Mag. 
(May 1833, p. 442) as ten miles long in a N. and S. line, and eight 
wide ; " in the inside of the reef there appears deep water ; " there is 
a passage near the S. W. corner : this therefore seems to be a submerged 
atoll, and is coloured blue. 

Savage Isld,, 19° S., 170° W., has been described by Cook and Forster. 
The younger Forster (vol. ii., p. 163) says it is about forty feet high : he 
suspects that it contains a low plain, which formerly was the lagoon. 
The Rev. J. Williams informs me that the reef fringing its shores, 
resembles that round Mangaia ; coloured r©d. 

Friendly Arch. — Pylstaart\^\^. Judging from the chart in Freycinet's 
"Atlas," I should have supposed that it had been regularly fringed; 
but as nothing is said in the " Hydrog. Memoir" (or in the "Voyage "of 
Tasman, the discoverer) about coral-reefs, I have left it uncoloured. — 
Tongatabou : In the " Atlas of the Voyage of the Astrolabe" the whole 
south side of the island is represented as narrowly fringed by the same 
reef which forms an extensive platform on the northern side. The origin 
of this latter reef, which might have been mistaken for a barrier-reef, has 
already been attempted to be explained, when giving the proofs of the 
recent elevation of this island. — In Cook's charts the little outlying 
island also of Eoaigee, is represented as fringed ; coloured red. — Eoua. 
I cannot make out from Captain Cook's charts and descriptions, that ' 
this island has any reef, although the bottom of the neighbouring sea 
seems to be corally, and the island itself is formed of coral-rock. 



PACIFIC OCEAN. 119 

Forster, however, distinctly (" Observations," p. 14) classes it with high 
islands having reefs, but it certainly is not encircled by a barrier-reef 
and the younger Forster (" Voyage," vol. i., p. 426) says, that "a bed 
of coral-rocks surrounded the coast towards the landing-place." I have 
tlierefore classed it with the fringed islands and coloured it red. The 
several islands lying N.W. of Tongatabou, namely, Anamoiika, Komango, 
Kototiy Lefonga, Foa, etc., are seen in Captain Cook's chart to be fringed 
by reefs, and several of them are connected together. From the various 
statements in the first volume of Cook's "Third Voyage," and especially 
in the fourth and sixth chapters, it appears that these reefs are of coral- 
formation, and certainly do not belong to the barrier class ; coloured red. 
— Toufoa and Kao^ forming the western part of the group, according to 
Forster have no reefs ; the former is an active volcano. — Vavao. There 
is a chart of this singularly formed island, by Espinoza : according to 
Mr. Williams it consists of coral-rock : the Chevalier Dillon informs me 
that it is not fringed ; not coloured. Nor are the islands of Latte and 
Afnargtcra, for I have not seen plans on a large scale of them, and do 
not know whether they are fringed. 

Nionha, 16° S., 174° W., or Keppel Island of Wallis, or Cocos Isld. From 
a view and chart of this island given in Wallis's '• Voyage " (4to edit.) 
it is evidently encircled by a reef; coloured blue : it is however remark- 
able that Boscaweii Island, immediately adjoining, has no reef of any 
kind ; uncoloured. 

Wallis Island, 13° S., 176° W., a chart and view of this island in 
Wallis's "Voyage " (4to edit.) shows that it is encircled. A view of it 
in the Naut. Mag., July 1833, p. 376, shows the same fact ; blue. 

Allotifatou, or Horn Island, Onouafu, or Proby Island, and Hunter 
Islands, lie between the Navigator and Fidji groups. I can find no 
distinct accounts of them. 

Fidji or Viti Group. — The best chart ot the numerous islands of this 
group, will be found in the " Atlas of the Astrolabe's Voyage." From this, 
and from the description given in the " Hydrog. Memoir," accompanying 
it, it appears that many of these islands are bold and mountainous, 
rising to the height of between 3,000 and 4,000 feet. Most of the islands 
are surrounded by reefs, lying far from the land, and outside of which 
the ocean appears very deep. The Astrolabe sounded with ninety 
fathoms in several places about a mile from the reefs, and found no 
bottom. Although the depth within the reef is not laid down, it is 
evident from several expressions, that Captain D'Urville believes that 
ships could anchor within, if passages existed through the outer barriers. 
The Chevallier Dillon informs me that this is the case : hence I have 
coloured this group blue. In the S.E. part lies Batoa, or Turtle Island 
of Cook ("Second Voyage," vol. ii., p. 23, and chart, 4to edit.) surrounded 
by a coral-reef, "which in some places extends two miles from the 
shore ; " within the reef the water appears to be deep, and outside it is 
unfathomable ; coloured pale blue. At the distance of a few miles, 
Captain Cook {Ibid., p. 24) found a circular coral-reef, four or five leagues 
in circuit, with deep water within ; " in short, the bank wants only a 
few little islets to make it exactly like one of tlie half-drowned isles so 



120 APPENDIX. 

often mentioned," — namely, atolls. South of Batoa, lies the high island 
of Ono, which appears in Bellinghauscn's "Atlas " to be encircled ; as do 
some other small islands to the south ; coloured pale blue ; near Ono, 
there is an annular reef, quite similar to the one just described in the 
words of Captain Cook ; coloured dark blue. 

Rotouviah, 13° S., 179° E.— From the chart in Duperrey's "Atlas," I 
thought this isld. was encircled, and had coloured it blue, but the 
Chevallier Dillon assures me that the reef is only a shore or fringing 
one ; red. 

Independence Isld., 10° S., 179° E., is described by Mr. G. Bennett, 
{United Service Journ., 1831, part ii., p. 197) as a low island of coral- 
formation ; it is small, and does not appear to contain a lagoon, although 
an opening through the reef is referred to. A lagoon probably once 
existed, and has since been filled up ; left uncoloured. 

Ellice Group. — Oscar, Peyster, and Ellice Islds. are figured in 
Arrowsmith's "Chart of the Pacific" (corrected to 1832) as atolls, and are 
said to be very low ; blue. — Nederlandisch Isld. I am greatly indebted 
to the kindness of Admiral Krusenstern, for sending me the original 
documents concerning this island. From the plans given by Captains 
Eeg and Khremtshenko, and from the detailed account given by the 
former, it appears that it is a narrow coral-island, about two miles long, 
containing a small lagoon. The sea is very deep close to the shore, 
which is fronted by sharp coral-rocks. Captain Eeg compares the 
lagoon with that of other coral-islands ; and he distinctly says, the land 
is "very low." I have therefore coloured it blue. Admiral Krusenstern 
("Memoir on the Pacific," Append., 1835) states that its shores are 
eighty feet high ; this probably arose from the height of the cocoa-nut 
trees, with which it is covered, being mistaken for land. — Gra7i Cocal 
is said in Krusenstern's " Memoir," to be low, and to be surrounded 
by a reef ; it is small, and therefore probably once contained a lagoon ; 
uncoloured. — St. Aiigiisiin. From a chart and view of it, given in 
the " Atlas of the Coqiiilles Vo3^age," it appears to be a small atoll, 
with its lagoon partly filled up ; coloured blue. 

Gilbert Group. — The chart of this group, given in the " Atlas of 
the Coqiiille's Voyage," at once shows that it is composed of ten well 
characterised atolls. In D'Urville and Lottin's chart, Sydenham is 
written with a capital letter, signifying that it is high ; but this certainly 
is not the case, for it is a perfectly characterised atoll, and a sketch, show- 
ing how low it is, is given in the " Coqidlle's Atlas." Some narrow strip- 
like reefs project from the southern side of D?um77iond atoll, and render 
it irregular. The southern island of the group is called Chase (in some 
charts, Rotches) ; of this I can find no account, but Mr. F. D. Bennett 
discovered {Geograph. Jourti.y vol. vii., p. 229), a low extensive island 
in nearly the same latitude, about three degrees westward of the 
longitude assigned to Rotches, but very probably it is the same island. 
Mr. Bennett informs me that the man at the masthead reported an 
appearance of lagoon-water in the centre ; and, therefore, considering 
its position, I have coloured it blue. — Pitt Isld., at the extreme northern 
point of the group, is left uncoloured, as its exact position and nature 



PACIFIC OCEAN. 121 

is not known. — Byron Isld., which lies a little to the eastward, does not 
appear to have been visited since Commodore Byron's voyage, and 
it was then seen only from a distance of eighteen miles ; it is said to 
be low ; uncoloured. 

Ocean, Pleasant, and Atla?ttic Islds. all lie considerably to the west 
of the Gilbert group : I have been unable to find any distinct account 
of them. Ocean Island is written with small letters in the French chart, 
but in Krusenstern's "Memoir" it is said to be high. 

Marshall Group. — We are well acquainted with this group from 
the excellent charts of the separate islands, made during the two 
voyages of Kotzebue : a reduced one of the whole group may be easily 
seen in Krusenstern's "Atlas," and in Kotzebue's " Second Voyage." The 
group consists (with the exception of two little islands which probably 
have had their lagoon filled up) of a double row of twenty-three large 
and well-characterised atolls, from the examination of which Chamisso 
has given us his well-known account of coral-formations. I include 
Caspar Rico, or Cornwallis Isld, in this group, which is described by 
Chamisso (Kotzebue's •' First Voyage," vol. iii., p. 179) "as a low sickle- 
formed group, with mould only on the windward side." Gaspard Island 
is considered by some geographers as a distinct island lying N.E. of the 
group, but it is not entered in the chart by Krusenstern ; left uncoloured. 
In the S.W. part of this group lies Baring Island, of which little is 
known (see Krusenstern's "Appendix," 1835, p. 149). I have left it un- 
coloured ; but Boston Isld. 1 have coloured blue, as it is described 
{Ibia) as consisting of fourteen small islands, which, no doubt, enclose a 
lagoon, as represented in a chart in the " Coquilles Atlas." — Two islands, 
Aicr Kawen and Caspar Rico, are written in the French chart with 
capital letters ; but this is an error, for froni the account given by 
Chamisso in Kotzebue's " First Voyage," they are certainly low. The 
nature, position, and even existence, of the shoals and small islands 
north of the Marshall group, are doubtful. 

New Hebrides. — Any chart, on even a small scale, of these islands, 
will show that their shores are almost without reefs, presenting a 
remarkable contrast with those of New Caledonia on the one hand, and 
the Fidji group on the other. Nevertheless, I have been assured by 
Mr. G. Bennett, that coral grows vigorously on their shores ; as indeed, 
will be further shown in some of the following notices. As, therefore, 
these islands are not encircled, and as coral grows vigorously on their 
shores, we might almost conclude, without further evidence, that they 
were fringed, and hence I have applied the red colour witli ratlier 
greater freedom than in other instances. — Matthew's Rock, an active 
volcano, some way south of the group (of which a plan is given in the 
" Atlas of the Astrolabe's Voyage ") does not appear to have reefs of any 
kind about it, — Annatom, the southernmost of the Hebrides; from a 
rough woodcut given in the United Sei vice Joiir7ial{\Z},\, part iii., p, 190), 
accompanying a paper by Mr. Bennett, it appears that the shore is 
fringed; coloured red, — Tanna, Forster, in his "Observations" (p, 22), 
says Tanna has on its shores coral-rock and madrepores ; and the 
younger Forster, in his account (vol. ii., p. 269) speaking of the harbour 



122 APPENDIX. 

says, the whole S.E. side consists of coral-reefs, which are overflowed 
at high-water ; part of the southern shore in Cook's chart is represented 
as fringed ; coloured red. — /ww^^ is described [United Service Journ., 
1 83 1, part iii., p. 192) by Mr. Bennett as being of moderate elevation, 
with cliffs appearing like sandstone : coral grows in patches on its 
shore, but I have not coloured it; and I mention these facts, because 
Immcr migiit have been thought from Forster's classification (" Observa- 
tions," p. 14), to have been a low island or even an atoll. — Erro?nango 
Isld. ; Cook (" Second Voyage," vol. ii., p. 45, 4to edit.) speaks of rocks 
everywhere linmg the coast, and the natives offered to haul his boat over 
the breakers to the sandy beach : Mr. Bennett, in a letter to the Editor 
of the Si7igapore Chron., alludes to the reefs on its shores. It may, I 
think, be safely inferred from these passages that the shore is fringed 
in parts by coral-reefs ; coloured red. — Saiidwich Isld., the east coast 
is said (Cook's " Second Voyage," vol. ii., p. 41) to be low, and to be 
guarded by a chain of breakers. In the accompanying chart it is seen to 
be fringed by a reef; coloured red. — Mallicollo ; Forster speaks of the 
reef-bounded shore : the reef is about thirty yards wide, and so shallow 
that a boat cannot pass over it. Forster also (" Observat.," p. 23) says, 
that the rocks of the sea-shore consist of madrepore. In the plan of 
Sandwich harbour, the headlands are represented as fringed ; coloured 
red. — Aurora and Pentecost\^\^s., according to Bougainville, apparently 
have no reefs ; nor has the large isld. of 6". Espiritu, nor Bligh Isld. 
or Banks' Islds., which latter lie to the N.E. of the Hebrides. But in 
none of these cases, have I met with any detailed account of tlieir 
shores, or seen plans on a large scale ; and it will be evident, that a 
fringing-reef of only thirty or even a few hundred yards in width, is of so 
little importance to navigation, that it will seldom be noticed, excepting 
by chance ; and hence I do not doubt that several of these islands, now 
left uncoloured, ought to be red. 

Santa-Cruz Group. — Vanikoro (Fig. i, PI. I.) offers a striking 
example of a barrier-reef: it was first described by the Chevalier Dillon, 
in his voyage, and was surveyed in the Astrolabe ; coloured pale blue. — 
Tikopia and Fataka Islands appear, from the descriptions of Dillon and 
D'Urville, to have no reefs; Aiioiida is a low, flat isld., surrounded by 
cliffs {^''Astrolabe Hydrog.," and Krusenstern, "Mem.," vol. ii., p. 432) ; 
these are uncoloured. Toupoua {Otooboa of Dillon) is stated by Captain 
Tromelin (" Annales Marit.," 1829, p. 289) to be almost entirely included 
in a reef, lying at the distance of two miles from the shore. There is a 
space of three miles without any reef, which, although indented with 
bays, offers no anchorage from the extreme depth of the water close to 
the shore : Captain Dillon also speaks of the reefs fronting this island ; 
coloured blue. — Santa-Otcz. I have carefully examined the works of 
Carteret, D'Entrecasteaux, W^ilson, and Tromehn, and I cannot discover 
any mention of reefs on its shores ; left uncoloured. — Tinako?'o is a 
constantly active volcano without reefs. — Mendana Isles (mentioned by 
Dillon under the name of Mavimee, etc.) ; said by Krusenstern to be low, 
and intertwined with reefs. I do not believe they include a lagoon ; I 
have left them uncoloured — Duff's Islds. compose a small group 



PACIFIC OCEAN. 123 

directed in a N.W. and S.E. band; they are described by Wilson, 
(p. 296, " Miss. Voy.," 4to edit.) as formed by bold-peaked land, with the 
i.clands surrounded by coral-reefs, extending about half a mile from the 
shore ; at a distance of a mile from the reefs he found only seven 
fathoms. As I have no reason for supposing there is deep water 
within these reefs, I have coloured them red. Kennedy Isld., N.E. of 
Duffs ; I have been unable to find any account of it. 

New Caledonia, — The great barrier-reefs on the shores of this island 
have already been described (Fig. 5, PL II.). They have been visited 
by Labillardiere, Cook, and the northern point by D'Urville ; this latter 
part so closely resembles an atoll that I have coloured it dark blue. 
The Loyalty group is situated eastward of this island ; from the chart 
and description given in the "Voyage of \h^ Astrolabe" they do not 
appear to have any reefs ; north of this group, there are some extensive 
low reefs (called Astrolabe and Beatcpre,) which do not seem to be 
atoll-formed ; these are left uncoloured. 

Australian Barrier-Reef. — The limits of this great reef, which 
has already been described, have been coloured from the charts of 
Flinders and King. In the northern parts, an atoll-formed reef, lying 
outside the barrier, has been described by Bligh, and is coloured dark 
blue. In the space between Austraha and New Caledonia, called by 
Flinders the Corallian Sea, there are numerous reefs. Of these, some 
are represented in Krusenstern's "Atlas " as having an atoll-like struc- 
ture ; namely, Bampiofi shoal, Frederic, Vine or Horse-shoe, and Alert 
reefs ; these have been coloured dark blue. 

Louisiade; the dangerous reefs which front and surround the 
western, southern, and northern coasts of this so-called peninsula and 
archipelago, seem evidently to belong to the barrier class. The land is 
lofty, with a low fringe on the coast ; the reefs are distant, and the sea 
outside them profoundly deep. Nearly all that is known of this group 
is derived from the labours of D'Entrecasteaux and Bougainville : the 
latter has represented one continuous reef ninety miles long, parallel 
to the shore, and in places as much as ten miles from it ; coloured pale 
blue. A little distance northward we have the Latighlait Islds., the 
reefs round which are engraved in the " Atlas of the Voyage of the 
Astrolabe" in the same manner as in the encircled islands of the 
Caroline Arch., the reef is, in parts, a mile and a half from the shore, 
to which it does not appear to be attached ; coloured blue. At some 
little distance from the extremity of the Louisiade lies the Wells reef, 
described in G. Hamilton's "Voyage in H.M.S. Pandora" (p. 100): 
it is said, "We found we had got embayed in a double reef, which will 
soon be an island." As this statement is only intelligible on the sup- 
position of the reef being crescent or horse-shoe formed, like so many 
other submerged annular reefs, I have ventured to colour it blue. 

Solomon Archipelago : the chart in Krusenstern's " Atlas " shows 
that these islands are not encircled, and as coral appears from the works 
f>f Surville, Bougainville, and Labillardiere, to grow on their shores, this 
rircumstance, as in the case of the New Hebrides, is a presumption 
'; hat they are fringed. I cannot find out anything from D'Entrecasteaux's 



124 APPENDIX. 

" Voyage," regarding the southern islds. of the group, so have left them 
uncoloured. — Malayta Isld. in a rough MS. chart in the Admiralty has 
its northern shore fringed. — Ysabel Isld., the N.E. part of this island, in 
the same chart, is also fringed : Mendana, speaking (Burney, vol. i., 
p. 280) of an islet adjoining the northern coast, says it is surrounded 
by reefs ; the shores, also of Port Praslin appear regularly fringed. — 
Choiseul Isld. ; in Bougainville's " Chart of Choiseul Bay," parts of the 
shores are fringed by coral-reefs. — Bougainville Isld. ; according to 
D'Entrecasteaux the western shore abounds with coral-reefs, and the 
smaller islands are said to be attached to the larger ones by reefs ; all 
the before-mentioned islands have been coloured red. Bouka Islds. ; 
Captain Duperrey has kindly informed me in a letter that he passed 
close round the northern side of this island (of which a plan is given 
in his " Atlas of the Coguilles Voyage "), and that it was " garnie d'unc 
bande de recifs k fleur d'eau adherentes au rivage ; " and he infers, 
from the abundance of coral on the islands north and south of Bouka, 
that the reef probably is of coral ; coloured red. 

Off the north coast of the Solomon Arch, there are several small 
groups which are little known ; they appear to be low, and of coral- 
formation ; and some of them probably have an atoll-like structure ; the 
Chevallier Dillon, however, informs me that this is not the case with the 
B. de Candclaria. — Oiiiong Java, according to the Spanish navigator, 
Maurelle, is thus characterised ; but this is the only one which 1 have 
ventured to colour blue. 

New Ireland. — The shores of the S.W. point of this island and some 
adjoining islets, are fringed by reefs, as may be seen in the " Atlases 
of the Voyages of the Coquille and Astrolabe." M. Lesson observes 
that the reefs are open in front of each streamlet. The Dtike of Yoj-k's 
Isld. is also fringed ; but with regard to the other parts of Neiu Ireland, 
New Hanover, and the small islands lying northward, I have been 
unable to obtain any information. I will only add that no part of New 
Ireland appears to be fronted by distant reefs. I have coloured red 
only the above specified portions. 

New Britain and the Northern Shore of New Guinea.— From 
the charts in the " Voyage of the Astrolabe," and from the " Hydrog. 
Memoir," it appears that these coasts are entirely without reefs, as are 
the Schouten Islands, lying close to the northern shore of New Guinea. 
The western and south-western parts of New Guinea, will be treated 
of when we come to the islands of the East Indian Archipelago. 

Admiralty Group. — From the accounts by Bougainville, Maurelle, 
D'Entrecasteaux, and the scattered notices collected by Horsburgh, it 
appears, that some of the many islands composing it, are high, with a bold 
outline ; and others are very low, small and interlaced with reefs. All 
the high islands appear to be fronted by distant reefs rising abruptly 
from the sea, and within some of which there is reason to believe that 
the water is deep. I have therefore little doubt they aie of the barrier 
class. — In the southern part of the group we have Elizabeth Isld., which 
is surrounded by a reef at the distance of a mile ; and two miles east- 
ward of it (Kruscnstern, "Append.," 1835, p. 42) there is a little island 



PACIFIC OCEAN, 125 

containing a lagoon. — Near here, also, lies Circular-reef (Horsburgh, 
** Direct.," vol. i., p, 691, 4th edit), "three or four miles in diameter 
having deep water inside with an opening at the N.N.W. part, and on 
the outside steep to." I have from these data, coloured the group pale 
blue, and drailar-7-eef dark blue. — The Anacho?ites, Echeqtiier, and 
Hermites, consist of innumerable low islands of coral-formation, which 
probably have atoll-like forms ; but not being able to ascertain this, I 
have not coloured them, nor Durour hid., which is described by Carteret 
as low. 

The Caroline Arch, is now well known, chiefly from the hydro- 
graphical labours of Lutke ; it contains about forty groups of atolls, 
and three encircled islands, two of which are engraved in Figs. 2 and 7, 
Plate I. Commencing with the eastern part ; the encircling reef round 
Ualen appears to be only about half a mile from the shore ; but as the 
land is low and covered with mangroves (" Voyage autour du Monde," 
par F. Lutke, vol. i., p. 339), the real margin has not probably been 
ascertained. The extreme depth in one of the harbours within tlie reef 
is thirty-thiree fathoms (see charts in " Atlas of Coquilles Voyage "), 
and outside at half a mile distant from the reef, no bottom was obtained 
with two hundred and fifty fathoms. The reef is surmounted by many 
islets, and the lagoon-like channel within is mostly shallow, and 
appears to have been much encroached on by the low land surrounding 
the central mountains ; these facts show that time has allowed much 
detritus to accumulate ; coloured pale blue. — Poiiyiiipete, or Seniavine. 
In the greater part of the circumference of this island, the reef is about 
one mile and three quarters distant ; on the north side it is five miles 
off the included high islets. The reef is broken in several places ; and 
just within it, the depth in one place is thirty fathoms, and in another, 
twenty-eight, beyond which, to all appearance, there was " un porte 
vaste et sur" (Lutke, vol. ii., p. 4); coloured pale blue. — Hogoleu or 
Roug. This wonderful group contains at least sixty-two islands, and 
its reef is one hundred and thirty-five miles in circuit. Of the islands, 
only a few, about six or eight (see " Hydrog. Descrip.," p. 428, of the 
"Voyage of the Asiivlabe," and the large accompanying chart taken 
chiefly from that given by Duperrey) are high, and the rest are all 
small, low, and formed on the reef. The depth of the great interior 
lake has not been ascertained; but Captain D'Urville appears to have 
entertained no doubt about the possibility of taking in a frigate. The 
reef lies no less than fourteen miles distant from the northern coasts 
of the interior high islds., seven from their western sides, and twenty 
from the southern ; the sea is deep outside. This island is a likeness 
on a grand scale to the Gambler group in the Low Archipelago. Of 
the groups of low * islands forming the chief part of the Caroline 
Archipelago, all those of larger size, have the true atoll-structure (as may 
be seen in the "Atlas" by Captain Lutke), and some even of the very 
small ones, as Macaskill and. Duperrey, of which plans are given in the 

* In D'Urville and Lottin's chart, Peserare is written with capital letters ; 
but this evidently is an error, for it is one of the low islets on the reef of 
Namonouyto (see Lutkc's charts) — a regular atoll. 



126 APPENDIX. 

" Atlas of the Coqiiihes Voyage." There are, however, some low 
small islands of coral-formation, namely, Ollap, Taf?iatam, Bigali, 
Satahonal, which do not contain lagoons ; but it is probable that 
lagoons originally existed, but have since filled up : Lutke (vol. ii., p. 
304) seems to have thought that all the low islands, with only one 
exception, contained lagoons. From the sketches, and from the 
manner in which the margins of these islands are engraved in the 
•' Atlas of the Voyage of the Coquille^' it might have been thought that 
they were not low ; but by a comparison with the remarks of Lutke 
(vol. ii., p. 107, regarding Bigali) and of Freycinet (" Hydrog. Memoir 
LUrajiie Voyage," p. 188, regarding Tamatam, Ollap, etc.), it will be 
seen that the aitist must have represented the land incorrectly. The 
most southern isld. in the group, namely Pigtiira?n, is not coloured, 
because I have found no account of it. Nougouor, or Moiite Vcrdison^ 
which was not visited by Lutke, is described and figured by Mr. 
Bennett {United Service Journal, Jan. 1832) as an atoll. All the 
above-mentioned islands have been coloured blue. 

Western Part of the Caroline Archipelago. — Fais Island is 
ninety feet high, and is surrounded, as I have been informed by 
Admiral Lutke, by a narrow reef of living coral, of which the broadest 
part, as represented in the charts, is only 150 yards; coloured red. 
— Philip Isld., I believe, is low ; but Hunter, in his *' Historical 
Journal," gives no clear account of it ; uncoloured. — Elivi; from the 
m.anner in which the islets on the reefs are engraved, in the " Atlas of 
the Astrolabe s Voyage,," I should have thought they were above the 
ordinary height, but Admiral Lutke assures me this is not the case : 
they form a regular atoll ; coloured blue. — Gouap {Eap of Chamisso), 
is a high island with a reef (see chart in "Voy. of Astrolabe"), more 
than a mile distant in most parts from the shore, and two miles in one 
part. Captain D'Urville thinks that there would be anchorage ("Hydrog. 
Descript. Astrolabe Voyage," p. 436) for ships within the reef, if a 
passage could be found ; coloured pale blue. — Goulou, from the chart 
in the ^'■Astrolabe's Atlas," appears to be an atoll. D'Urville (" Hydrog. 
Descript.," p. 437) speaks of the low islets on the reef ; coloured dark 
blue. 

Pelev(^ Islds. — Krusenstern speaks of some of the islands being 
mountainous ; the reefs are distant from the shore, and there are spaces 
within them, and not opposite valleys, with from ten to fifteen fathoms. 
According to a MS. chart of the group by Lieutenant Elmer in the 
Admiralty, there is a large space within the reef with deepish water ; 
although the high land does not hold a central position with respect to 
the reefs, as is generally the case, I have little doubt that the reefs of 
the Pelew Islands ought to be ranked with the barrier class, and I have 
coloured them pale blue. In Lieutenant Elmer's chart there is a horse- 
shoe-formed shoal, laid down thirteen miles N.W. of Pelew, with 
fifteen fathoms within the reef, and some dry banks on it ; coloured dark 
blue. — Spanish, Martires, Sanserot, Fulo A?ma, and Mariere Islands 
are not coloured, because I know nothing about them, excepting that 
according to Krusenstern, the second, third, and fourth mentioned, are 



PACIFIC OCEAN. 127 

low, placed on coral-reefs, and therefore, perhaps, contain lagoons ; but 
Pulo Mariere is a little higher, 

Mariana Archipelago, or Ladrones. — Guahatt. Almost the whole 
of this island is fringed by reefs, which extend in most parts about a 
third of a mile from the land. Even where the reefs are most extensive, 
the water within them is shallow. In several parts there is a navigable 
channel for boats and canoes within the reefs. In Freycinet's " Hydrog. 
Mem." there is an account of these reels, and in the " Atlas," a map 
on a large scale ; coloured red. — Rota. " L'ile est presque entierement 
entouree des recifs " (p. 212, Freycinet's "Hydrog. Mem."). These 
reefs project about a quarter of a mile from the shore ; coloured red. — 
Tinian. The eastern coast is precipitous, and is without reefs ; but the 
western side is fringed like the last island ; coloured red. — Saypan. 
The N.E. coast, and likewise the western shores appear to be fringed ; 
but there is a great, irregular, horn-like reef projecting far from this 
side ; coloured red. — Farallon de Medinilla, appears so regularly and 
closely fringed in Freycinet's charts, that I have ventured to colour it red, 
although nothing is said about reefs in the " Hydrographical Memoir." 
The several islands which form the northern part of the group are 
volcanic (with the exception perhaps of Torres, which resembles in 
form the madreporitic island of Medinilla), and appear to be without 
reefs. — Mangs, however, is described (by Freycinet, p. 219, " Hydrog.") 
from some Spanish charts, as formed of small islands placed " an 
milieu des nombreux recifs ; " and as these reefs in the general chart 
of the group do not project so much as a mile; and as there is no 
appearance from a double line, of the existence of deep water within, 
I have ventured, although with much hesitation, to colour them red. 
Respecting Folger and Marshall Islds. which lie some way east of the 
Marianas, I can find out nothing, excepting that they are probably low. 
Krusenstern says this of Marshall Isld. ; and Folger Isld. is written 
with small letters in D'Urville's chart ; uncoloured. 

BoNiN OR Arzobispo Group. — Peel Isld. has been examined by 
Captain Beechey, to whose kindness I am much indebted for giving me 
information regarding it : " At Port Lloyd there is a great deal of coral ; 
and the inner harbour is entirely formed by coral-reefs, which extend 
outside the port along the coast." Captain Beechey, in another part 
(jf his letter to me, alludes to the reefs fringing the island in all direc- 
tions ; but at the same time it must be observed that the surf washes 
the volcanic rocks of the coast in the greater part of its circumference. 
I do not know whether the other islands of the Archipelago are fringed ; I 
have coloured Peel Isld. red. — Grampus Isld. to the eastward, does not 
appear (Meare's "Voyage," p. 95) to have any reefs, nor does Rosatio 
Isld. (from Lutk6's chart), which lies to the westward. Respecting 
the few other islds. in this part of the sea, namely the Sulphur Islds., 
with an active volcano, and those lying between Bonin and Japan 
(which are situated near the extreme limit in latitude, at which reefs 
are formed), I have not been able to find any clear account. 

West End of New Guinea. — Port Dory. From the charts in the 
''Voyage of the Coqtcillc" it would appear that the coast in this part 



128 APPENDIX. 

is fringed by coral-reefs ; M. Lesson, however, remarks that the coral 
is sickly ; coloured red. — Waigmi. A considerable portion of the 
northern shores of these islands is seen in the charts (on a large scale) in 
Freycinet's "Atlas" to be fringed by coral-reefs. Forrest (p. 21, " Voyage 
to New Guinea ") alludes to the coral-reefs lining the heads of Piapis 
Bay; and Horsburgh (vol. ii., p. 599, 4th edit.), speaking of the islands 
in Dampier Strait, says " sharp coral-rocks line their shores ; " coloured 
red. — In the sea north of these islands, we have Gicedes (or Freewill, 
or St. David's), which from the chart given in the 4to edit, of Carteret's 
"Voyage," must be an atoll. Krusenstcrn says the islets are very low ; 
coloured blue. — Carteret's Shoals, in 2° 53' N., are described as circular, 
with stony points showing all round, with deeper water in the middle ; 
coloured blue. — Aioic ; the plan of this group, given in the "Atlas of 
the Voyage of the Astrolabe," shows that it is an atoll ; and, from a 
chart in Forrest's "Voyage," it appears that there is twelve fathoms 
within the circular reef ; coloured blue. — The S.W. coast of New Guinea 
appears to be low, muddy, and devoid of reefs. The Arm, Timor-laiit, 
and Tc7ii7nber groups have lately been examhicd by Captain Kolff, the 
MS. translation of which, by Mr. W. Earl, I have been permitted to read, 
through the kindness of Captain Washington, R.N. These islands are 
mostly rather low, and are surrounded by distant reefs (the Ki Islands, 
however, are lofty, and, from Mr. Stanley's survey, appear without 
reefs) ; the sea in some parts is shallow, in others profoundly deep (as 
near Larrat). From the imperfection of the published charts, I have 
been unable to decide to which class these reefs belong. From the 
distance to which they extend from the land, where the sea is very deep, 
I am strongly inclined to believe they ought to come within the barrier 
class, and be coloured blue ; but I have been forced to leave them 
uncoloured. — The last-mentioned groups are connected with the east 
end of Coram by a chain of small islands, of which the small groups of 
Ceram-laut, Goi-ain, and Keffiiig are surrounded by very extensive reefs, 
projecting into deep water, which, as in the last case^ I strongly suspect 
belong to the barrier class ; but I have not coloured them. From the 
south side of Keffing, the reefs project five miles (Windsor Earl's 
" Sailing Direct, for the Arafura Sea," p. 9). 

Ceram. — In various charts which I have examined, several parts of 
the coast are represented as fringed by reefs. — Manipa Island, between 
Ceram and Bourou, in an old MS. chart in the Admiralty, is fringed by 
a very irregular reef, partly dry at low water, which I do not doubt is of 
coral-formation ; both islands coloured red. — Bouroti; parts of this 
island appear fringed by coral-reefs, namely, the eastern coast, as seen 
in Freycinet's chart ; and Cajeli Bay, which is said by Horsburgh (vol. ii., 
p. 630) to be lined by coral-reefs, that stretch out a little way, and have 
only a few feet water on them. In several charts, portions of the 
islands forming the Amboina Group are fringed by reefs ; for instance, 
Noessa, Hare?ica, and Ucaster, in Freycinet's charts. The above-men- 
tioned islands have been coloured red, although the evidence is not very 
satisfactory. — North of Bourou the parallel line of the Xidla Isles 
extends : I have not been able to find out anything about them, excepting 



EAST-INDIAN ARCHIPELAGO, 129 

that Horsburgh (vol. ii., p. 543) says that the northern shore is sur- 
rounded by a reef at the distance of two or three miles ; uncoloured. — 
Mysol Group ; the Kanary Islands are said by Forrest (" Voyage/' p. 130) 
to be divided from each other by deep straits, and are lined with coral- 
rocks ; coloured red. — Guebe, lying between Waigiou and Gilolo, is 
engraved as if fringed ; and it is said by Freycinet, that all the soundings 
under five fathoms were on coral ; coloured red. — Gilolo. In a chart 
published by Dalrymple, the numerous islands on the western, southern 
{Balchlan and t\\Q- Strait of Patte7itia), and eastern sides appear fringed 
by narrow reefs ; these reefs, I suppose, are of coral, for it is said in 
"Malte Brun" (vol. xii,, p. 156), " Sur les cotes (of Batchian) comme 
dafis les pliipart des iles de cet archipel, il y a de rocs de medrepores 
d'une beaute et d'une variete infimies." Forrest, also (p. 50), says 
Seland, near Batchian, is a little island with reefs of coral ; coloured 
red. — Morty Island (north of Gilolo) ; Horsburgh (vol. ii., p. 506) says 
the northern coast is lined by reefs, projecting one or two miles, and 
having no soundings close to them ; I have left it uncoloured, although, 
as in some former cases, it ought probably to be pale blue. — Celebes. The 
western and northern coasts appear in the charts to be bold and without 
reefs. Near the extreme northern point, however, an islet in the Straits 
of Limb e, and parts of the adjoining shore, appear to be fringed : the cast 
side of the bay of Manado, has deep water, and is fringed by sand and 
coral {'' Astrol. Voyage," Hydrog. Part, pp. 453-4); this extreme point, 
therefore, I have coloured red. — Of the islands leading from this point 
to Magindanao, 1 have not been able to find any account, except of 
Serangani, which appears surrounded by narrow reefs ; and Forrest 
('• Voyage," p. 164) speaks of coral on its shores; I have, therefore, 
coloured this island red. To the eastward of this chain lie several 
islands ; of which I cannot find any account, except of Karkalang, which 
is said by Horsburgli (vol. ii., p. 504) to be lined by a dangerous reef, 
projecting several miles from the northern shore ; not coloured. 

Islands near Timor. — The account of the following islands is taken 
from Captain D. Kolffs "Voyage," in 1825, translated by Mr. W. Earl, 
from the Dutch. — Lxtte has "reefs extending along shore at the distance 
of half a mile from the land." — Moa has reefs on the S.W. part. — Lakor 
has a reef lining its shore; these islands are coloured red. — Still more 
eastward, Luan has, differently from the last-mentioned islands, an 
extensive reef; it is steep outside, and within there is a depth of twelve 
feet ; from these facts, it is impossible to decide to which class this 
island belongs. — Ivtssa, off the point of Timor, has its " shore fronted 
by a reef, steep too on the outer side, over which small proahs can go 
at the time of high water; " coloured red. — Timor; most of the points, 
and some considerable spaces of the northern shore, are seen in Frey- 
cinet's chart to be fringed by coral-reefs ; and mention is made of them 
in the accompanying "Hydrog. Memoir;" coloured red. — Savti, S.E. of 
Timor, appears in Flinders' chart to be fringed ; but I have not coloured 
it, as I do not know that the reefs are of coral. — Sandalivood Isld. 
l:as, according to Horsburgh (vol. ii., p. 607), a reef on its southern 
shore, four miles distant from the land ; as the neighbouring sea is deep, 

9 



I30 APPENDIX. 

and generally bold, this probably is a barricr-rcef, but I have not ven- 
tured to colour it. 

N.VV. Coast of Australia. — It appears, in Captain King's Sailing 
Directions ("Narrative of Survey," vol. ii., pp. 325-369), that there are 
many extensive coral-reefs skirting, often at considerable distances, the 
N.W. shores, and encompassing the small adjoining islets. Deep 
water, in no instance, is represented in the charts between these reefs 
and the land ; and, therefore, they probably belong to the fringing class. 
But as they extend far into the sea, which is generally shallow, even in 
places where the land seems to be somewhat precipitous ; I have not 
coloured them. Houtman's Abrolhos (lat. 28° S. on west coast) have 
lately been surveyed by Captain Wickham (as described in Naitt Mag., 
1841, p. 511): they lie on the edge of a steeply shelving bank, which 
extends about thirty miles seaward, along the whole line of coast. The 
two southern reefs, or islands, enclose a lagoon-like space of water, 
varying in depth from five to fifteen fathoms, and in one spot with twenty- 
three fathoms. The greater part of the island has been formed on their 
inland sides, by the accumulation of fragments of coral ; the seaward face 
consisting of nearly bare ledges of rock. Some of the specimens, brought 
home by Captain Wickham, contained fragments of marine shells, 
but others did not ; and these closely resembled a formation at King 
George's Sound, principally due to the action of the wind on calcareous 
dust, which I shall describe in a forthcoming part. From the extreme 
irregularity of these reefs with their lagoons, and from their position on 
a bank, the usual depth of which is only thirty fathoms, I have not 
ventured to class them with atolls, and hence have left them uncoloured. 
— Rowley Shoals. These lie some way from the N.W. coast of Aus- 
tralia : according to Captain King ("Narrative of Survey," vol. i., p. 60), 
they are of coral-formation. They rise abruptly from the sea, and 
Captain King had no bottom with 170 fathoms close to them. Three of 
them are crescent-shaped ; they are mentioned by Mr. Lycll, on the 
authority of Captain King, with reference to the direction of their open 
sides. " A third oval reef of the same group is entirely submerged " 
(" Principles of Geolog.," book iii., chap, xviii.) ; coloured blue. — Scoffs 
Reefs, lying north of Rowley Shoals, are briefly described by Captain 
Wickham {Naiit. Mag., 1841, p. 440): they appear to be of great size, 
of a circular form, and "with smooth water within, forming probably a 
lagoon of great extent." There is a break on the western side, where 
there probably is an entrance : the water is very deep off these reefs ; 
coloured blue. 

Proceeding westward along the great volcanic chain of the East 
Indian Archipelago, Solar Strait is represented in a chart published by 
Dairy mple from a Dutch MS., as fringed ; as are parts of Flo res, of 
Adenara, and of Solor. Horsburgh speaks of coral growing on these 
shores ; and therefore I have no doubt that the reefs are of coral, and 
accordingly have coloured them red. We hear from Horsburgh (vol. 
ii., p. 602) that a coral-flat bounds the shores of Sapy Bay. From the 
same authority it appears (p. 610) that reefs fringe the island of 
Timor- Young, on the N. shore of Sumbawa ; and, likewise (p. 600), 



EAST-INDIAN ARCHIPELAGO. 131 

that Bally town in Lo?7ibock, is fronted by a reef, stretching along the 
shore at a distance of a hundred fathoms, with channels through it for 
boats ; these places, therefore, have been coloured red. — Bally Isld. 
In a Dutch MS. chart on a large scale of Java, which was brought from 
that island by Dr. Horsfield, who had the kindness to show it me at 
the India House, its western, northern, and southern shores appear 
very regularly fringed by a reef (see also Horsburgh, vol. ii., p. 593) ; 
and as coral is found abundantly there, I have not the least doubt that 
the reef is of coral, and therefore have coloured it red. 

Java. — My information regarding the reefs of this great island is 
derived from the chart just mentioned. The greater part of Madiiara 
is represented in it as regularly fringed, and likewise portions of the 
coast of Java immediately south of it. Dr. Horsfield informs me that 
coral is very abundant near Sourabaya. The islets and parts of the 
N. coast of Java, west of Point Buang, or Japara, are fringed by 
reefs, said to be of coral. Liibeck, or Bavian Islands, lying at some 
distance from the shore of Java, are regularly fringed by coral-reefs. 
Carimon Java appears equally so, though it is not directly said that the 
reefs are of coral ; there is a depth between thirty and forty fathoms 
round these islands. Parts of the shores of Simda Sir., where the water 
is from forty to eighty fathoms deep, and the islets near Batavia appear 
in several charts to be fringed. In the Dutch chart the southern shore, 
in the narrowest part of the island, is in two places fringed by reefs of 
coral. West of Segorrowodee Bay, and the extreme S.E. and E. portions 
are likewise fringed by coral-reefs ; all the above-mentioned places 
coloured red. 

Macassar Sir.; the east coast of Borneo appears, in most parts, free 
from reefs, and where they occur, as on the east coast of Pamaroong, 
the sea is very shallow; hence no part is coloured. In Macassar Str., 
itself, in about lat. 2° S., there are many small islands with coral-shoals 
projecting far from them. There are also (old charts by Dalrymple) 
numerous little flats of coral, not rising to the surface of the water, and 
shelving suddenly from five fathoms to no bottom with fifty fathoms ; 
they do not appear to have a lagoon-like structure. There are similar 
coral-shoals a little farther south ; and in lat. 4° 55' there are two, which 
are engraved from modern surveys, in a manner which might represent 
an annular reef with deep water inside : Captain Moresby, however, 
who was formerly in this sea, doubts this fact, so that I have left them 
uncoloured: at the same time I may remark, that these two shoals 
make a nearer approach to the atoll-like structure than any other within 
the E. Indian Arch. Southward of these shoals there are other low 
islands and irregular coral-reefs ; and in the space of sea, north of the 
;;reat volcanic chain, from Timor to Java, we have also other islands, 
such as the Postillions, Kalatoa, Tokan-Bessees, etc., which are chiefly 
low, and are surrounded by very irregular and distant reefs. From the 
imperfect charts I have seen, I have not been able to decide whether 
they belong to the atoll or barrier-classes, or whether they merely fringe 
submarine banks, and gently sloping land. In the Bay of Bonin, 
between the two southern arms of Celebes, there arc numerous coral- 



132 APPENDIX. 

reefs ; but none of them seem to have an atoll-like structure. I have, 
therefore, not coloured any of the islands in this part of the sea ; I 
think it, hovt^ever, exceedingly probable that some of them ought to be 
blue. I may add that there is a harbour on the S.E. coast of Boicton 
which, according to an old chart, is formed by a reef, parallel to the 
shore, with deep water within ; and in the " Voyage of the Coquille" 
some neighbouring islands are represented with reefs a good way dis- 
tant, but I do not know whether with deep water within. I have not 
thought the evidence sufficient to permit me to colour them. 

Sumatra. — Commencing with the west coast and outlying islands, 
Engano hid. is represented in the published chart as surrounded by a 
narrow reef, and Napier, in his " Sailing Directions," speaks of the reef 
being of coral (also Horsburgh, vol. ii., p. 115) ; coloured red. — Ratlsld. 
(3° 51' S.) is surrounded by reefs of coral, partly dry at low water, 
(Horsburgh, vol. ii., p. 96). — Trieste Island (4° 2' S.). The shore is 
represented in a chart which I saw at the India House, as fringed in 
such a manner, that I feel sure the fringe consists of coral ; but as the 
island is so low, that the sea sometimes flows quite over it (Dampier, 
"Voyage," vol. i., p. 474), I have not coloured it. — Pulo Dooa (lat. 3°). 
In an old chart it is said there are chasms in the reefs round the island, 
admitting boats to the watering-place, and that the soutliern islet con- 
sists of a mass of sand and coral. — Pulo Pisa?tg ; Horsburgh (vol, ii., 
p. 86) says that the rocky coral-bank, which stretches about forty yards 
from the shore, is steep to all round : in a chart, also, which I have 
seen, the island is represented as regularly fringed. — Ptilo Mintao is 
lined with reefs on its west side (Horsburgh, vol. ii., p. 107). — Pulo 
Baniak ; the same authority (vol. ii., p. 105), speaking of a part, says 
it is faced \\\\\\ coral-rocks. — Mi?7gum (3° 36' N.), A coral-reef fronts 
this place, and projects into the sea nearly a quarter of a mile ("Notices 
of the Indian Arch.," published at Singapore, p. 105). — Pulo Brassa 
(5° 46' N.). A reef surrounds it at a cable's length (Horsburgh, vol. 
ii., p. 60). I have coloured all the above-specified points red. I 
may here add, that both Horsburgh and Mr. Moor (in the "Notices" 
just alluded to) frequently speak of the numerous reefs and banks of 
coral on the west coast of Sumatra ; but these nowhere have the struc- 
ture of a barrier-reef, and Marsden (" History of Sumatra ") states, that 
where the coast is flat, the fringing-reefs extend furthest from it. The 
northern and southern points, and the greater part of the east coast, 
are low, and faced with mud banks, and therefore without coral. 

N ICO BAR Islands. — The chart represents the islands of this group 
as fringed by reefs. With regard to Great Nicobar, Captain Moresby 
informs me, that it is fringed by reefs of coral, extending between two 
and three hundred yards from the shore. The Northern Nicobars 
appear so regularly fringed in the published charts, that I have no 
doubt the reefs are of coral. This group, therefore, is coloured red. 

Andaman Islands. — From an examination of the MS. chart, on a 
large scale, of this island, by Captain Arch, Blair, in the Admiralty, several 
portions of the coast appear fringed ; and as Horsburgh speaks of coral- 
reefs being numerous in the vicinity of these islands, I should have 



l: AST-INDIAN ARCHIPELAGO. 133 

coloured them red, had not some expressions in a paper in the '• Asiatic 
Researches" (vol. iv., p. 402) led me to doubt the existence of reefs; 
uncoloured. 

The coast of Malacca, Te7tasserim, and the coasts northward, appear 
in the greater part to be low and muddy : where reefs occur, as in parts 
oi Malacca Straits, and uq^x Singapore, they are of the fringing kind ; but 
the water is so shoal, that I have not coloured them. In the sea, how- 
ever, between Malacca and the west coast of Borneo, where there is a 
greater depth from forty to fifty fathoms, I have coloured red some of 
the groups, which are regularly fringed. The northern Nattmas and the 
Ajiajnbas Islds, are represented in the charts on a large scale, published 
in the " Atlas of the Voyage of the Favojtritc,'' as fringed by reefs of 
coral, with very shoal water within them. — Tumbelait and Bu7ioa Islds. 
(1° N.) are represented in the English charts as surrounded by a very 
regular fringe. — St. Barbes {p° 15' N.) is said by Horsburgh (vol. ii., 
p. 279) to be fronted by a reef, over which boats can land only at high 
water. — The shore of Borneo at Tunjong Apee is also fronted by a reef, 
extending not far from the land (Horsburgh, vol. ii., p. 468), These 
places I have coloured red ; although with some hesitation, as the water 
is shallow. I might perhaps have added Ftclo Leaf, in Caspar Str., 
Lucepara, and Cai'imata ; but as the sea is confined and shallow, and 
the reefs not very regular, I have left them uncoloured. 

The water shoals gradually towards the whole west coast of Borneo : 
I cannot make out that it has any reefs of coral. The islands, however, 
off the northern extremity, and near the S.W. end of Palawan, are 
fringed by very distant coral-reefs ; thus the reefs in the case of Balabac 
are no less than five miles from the land ; but the sea, in the whole of 
this district, is so shallow, that the reefs might be expected to extend 
very far from the land. I have not, therefore, thought myself authorised 
to colour them. The N.E. point of Borneo, where the water is very 
shoal, is connected with Magindanao by a chain of islands called the 
Sooloo Archipelago, about which I have been able to obtain very little 
information ; Pangoota7-an, although ten miles long, entirely consists of 
a bed of coral-rock (" Notices of E. Indian Arch.," p. 58) : I believe from 
Horsburgh that the island is low; not coloured. — Tahow bank, in some 
old charts, appears like a submerged atoll ; not coloured. Forrest 
("Voyage," p. 21) states that one of the islands near Sooloo is sur- 
rounded by coral-rocks ; but there is no distant reef. Near the S. end 
of Bassela7t, some of the islets in the chart accompanying Forrest's 
" Voyage," appear fringed with reefs; hence I have coloured, though 
unwillingly, parts of the Sooloo group red. The sea between Sooloo 
and Palawan, near the shoal coast of Borneo, is interspersed with 
irregular reefs and shoal patches ; not coloured : but in the northern 
part of this sea, there are two low islets, Cagaya7ies and Cavilli, sur- 
rounded by extensive coral-reefs ; the breakers round the latter (Hors- 
burgh, vol. ii., p. 513) extend five or six miles from a sandbank, which 
forms the only dry part ; these breakers are steep to outside ; there 
appears to be an opening through them on one side, with four or five 
fathoms within : from this description, I strongly suspect that Cavilli 



134 APPENDIX. 

ought to be considered an atoll ; but, as I have not seen any chart of 
it, on even a moderately large scale, I have not coloured it. The islets 
off the northern end of Palawan, are in the same case as those off the 
southern end, namely they are fringed by reefs, some way distant from 
the shore, but the water is exceedingly shallow ; uncoloured. The 
western shore of Palawan will be treated of under the head of China 
Sea. 

Philippine Archipelago. — A chart on a large scale of Appoo Shoal, 
which Hes near the S.E. coast of Mindoro, has been executed by Captain 
D. Ross : it appears atoll-formed, but with rather an irregular outline ; 
its diameter is about ten miles ; there are two well-defined passages 
leading into the interior lagoon, which appears open ; close outside the 
reef all round, there is no bottom with seventy fathoms ; coloured blue. 
— Mindoro : the N.W. coast is represented in several charts, as fringed 
by a reef, and Luban Isld. is said, by Horsburgh (vol. ii., p. 436), to be 
'* lined by a reef." — Luzon : Mr. Cuming, who has lately investigated 
with so much success the Natural History of the Philippines, informs 
me, that about three miles of the shore north of Point St. Jago, is 
fringed by a reef ; as are (Horsburgh, vol. ii., p. 437) the Three Friars 
off Silanguin Bay. Between Point Capones and Playa Honda, the coast 
is "lined by a coral-reef, stretching out nearly a mile in some places," 
(Horsburgh) ; and Mr. Cuming visited some fringing-reefs on parts 
of this coast, namely, near Puebla, Iba, and Mansinglor. In the neigh- 
bourhood of Solon-solon Bay, the shore is lined (Horsburgh, ii., 
p. 439) by coral-reefs, stretching out a great way : there are also reefs 
about the islets off Solamague ; and as I am informed by Mr. Cuming, 
near St. Catalina, and a little north of it. The same gentleman informs 
me there are reefs on the S.E. point of this island in front of Samar, 
extending from Malalabon to Bulusan. These appear to be the principal 
fringing-reefs on the coasts of Luzon ; and they have all been coloured 
red. Mr. Cuming informs me that none of them have deep water 
within ; although it appears from Horsburgh that some few extend to 
a considerable distance from the shore. Within the Philippine Archi- 
pelago, the shores of the islands do not appear to be commonly fringed, 
with the exception of the S. shore of Masbate, and nearly the whole of 
Bohol; which are both coloured red. On the S. shore of Magindanao, 
Bunwoot Isld. is surrounded (according to Forrest, " Voyage," p. 253), 
by a coral-reef, which in the chart appears one of the fringing class. 
With respect to the eastern coasts of the whole Archipelago, I have not 
been able to obtain any account. 

Babuyan Islands. — Horsburgh says (vol. ii., p. 442), coral-reefs 
line the shores of the harbour in Fuga ; and the charts show there are 
other reefs about these islands. Camiguin has its shore in parts lined 
by coral-rock (Horsburgh, p. 443); about a mile off shore there is 
between thirty and thirty-five fathoms. The plan of Port San Pio 
Quinto shows that its shores are fringed with coral ; coloured red. — 
Bashee Islands : Horsburgh, speaking of the southern part of the 
group (vol. ii., p. 445) says the shores of both islands are fortified by a 
reef, and through some of the gaps in it, the natives can pass in their 



EAST-INDIAN ARCHIPELAGO. 135 

boats in fine weather ; the bottom near the land is coral-rock. From 
the published charts, it is evident that several of these islands are most 
regularly fringed ; coloured red. The northern islands are left 
uncoloured, as I have been unable to find any account of them. — 
Formosa. The shores, especially the western one, seem chiefly com- 
posed of mud and sand, and I cannot make out that they are anywhere 
lined by reefs ; except in a harbour (Horsburgh, vol. ii., p. 449) at 
the extreme northern point: hence, of course, the whole of this island 
is left uncolcured. The small adjoining islands are in the same case. 
— Patchow, or Madjiko-Sima Groups. Patchuson has been described 
by Captain Broughton ("Voy. to the N. Pacific," p. 191); he says, the 
boats, with some difficulty, found a passage through the coral-reefs, 
which extend along the coast, nearly half a mile off it. The boats were 
well sheltered within the reef ; but it does not appear that the water is 
deep there. Outside the reef the depth is very irregular, varying from 
five to fifty fathoms ; the form of the land is not very abrupt ; coloured 
red. — Taypm-san ; from the description given (p. 195) by the same 
author, it appears that a very irregular reef extends, to the distance of 
several miles, from the southern island ; but whether it encircles a 
space of deep water is not evident ; nor, indeed, whether these outlying 
reefs are connected with those more immediately adjoining the land ; 
left uncoloured. I may here just add that the shore of Kumi (lying 
west of Patchow), has a narrow reef attached to it in the plan of it, 
in La Peyrouse's " Atlas ; " but it does not appear in the account of the 
voyage that it is of coral; uncoloured. — Loo Choo. The i greater part 
of the coast of this moderately hilly island, is skirted by reefs, which 
do not extend far from the shore, and which do not leave a channel of 
deep water within them, as may be seen in the charts accompanying 
Captain B. Hall's voyage to Loo Choo (see also remarks in Appendix, 
pp. xxi. and xxv.). There are, however, some ports with deep water, 
formed by reefs in front of valle3^s, in the same manner as happens at 
Mauritius. Captain Beechey, in a letter to me, compares these reefs 
with those encircling the Society Islands ; but there appears to me a 
marked difference between them, in the less distance at which the 
Loo Choo reefs lie from the land with relation to the probable sub- 
marine inclination, and in the absence of an interior deep water-moat 
or channel, parallel to the land. Hence, I have classed these reefs 
with fringing-reefs, and coloured them red. — Pescadores (west of 
Formosa). Dampier (vol. i,, p. 416), has compared the appearance of 
the land to the southern parts of England. The islands are interlaced 
with coral-reefs ; but as the water is very shoal, and as spits of sand 
and gravel (Horsburgh, vol. ii., p. 450) extend far out from them, it is 
impossible to draw any inferences regarding the nature of the reefs. 

China vSea. — Proceeding from north to south, we first meet the 
Piatas Shoal (lat. 20° N.) which, according to Horsburgh (vol. ii., 
P- 335)» is composed of coral, is of a circular form, and has a low islet on 
it. The reef is on a level with the water s edge, and when the sea runs 
high, there are breakers mostly all round, ** but the water within seems 
pretty deep in some places ; although steep-to in most parts outside, 



136 APPENDIX. 

there appear to be several parts where a ship might find anchorage 
outside the breakers ; " coloured blue. — The Paracells have been 
accurately surveyed by Captain D. Ross, and charts on a large scale 
published : but few low islets have been formed on these shoals, and 
this seems to be a general circumstance in the China Sea ; the sea 
close outside the reefs is very deep ; several of them have a lagoon- 
like structure ; or separate islets {Prattle, Robert, Dru7n7?iond, etc.) are 
so arranged round a moderately shallow space, as to appear as if they 
had once formed one large atoll. — Bombay Shoal (one of the Paracells) 
has the form of an annular reef, and is •' apparently deep within ; " it 
seems to have an entrance (Horsburgh, vol. ii., p. 332) on its west side ; 
it is very steep outside. — Discovery Shoal, also, is of an oval form, with 
a lagoon-like space within, and three openings leading into it, in which 
there is a depth from two to twenty fathoms. Outside, at the dis- 
tance (Horsburgh, vol. ii., p. 333) of only twenty yards from the reef, 
soundings could not be obtained. The Paracells are coloured blue. — ■ 
Macclesfield Bank : this is a coral-bank of great size, lying east of the 
Paracells ; some parts of the bank are level, with a sandy bottom, but, 
generally, the depth is very irregular. It is intersected by deep cuts 
or channels. I am not able to perceive in the published charts (its 
limits, however, are not very accurately known) whether the central 
part is deeper, which I suspect is the case, as in the Great Chagos 
Bank, in the Indian Ocean; not coloured. — Scarborough Shoal: this 
coral-shoal is engraved with a double row of crosses, forming a circle, 
as if there was deep water within the reef : close outside there was no 
bottom, with a hundred fathoms ; coloured blue. — The sea off the west 
coast of Palawan and the northern part of Borneo is strewed with 
shoals: Swallow Shoal, according to Horsburgh (vol. ii,, p. 431) "is 
formed, like ??tost of the shoals hereabouts, of a belt of coral-rocks, 
" with a basin of deep water within." — Half-Moon Shoal has a similar 
structure ; Captain D. Ross describes it, as a narrow belt of coral-rock, 
" with a basin of deep water in the centre," and deep sea close outside. 
— Bombay Shoal appears (Horsburgh, vol. ii., p. 432) " to be a basin of 
smooth water surrounded by breakers." These three shoals I have 
coloured blue. — The Paraqiias Shoals are of a circular form, with deep 
gaps running through them ; not coloured. — A bank gradually shoaling 
to the depth of thirty fathoms, extends to a distance of about twenty 
miles from the northern part of Borneo, and to thirty miles from the 
northern part of Palawan. Near the land this bank appears tolerably 
free from danger, but a little further out it is thickly studded with coral- 
shoals, which do not generally rise quite to the surface ; some of them 
are very steep-to, and others have a fringe of shoal-water round them. 
I should have thought that these shoals had level surfaces, had it not 
been for the statement made by Horsburgh " that most of the shoals 
hereabouts are formed of a belt of coral." But, perhaps that expres- 
sion was more particularly applied to the shoals further in the offing. 
If these reefs of coral have a lagoon-like structure, they should have 
been coloured blue, and they would have formed an imperfect barrier 
in front of Palawan and the northern part of Borneo. But, as the water 



INDIAN OCEAN- 137 

is not very deep, these reefs may have grown up from ineqiiaHties on 
the bank : I have not coloured them. — The coast of China, To?iqimi, and 
Cochi7i-Chi7ta, forming the u^estern boundary of the China Sea, appear 
to be without reefs : with regard to the two last-mentioned coasts, I 
speak after examining the charts on a large scale in the " Atlas of the 
Voyage of the Favourite^ 

Indian Ocean. — South Keeliiig atoll has been specially described 
nine miles north of it lies North Keeling, a very small atoll, surveyed 
by the Beagle^ the lagoon of which is dry at low water. — Christmas 
Island, lying to the east, is a high island, without, as I have been in- 
formed by a person who passed it, any reefs at all. — Ceylon : a space 
about eighty miles in length of the south-western and southern shores 
of these islands has been described by Mr. Twynam {Naut. Mag., 1836, 
pp. 365 and 518); parts of this space appear to be very regularly 
fringed by coral-reefs, which extend from a quarter to half a mile from 
the shore. These reefs are in places breached, and afford safe anchorage 
for the small trading craft. Outside, the sea gradually deepens ; there 
is forty fathoms about six miles off shore : this part I have coloured 
red. In the published charts of Ceylon there appear to be fringing- 
reefs in severed parts of the south-eastern shores, which I have also 
coloured red. — At Venloos Bay the shore is likewise fringed. North of 
Trincomalee there are also reefs of the same kind. The sea off the 
northern part of Ceylon is exceedingly shallow ; and therefore I have 
not coloured the reefs which fringe portions of its shores, and the 
adjoining islets, as well as the Indian promontory of Madura. 

Chagos, Maldiva, and Laccadive Archipelagoes. — These three 
great groups which have already been often noticed, are now well known 
from the admirable surveys of Captain Moresby and Lieutenant Powell. 
The published charts, which are worthy of the most attentive examina- 
tion, at once show that the Chagos and Maldiva groups are entirely 
formed of great atolls, or lagoon-formed reefs, surmounted by islets. 
In the Laccadive group, this structure is less evident ; the islets are 
low, not exceeding the usual height of coral-formations (see Lieutenant 
Wood's account, Geograph. Journ., vol, vi., p. 29), and most of the reefs 
are circular, as may be seen in the published charts ; and within several 
of them, as I am informed by Captain Moresby, there is decpish water; 
these, therefore, have been coloured blue. Directly north, and almost 
forming part of this group, there is a long, narrow, slightly curved bank, 
rising out of the depths of the ocean, composed of sand, shells, and 
decayed coral, with from twenty-three to thirty fathoms on it. I have 
no doubt that it has had the same origin with the other Laccadive banks ; 
but as it does not deepen towards the centre I have not coloured it. I 
might have referred to other authorities regarding these three archi- 
pelagoes ; but after the publication of the charts by Captain Moresby, 
to whose personal kindness in giving me much information I am 
exceedingly indebted, it would have been superfluous. 

Sahia de Malha bank consists of a series of narrow banks, with from 
eight to sixteen fathoms on them ; they are arranged in a semicircular 
manner, round a space about forty fathoms deep, which slopes on the 



138 APPENDIX. 

S.E. quarter to unfathomable depths ; they are steep-to on both sides, 
but more especially on the ocean-side. Hence this bank closely re- 
sembles in structure, and I may add from Captain Moresby's informa- 
tion in composition, the Pitt's Bank in the Chagos group; and the 
Pitt's Bank, must, after what has been shown of the Great Chagos 
Bank, be considered as a sunken, half-destroyed atoll ; hence coloured 
blue. — Cargados Carajos Bank. Its southern portion consists of a large, 
curved, coral-shoal, with some low islets on its eastern edge, and like- 
wise some on the western side, between which there is a depth of about 
twelve fathoms. Northward, a great bank extends. I cannot (probably 
owing to the want of perfect charts) refer this reef and bank to any 
class ; — therefore not coloured. — He de Sable is a little island, lying 
west of C. Carajos, only some toises in height (" Voyage of the 
Favourite" vol. i., p. 130) ; it is surrounded by reefs ; but its structure 
is unintelligible to me. There are some small banks north of it, of 
which I can find no clear account. — Maio'itius. The reefs round this 
island have been described in the chapter on fringing-reefs ; coloured 
red. — Rodfiguez. The coral-reefs here are exceedingly extensive; in 
one part they project even five miles from the shore. As far as 1 can 
Inake out, there is no deep-water moat within them ; and the sea outside 
does not deepen very suddenly. The outline, however, of the land 
appears to be ("Life of Sir J. Makintosh," vol. ii., p. 165) hilly and 
rugged. I am unable to decide whether these reefs belong to the 
barrier class, as seems probable from their great extension, or to the 
fringing class; uncoloured. — Bourbon. The greater part of the shores 
of this island are without reefs ; but Captain Carmichael (Hooker's " Bot. 
Misc.") states that a portion, fifteen miles in length, on the S.E. side, is 
imperfectly fringed with coral-reefs : I have not thought this sufficient 
to colour the island. 

Seychelles. — The rocky islands of primary formation, composing 
this group, rise from a very extensive and tolerably level bank, having 
a depth between twenty and forty fathoms. In Captain Owen's chart, 
and in that in the " Atlas of the Voyage of the Favourite" it appears 
that the east side of Make and the adjoining islands of St. Anne and 
Cerf, are regularly fringed by coral-reefs. A portion of the S.E. part 
of Cuiieuse Isld., the N., and part of the S.W. shore of Praslin Isld., 
and the whole west side of Digue Isld., appear fringed. From a MS. 
account of these islands by Captain F. Moresby, in the Admiralty, it 
appears that Silhouette is also fringed ; he states that all these islands 
are formed of granite and quartz, that they rise abruptly from the sea, 
and that "coral-reefs have grown round them, and project for some 
distance." Dr. Allan, of Forres, who visited these islands, informs me 
that there is no deep water between the reefs and the shore. The above 
specified points have been coloured red. Aviirantes Islands : The small 
islands of this neighbouring group, according to the MS. account ol 
them by Captain F. Moresby, are situated on an extensive bank ; they 
consist of the debris of corals and shells ; are only about twenty feet in 
height, and are environed by reefs, some attached to the shore, and some 
rather distant from it. — I have taken great pains to procure plans and 



INDIAN OCEAN 139 

information regarding the several islands lying between S.E. and S.W. 
of the Amirantes, and the Seychelles ; relying chiefly on Captain 
F. Moresby and Dr. Allan, it appears that the greater number, namely — 
Platte, Alphonse, Coetivi, Galega, Providence, St. Pierre, Astova, 
Assojnption, and Glorioso, are low, formed of sand or coral-rock, and 
irregularly shaped ; they are situated on very extensive banks, and are 
connected with great coral-reefs. Galega is said by Dr. Allan, to be 
rather higher than the other islands; and St. Pierre is described by 
Captain F. Moresby, as being cavernous throughout, and as not con- 
sisting of either limestone or granite. These islands, as well as the 
Amirantes, certainly are not atoll-formed, and they differ as a group 
from every other group with which I am acquainted ; I have not coloured 
them ; but probably the reefs belong to the fringing class. Their forma- 
tion is attributed, both by Dr. Allan and Captain F. Moresby, to the 
action of the currents, here exceedingly violent, on banks, which no 
doubt have had an independent geological origin. They resemble in 
many respects some islands and banks in the West Indies, which owe 
their origin to a similar agency, in conjunction with an elevation of the 
entire area. In close vicinity to the several islands, there are three 
others of an apparently different nature : first, Juan de Nova, which 
appears from some plans and accounts to be an atoll ; but from others 
does not appear to be so ; not coloured. Secondly, Cosmoledo ; " this 
group consists of a ring of coral, ten leagues in circumference, and a 
quarter of a mile broad in some places, enclosing a magnificent lagoon, 
into which there did not appear a single opening " (Horsburgh, vol. i., 
p. 151)-; coloured blue. Thirdly, Aldabra; it consists of three islets, 
about twenty-five feet in height, with red cliffs (Horsburgh, vol. i., 
p. 176) surrounding a very shallow basin or lagoon. The sea is pro- 
foundly deep close to the shore. Viewing this island in a chart, it 
would be thought an atoll; but the foregoing description shows that 
there is something different in its nature ; Dr. Allan also states that it 
is cavernous, and that the coral-rock has a vitrified appearance. Is it 
an upheaved atoll, or the crater of a volcano ? — uncoloured. 

Comoro Group. — Mayotta, according to Horsburgh (vol. i., p. 216, 
4th edit.), is completely surrounded by a reef, which runs at the distance 
of three, four, and in some places even five miles from the land ; in an 
old chart, published by Dalrymple, a depth in many places of thirty-six 
and thirty-eight fathoms is laid down within the reef. In the same 
chart, the space of open water within the reef in some parts is even 
more than three miles wide : the land is bold and peaked ; this island, 
therefore, is encircled by a well-characterised barrier-reef, and is 
coloured pale \A\ie.— Johanna ; Horsburgh says (vol. i., p. 217) this 
island from the N.W. to the S.W. point, is bounded by a reef, at the 
distance of two miles from the shore ; in some parts, however, the reef 
must be attached, since Lieutenant Boteler ("Nam," vol. i., p. 161) 
describes a passage through it, within which there is room only for a 
few boats. Its height, as I am informed by Dr. Allan, is about 3,500 
feet ; it is very precipitous, and is composed of granite, greenstone, and 
quartz; coloured blue. — Mohilla; on the S. side of this island there is 



140 APPENDIX. 

anchorage, in from thirty to forty-five fathoms, between a reef and the 
shore (Horsburgh, vol. i., p, 214) ; in Captain Owen's chart of Madagascar, 
this island is represented as encircled ; coloured blue. — Great Comoro 
Isld. is, as I am informed by Dr. Allan, about 8,000 feet high, and 
apparently volcanic ; it is not regularly encircled ; but reefs of various 
shapes and dimensions, jut out from every headland on the W., S., and 
S.E. coasts, inside of which reefs there are channels, often parallel 
with the shore, with deep water. On the north-western coasts the 
reefs appear attached to the shores. The land near the coast is in 
some places bold, but generally speaking it is flat ; Horsburgh says 
(vol. i., p. 214) the water is profoundly deep close to W\^ shore, ixovsx 
which expression I presume some parts are without reefs. From this 
description I apprehend the reef belongs to the barrier class ; but I 
have not coloured it, as most of the charts which I have seen, represent 
the reefs round it as very much less extensive than round the other 
islands in the group. 

Madagascar. — My information is chiefly derived from the published 
charts by Captain Owen, and the accounts given by him and by Lieu- 
tenant Boteler. Commencing at the S.W. extremity of the island ; 
towards the northern part of the Star Bank (in lat. 25° S.) the coast 
for ten miles is fringed by a reef; coloured red. The shore immediately 
S. of St. AvgiistUie's Bay appears fringed ; but Tiillcar Harbour, directly 
N. of it, is formed by a narrow reef ten miles long, extending parallel 
to the shore, with from four to ten fathoms within it. If this reef had 
been more extensive, it must have been classed as a barrier-reef; but 
as the line of coast falls inwards here, a submarine bank perhaps extends 
parallel to the shore, which has offered a foundation for the growth of 
the coral ; I have left this part uncoloured. From lat. 22° 16' to 21° 37', 
the shore is fringed by coral-reefs (see Lieutenant Boteler's " Narrative," 
vol. ii., p. 106), less than a mile in width, and with sliallow water within. 
There are outlying coral-shoals in several parts of the offing, with about 
ten fathoms between them and the shore, and the depth of the sea one 
mile and a half seaward, is about thirty fathoms. The part above 
specified is engraved on a large scale ; and as in the charts on rather 
a smaller scale the same fringe of reef extends as far as lat. 33° 15' ; I 
have coloured the whole of this part of the coast red. The islands of 
Juan de Nova (in lat. 17° S.) appear in the charts on a large scale to be 
fringed, but I have not been able to ascertain whether the reefs are of 
coral ; uncoloured. The main part of the west coast appears to be low, 
with outlying sandbanks, which, Lieutenant Boteler (vol. ii., p. 106) says, 
" are faced on the edge of deep water by a line of sharp-pointed coral- 
rocks." Nevertheless I have not coloured this part, as I cannot make 
out by the charts that the coast itself is fringed. The headlands of 
Narrenda and Passandava Bays (14° 40') and the islands in front of 
Radama Harbour are represented in the plans as regularly fringed, and 
have accordingly been coloured red. With respect to the East coast of 
Madagascar, Dr. Allan informs me in a letter, that the whole line of 
coast, from Ta7natave, in 18° 12', to C. A7nber, at the extreme northern 
point of the island, is bordered by coral-reefs. The land is low, uneven, 



EAST COAST OF AFRICA, 141 

and gradually rising from the coast. From Captain Owen's charts, also, 
the existence of these reefs, which evidently belong to the fringing class, 
on some parts, namely, N. of British Sound and near Ngoncy, of the 
above line of coast might have been inferred. Lieutenant Boteler (vol. i., 
p, 155) speaks of "the reef surrounding the island of St. Mary's at a 
small distance from the shore." In a previous chapter I have described, 
from the information of Dr. Allan, the manner in which the reefs extend 
in N.E. hnes from the headlands on this coast, thus sometimes forming 
rather deep channels within them ; this seems caused by the action of 
the currents, and the reefs spring up from the submarine prolongations 
of the sandy headlands. The above specified portion of the coast is 
coloured red. The remaining S.E. portions do not appear in any pub- 
lished chart to possess reefs of any kind ; and the Rev. W. Ellis, whose 
means of information regarding this side of Madagascar have been 
extensive, informs me he believes there are none. 

East Coast of Africa. — Proceeding from the northern part, the 
roast appears, for a considerable space, without reefs. My information, 
I may here observe, is derived from the survey by Captain Owen, 
together with his narrative ; and that by Lieutenant Boteler. At Mukdee- 
sha (10° 1' N.) there is a coral-reef extending four or five miles along 
the shore (Owen's *' Narr.," vol. i., p. 357) which in the chart lies at 
the distance of a quarter of a mile from the shore, and has within it 
from six to ten feet water : this then is a fringing-reef, and is coloured 
red. From Juba, a little S. of the equator, to La?noo (in 2° 20' S.) " the 
coast and islands are formed of madrepore " (Owen's " Narrative," vol. i., 
p. 363). The chart of this part (entitled Dimdas Islds.'), presents an 
extraordinary appearance ; the coast of the mainland is quite straight 
and it is fronted at the average distance of two miles, by exceedingly 
narrow, straight islets, fringed with reefs. Within the chain of islets, 
there are extensive tidal flats and muddy bays, into which many rivers 
enter ; the depths of these spaces varies from one to four fathoms — the 
latter depth not being common, and about twelve feet the average. Out- 
side the chain of islets, the sea, at the distance of a mile, varies in depth 
from eight to fifteen fathoms. Lieutenant Boteler ( " Narr.," vol. i., p. 
369) describes the muddy bay of Patta, which seems to resemble other 
];arts of this coast, as fronted by small, narrow, level islets lormed of 
c'ecomposing coral, the margin of which is seldom of greater height 
than twelve feet, overhanging the rocky surface from which the islets 
rise. Knowing that the islets are formed of coral, it is, I think, scarcely 
] ossible to view the coast, and not at once conclude that we here see 
a Iringing-reef, which has been upraised a few feet : the unusual depth 
of from two to four fathoms within some of these islets, is probably due 
to muddy rivers having prevented the growth of coral near the shore. 
There is, however, one difficulty on this view, namely, that before the 
elevation took place, which converted the reef into a chain of islets, the 
water must apparently have been still deeper ; on the other hand it 
may be supposed that the formation of a nearly perfect barrier in front, 
of so large an extent of coast, would cause the currents (especially in 
front of the rivers), to deepen their muddy beds. When describing in 



142 APPENDIX. 

the chapter on fringing-reefs, those of Mauritius, I have given my 
reasons for believing that the shoal spaces within reefs of this kind, 
must, in many instances, have been deepened. However this may be, 
as several parts of this line of coast are undoubtedly fringed by living 
reefs, I have coloured it red. — Malcenda (3° 20' S.). In the plan of 
the harbour, the south headland appears fringed ; and in Owen's chart 
on a larger scale, the reefs are seen to extend nearly thirty miles 
southward ; coloured red. — Mojjibas (4° 5' S.). The island wliich 
forms the harbour, "is surrounded by cliffs of madrepore, capable 
of being rendered almost impregnable " (Owen's " Nam," vol. i., 
p. 412). The shore of the mainland, N. and S. of the harbour, is most 
regularly fringed by a coral-reef at a distance from half a mile to one 
mile and a quarter from the land ; within the reef the depth is from 
nine to fifteen feet ; outside the reef the depth at rather less than half 
a mile is thirty fathoms. From the charts it appears that a space about 
thirty-six miles in length, is here fringed ; coloured red. — Pemba (5° S.) 
is an isld. of coral-formation, level, and about two hundred feet in height 
(Owen's "Narr.," vol. i., p. 425); it is thirty-five miles long, and is 
separated from the mainland by a deep sea. The outer coast is repre- 
sented in the chart as regularly fringed ; coloured red. The mainland 
in front of Pemba is likewise fringed ; but there also appear to be 
some outlying reefs with deep water between them and the shore. 
I do not understand their structure, either from the charts or the descrip- 
tion, therefore have not coloured them. — Zanzibar resembles Pemba in 
most respects ; its southern half on the western side and the neighbour- 
mg islets are fringed ; coloured red. On the mainland, a little S. of 
Zanzibar, there are some banks parallel to the coast, which I should 
have thouglit had been formed of coral, had it not been said (Boteler's 
" Narr.," vol. ii., p. 39) that they were composed of sand ; not coloured. — 
Latham's Bank is a small island, fringed by coral-reefs ; but being only 
ten feet high, it has not been coloured. — Mo7tfeea is an island of the same 
character as Pemba ; its outer shore is fringed, and its southern ex- 
tremity is connected with Keelwa Point on the mainland by a chain of 
islands fringed by reefs ; coloured red. The four last-mentioned islands 
resemble in many respects some of the islands in the Red Sea, which 
will presently be described. — Keelwa. In a plan of the shore, a space 
of twenty miles N. and S. of this place is fringed by reefs, apparently of 
coral : these reefs are prolonged still further southward in Owen's 
general chart. The coast in the plans of the rivers Lindy^vA Monghow 
(9° 59' and 10° 7' S.) has the same structure ; coloured red. — Qiierimba 
Islands (from 10° 40' to 13° S.). A chart on a large scale is given of 
these islands ; they are low, and of coral-formation (Boteler's " Narr.," 
vol. ii., p. 54) ; and generally have extensive reefs projecting from them 
which are dry at low water, and which on the outside rise abruptly 
from a deep sea : on their insides they are separated from the continent 
by a channel, or rather a succession of bays, with an average depth of 
ten fathoms. The small headlands on the continent also have cOral- 
banks attached to them ; and the Querimba islands and banks are 
placed on the lines of prolongation of these headlands, and are 



RED SEA. 143 

separated from them by very shallow channels. It is evident that 
whatever cause, whether the drifting of sediment or subterranean 
movements, produced the headlands, likewise produced, as might have 
been expected, submarine prolongations to them ; and these towards 
their outer extremities, have since afforded a favourable basis for the 
growth of coral-reefs, and subsequently for the formation of islets. As 
these reefs clearly belong to the fringing class, the Querimba islands 
have been coloured red. — Monabila (13° 32' S.). In the plan of this 
harbour, the headlands outside are fringed by reefs apparently of 
coral; coloured red. — Mozajuhiqite (150'^ S.). The outer part of the 
island on which the city is built, and the neighbouring islands, are 
fringed by coral-reefs ; coloured red. From the description given in 
Owen's "Narr." (vol. i., p. 162), the shore from Mozambique to Delagoa 
Bay appears to be low and sandy; many of the shoals and islets 
off this line of coast are. of coral-formation; but from their small size 
and lowness, it is not possible, from the charts, to know whether they 
are truly fringed. Hence this portion of coast is left uncoloured, as are 
likewise those parts more northward, of which no mention has been 
made in the foregoing pages from the want of information. 

Persian Gulf. — From the charts lately published on a large scale 
by the East India Company, it appears that several parts, especially 
the southern shores of this gulf, are fringed by coral-reefs ; but as the 
water is very shallow, and as there are numerous sandbanks, which are 
difficult to distinguish on the chart from reefs, I have not coloured the 
upper part red. Towards the mouth, however, where the water is rather 
deeper, the islands of Onmiz and Larrack, appear so regularly fringed, 
that I have coloured them red. There are certainly no atolls in the 
Persian Gulf. The shores of Immatmi, and of the promontory forming 
the southern headland of the Persian Gulf, seem to be without reefs. 
The whole S.W. part (except one or two small patches) of Arabia Felix, 
and the shores oiSocotra appear from the charts and memoir of Captain 
Haines {Geograph. Journ., 1839, p. 125) to be without any reefs. I 
believe there are no extensive coral-reefs on any part of the coasts of 
hidia, except on the low promontory oi Madura (as already mentioned) 
in front of Ceylon. 

Red Sea. — My information is thiefly derived from the admirable 
charts pubhshed by the East India Company in 1836, from personal 
communication with Captain Moresby, one of the surveyors, and from 
the excellent memoir, " Uber die Niitur der Corallen-Banken des Rothen 
Meeres," by Ehrenberg. The plains immediately bordering the Red 
Sea seem chiefly to consist of a sedimentary formation of the newer 
tertiary period. The shore is, with the exception of a few parts, fringed 
by coral-reefs. The water is generally profoundly deep close to the 
shore ; but this fact, which has attracted the attention of most voyagers, 
seems to have no necessary connection with the presence of reefs ; for 
Captain Moresby particularly observed to me, that, in lat. 24° 10' on the 
eastern side, there is a piece of coast, with very deep water close to it, 
without any reefs, but not differing in other respects from the usual 
nature of the coast-line. The most remarkable feature in the Red Sea 



144 APPENDIX, 

is the chain of submerged banks, reefs, and islands, lying some way 
from the shore, chiefly on the eastern side ; the space within being deep 
enough to admit a safe navigation in small vessels. The banks are 
generally of an oval form, and some miles in width ; but some of them 
are very long in proportion to their width. Captain Moresby informs 
me that any one, who had not made actual plans of them, would be apt 
to think that they were much more elongated than they really are. 
Many of them rise to the surface, but the greater number lie from five 
to thirty fathoms beneath it, with irregular soundings on them. They 
consist of sand and living coral ; coral on most of them, according to 
Captain Moresby, covering the greater part of their surface. They 
extend parallel to the shore, and they are not unfrequently connected 
in their middle parts by short transverse banks with the mainland. 
The sea is generally profoundly deep quite close to them, as it is near 
most parts of the coast of the mainland; but this is not universally the 
case, for between lat. 1 5° and 1 7° the water deepens quite gradually 
from the banks, both on the eastern and western shores, towards the 
middle of the sea. Islands in many parts arise from these banks ; they 
are low, flat-topped, and consist of the same horizontally stratified 
formation with that forming the plain-like margin of the mainland. 
Some of the smaller and lower islands consist of mere sand. Captain 
Moresby informs me, that small masses of rock, the remnants of islands, 
are left on many banks where there is now no dry land. Ehrcnberg 
also asserts that most of the islets, even the lowest, have a flat abraded 
basis, composed of the same tertiary formation : he believes that as 
soon as the surf wears down the protuberant parts of a bank, just 
beneath the level of the sea, the surface becomes protected from 
further abrasion by the growth of coral, and he thus accounts for the 
existence of so many banks standing on a level with the surface of 
this sea. It appears that most of the islands are certainly decreasing 
in size. 

The form of the banks and islands is most singular in the part just 
referred to, namely, from lat. 15" to 17°, where the sea deepens quite 
gradually : the Dhalac group, on the western coast, is surrounded by 
an intricate archipelago of islets and shoals ; the main island is very 
irregularly shaped, and it includes a bay seven miles long, by four 
across, in which no bottom was found with 252 feet: there is only one 
entrance into this bay, half a mile wide, and with an island in front of 
it. The submerged banks on the eastern coast, within the same lati- 
tudes, round Farsa?i Isld,, are, likewise, penetrated by many narrow 
creeks of deep water ; one is twelve miles long, in the form of a hatchet, 
in which, close to its broad upper end, soundings were not struck with 
360 feet, and its entrance is only half a mile wide : in another creek of 
the same nature, but even with a more irregular outline, there was no 
bottom with 480 feet. The island of Farsan, itself, has as singular a 
form as any of its surrounding banks. The bottom of the sea round the 
Dhalac and Farsan Islands consists chiefly of sand and agglutinated 
fragments, but, in the deep and narrow creeks, it consists of mud ; the 
islands themselves consist of thin, horizontally stratified, modern tertiary 



RED SEA. 145 

beds, containing but little broken coral,* their shores are fringed by 
living coral-reefs. 

From the account given by Riippell f of the manner in which Dhalac 
has been rent by fissures, the opposite sides of which have been 
unequally elevated (in one instance to the amount of fifty feet), it seems 
probable that its irregular form, as well as probably that of Farsan, may 
have been partly caused by unequal elevations ; but, considering the 
general form of the banks, and of the deep-water creeks, together with 
the composition of the land, I think their configuration is more probably 
due in great part to strong currents having drifted sediment over an 
uneven bottom : it is almost certain that their form cannot be attributed 
to the growth of coral. Whatever may have been the precise origin of 
the Dhalac and Farsan Archipelagoes, the greater number of the banks 
on the eastern side of the Red Sea seem to have originated through 
nearly similar means. I judge of this from their similarity in configura- 
tion (in proof of which I may instance a bank on the east coast in 
lat. 22° ; and although it is true that the northern banks generally have 
a less complicated outline), and from their similarity in composition, as 
may be observed in their upraised portions. The depth within the 
banks northward of lat. 17°, is usually greater, and their outer sides 
shelve more abruptly (circumstances which seem to go together) than in 
the Dhalac and Farsan Archipelagoes ; but this might easily have been 
caused by a difference in the action of the currents during their forma- 
tion : moreover, the greater quantity of living coral, which, according to 
Captain Moresby, exists on the northern banks, would tend to give 
them steeper margins. 

From this account, brief and imperfect as it is, we can see that the 
great chain of banks on the eastern coast, and on the western side in 
the southern portion, differ greatly from true barrier-reefs wholly formed 
by the growth of coral. It is indeed the direct conclusion of Ehrenberg 
("Ober die," etc., pp. 45 and 51), that they are connected in their origin 
quite secondarily with the growth of coral ; and he remarks that tlie 
islands off the coast of Norway, if worn down level with the sea, and 
merely coated with living coral, would present a nearly similar appear- 
ance. I cannot, however, avoid suspecting, from information given me 
by Dr. Malcolmson and Captain Moresby, that Ehrenberg has rather 
under-rated the influence of corals, in some places at least, on the 
formation of the tertiary deposits of the Red Sea. 

The west coast of the Red Sea between lat. 19° and 22°. — There are, 
in this space, reefs, which, if I had known notliing of those in other 
parts of the Red Sea, I should unhesitatingly have considered as 
barrier-reefs; and, after deliberation, I have come to the same con- 
clusion. One of these reefs, in 20« 1 5', is twenty miles long, less than a 
mile in width (but expanding at the northern end into a disc), slightly 
sinuous, and extending parallel to the mainland at the distance of five 
miles from it, with very deep water within ; in one spot soundings were 
not obtained with 205 fathoms. Some leagues further south, there is 
another linear reef, very narrow, ten miles long, with other small portions 
♦ Riippell, " Rcisc in Abyssinic," Band, i., S. 247. f Ibid., S. 245. 

10 



146 APPENDIX. 

of reef, north and south, ahnost connected with it ; and within this Hne 
of reefs (as well as outside) the water is profoundly deep. There are 
also some small linear and sickle-formed reefs, lying a little way out at 
sea. All these reefs are covered, as I am informed by Captain Moresby, 
by living corals. Here, then, we have all the characters of reefs of the 
barrier class ; and in some outlying reefs we have an approach to the 
structure of atolls. The source of my doubts about the classification of 
these reefs, arises from having observed in the Dhalac and Farsan 
groups the narrowness and straightness of several spits of sand and 
rock: one of these spits in the Dhalac group is nearly fifteen miles 
long, only two broad, and it is bordered on each side with deep water ; 
so that, if worn down by the surf, and coated with living corals, it would 
form a reef nearly similar to those within the space under consideration. 
There is, also, in this space (lat. 21°) a peninsula, bordered by cliffs, with 
its extremity worn down to the level of the sea, and its basis fringed 
with reefs : in the line of prolongation of this peninsula, there lies the 
island of Macowa (formed, according to Captain Moresby, of the usual 
tertiary deposit), and some smaller islands, large parts of which likewise 
appear to have been worn down, and are now coated with living corals. 
If the removal of the strata in these several cases had been more com- 
plete, the reefs thus formed would have nearly resembled those barrier- 
like ones now under discussion. Notwithstanding these facts, I cannot 
persuade myself that the many very small, isolated, and sickle-formed 
reefs and others, long, nearly straight, and very narrow, with the water 
unfathomably deep close round them, could possibly have been formed 
by corals merely coating banks of sediment, or the abraded surfaces of 
irregularly shaped islands. I feel compelled to believe that the founda- 
tions of these reels have subsided, and that the corals, during their 
upward growth, have given to these reefs their present forms : I may 
remark that the subsidence of narrow and irregularly-shaped peninsulas 
and islands, such as those existing on the coasts of the Red Sea, would 
afford the requisite foundations for the reefs in question. 

The west coast from lat. 22° to 24°. — This part of the coast (north of 
the space coloured blue on the map) is fronted by an irregularly shelving 
bank, from about ten to thirty fathoms deep ; numerous little reefs, 
some of which have the most singular shapes, rise from this bank. It 
may be observed, respecting one of them, in lat. 23° 10', that if the 
promontory in lat. 24° were worn down to the level of the sea, and 
coated with corals, a very similar and grotesquely formed reef would be 
produced. Many of the reefs on this part of the coast may thus have 
originated ; but there are some sickle, and almost atoll-formed reefs 
lying in deep water off the promontory in lat. 24°, which lead me to 
suppose that all these reels are more probably allied to the barrier or 
atoll classes. I have not, however, ventured to colour this portion of 
coast. On the west coast from lat. 19° to 17° (south of space coloured 
blue on the map), there are many low islets of very small dimensions, 
not much elongated, and rising out of great depths at a distance from 
the coast; these cannot be classed cither with atolls, or barrier- or 
fringing-reefs. I may here remark that the outlying reefs on the west 



WEST INDIES. 147 

coast, between lat. 19° and 24"", are the only ones in the Red Sea, which 
approach in structure to the true atolls of the Indian and Pacific Oceans, 
but they present only imperfect miniature likenesses of them. 

Eastern coast. — I have felt the greatest doubt about colouring any 
portion of this coast, north of the fringing-reefs round the Farsan Islands 
in 16° 10'. There are many small outlying coral-reefs along the whole 
line of coast ; but as the greater number rise from banks not very deeply 
submerged (the formation of which has been shown to be only 
secondarily connected with the growth of coral), their origin may be due 
simply to the growth of knolls of corals, from an irregular foundation 
situated v^ithin a limited depth. But between lat. 18° and 20°, there are 
so many linear, elhptic, and extremely small reefs, rising abruptly out of 
profound depths, that the same reasons, which led me to colour blue a 
portion of the west coast, have induced me to do the same in this part. 
There exist some small outlying reefs rising from deep water, north of 
lat. 20° (the northern limit coloured blue), on the east coast ; but as 
they are not very numerous and scarcely any of them linear, I have 
thought it right to leave them uncoloured. 

In the southern parts of the Red Sea, considerable spaces of the main- 
land, and of some of the Dhalac islands, are skirted by reefs, which, as 
I am informed by Captain Moresby, are of living coral, and have all the 
characters of the fringing class. As in these latitudes, there are no 
outlying linear or sickle-formed reefs, rising out of unfathomable depths, 
I have coloured these parts of the coast red. On similar grounds, 1 have 
coloured red the northe7'7i parts of the wester}! coast {viox\h of lat. 24° 30'), 
and likewise the shores of the chief part of the Gulf of Suez. In the 
Gulf of Acaba, as I am informed by Captain Moresby there are no 
coral-reefs, and the water is profoundly deep. 

West Indies. — My information regarding the reefs of this area, is 
derived from various sources, and from an examination of numerous 
charts ; especially of those lately executed during the survey under 
Captain Owen, R.N. I lay under particular obligation to Captain Bird 
Allen, R.N., one of the members of the late survey, for many personal 
communications on this subject. As in the case of the Red Sea, it is 
necessary to make some preliminary remarks on the submerged banks 
of the West Indies, which are in some degree connected with coral- 
reefs, and cause considerable doubts in their classification. That 
large accumulations of sediment are in progress on the West Indian 
shores, will be evident to any one who examines the charts of that sea, 
especially of the portion north of a line joining Yucutan and Florida. 
The area of deposition seems less intimately connected with the 
debouchement of the great rivers, than with the course of the sea- 
currents ; as is evident from the vast extension of the banks from the 
promontories of Yucutan and Mosquito. 

Besides the coast-banks, there are many of various dimensions which 
stand quite isolated ; these closely resemble each other ; they lie from 
two or three to twenty or thirty fathoms under water, and are composed 
of sand, sometimes firmly agglutinated, with little or no coral; their 
surfaces are smooth and nearly levd, shelving only to the amount of a 



148 APPENDIX, 

few fathoms, very gradually all round towards their edges, where they 
plunge abruptly into the unfathomable sea. This steep inclination of 
their sides, which is likewise characteristic of the coast-banks, is very 
remarkable : I may give as an instance, the Misteriosa Bank, on the 
edges of which the soundings change in 250 fathoms horizontal distance, 
from II to 210 fathoms; off the northern point of the bank of Old 
Providence, in 200 fathoms horizontal distance, the change is from 
19 to 152 fathoms; off the Great Bahama Bank, in 160 fathoms 
horizontal distance, the inclination is in many places from 10 fatlioms 
to no bottom with 190 fathoms. On coasts in all parts of the world, 
where sediment is accumulating, something of this kind may be ob- 
served ; the banks shelve very gently far out to sea, and then terminate 
abruptly. The form and composition of the banks standing in the 
middle parts of the W. Indian Sea, clearly show that their origin must 
be chiefly attributed to the accumulation of sediment ; and the only 
obvious explanation of their isolated position is the presence of a 
nucleus, round which the currents have collected fine drift matter. 
Any one who will compare the character of the bank surrounding the 
hilly island of Old Providence, with those banks in its neighbourhood 
which stand isolated, will scarcely doubt that they surround submerged 
mountains. We are led to the same conclusion by examining the 
bank called Thunder Knoll, which is separated from the Great Mosquito 
Bank by a channel only seven miles wide, and 145 fathoms deep. 
There cannot be any doubt that the Mosquito Bank has been formed 
by the accumulation of sediment round the promontory of the same 
name ; and Thunder Knoll resembles the Mosquito Bank, in the state 
of its surface submerged twenty fathoms, in the inclinations of its sides, 
in composition, and in every other respect. I may observe, although 
the remark is here irrelevant, that geologists should be cautious in 
concluding that all the outlyers of any formation have once been 
connected together, for we here see that deposits, doubtless of exactly 
the same nature, may be deposited with large valley-like spaces 
between them. 

Linear strips of coral-reefs and small knolls project from many of the 
isolated, as well as coast-banks ; sometimes they occur quite irregularly 
placed, as on the Mosquito Bank, but more generally they form cres- 
cents on the windward side, situated some little distance within the 
outer edge of the banks : — thus on the Serranilla Bank they form an 
interrupted chain which ranges between two and three miles within the 
windward margin : generally they occur, as on Roncador, Courtown, 
and Anegada Banks, nearer the line of deep water. Their occurrence 
on the windward side is conformable to the general rule, of the effi- 
cient kinds of corals flourishing best where most exposed ; but their 
position some way within the line of deep water I cannot explain, 
without it be, that a depth somewhat less than that close to the outer 
margin of the banks, is most favourable to their growth. Where the 
corals have formed a nearly continuous rim, close to the windward 
edge of a bank some fathoms submerged, the reef closely resembles an 
'atoll ; but if the bank surrounds an island (as in the case of Old Pro- 



tVEST INDIES. 149 

vidence), the reef resembles an encircling barriei--teef. I should 
undoubtedly have classed some of these fringed banks as imperfect 
atolls, or barrier-reefs, if the sedimentary nature of their foundations 
had not been evident from the presence of other neighbouring banks, 
of similar forms and of similar composition, but without the crescent- 
like marginal reef : in the third chapter, I observed that probably some 
atoll-like reefs did exist, which had originated in the manner here 
supposed 

Proofs of elevation within recent tertiary periods abound, as referred 
to in the sixth chapter, over nearly the whole area of the West Indies. 
Hence it is easy to understand the origin of the low land on the coasts, 
where sediment is now accumulating ; for instance on the northern 
part of Yucutan, and on the N.E. part of Mosquito, where the land is 
low, and where extensive banks appear to be in progressive formation. 
Hence, also, the origin of the Great Bahama Banks, which are bordered 
on their western and southern edges by very narrow, long, singularly 
shaped islands, formed of sand, shells, and coral-rock, and some of 
them about a hundred feet in height, is easily explained by the eleva- 
tion of banks fringed on their windward (western and southern) sides by 
coral-reefs. On this view, however, we must suppose either that the 
chief part of the surfaces of the great Bahama sandbanks were all 
originally deeply submerged, and were brought up to their present 
level by the same elevatory action, which formed the linear islands ; 
or that during the elevation of the banks, the superficial currents and 
swell of the waves continued wearing them down and keeping them 
at a nearly uniform level : the level is not quite uniform ; for, in 
proceeding from the N.W. end of the Bahama group towards the S.E. 
end, the depth of the banks increases, and the area of land decreases, 
in a very gradual and remarkable manner. The latter view, namely, 
that these banks have been worn down by the currents and swell dur- 
ing their elevation, seems to me the most probable one. It is, also, I 
believe, applicable to many banks, situated in widely distant parts of 
the West Indian Sea, w4iich are wholly submerged ; for, on any other 
view, we must suppose, that the elevatory forces have acted with 
astonishing uniformity. 

The shores of the Gulf of Mexico, for the space of many hundred 
miles, is formed by a chain of lagoons, from one to twenty miles in 
breadth (" Columbian Navigator," p. 178, etc.), containing either fresh 
or salt water, and separated from the sea by linear strips of sand. 
Great spaces of the shores of Southern Brazil,* and of the United 
States from Long Island (as observed by Professor Rogers) to Florida 
have the same character. Professor Rogers, in his " Report to the 
British Association " (vol. iii., p. 13), speculates on the origin of these 
low, sandy, linear islets ; he states that the layers of which they are 
composed are too homogeneous, and contain too large a proportion of 

* In the London and Edinhirgh Philosophical Journal, 1841, p. 257, I have 
described a singular bar of sandstone lying parallel to the coast oft" Pcrnam- 
buco in Biazil, which probably is an analogous formation. 



150 APPENDIX. 

shells, to permit the common supposition of their formation being 
simply due to matter thrown up, where it now lies, by the surf: he 
considers these islands as upheaved bars or shoals, which were deposited 
in lines where opposed currents met. It is evident that these islands 
and spits of sand parallel to the coast, and separated from it by shallow 
lagoons, have no necessary connection with coral-formations. But 
in Southern Florida, from the accounts I have received from persons 
who have resided there, the upraised islands seem to be formed of 
strata, containing a good deal of coral, and they are extensively fringed 
by living reefs ; the channels within these islands are in some places 
between two and three miles wide, and five or six fathoms deep, though 
generally * they are less in depth than width. After having seen how 
frequently banks of sediment in the West Indian Sea are fringed by 
reefs, we can readily conceive that bars of sediment might be greatly 
aided in their formation along a line of coast, by the growth of corals ; 
and such bars would, in that case, have a deceptive resemblance with 
true barrier-reefs. 

Having now endeavoured to remove some sources of doubt in classi- 
fying the reefs of the West Indies, I will give my authorities for colour- 
ing such portions of the coast as I have thought myself warranted in 
doing. Captain Bird Allen informs me, that most of the islands on the 
Bahama Banks are fringed, especially on their windward sides, with 
living reefs ; and hence I have coloured those, which are thus repre- 
sented in Captain Owen's late chart, red. The same officer informs me, 
that the islands along the southern part of Florida are similarly fringed ; 
coloured red. Cuba : Proceeding along the northern coast, at the dis- 
tance of forty miles from the extreme S.E. point, the shores are fringed by 
reefs, which extend westward for a space of i6o miles, with only a few 
breaks. Parts of these reefs are represented in the plans of the harbours 
on this coast by Captain Owen ; and an excellent description is given of 
them by Mr. Taylor (Loudon's " Mag. of Nat. Hist.," vol. ix., p. 449) ; he 
states that they enclosed a space called the '' baxo" from half to three- 
quarters of a mile in width, with a sandy bottom, and a little coral. In 
most parts people can wade, at low water, to the reef; but in some 
parts the depth is between two and three fathoms. Close outside the 
reef, the depth is between six and seven fathoms ; these well-charac- 
terised fringing-reefs are coloured red. Westward of long. 77° 30', on the 
northern side of Cuba, a great bank commences, which extends along 
the coast for nearly four degrees of longitude. In the place of its 
commencement, in its structure, and in the ^* cays" or low islands on 
its edge, there is a marked correspondence (as observed by Humboldt, 
" Pers. Narr.," vol. vii., p. 88) between it and the Great Bahama and Sal 
Banks, which lie directly in front. Hence one is led to attribute the 
same origin to both these sets of banks ; namely, the accumulation of 
sediment, conjoined with an elevatory movement, and the growth of 

* In the ordinary sea-charts, no lagoons appear on the coast of Florida, 
north of 26° ; but Major Whiting {SiUiinan''s Joiirncd, vol. xxxv., p. 54) says 
that many are formed by sand thrown up along the whole fine of coast from 
St. Augustine's to Jupiter Inlet. 



WEST INDIES. 151 

coral on their outward edges ; those parts which appear fringed by 
living reefs are coloured red. Westward of these banks, there is a 
portion of coast apparently without reefs, except in the harbours, the 
shores of which seem in the published plans to be fringed. The 
Colorado Shoals (see Captain Owen's charts), and the low land at the 
western end of Cuba, correspond as closely in relative position and 
structure to the banks at the extreme point of Florida, as the banks 
above described on the north side of Cuba, do to the Bahamas. The 
depth within the islets and reefs on the outer edge of the Colorados^ is 
generally between two and three fathoms, increasing to twelve fathoms 
in the southern part, where the bank becomes nearly open, without 
islets or coral-reefs ; the portions which are fringed are coloured red. 
The southern shore of Cuba is deeply concave, and the included space 
is filled up with mud and sandbanks, low islands and coral-reefs. 
Between the mountainous Isle of Pines and the southern shore of Cuba, 
the general depth is only between two and three fathoms ; and in this 
part small islands, formed of fragmentary rock and broken madrepores 
(Humboldt, " Pers. Narr.," vol. vii., pp. 51, 86 to 90, 291, 309, 320), rise 
abruptly, and just reach the surface of the sea. From some expressions 
used in the " Columbian Navigator" (vol. i., pt. ii., p. 94), it appears that 
considerable spaces along the outer coast of Southern Cuba are bounded 
by cliffs of coral-rock, formed probably by the upheaval of coral-reefs 
and sandbanks. The charts represent the southern part of the Isle of 
Pines as fringed by reefs, which the " Columb. Navig." says extend 
some way from the coast, but have only from nine to twelve feet water 
on them ; these are coloured red. — I have not been able to procure any 
detailed description of the large groups of banks and "cays" further 
eastward on the southern side of Cuba ; within them there is a large 
expanse, with a muddy bottom, from eight to twelve fathoms deep ; 
although some parts of this line of coast are represented in the general 
charts of the West Indies, as fringed, I have not thought it prudent to 
colour them. The remaining portion of the south coast of Cuba appears 
to be without coral-reefs. 

YucuTAN. — The N.E. part of the promontory appears in Captain 
Owen's charts to be fringed ; coloured red. The eastern coast, from 20" 
to 1 8° is fringed. South of lat. 1 8°, there commences the most remarkable 
reef in the West Indies : it is about one hundred and thirty miles in 
length, ranging in a N. and S. line, at an average distance of fifteen miles 
from the coast. The islets on it are all low, as I have been informed 
by Captain B. Allen ; the water deepens suddenly on the outside of the 
reef, but not more abruptly than off many of the sedimentary banks : 
within its southern extremity (off Ho?tduras) the depth is twenty-five 
fathoms ; but in the more northern parts, the depth soon increases to ten 
fathoms, and within the northernmost part, for a space of twenty miles, 
the depth is only from one to two fathoms. In most of these respects 
we have the characteristics of a barrier-reef ; nevertheless, from observ- 
ing, first, that the channel within the reef is a continuation of a great 
irregular bay, which penetrates the mainland to the depth of fifty miles ; 
and secondly, that considerable spaces of this barrier-like reef are 



152 APPENDIX. 

described in the charts (for instance, inlat. i6° 45' and 16^ i2') as formed 
of pure sand ; and thirdly, from knowing that sediment is accumulating 
in many parts of the West Indies in banks parallel to the shore ; I have 
not ventured to colour this reef as a barrier, without further evidence 
that it has really been formed by the growth of corals, and that it is not 
merely in parts a spit of sand, and in other parts a worn-down promon- 
tory, partially coated and fringed by reefs ; I lean, however, to the 
probability of its being a barrier-reef, produced by subsidence. To add 
to my doubts, immediately on the outside of this barrier-like reef, 
Ttirneffe, Lighthouse^ and Glover reefs are situated, and these reefs 
have so completely the form of atolls, that if they had occurred in the 
Pacific, I should not have hesitated about colouring them blue. Tiirneffe 
Reef SGQms, almost entirely filled up with low mud islets ; and the depth 
wilhin the other two reefs is only from one to three fathoms. From 
this circumstance and from their similarity in form, structure, and relative 
position, both to the bank called Northern Ttianglcs, on which there is 
an islet between seventy and eighty feet, and to Cozuviel Island, the 
level surface of which is likewise between seventy and eighty feet in 
height, I consider it more probable that the three foregoing banks are 
the worn-down bases of upheaved shoals, fringed with corals, than that 
they are true atolls, wholly produced by the growth of coral during 
subsidence ; left uncoloured. 

In front of the eastern Mosquito coast, there are between lat, 12° and 
16° some extensive banks (already mentioned, p. 148), with high islands 
rising from their centres ; and there are other banks wholly submerged, 
both of which kinds of banks are bordered, near their windward margins, 
by crescent-shaped coral-reefs. But it can hardly be doubted, as was 
observed in the preliminary remarks, that these banks owe their origin, 
like the great bank extending from the Mosquito promontory, almost 
entirely to the accumulation of sediment, and not to the growth of 
corals ; hence I have not coloured them. 

Cay77ian Island: this island appears in the charts to be fringed ; and 
Captain B. Allen informs me that the reefs extend about a mile from 
the shore, and have only from five to twelve feet water within them ; 
coloured red. — Jamaica: judging from the charts, about fifteen miles of 
the S.E. extremity, and about twice that length on the S.W. extremity, 
and some portions on the S. side near Kingston and Port Royal, are 
regularly fringed, and therefore are coloured red. From the plans of 
some harbours on the N. side of Jamaica, parts of the coast appear 
to be fringed ; but as these are not represented in the charts of 
the whole island, I have not coloured them. — St. Domingo : I have not 
been able to obtain sufficient information, either from plans of the 
harbours, or from general charts, to enable me to colour any part of the 
coast, except sixty miles from Port de Plata westward, which seems 
very regularly fringed ; many other parts, however, of the coast are 
probably fringed, especially towards the eastern end of the island. — 
Puerto Rico: considerable portions of the southern, western, and 
eastern coasts, and some parts of the northern coast, appear in the 
charts to be fringed; coloured red. — Some miles in length of the 



BERMUDA ISLANDS. 153 

southern side of the Island of St. Thomas is fringed ; most of the Virgin 
Gorda Islands, as I am informed by Mr. Schomburgk, are fringed ; the 
shores of Anegada, as well as the bank on which it stands, are likewise 
fringed ; these islands have been coloured red. The greater part of 
the southern side of Santa Cruz appears in the Danish survey to be 
fringed (see also Prof. Hovey's account of this island, in Silhman's 
Jou7'nal, vol. xxxv., p. 74) ; the reefs extend along the shore for a con- 
siderable space, and project rather more than a mile ; the depth within 
the reef is three fathoms ; coloured red. — The Antilles, as remarked 
by Von Buch (" Descrip. lies Canaries," p. 494), may be divided into 
two linear groups, the western row being volcanic, and the eastern of 
modern calcareous origin ; my information is very defective on the whole 
group. Of the eastern islands, Barbuda and the western coasts of 
Antigua and Mariagalante appear to be fringed : this is also the case 
with Barhadoes, as I have been inforn^ed by a resident ; these islands 
are coloured red. On the shores of the Western Antilles, of volcanic 
origin, very few coral-reefs appear to exist. The island of Martinique, 
of which there are beautifully executed French charts, on a very large 
scale, alone presents any appearance worthy of special notice. The 
south-w^estern, southern, and eastern coasts, together forming about 
half the circumference of the island, are skirted by very irregular banks, 
projecting generally rather less than a mile from the shore, and lying 
from two to five fathoms submerged. In front of almost every valley, 
they are breached by narrow, crooked, steep-sided passages. The 
French engineers ascertained by boring, that these submerged banks 
consisted of madreporitic rocks, which were covered in many parts by 
thin layers of mud or sand. From this fact, and especially from the 
structure of the narrow breaches, I think there can be little doubt that 
these banks once formed living reefs, which fringed the shores of the 
island, and like other reefs probably reached the surface. From some 
of these submerged banks reefs of living coral rise abruptly, either in 
small detached patches, or in lines parallel to, but some way within the 
outer edges of the banks on which they are based. Besides the above 
banks which skirt the shores of the island, there is on the eastern side 
a range of linear banks, similarly constituted, twenty miles in length, 
extending parallel to the coast line, and separated from it by a space 
between two and four miles in width, and from five to fifteen fathoms 
in depth. From this range of detached banks, some linear reefs of 
living coral likewise rise abruptly; and if they had been of greater 
length (for they do not front more than a sixth part of the circumference 
of the island), they would necessarily from their position have been 
coloured as barrier-reefs ; as the case stands they are left uncoloured. 
I suspect that after a small amount of subsidence, the corals were killed 
by sand and mud being deposited on them, and the reefs being thus 
prevented from growing upwards, the banks of madreporitic rock were 
left in their present submerged condition. 

The Bermuda Islands have been carefully described by Lieutenant 
Nelson, in an excellent Memoir in the " Geol. Transactions " (vol. v., 
part i., p. 103). In the form of the bank or reef, on one side of which 



154 APPENDIX. 

the islands stand, there is a close general resemblance to an atoll ; but 
in the following respects there is a considerable difference, — first, in the 
margin of the reef not forming (as I have been informed by Mr. Chaffers, 
R.N.) a flat, solid surface, laid bare at low water, and regularly bounding 
the internal space of shallow water or lagoon ; secondly, in the border 
of gradually shoaling water, nearly a mile and a half in width, which 
surrounds the entire outside of the reef (as is laid down in Captain 
Hurd's chart) ; and thirdly, in the size, height, and extraordinary form 
of the islands, which present little resemblance to the long, narrow, 
simple islets, seldom exceeding half a mile in breadth, which surmount 
the annular reefs of almost all the atolls in the Indian and Pacific 
Oceans. Moreover, there are evident proofs (Nelson, Ibid., p. ii8), 
that islands similar to the existing ones, formerly extended over other 
parts of the reef. It would, I believe, be difficult to find a true atoll 
with land exceeding thirty feet in height; whereas, Mr. Nelson estimates 
the highest point of the Bermuda islands to be 260 feet ; if, however, 
Mr. Nelson's view, that the whole of the land consists of sand drifted by 
the winds, and agglutinated together, were proved correct, this differ- 
ence would be immaterial; but, from his own account (p. 118), there 
occur in one place, five or six layers of red earth, interstratified with 
the ordinary calcareous rock, and including stones too heavy for the 
wind to have moved, without having at the same time utterly dispersed 
every grain of the accompanying drifted matter. Mr. Nelson attributes 
the origin of these several layers, with their embedded stones, to as 
many violent catastrophes ; but further investigation in such cases has 
generally succeeded in explaining phenomena of this kind by ordinary 
and simpler means. Finally, I may remark, that these islands have a 
considerable resemblance in shape to Barbuda in the West Indies, and 
to Pemba on the eastern coast of Africa, which latter island is about two 
hundred feet in height, and consists of coral-rock. I beheve that the 
Bermuda Islands, from being fringed by living reefs, ought to have been 
coloured red ; but I have left them uncoloured, on account of their 
general resemblance in external form to a lagoon-island or atoll. 



GEOLOGICAL OBSERVATIONS 



VOLCANIC ISLANDS. 



CRITICAL INTRODUCTION. 



THE preparation of the series of works published under the 
general title " Geology of the Voyage of the Beagle " occu- 
pied a great part of Darwin's time during the ten years that 
followed his return to England. The second volume of the 
series, entitled " Geological Observations on Volcanic Islands, 
with Brief Notices on the Geology of Australia and the Cape of 
Good Hope," made its appearance in 1844. The materials for 
this volume were collected in part during the outward voyage, 
when the Beagle called at St. Jago in the Cape de Verde Islands, 
and St. Paul's Rocks, and at Fernando Noronha, but mainly 
during the homeward cruise ; then it was that the Galapagos 
Islands were surveyed, the Low Archipelago passed through, and 
Tahiti visited ; after making calls at the Bay of Islands, in New 
Zealand, and also at Sydney, Hobart Town and King George's 
Sound in Australia, the Beagle sailed across the Indian Ocean to 
the little group of the Keeling or Cocos Islands, which Darwin 
has rendered famous by his observations, and thence to Mauritius ; 
calling at the Cape of Good Hope on her way, the ship then 
proceeded successively to St. Helena and Ascension, and re- 
visited the Cape de Verde Islands before finally reaching England. 
Although Darwin was thus able to gratify his curiosity by 
visits to a great number of very interesting volcanic districts, the 
voyage opened for him with a bitter disappointment. He had 
been reading Humboldt's "Personal Narrative" during his last 
year's residence in Cambridge, and had copied out from it long 
passages about Teneriffe. He was actually making inquiries as 
to the best means of visiting that island, when the offer was made 
to him to accompany Captain Fitzroy in the Beagle. His friend 
Henslow too, on parting with him, had given him the advice to 
procure and read the recently published first volume of the 



158 CRITICAL INTRODUCTION, 

" Principles of Geology," though he warned him against accepting 
the views advocated by its author. During the time the Beagle 
was beating backwards and forwards when the voyage commenced, 
Darwin, although hardly ever able to leave his berth, was employ- 
ing all the opportunities which the terrible sea-sickness left him, 
in studying Humboldt and Lyell. We may therefore form an 
idea of his feelings when, on the ship reaching Santa Cruz, and 
the Peak of Teneriffe making its appearance among the clouds, 
they were suddenly informed that an outbreak of cholera would 
prevent any landing ! 

Ample compensation for this disappointment was found, how- 
ever, when the ship reached Porta Praya in St. Jago, the largest 
of the Cape de Verde Islands. Here he spent three most delight- 
ful weeks, and really commenced his work as a geologist and 
naturalist. Writing to his father he says, " Geologising in a 
volcanic country is most delightful ; besides the interest attached 
to itself, it leads you into most beautiful and retired spots. 
Nobody but a person fond of Natural History can imagine the 
pleasure of strolling under cocoa-nuts in a thicket of bananas and 
coffee-plants, and an endless number of wild flowers. And this 
island, that has given me so much instruction and delight, is 
reckoned the most uninteresting place that we perhaps shall touch 
at during our voyage. It certainly is generally very barren, but 
the valleys are more exquisitely beautiful, from the very contrast. 
It is utterly useless to say anything about the scenery ; it would 
be as profitable to explain to a blind man colours, as to a person 
who has not been out of Europe, the total dissimilarity of a 
tropical view. Whenever I enjoy anything, I always look forward 
to writing it down, either in my log-book (which increases in 
bulk), or in a letter ; so you must excuse raptures, and those 
raptures badly expressed. I find my collections are increasing 
wonderfully, and from Rio I think I shall be obliged to send a 
cargo home." 

The indelible impression made on Darwin's mind by this first 
visit to a volcanic island, is borne witness to by a remarkable 
passage in the " Autobiography" written by him in 1876. "The 
geology of St. Jago is very striking, yet simple ; a stream of lava 
formerly flowed over the bed of the sea, formed of triturated recent 
shells and corals, which it has baked into a hard white rock. 
Since then the whole island has been upheaved. But the line of 
white rock revealed to me a new and important fact, namely that 
there had been afterwards subsidence round the craters which had 



CRITICAL INTRODUCTION. 159 

since been in action, and had poured forth lava. It then first 
dawned on me that I might perhaps write a book on the geology 
of the various countries visited, and this made me thrill with 
delight. That was a memorable hour to me, and how distinctly 
I can call to mind the low cliff of lava beneath which I rested, 
with the sun glaring hot, a few strange desert plants growing near 
and with living corals in the tidal pools at my feet." 

Only five years before, when listening to poor Professor Jame- 
son's lectures on the effete Wernerianism, which at that time did 
duty for geological teaching, Darwin had found them " incredibly 
dull," and he declared that " the sole effect they produced on me 
was a determination never so long as I lived to read a book on 
Geology, or in any way to study the science." 

What a contrast we find in the expressions which he makes use 
of in referring to Geological Science, in his letters written home 
from the Beagle ! After alluding to the delight of collecting and 
studying marine animals, he exclaims, " But Geology carries the 
day ! " Writing to Henslow he says, " I am quite charmed with 
Geology, but, like the wise animal betw^een two bundles of hay, 
I do not know which to like best ; the old crystalline group of 
rocks, or the softer and more fossiliferous beds." And just as the 
long voyage is about to come to a close he again writes, " I find, 
in Geology a never-failing interest ; as it has been remarked, it 
creates the same grand ideas respecting this world w^hich Astronomy 
does for the Universe." In this passage Darwin doubtless refers 
to a remark of Sir John Herschel's in his admirable " Preliminary 
Discourse on the Study of Natural Philosophy," — a book which 
exercised a most remarkable and beneficial influence on the mind 
of the young naturalist. 

If there cannot be any doubt as to the strong predilection in 
Darwin's mind for geological studies, both during and after the 
memorable voyage, there is equally little difficulty in perceiving 
the school of geological thought which, in spite of the warnings of 
Sedgwick and Henslow, had obtained complete ascendency over 
his mind. He writes in 1876: "The very first place which I 
examined, namely St. Jago in the Cape de Verde Islands, showed 
me clearly the wonderful superiority of Lyell's manner of treating 
Geology, compared with that of any other author, whose works I 
had with me, or ever afterwards read." And again, " The science 
of Geology is enormously indebted to Lyell — more so, as I believe, 
than to any other man who ever lived. ... I am proud to 
remember that the first place, namely, St. Jago, in the Cape de 



i6o CRITICAL INTRODUCTION. 

Verde Archipelago, in which I geologised, convinced me of the 
infinite superiority of Lyell's views over those advocated in any 
other work known to me." 

The passages I have cited will serve to show the spirit in which 
Darwin entered upon his geological studies, and the perusal of 
the following pages will furnish abundant proofs of the enthu- 
siasm, acumen, and caution with which his researches were 
pursued. 

Large collections of rocks and minerals were made by Darwin 
during his researches, and sent home to Cambridge, to be kept 
under the care of his faithful friend Henslow. After visiting his 
relations and friends, Darwin's first care on his return to England 
was to unpack and examine these collections. He accordingly, at 
the end of 1836, took lodgings for three months in Fitzwilliam 
Street, Cambridge, so as to be near Henslow ; and in studying 
and determining his geological specimens received much valuable 
aid from the eminent crystallographer and mineralogist. Professor 
William Hallows Miller. 

The actual writing of the volume upon volcanic islands was not 
commenced till 1843, when Darwin had settled in the spot which 
became his home for the rest of his life — the famous house at 
Down, in Kent. Writing to his friend Mr. Fox, on March 28th, 
1843, ^^ says, "I am very slowly progressing with a volume, or 
rather pamphlet, on the volcanic islands which we visited : I 
manage only a couple of hours per day, and that not very regu- 
larly. It is uphill work writing books, which cost money in 
publishing, and which are not read even by geologists." 

The work occupied Darwin during the whole of the year 1843, 
and was issued in the spring of the following year, the actual time 
engaged in preparing it being recorded in his diary as " from the 
summer of 1842 to January 1844;" but the author does not 
appear to have been by any means satisfied with the result when 
the book was finished. He wrote to Lyell, " You have pleased 
me much by saying that you intend looking through my ' Volcanic 
Islands ; ' it cost me eighteen months ! ! ! and I have heard of very 
few who have read it. Now I shall feel, whatever little (and little 
it is) there is confirmatory of old work, or new, will work its effect 
and not be lost." To Sir Joseph Hooker he wrote, " I have just 
finished a little volume on the volcanic islands which we visited. 
I do not know how far you care for dry simple geology, but I hope 
you will let me send you a copy." 

Every geologist knows how full of interest and suggestiveness is 



CRITICAL INTRODUCTION. i6i 

this book of Darwin's on volcanic islands. Probably the scant 
satisfaction which its author seemed to find in it may be traced 
to the effect of a contrast which he felt between the memory of 
glowing delights he had experienced when, hammer in hand, he 
roamed over new and interesting scenes, and the slow, laborious, 
and less congenial task of re-writing and arranging his notes in 
book-form. 

In 1874, in writing an account of the ancient volcanoes of the 
Hebrides, I had frequent occasion to quote Mr. Darwin's observa- 
tions on the Atlantic volcanoes, in illustration of the phenomena 
exhibited by the relics of still older volcanoes in our own islands. 
Darwin, in writing to his old friend Sir Charles Lyell upon the 
subject, says, " I was not a little pleased to see my volcanic book 
quoted, for I thought it was completely dead and forgotten." 

Two years later the original publishers of this book and of that 
on South America proposed to re-issue them. Darwin at first 
hesitated, for he seemed to think there could be little of abiding 
interest in them ; he consulted me upon the subject in one of the 
conversations which I used to have with him at that time, and I 
strongly urged upon him the reprint of the works. I was much 
gratified when he gave way upon the point, and consented to their 
appearing just as originally issued. In his preface he says, 
" Owing to the great progress which Geology has made in recent 
times, my views on some few points may be somewhat antiquated, 
but I have thought it best to leave them as they originally 
appeared." 

It may be interesting to indicate, as briefly as possible, the 
chief geological problem upon which the publication of Darwin's 
" Volcanic Islands " threw new and important light. The merit 
of the work consisted in supplying interesting observations, which 
in some cases have proved of crucial value in exploding prevalent 
fallacies ; in calling attention to phenomena and considerations 
that had been quite overlooked by geologists, but have since 
exercised an important influence in moulding geological specu- 
lation; and lastly in showing the importance which attaches to 
small and seemingly insignificant causes, some of which afford a 
key to the explanation of very curious geological problems. 

Visiting as he did the districts in which Von Buch and others 
had found what they thought to be evidence of the truth of 
" Elevation-craters," Darwin was able to show that the facts were 
capable of a totally different interpretation. The views originally 
put forward by the old German geologist and traveller, and almost 

II 



1 62 CRITICAL INTRODUCTION. 

universally accepted by his countrymen, had met with much 
support from Elie de Beaumont and Dufr(fnoy, the leaders of 
geological thought in France. They were, however, stoutly 
opposed by Scrope and Lyell in this country, and by Constant 
Prevost and Virlet on the other side of the channel. Darwin, in 
the work before us, shows how little ground there is for the 
assumption that the great ring-craters of the Atlantic islands have 
originated in gigantic blisters of the earth's surface which, opening 
at the top, have given origin to the craters. Admitting the in- 
fluence of the injection of lava into the structure of the volcanic 
cones, in increasing their bulk and elevation, he shows that, in 
the main, the volcanoes are built up by repeated ejections causing 
an accumulation of materials around the vent. 

While, however, agreeing on the whole with Scrope and Lyell, 
as to the explosive origin of ordinary volcanic craters, Darwin 
clearly saw that, in some cases, great craters might be formed or 
enlarged, by the subsidence of the floors after eruptions. The 
importance of this agency, to which too little attention has been 
directed by geologists, has recently been shown by Professor Dana, 
in his admirable work on Kilauea and the other great volcanoes 
of the Hawaiian Archipelago. 

The effects of subsidence at a volcanic centre in producing a 
downward dip of the strata around it, was first pointed out by 
Darwin, as the result of his earliest work in the Cape de Verde 
Islands. Striking illustrations of the same principle have since 
been pointed out by M. Robert and others in Iceland, by Mr. 
Heaphy in New Zealand, and by myself in the Western Isles of 
Scotland. 

Darwin again and again called attention to the evidence that 
volcanic vents exhibit relations to one another which can only 
be explained by assuming the existence of lines of fissure in the 
earth's crust, along which the lavas have made their way to the 
surface. But he, at the same time, clearly saw that there was no 
evidence of the occurrence of great deluges of lava along such 
fissures ; he showed how the most remarkable plateaux, composed 
of successive lava sheets, might be built up by repeated and 
moderate ejections from numerous isolated vents; and he ex- 
pressly insists upon the rapidity with which the cinder-cones 
around the orifices of ejection and the evidences of successive 
outflows of lava would be obliterated by denudation. 

One of the most striking parts of the book is that in which he 
deals with the effects of denudation in producing " basal wrecks " 



CRITICAL INTRODUCTION. 163 

or worn down stumps of volcanoes. He was enabled to examine 
a series of cases in which could be traced every gradation, from 
perfect volcanic cones down to the solidified plugs which had 
consolidated in the vents from which ejections had taken place. 
Darwin's observations on these points have been of the greatest 
value and assistance to all who have essayed to study the effects 
of volcanic action during earlier periods of the earth's history. 
Like Lyell, he was firmly persuaded of the continuity of geological 
history, and ever delighted in finding indications, in the present 
order of nature, that the phenomena of the past could be ac- 
counted for by means of causes which are still in operation. 
Lyell's last work in the field was carried on about his home in 
Forfarshire, and only a few months before his death he wrote to 
Darwin : " All the work which I have done has confirmed me in 
the belief that the only difference between Palaeozoic and recent 
volcanic rocks is no more than we must allow for, by the 
enormous time to which the products of the oldest volcanoes have 
been subjected to chemical changes." 

Darwin was greatly impressed, as the result of his studies of 
volcanic phenomena, followed by an examination of the great 
granite-masses of the Andes, with the relations between the so- 
called Plutonic rocks and those of undoubtedly volcanic origin. 
It was indeed a fortunate circumstance, that after studying some 
excellent examples of recent volcanic rocks, he proceeded to 
examine in South America many fine illustrations of the older 
igneous rock-masses, and especially of the most highly crystalline 
types of the same, and then on his way home had opportunities 
of reviving the impression made upon him by the fresh and un- 
altered volcanic rocks. Some of the general considerations 
suggested by these observations were discussed in a paper read by 
him before the Geological Society, on March 7th, 1838, under the 
title " On the Connection of Certain Volcanic Phenomena, and On 
the Formation of Mountain-chains, and the Effect of Continental 
Elevations." The exact bearing of these two classes of facts upon 
one another are more fully discussed in his book on South 
American geology. 

The proofs of recent elevation around many of the volcanic 
islands led ]3arwin to conclude that volcanic areas were, as a 
rule, legions in which upward movements were taking place, and 
he was naturally led to contrast them with the areas in which, as 
he showed, the occurrence of atolls, encircling reefs, and barrier- 
reefs afford indication of subsidence. In this way he was able to 



1^4 CRITICAL tNTRODUCflON. 

map out the oceanic areas in different zones, along which opposite 
kinds of movement were taking place. His conclusions on this 
subject were full of novelty and suggestiveness. 

Very clearly did Darwin recognise the importance of the fact 
that most of the oceanic islands appear to be of volcanic origin, 
though he was careful to point out the remarkable exceptions 
which somewhat invalidate the generalisation. In his " Origin of 
Species " he has elaborated the idea and suggested the theory of 
the permanence of ocean-basins, a suggestion which has been 
adopted and pushed farther by subsequent authors, than we think 
its originator would have approved. His caution and fairness of 
mind on this and similar speculative questions was well known to 
all who w^ere in the habit of discussing them with him. 

Some years before the voyage of the Beagle, Mr. Poulett 
Scrope had pointed out the remarkable analogies that exist 
between certain igneous rocks of banded structure, as seen in the 
Ponza Islands, and the foliated crystalline schists. It does not 
appear that Darwin was acquainted with this remarkable memoir, 
but quite independently he called attention to the same phe- 
nomena when he came to study some very similar rocks 
which occur in the island of Ascension. Coming fresh from the 
study of the great masses of crystalline schist in the South 
American continent, he was struck by the circumstance that in 
the undoubtedly igneous rocks of Ascension we find a similar 
separation of the constituent minerals along parallel "folia." 
These observations led Darwin to the same conclusion as that 
arrived at some time before by Scrope — namely that when crystal- 
lisation takes place in rock masses under the influence of great 
deforming stresses, a separation and parallel arrangement of the 
constituent minerals will result. This is a process which is now 
fully recognised as having been a potent factor in the production 
of the metamorphic rock, and has been called by more recent 
writers " dynamo-metamorphism." 

In this, and in many similar discussions, in which exact minera- 
logical knowledge was required, it is remarkable how successful 
Darwin was in making out the true facts with regard to the rocks 
he studied by the simple aid of a penknife and pocket-lens, 
supplemented by a few chemical tests and the constant use of the 
blowpipe. Since his day, the method of study of rocks by thin 
sections under the microscope has been devised, and has become 
a most efficient aid in all petrographical inquiries. During the 
voyage of H.M.S. Challenger^ many of the islands studied by 



CRITICAL INTRODUCTION. 165 

Darwin have been revisited and their rocks collected. The 
results of their study by one of the greatest masters of the science 
of micropetrography — Professor Renard of Brussels — have been 
recently published in one of the volumes of "Reports on the 
Challenger Expedition." While much that is new and valuable 
has been contributed to geological science by these more recent 
investigations, and many changes have been made in nomencla- 
ture and other points of detail, it is interesting to tind that all the 
chief facts described by Darwin and his friend Professor Miller 
have stood the test of time and further study, and remain as a 
monument of the acumen and accuracy in minute observation of 
these pioneers in geological research. 

John W. Judd. 



CHAPTER I. 

ST. JAGO, IN THE CAPE DE VERDE ARCHIPtLAGO. 

Rocks of the lowest series. — A calcareous sedimentary deposit, with recent 
shell?, altered by the contact of superincumbent lava, its horizontality 
and extent. — Subsequent volcanic eruptions, associated with calcareous 
matter in an earthy and fibrous form, and often enclosed within the 
separate cells of the scoriae.— Ancient and obliterated orifices of eruption 
, of small size.^ — Difficulty of tracing over a bare plain recent streams of 
lava. — Inland hills of more ancient volcanic rock. — Decomposed olivine in 
large masses. — Feldspathic recks beneath the upper crystalline basaltic 
strata.— Uniform structure and form of the more ancient volcanic hills. — 
Form of the valleys near the coast. — Conglomerate now forming on the 
sea beach. 

The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty 
miles in length by about twel<ve in breadth. My observations, made 
during two visits, were confined to the southern portion within the 
distance of a few leagues from Porto Praya. The country, viewed from 
the sea, presents a varied outline : smooth conical hills of a reddish 
colour (like Red Hill in the accompanying woodcut),* and others less 
regular, flat-topped, and of a blackish colour (like A, B, C,) rise from 
successive, step-formed plains of lava. At a distance, a chain of 
mountains, many thousand feet in height, traverses the interior of the 
island. There is no active volcano in St. Jago, and only one in the 
group, namely at Fogo. The island since being inhabited has not 
suffered from destructive earthquakes. 

The lowest rocks exposed on the coast near Porto Praya, arc highly 
crystalline and compact ; they appear to be of ancient, submarine, 
volcanic origin ; they are unconformably covered by a thin, irregular, 
calcareous deposit, abounding with shells of a late tertiary period ; and 
this again is capped by a wide sheet of basaltic lava, which has flowed 
in successive streams from the interior of the island, between the 
square-topped hills marked A, B, C, etc. Still more recent streams of 
lava have been erupted from the scattered cones, such as Red and 
Signal Post Hills. The upper strata of the square-topped hills are 

* The outline of the coast, the position of the villages, streamlets, and of 
most of the hills in this woodcut, are copied from the chart made on board 
H.M.S. Leven. Ihe square-topped hills (A, B, C, etc.,) are put in merely 
by eye, to illustrate my description. 



i68 



ST. J AGO 



intimately related in mineralogical composition, and in other respects, 
with the lowest series of the coast-rocks, with which they seem to be 
continuous. 

Mineralogical description of the rocks of the lowest series. — These 
rocks possess an extremely varying character ; they consist of black, 
brown, and grey, compact, basaltic bases, with numerous crystals of 
augite, hornblende, olivine, mica, and sometimes glassy feldspar. A 
common variety is almost entirely composed of crystals of augite with 
olivine. Mica, it is known, seldom occurs where augite abounds ; nor 
probably does the present case offer a real exception, for the mica (at 
least in my best characterised specimen, in which one nodule of this 
mineral is nearly half an inch in length) is as perfectly rounded as a 
pebble in a conglomerate, and evidently has not been crystallised in 
the base, in which it is now enclosed, but has proceeded from the fusion 

Miles. 




Porto rraya. 

PART OF ST. JAGG, GNE OF THE CAPE DE VERDE ISLANDS. 

of some pre-existing rock. These compact lavas alternate with tuffs, 
amygdaloids, and wacke, and in some places with coarse conglomerate. 
Some of the argillaceous wackes are of a dark green colour, others, 
pale yellowish-green, and others nearly white ; I was surprised to find 
that some of the latter varieties, even where whitest, fused into a jet 
black enamel, whilst some of the green varieties afforded only a pale 
gray bead. Numerous dikes, consisting chiefly of highly compact 
augitic rocks, and of gray amygdaloidal varieties, intersect the strata, 
which have in several places been dislocated with considerable violence, 
and thrown into highly inclined positions. One line of disturbance 
crosses the northern end of Quail Island (an islet in the Bay of Porto 
Praya), and can be followed to the mainland. These disturbances took 
place before the deposition of the recent gedimcntary bed ; and the 



CALCAREOUS DEPOSIT. 169 

surface, also, had previously been denuded to a great extent, as is 
shown by many truncated dikes. 

Descfiption of the calcareous deposit overlying the foregoing volca7iic 
rocks. — This stratum is very conspicuous from its white colour, and 
from the extreme regularity with which it ranges in a horizontal line 
for some miles along the coast. Its average height above the sea, 
measured from the upper line of junction with the superincumbent 
basaltic lava, is about sixty feet ; and its thickness, although varying 
much from the inequalities of the underlying formation, may be 
estimated at about twenty feet. It consists of quite white calcareous 
matter, partly composed of organic debris, and partly of a substance 
which may be aptly compared in appearance with mortar. Fragments 
of rock and pebbles are scattered throughout this bed, often forming, 
especially in the lower part, a conglomerate. Many of the fragments 
of rock are whitewashed with a thin coating of calcareous matter. At 
Quail Island, the calcareous deposit is replaced in its lowest part by a 
soft, brown, earthy tuff, full of Turritellae ; this is covered by a bed of 
pebbles, passing into Sandstone, and mixed with fragments of echini, 
claws of crabs, and shells ; the oyster-shells still adhering to the rock 
on which they grew. Numerous white balls appearing like pisolitic 
concretions, from the size of a walnut to that of an apple, are embedded 
in this deposit ; they usually have a small pebble in their centres. 
Although so like concretions, a close examination convinced me that 
they were Nulliporae, retaining their proper forms, but with their 
surfaces slightly abraded : these bodies (plants as they are now generally 
considered to be) exhibit under a microscope of ordinary power, no 
traces of organisation in their internal structure. Mr. George R. 
Sowerby has been so good as to examine the shells which I collected ; 
there are fourteen species in a sufficiently perfect condition for their 
characters to be made out with some degree of certainty, and four 
which can be referred only to their genera. Of the fourteen shells, of 
which a list is given in the Appendix, eleven are recent species ; one, 
though undescribed, is perhaps identical with a species which I found 
living in the harbour of Porto Praya ; the two remaining species are 
unknown, and have been described by Mr. Sowerby. Until the shells 
of this Archipelago and of the neighbouring coasts are better known, it 
would be rash to assert that even these two latter shells are extinct. 
The number of species which certainly belong to existing kinds, 
although few in number, are sufficient to show that the deposit belongs 
to a late tertiary period. From its mineralogical character, from the 
number and size of the embedded fragments, and from the abundance 
of Patellae, and other littoral shells, it is evident that the whole was 
accumulated in a shallow sea, near an ancient coast-hne. 

Effects prodtcced by the flowing of the stcperiiiaimbent basaltic lava 
over the calcareous deposit. — These effects are very curious. The 
calcareous matter is altered to the depth of about a foot beneath the 
line of junction; and a most perfect gradation can be traced, from 
loosely aggregated, small, particles of shells, corallines, and Nulliporae, 
intP 3 rocl<, in whjch not a trace of mechanical origin can be discovered, 



170 ST. J A GO. 

even with a microscope. Where tlie mctamorphic change has been 
greatest, two varieties occur. The first is a hard, compact, white, fine- 
grained rock, striped with a few parallel lines of black volcanic particles, 
and resembling a sandstone, but which, upon close examination, is seen 
to be crystallised throughout, with the cleavages so perfect that they 
can be readily measured by the reflecting goniometer. In specimens, 
where the change has been less complete, when moistened and 
examined under a strong lens, the most interesting gradation can be 
traced, some of the rounded particles retaining their proper forms, and 
others insensibly melting into the granulo-crystalline paste. The 
weathered surface of this stone, as is so frequently the case with 
ordinary limestones, assumes a brick-red colour. 

The second metamorphosed variety is likewise a hard rock, but with- 
out any crystalline structure. It consists of a wliite, opaque, compact, 
calcareous stone, thickly mottled with rounded, though regular, spots of 
a soft, earthy, ochraceous substance. This earthy matter is of a pale 
yellowish-brown colour, and appears to be a mixture of carbonate of 
lime with iron ; it effervesces with acids, is infusible, but blackens 
under the blowpipe, and becomes magnetic. The rounded form of the 
minute patches of earthy substance, and the steps in the progress of 
their perfect formation, which can be followed in a suit of specimens, 
clearly show that they are due either to some power of aggregation in 
the earthy particles amongst themselves, or more probably to a strong 
attraction between the atoms of the carbonate of lime, and consequently 
to the segregation of the earthy extraneous matter. I was much 
interested by this fact, because I have often seen quartz rocks (for 
instance, in the Falkland Islands, and in the lower Silurian strata of 
the Stiper-stones in Shropshire), mottled in a precisely analogous 
manner, with little spots of a white, earthy substance (earthy feldspar ?) ; 
and these rocks, there was good reason to suppose, had undergone the 
action of heat, — a view which thus receives confirmation. This spotted 
structure may possibly afford some indication in distinguishing those 
formations of quartz, which owe their present structure to igneous 
action, from those produced by the agency of water alone ; a source of 
doubt, which I should think from my own experience, that most 
geologists, when examining arenaceo-quartzose districts must have 
experienced. 

The lowest and most scoriaceous part of the lava, in rolhng over the 
sedimentary deposit at the bottom of the sea, has caught up large 
quantities of calcareous matter, which now forms a snow-white, highly 
crystalline basis to a breccia, including small pieces of black, glossy 
scoriee. A little above this, where the lime is less abundant, and the 
lava more compact, numerous little balls, composed of spicula ot 
calcareous spar, radiating from common centres, occupy the interstices. 
In one part of Quail Island, the Hme has thus been crystallised by the 
heat of the superincumbent lava, where it is only thirteen feet in 
thickness ; nor had the lava been originally thicker, and since reduced 
by degradation, as could be told from the degree of cellularity of its 
surface. I have already observed that the sea must have been shallow 



CALCAREOUS HORIZONTAL DEPOSIT. 171 

in which the calcareous deposit was accumulated. In this case, there- 
fore, the carbonic acid gas has been retained under a pressure, 
insignificant compared wdth that (a column of water, 1,708 feet in 
height) originally supposed by Sir James Hall to be requisite for 
this end : but since his experiments, it has been discovered that 
pressure has less to do with the retention of carbonic acid gas, 
than the nature of the circumjacent atmosphere ; and hence, as is 
stated to be the case by Mr. Faraday,* masses of limestone are some- 
times fused and crystallised even in common limekilns. Carbonate 
of lime can be heated to almost any degree, according to Faraday, 
in an atmosphere of carbonic acid gas, without being decomposed ; 
and Gay-Lussac found that fragments of limestone, placed in a tube 
and heated to a degree, not sufficient by itself to cause their decom- 
position, yet immediately evolved their carbonic acid, when a stream of 
common air or steam was passed over them : Gay-Lussac attributes this 
to the mechanical displacement of the nascent carbonic acid gas. The 
calcareous matter beneath the lava, and especially that forming the 
crystalline spicula between the interstices of the scoriae, although 
heated in an atmosphere probably composed chiefly of steam, could 
not have been subjected to the effects of a passing stream ; and hence 
it is, perhaps, that they have retained their carbonic acid, under a small 
amount of pressure. 

The fragments of scoriae, embedded in the crystalline calcareous 
basis, are of a jet black colour, with a glossy fracture like pitchstone. 
Their surfaces, however, are coated with a layer of a reddish-orange, 
translucent substance, which can easily be scratched with a knife ; 
hence they appear as if overlaid by a thin layer of rosin. Some of the 
smaller fragments are partially changed throughout into this substance : 
a change which appears quite different from ordinary decomposition. 
At the Galapagos Archipelago (as will be described in a future chapter), 
great beds are formed of volcanic ashes and particles of scoriae, which 
have undergone a closely similar change. 

The exte?it and horizontality of the calcareous stratum. — The upper 
line of surface of the calcareous stratum, which is so conspicuous from 
being quite white and so nearly horizontal, ranges for miles along the 
coast, at the height of about sixty feet above the sea. The sheet of 
basalt, by which it is capped, is on an average eighty feet in thickness. 
Westward of Porto Praya beyond Red Hill, the white stratum with 
the superincumbent basalt is covered up by more recent streams. 
Northward of Signal Post Hill, I could follow it with my eye, trending 
away for several miles along the sea cliffs. The distance thus observed 
is about seven miles ; but I cannot doubt from its regularity that it 
extcnc^s much farther. In some ravines at right angles to the coast, it is 
seen gently dipping towards the sea, probably with the same inclination 

* I am much indebted to Mr. E. W. Brayley in having given me the 
following references to papers on this subject : Faraday, in the Edinburgh 
New Philosophical Journal, vol. xv., p. 398; Gay-Lussac, in "Annales de 
Chem. et Phys.," torn. Ixiii., p. 219, translated in the London and Edinburgh 
Philosophical Magazine, vol. x., p. 496. 



172 ST. J AGO. 

as when deposited round the ancient shores oi the island. I found 
only one inland section, namely, at the base of the hill marked A, 
where, at the height of some hundred feet, this bed was exposed ; it 
here rested on the usual compact augitic rock associated with wacke, 
and was covered by the widespread sheet of modern basaltic lava. 
Some exceptions occur to the horizontality of the white stratum : at 
Quail Island, its upper surface is only forty feet above the level of the 
sea ; here also the capping of lava is only between twelve and fifteen 
feet in thickness ; on the other hand, at the north-east side of Porto Praya 
harbour, the calcareous stratum, as well as the rock on which it rests, 
attain a height above the average level : the inequality of level in these 
two cases is not, as I believe, owing to unequal elevation, but to 
original irregularities at the bottom of the sea. Of this fact, at Quail 
Island, there was clear evidence in the calcareous deposit being in one 
part of much greater than the average thickness, and in another part 
being entirely absent ; in this latter case, the modern basaltic lavas 
rested directly on those of more ancient origin. 

Under Signal Post Hill, the white stratum dips into the sea in a 
remarkable manner. This hill is conical, 450 feet in height, and 
retains some traces of having had a crateriform structure ; it is com- 

No. a. 




SIGNAL POST HIIX. 
A— Ancient volcanic rocks. B -Calcareous stratum. C— Upper basaltic lava. 

posed chiefly of matter erupted posteriorly to the elevation of the 
great basaltic plain, but partly of lava of apparently submarine origin 
and of considerable antiquity. The surrounding plain, as well as the 
eastern flank of this hill, has been worn into steep precipices, over- 
banging the sea. In these precipices, the white calcareous stratum 
may be seen, at the height of about seventy feet above the beach, 
running for some miles both northward and southward of the hill, in a 
line appearing to be perfectly horizontal ; but for a space of a quarter 
of a mile directly under the hill, it dips into the sea and disappears. On 
the south side the dip is gradual, on the north side it is more abrupt, as 
is shown in the woodcut. As neither the calcareous stratum, nor the 
superincumbent basaltic lava (as far as the latter can be distinguished 
from the more modern ejections), appears to thicken as it dips, I infer 
that these strata were not originally accumulated in a trough, the centre 
of which afterwards became a point of eruption ; but that they have 
subsequently been disturbed and bent. We may suppose either that 
Signal Post Hill subsided after its elevation with the surrounding 
country, or that it never was uplifted to the same height with it. This 
latter seems to me the most probable alternative, for during the slow and 
equable elevation of this portion of the island, the v^ubterr^neari motive 



CALCAREOUS MATTER ENTANGLED IN LAVA. 173 

power, from expending part of its force in repeatedly erupting volcanic 
matter from beneath tliis poitit, would, it is likely, have less force to 
uplift it. Something of the same kind seems to have occurred near Red 
Hill, fol: when tracing upwards the rxaked streams of lava from near 
Porto Praya towards the interior of the island, I was strongly induced 
to suspect, that since the lava had flowed, the slope of the land had 
been slightly modified, either by a small subsidence near Red Hill, or 
by that portion of the plain having been uplifted to a less height during 
the elevation of the whole areai 

The basaltic lava, superi7idcmhenf on the calcareous deposit — This 
lava is of a pale grey colour, fusing into a black enamel ; its fracture is 
rather earthy and concretionary; it contains olivine in small grains. 
The central parts of the mass are compact, or at most crenulated with 
a few minute cavities, and are often columnar. At Quail Island this 
structure was assumed in a striking manner ; the lava in one part being 
divided into horizontal laminae, which became in another part split by 
vertical fissures into five-sided plates ; and these again, being piled on 
each other, insensibly became soldered together, forming fine sym- 
metrical columns. The lower surface of the lava is vesicular, but some- 
times only to the thickness of a few inches ; the upper surface, which is 
likewise vesicular, is divided into balls, frequently as much as three 
feet in diameter, made up of concentric layers. The mass is composed 
of more than one stream ; its total thickness being, on an average, 
about eighty feet : the lower portion has certainly flowed beneath the 
sea, and probably likewise the upper portion. The chief part of this 
lava has flowed from the central districts, between the hills marked 
A, B, C, etc., in the woodcut-map. The surface of the country, near 
the coast, is level and barren ; towards the interior, the land rises by 
successive terraces, of which four, when viewed from a distance, could 
be distinctly counted. 

Volcanic eruptions subsequent to the elevation of the coastland ; the 
ejected matte? associated with earthy lime. — These recent lavas have 
proceeded from those scattered, conical, reddish-coloured hills, which 
rise abruptly from the plain-country near the coast. I ascended some 
of them, but will describe only one, namely, Red Hill, which may serve 
as a type of its class, and is remarkable in some especial respects. Its 
height is about six hundred feet ; it is composed of bright red, highly 
scoriaceous rock of a basaltic nature ; on one side of its summit there 
is a hollow, probably the last remnant of a crater. Several of the other 
hills of this class, judging from their external forms, are surmounted by 
much more perfect craters. When sailing along the coast, it was 
evident that a considerable body of lava had flowed from Red Hill, 
over a line of cliff about one hundred and twenty feet in height, into 
the sea : this line of cliff is continuous with that forming the coast, and 
bounding the plain on both sides of this hill ; these streams, therefore, 
were erupted, after the formation of the coast-clifTs, from Red Hill, 
when it must have stood, as it now does, above the level of the sea. 
This conclusion accords with the highly scoriaceous condition of all 
the rock on it, appearing to be of subaerial formation ; and this is 



174 ST. J AGO. 

important, as there are some beds of calcareous matter near its summit, 
which might, at a hasty glance, have been mistaken for a submarine 
deposit. These beds consist of white, earthy, carbonate of lime, ex- 
tremely friable so as to be crushed with the least pressure ; the most 
compact specimens not resisting the strength of the fingers. Some of 
the masses are as white as quicklime, and appear absolutely pure ; but 
on examining them with a lens, minute particles of scoriae can always 
be seen, and I could find none which, when dissolved in acids, did not 
leave a residue of this nature. It is, moreover, difficult to find a particle 
of the lime which does not change colour under the blowpipe, most of 
them even becoming glazed. The scoriaceous fragments and the cal- 
careous matter are associated in the most irregular manner, sometimes 
in obscure beds, but more generally as a confused breccia, the lime in 
some parts and the scoriae in others being most abundant. Sir H. De 
la Bcche has been so kind as to have some of the purest specimens 
analysed, with a view to discover, considering their volcanic origin, 
whether they contained much magnesia ; but only a small portion was 
found, such as is present in most limestones. 

Fragments of the scoriae embedded in the calcareous mass, when 
broken, exhibit many of their cells lined and partly filled with a white, 
delicate, excessively frapile, moss-like, or rather conferva-like, reticulation 
of carbonate of lime. These fibres, examined under a lens of one-tenth 
of an inch focal distance, appear cylindrical ; they are rather above 
one-thousandth of an inch in diameter ; they are either simply branched, 
or more commonly united into an irregular mass of network, with the 
meshes of very unequal sizes and of unequal numbers of sides. Some 
of the fibres are thickly covered with extremely minute spicula, occa- 
sionally aggregated into little tuffs ; and hence they have a hairy 
appearance. These spicula are of the same diameter throughout their 
length; they are easil}'' detached, so that the object-glass of the micro- 
scope soon becomes scattered over with them. Within the cells of 
many fragments of the scoriae, the lime exhibits this fibrous structure, 
but generally in a less perfect degree. These cells do not appear to 
be connected with one another. There can be no doubt, as will 
presently be shown, that the lime was erupted, mingled with the lava 
in its fluid state ; and therefore I have thought it worth while to describe 
minutely this curious fibrous structure, of which I know nothing 
analogous. From the earthy condition of the fibres, this structure does 
not appear to be related to crystallisation. 

Other fragments of the scoriaceous rock from this hill, v.'hen broken, 
are often seen marked with short and irregular white streaks, which 
are owing to a row of separate cells being partly, or quite, filled with 
white calcareous powder. This structure immediately reminded me 
of the appearance in badly kneaded dough, of balls and drawn-out 
streaks of flour, which have remained unmixed with the paste ; and I 
cannot doubt that small masses of the lime, in the same manner re- 
maining unmixed with the fluid lava, have been drawn out when the 
whole was in motion. I carefully examined, by trituration and solution 
in acids, pieces of the scoriae, taken from within half-an-inch of those 



CALCAREOUS MATTER ENTANGLED IN LAVA. 175 

cells which were filled with the calcareous powder, and they did not 
contain an atom of free lime. It is obvious that the lava and lime have 
on a large scale been very imperfectly mingled ; and where small 
portions of the lime have been entangled within a piece of the viscid 
lava, the cause of their now occupying,' in the form of a powder or of a 
fibrous reticulation, the vesicular cavities, is, I think, evidently due to 
the confined gases having most readily expanded at the points where 
the incoherent lime rendered the lava less adhesive. 

A mile eastward of the town of Praya, there is a steep-sided gorge, 
about one hundred and fifty yards in width, cutting through the basaltic 
plain and underlying beds, but since filled up by a stream of more 
modern lava. This lava is dark grey, and in most parts compact and 
rudely columnar ; but at a little distance from the coast, it includes in 
an irregular manner a brecciated mass of red scoriae mingled with a 
considerable quantity of white, friable, and in some parts, nearly pure 
earthy lime, like that on the summit of Red Hill. This lava, with its 
entangled lime, has certainly flowed in the form of a regular stream ; 
and, judging from the shape of the gorge, towards which the drainage 
of the country (feeble though it now be) still is directed, and from the 
appearance of the bed of loose water-worn blocks with their interstices 
unfilled, like those in the bed of a torrent, on which the lava rests, we 
may conclude that the stream was of subaerial origin. I was unable to 
trace it to its source, but, from its direction, it seemed to have come 
from Signal Post Hill, distant one mile and a quarter, which, like Red 
Hill, has been a point of eruption subsequent to the elevation of the 
great basaltic plain. It accords with this view, that I found on Signal 
Post Hill, a mass of earthy, calcareous matter of the same nature, 
mingled with scoriae. I may here observe that part of the calcareous 
matter forming the horizontal sedimentary bed, especially the finer 
matter with which the embedded fragments of rock are whitewashed, 
has probably been derived from similar volcanic eruptions, as well as 
from trit\irated organic remains : the underlying, ancient, crystalline 
rocks, also, are associated with much carbonate of lime, filling amygda- 
loidal cavities, and forming irregular masses, the nature of which latter 
I was unable to understand. 

Considering the abundance of earthy lime near the summit of Red 
Hill, a volcanic cone six hundred feet in height, of subaerial growth, — 
considering the intimate manner in which minute particles and large 
masses of scoriae are embedded in the masses of nearly pure lime, and 
on the other hand, the manner in which small kernels and streaks of 
the calcareous powder are included in solid pieces of the scoriae, — con- 
sidering, also, the similar occurrence of lime and scoriae within a stream 
of lava, also supposed, with good reason, to have been of modern 
subaerial origin, and to have flowed from a hill, vi^here earthy lime also 
occurs : 1 think, considering these facts, there can be no doubt that the 
lime has been erupted, mingled with the molten lava. I am not aware 
that any similar case has been described : it appears to me an interest- 
ing one, inasmuch as most geologists must have speculated on the 
probable effects of a volcanic focus, bursting through deep-seated beds 



176 ST. JAGO. 

of different mineralogical composition. The great abundance of frCe 
silex in the trachytes of some countries (as described by Beudant in 
Hungary, and by P. Scrope in the Panza Islands), perhaps solves the 
inquiry with respect to deep-seated beds of quartz ; and we probably 
here see it answered, where the volcanic action has invaded subjacent 
masses of limestone. One is naturally led to conjecture in what state 
the now earthy carbonate of lime existed, when ejected with the in- 
tensely heated lava : from the extreme cellularity of the scoriae on Red 
Hill, the pressure cannot have been great, and as most volcanic erup- 
tions are accompanied by the emission of large quantities of steam and 
other gases, we here have the most favourable conditions, according to 
the views at present entertained by chemists, for the expulsion of the 
carbonic acid.* Has the slow re-absorption of this gas, it may be 
asked, given to the lime in the cells of the lava, that peculiar fibrous 
structure, like that of an efflorescing salt? Finally, I may remark on 
the great contrast in appearance between this earthy lime, which must 
have been heated in a free atmosphere of steam and other gases, while 
the white, crystalline, calcareous spar, produced by a single thin sheet 
of lava (as at Quail Island) rolling over similar earthy lime and the 
debris of organic remains, at the bottom of a shallow sea. 

Signal Post Hill. — This hill has already been several times men- 
tioned, especially with reference to the remarkable manner in which the 
white calcareous stratum, in other parts so horizontal (Woodcut No. 
2), dips under it into the sea. It has a broad summit, with obscure 
traces of a crateriform structure, and is composed of basaltic rocks,t 
some compact, others highly cellular with inclined beds of loose scoriae, 
of which some are associated with earthy lime. Like Red Hill, it has 
been the source of eruptions, subsequently to the elevation of the 
surrounding basaltic plain ; but unlike that hill, it has undergone con- 
siderable denudation, and has been the seat of volcanic action at a 
remote period, when beneatli the sea. I judge of this latter circum- 
stance from finding on its inland flank the last remnants of three small 

* Whilst deep beneath the surface, the carbonate of lime was, I presume, 
in a fluid state. Hutton, it is known, thought that all amygdaloids were 
produced by drops of molten limestone floating in the trap, like oil in water : 
this no doubt is erroneous, but if the matter forming the summit of Red Hill 
had been cooled under the pressure of a moderately deep sea, or within the 
walls of a dike, we should, in all probability, have had a trap rock associated 
with large masses of compact, crystalline, calcareous spar, which, according 
to the views entertained by many geologists, would have been wrongly 
attributed to subsequent infiltration. 

f Of these, one common variety is remarkable for being full of small frag- 
ments of a dark jasper-red earthy mineral, which, when examined carefully, 
shows an indistinct cleavage ; the little fragments are elongated in form, are 
soft, are magnetic before and after being heated, and fuse with difficult}' into 
a dull enamel. This mineral is evidently closely related to the oxides cf 
iron, but I cannot ascertain what it exactly is. The rock containing this 
mineral is crenulated with small angular cavities, which are lined and filled 
with yellowish crystals of carbonate of Hme. 



SMALL ORIFICES OF ERUPTION. 177 

points of eruption. These points are composed of glossy scoricc, 
cemented by crystalline calcareous spar, exactly like the great sub- 
marine calcareous deposit, where the heated lava has rolled over it : 
tiieir demolished state can, I think, be explained only by the denuding 
action of the Waves of the sea. I was guided to the first orifice by 
observing a sheet of lava, about two hundred yards square, with 
steepish sides, superimposed on the basaltic plain with no adjoining 
hillock, whence it could have been erupted ; and the only trace of a 
crater which I was able to discover, consisted of some inclined beds of 
scoriae at one of its corners. At the distance of fifty yards from a 
second level-topped patch of lava, but of much smaller size, I found an 
irregular circular group of masses of cemented, scoriaceous breccia, 
about six feet in height, which doubtless had once formed the point of 
eruption. The third orifice is now marked only by an irregular circle of 
cemented scoriae, about four yards in diameter, and rising in its highest 
point scarcely three feet above the level of the plain, the surface of 
which, close all round, exhibits its usual appearance : here we have 
a horizontal basal section of a volcanic spiracle, which, together with 
all its ejected matter, has been almost totally obliterated. 

The stream of lava, which fills the narrow gorge * eastward of the 
town of Praya, judging from its course, seems, as before remarked, to 
have come from Signal Post Hill, and to have flowed over the plain, 
after its elevation : the same observation applies to a stream (possibly 
part of tlie same one) capping the sea cliffs, a little eastward of the 
gorge. When I endeavoured to follow these streams over the stony 
level plain, which is almost destitute of soil and vegetation, I was much 
surprised to find, that although composed of hard basaltic matter, 
and not having been exposed to marine denudation, all distinct 
traces of them soon became utterly lost. But I have since observed 
at the Galapagos Archipelago, that it is often impossible to follow even 
great deluges of quite recent lava across older streams, except by the 
size of the bushes growing on them, or by the comparative states of 
glossiness of their surfaces, — characters which a short lapse of time would 
be sufficient quite to obscure. I may remark, that in a level country, 
with a dry climate, and with the wind blowing always in one direction 
(as at the Cape de Verde Archipelago), the effects of atmospheric 
degradation are probably much greater than would at first be expected ; 
for soil in this case accumulates only in a few protected hollows, and 
being blown in one direction, it is always travelling towards the sea in 
the lorm of the finest dust, leaving the surface of the rocks bare, and 
exposed to the full effects of renewed meteoric action. 

Inland hills of more ande7tt volcanic rocks. — These hills are laid down 
by eye, and marked as A, B, C, etc., in the woodcut-map. They 
are related in mineralogical composition, and are probably directly 

* The sides of this gorge, where the upper basaltic stratum is intersected, 
are almost perpendicular. The lava, which has since filled it up, is attached 
to these sides, almost as firmly as a dike is to its walls. In most cases, 
where a stream of lava has flowed down a valley, it is bounded on each side 
by loose scoriaceous masses. 

12 



178 ST. J AGO. 

continuous with the lowest rocks exposed on the coast. These hills, 
viewed from a distance, appear as if they had once formed part of an 
irregular tableland, and from their corresponding structure and composi- 
tion this probably has been the case. They have flat, shghtly inclined 
summits ; and are, on an average, about six hundred feet in height ; 
tlicy present their steepest slope towards the interior of the island, 
from which point they radiate outwards, and are separated from each 
other by broad and deep valleys, through which the great streams of 
lava, forming the coast-plains, have descended. Their inner and 
steeper escarpments are ranged in an irregular curve, which rudely 
follows the line of the shore, two or three miles inland from it. I 
ascended a few of these hills, and from others, which I was able to 
examine with a telescope, I obtained specimens, through the kindness 
of Mr. Kent, the assistant-surgeon of the Beagle ; although by these 
means I am acquainted with only a part of the range, five or six miles 
in length, yet I scarcely hesitate, from their uniform structure, to affirm 
that they are parts of one great formation, stretching round much of the 
circumference of the island. 

The upper and lower strata of these hills differ greatly in composi- 
tion. The upper are basaltic, generally compact, but sometimes scoria- 
ceous and amygdaloidal, with associated masses of wacke : where the 
basalt is compact, it is either fine-grained or very coarsely crystalhsed; 
in the latter case it passes into an angiticrock, containing much olivine; 
the olivine is either colourless, or of the usual yellow and dull reddish 
shades. On some of the hills, beds of calcareous matter, both in an 
earthy and in a crystalline form, including fragments of glossy scoriae, 
are associated with the basaltic strata. These strata differ from the 
streams of basaltic lava forming the coast-plains, only in being more 
compact, and in the crystals of augite, and in the grains of olivine 
being of much greater ' size ; — characters which, together with the 
appearance of the associated calcareous beds, induce me to believe that 
tiiey are of submarine formation. 

Some considerable masses of wacke, which are associated with these 
basaltic strata, and which likewise occur in the basal series on the 
coast, especially at Quail Island, are curious. They consist of a pale 
yellowish-green argillaceous substance, of a crumbling texture when 
dry, but unctuous when moist : in its purest form, it is of a beautiful 
green tint, with translucent edges, and occasionally wdth obscure traces 
of an original cleavage. Under the blowpipe it fuses very readily into 
a dark grey, and sometimes even black bead, which is slightly magnetic. 
From these characters, I naturally thought that it was one of the pale 
species, decomposed, of the genus augite ; — a conclusion supported by 
the unaltered rock being full of large separate crystals of black augite, 
and of balls and irregular streaks of dark grey augitic rock. As the 
basalt ordinarily consists of augite, and of olivine often tarnished and 
of a dull red colour, I was led to examine the stages of decomposition 
of this latter mineral, and I found, to my surprise, that I could trace a 
nearly perfect gradation from unaltered olivine to the green wacke. 
Part of the same grain under the blowpipe would in some instances 



ANCIENT VOLCANIC HILLS. 179 

behave like olivine, its colour being only slightly changed, and part 
would give a black magnetic bead. Hence I can have no doubt that 
the greenish vvacke originally existed as olivine ; but great chemical 
changes must have been effected during the act of decomposition thus 
to have altered a very hard, transparent, infusible mineral, into a soft, 
unctuous, easily melted, argillaceous substance.* 

The basal strata of these hills, as well as some neighbouring, separ- 
ate, bare, rounded hillocks, consist of compact, fine-grained, non- 
crystalline (or so slightly as scarcely to be perceptible), ferruginous, 
feldspathic rocks, and generally in a state of semi-decomposition. 
Their fracture is exceedingly irregular, and splintery ; yet small frag- 
ments are often very tough. They contain much ferruginous matter, 
either in the form of minute grains with a metallic lustre, or of brown 
hair-hke threads : the rock in this latter case assuming a pseudo- 
brecciated structure. These rocks sometimes contain mica and veins ol 
agate. Their rusty brown or yellowish colour is partly due to the 
oxides of iron, but chiefly to innumerable, microscopically minute, 
black specks, which, when a fragment is heated, are easily fused, and 
evidently are either hornblende or augite. These rocks, therefore, 
although at first appearing like baked clay or some altered sedimentary 
deposit, contain all the essential ingredients of trachyte ; from which 
they differ only in not being harsh, and in not containing crystals of 
glassy feldspar. As is so often the case with trachytic formation, no 
stratification is here apparent. A person would not readily believe 
that these rocks could have flowed as lava ; yet at St. Helena there are 
well- characterised streams (as will be described in an ensuing chapter) 
of nearly similar composition. Amidst the hillocks composed of these 
rocks, I found in three places, smooth conical hills of phonolite, abound- 
ing with fine crystals of glassy feldspar, and with needles of horn- 
blende. These cones of phonohte, I believe, bear the same relation to the 
surrounding feldspathic strata which some masses of coarsely crystal- 
lised augitic rock, in another part of the island, bear to the surrounding 
basalt, namely, that both have been injected. The rocks of a feldspathic 
nature being anterior in origin to the basaltic strata, which cap them, as 
well as to the basaltic streams of the coast-plains, accords with the usual 
order of succession of these two grand divisions of the volcanic series. 

* D'Aubuisson, " Traite de Geognosie " (torn, ii., p. 569), mentions, on the 
authority of M. Marcel de Serres, masses of green earth near Montpellier, 
which are supposed to be due to the decomposition of olivine. I do not, 
however, find, that the action of this mineral under the blowpipe being 
( ntirely altered, as it becomes decomposed, has been noticed ; and the know- 
ledge of this fact is important, as at first it appears highly improbable that a 
hard, transparent, refractory mineral should be changed into a soft, easily 
fused clay, like this of St. Jago. I shall hereafter describe a green sub- 
stance, forming threads within the cells of some vesicular basaltic rocks in 
Van Diemen's Land, which behave under the blowpipe like the green wacke 
of St. Jago ; but its occurrence in cylindrical threads, shows it cannot have 
resulted from the decomposition of olivine, a mineral always existing in the 
form of grains or crystals. 



i8o ST. J A GO. 

The strata of most of these hills in the upper part, where alone the 
planes of division are distinguishable, are inclined at a small angle 
from the interior of the island towards the sea-coast. The inclination 
is not the same in each hill ; in that marked A it is less than in B, D, 
or E ; in C the strata are scarcely deflected from a horizontal plane, 
and in F (as far as I could judge without ascending it) they are slightly 
inclined in a reverse direction, that is, inwards and towards the centre 
of the island. Notwithstanding these differences of inchnation, their 
correspondence in external form, and in the composition both of their 
upper and lower parts, — their relative position in one curved line, with 
their steepest sides turned inwards, — all seem to show that they 
originally formed parts of one platform ; which platform, as before 
remarked, probably extended round a considerable portion of the cir- 
cumference of the island. The upper strata certainly flowed as lava, 
and probably beneath the sea, as perhaps did the lower feldspathic 
masses : how then come these strata to hold their present position, and 
whence were they erupted ? 

In the centre of the island * there are lofty mountains, but they are 
separated from the steep inland flanks of these hills by a wide space of 
lower country: the interior mountains, moreover, seem to have been 
the source of those great streams of basaltic lava which, contracting as 
they pass between the bases of the hills in question, expand into the 
coast-plains. Round the shores of St. Helena there is a rudely formed 
ring of basaltic rocks, and at Mauritius there are remnants of another 
such a ring round part, if not round the whole, of the island ; here 
again the same question immediately occurs, how came these masses 
to hold their present position, and whence were they erupted ? The 
same answer, whatever it may be, probably applies in these three 
cases ; and in a future chapter we shall recur to this subject. 

Valleys near the coast, — These are broad, very flat, and generally 
bounded by low cliff-formed sides. Portions of the basaltic plain are 
sometimes nearly or quite isolated by them ; of which fact, the space 
on which the town of Praya stands offers an instance. The great 
valley west of the town has its bottom filled up to a depth of more than 
twenty feet by well-rounded pebbles, which in some parts are firmly 
cemented together by white calcareous matter. There can be no 
doubt, from the form of these valleys, that they were scooped out by 
the waves of the sea, during that equable elevation of the land, of 
which the horizontal calcareous deposit, with its existing species of 
marine remains, gives evidence. Considering how well shells have 
been preserved in this stratum, it is singular that I could not find even 
a single small fragment of shell in the conglomerate at the bottom of 
the valleys. The bed of pebbles in the valley west of the town is 

* I saw very little of the inland parts of the island. Near the village 
of St. Domingo, there are magnificent cliffs of rather coarsely crystallised 
basaltic lava. Following the little stream in this valley, about a mile above 
the village, the base of the great cliff was formed of a compact fine-grained 
basalt, conformably covered by a bed of pebbles. Near Fuentes, I met with 
pap-formed hills of the compact feldspathic series of rocks, 



FERNANDO NORONHA. i8i 

intersected by a second valley joining it as a tributary, but even this 
valley appears much too wide and flat-bottomed to have been formed 
by the small quantity of water, which falls only during one short wet 
season ; for at other times of the year these valleys are absolutely dry. 
Recent conglo??terate. — On the shores of Quail Island, I found frag- 
ments of brick, bolts of iron, pebbles, and large fragments of basalt, 
united by a scanty base of impure calcareous matter into a firm con- 
glomerate. To show how exceedingly firm this recent conglomerate is, 
I may mention, that I endeavoured with a heavy geological hammer to 
knock out a thick bolt of iron, which was embedded a little above low- 
water mark, but was quite unable to succeed. 



- CHAPTER II. 

Fernando Noronha. — Precipitous hill of phonolite. Tercehia. — ^Trachytic 
rocks ; their singular decomposition by steam of high temperature. 
Tahiti. — Passage from wacke into trap ; singular volcanic rock with the- 
vesicles half filled with mesotype. Mauritius. — Proofs of its recent 
elevation. — Structure of its more ancient mountains; similarity with 
St. Jago, St. Paul's Rocks. — Not of volcanic origui — their singular 
mineralogical composition. 

Ferna7ido Noronha. — During our short visit at this and the four follow- 
ing islands, I observed very little worthy of description. Fernando 
Noronha is situated in the Atlantic Ocean, in lat. 3° 50' S., and 230 
miles distant from the coast of South America. It consists of several 
islets, together nine miles in length by three in breadth. The whole 
seems to be of volcanic origin ; although there is no appearance of any 
crater, or of any one central eminence. The most remarkable feature 
is a hill 1,000 feet high, of which the upper 400 feet consist of a 
precipitous, singularly shaped pinnacle, formed of columnar phonolite, 
containing numerous crystals of glassy feldspar, and a few needles of 
hornblende. From the highest accessible point of this hill, I could 
distinguish in different parts of the group several other conical hills, 
apparently of the same nature. At St. Helena there are similar, great, - 
conical, protuberant masses of phonolite, nearly one thousand feet in 
height, which hav^e been formed by the injection of fluid fcldspathic lava 
into yielding strata. If this hill has had, as is probable, a similar origin, 
denudation has been here effected on an enormous scale. Near the 
base of this hill, I observed beds of white tuff, intersected by numerous 
dikes, some of amygdaloidal basalt and others of trachyte ; and beds 
of slaty phonolite with the planes of cleavage directed N.W. and S.E. 
Parts of this rock, where the crystals were scanty, closely resembled 
common clay-slate, altered by the contact of a trap-dike. The lamina- 
tion of rocks, which undoubtedly have once been fluid, appears to me a 
subject wpll dcscj-ving attention. On thp bei^cfi there were numerous 



1 82 TERCEIRA. 

fragments of compact basalt, of which rock a distant facade of columns 
seemed to be formed. 

Terceira in the Azores. — The central parts of this island consist of 
irregularly rounded mountains of no great elevation, composed of 
trachyte, which closely resembles in general character the trachyte of 
Ascension, presently to be described. This formation is in many parts 
overlaid, in the usual order of superposition, by streams of basaltic lava, 
which near the coast compose nearly the whole surface. The course 
which these streams have followed from their craters, can often be 
followed by the eye. The town of Angra is overlooked by a crateriform 
hill (Mount Brazil), entirely built of thin strata of fine-grained, harsh, 
brown-coloured tuff. The upper beds are seen to overlap the basaltic 
streams on which the town stands. This hill is almost identical in 
structure and composition with numerous crateriformed hills in the 
Galapagos Archipelago. 

Effects of steam on the trachytic rocks. — In the central part of the 
island there is a spot, where steam is constantly issuing in jets from 
the bottom of a small ravine-like hollow, which has no exit, and which 
abuts against a range of trachytic mountains. The steam is emitted 
from several irregular fissures : it is scentless, soon blackens iron, and 
is of much too high temperature to be endured by the hand. The 
manner in which the solid trachyte is changed on the borders of these 
orifices is curious: first, the base becomes earthy, with red freckles 
evidently due to the oxidation of particles of iron ; then it becomes 
soft; and lastly, even the crystals of glassy feldspar yield to the dis- 
solving agent. After the mass is converted into clay, the oxide of iron 
seems to be entirely removed from some parts, which are left perfectly 
white, whilst in other neighbouring parts, which are of the brightest red 
colour, it seems to be deposited in greater quantity ; some other masses 
are marbled with two distinct colours. Portions of the white clay, now 
that they are dry, cannot be distinguished by the eye from the finest 
prepared chalk ; and when placed between the teeth they feel equally 
soft-grained; the inhabitants use this substance for white-washing 
their houses. The cause of the iron being dissolved in one part, and 
close by being again deposited, is obscure ; but the fact has been 
observed in several other places.* In some half-decayed specimens, I 
found small, globular aggregations of yellow hyalite, resembling gum- 
arabic, which no doubt had been deposited by the steam. 

As there is no escape for the rain-water, which trickles down the 
sides of the ravine-like hollow, whence the steam issues, it must all 
percolate downwards through the fissures at its bottom. Some of the 
inhabitants informed me that it was on record that flames (some 
luminous appearance?) had originally proceeded from these cracks, 

* Spallanzani, Dolomieu, and Hoffman have described similar cases in the 
Italian volcanic islands. Dolomieu says the iron at the Panza Islands is 
redeposited in the form of veins (p. 86, " Memoire sur les Isles Ponces "). 
These authors likewise believe that the steam deposits siHca : it is now 
experimentally known that vapour of a high temperature is able to dissolve 
silica. 



TAHITI. 183 

and that the flames had been succeeded by the steam ; but I was not 
able to ascertain how long this was ago, or anything certain on the 
subject. When viewing the spot, I imagined that the injection of a 
large mass of rock, like the cone of phonolite at Fernando Noronha, 
in a semi-fluid state, by arching the surface might have caused a 
wedge-shaped hollow with cracks at the bottom, and that the 
rain-water percolating to the neighbourhood of the heated mass, 
would during many succeeding years be driven back in the form of 
steam. 

Tahiti {Otaheite). — I visited only a part of the north-western side of 
this island, and this part is entirely composed of volcanic rocks. Near 
the coast there are several varieties of basalt, some abounding with 
large crystals of augite and tarnished olivine, others compact and 
earthy, — some slightly vesicular, and others occasionally amygdaloidal. 
These rocks are generally much decomposed, and to my surprise, I 
found in several sections that it was impossible to distinguish, even 
approximately, the line of separation between the decayed lava and 
the alternating beds of tuff. Since the specimens have become dry, 
it is rather more easy to distinguish the decomposed igneous rocks 
from the sedimentary tuffs. This gradation in character between rocks 
having such widely different origins, may I think be explained by the 
yielding under pressure of the softened sides of the vesicular cavities, 
which in many volcanic rocks occupy a large proportion of their bulk. 
As the vesicles generally increase in size and number in the upper parts 
of a stream of lava, so would the effects of their compression increase ; 
the yielding, moreover, of each lower vesicle must tend to disturb all 
the softened matter above it. Hence we might expect to trace a 
perfect gradation from an unaltered crystalline rock to one in which all 
the particles (although originally forming part of the same solid mass) 
had undergone mechanical displacement ; and such particles could hardly 
be distinguished from others of similar composition, which had been 
deposited as sediment. As lavas are sometimes laminated in their 
upper parts even horizontal lines, appearing like those of aqueous 
deposition, could not in all cases be rehed on as a criterion of 
sedimentary origin. From these considerations it is not surprising that 
fcrmerly many geologists believed in real transitions from aqueous 
deposits, through wacke, into igneous traps. 

in the valley of Tia-auru, the commonest rocks are basalts with mucli 
olivine, and in some cases almost composed of large crystals of augito. 
I picked up some specimens, with much glassy feldspar, approaching in 
character to trachyte. There were also many large blocks of vesicular 
basalt, with the cavities beautifully lined with chabasie (?), and radiat 
ing bundles of mesotype. Some of these specimens presented a 
curious appearance, owing to a number of the vesicles being half filled 
up with a white, soft, earthy mesotypic mineral, which intumesced 
under the blowpipe in a remarkable manner. As the upper surfaces 
in all the half-filled cells are exactly parallel, it is evident that this 
substance has sunk to the bottom of each cell from its weight. Some- 
times, however, it entirely fills the cells. Other cells are either quite 



1 84 MAURITIUS. 

filled, or lined, with small crystals, apparently of chabasie ; these 
crystals, also, frequently line the upper half of the cells partly filled 
with the earthy mineral, as well as the upper surface of this substance 
itself, in which case the two minerals appear to blend into each other. 
I have never seen any other amygdaloid * with the cells half filled in 
the manner here described ; and it is difficult to imagine the causes 
which determined tlie earthy mineral to sink from its gravity to the 
bottom of the cells, and the crystalline mineral to adhere in a coating of 
equal thickness round the sides of the cells. 

The basaltic strata on the sides of the valley are gently inclined 
seaward, and I nowhere observed any sign of disturbance ; the strata 
are separated from each other by thick, compact beds of conglomerate, 
in which the fragments are large, some being rounded, but most angular. 
From the character of these beds, from tlie compact and crystalline 
condition of most of the lavas, and from the nature of the infiltrated 
minerals, I was led to conjecture that they had originally flowed 
beneath the sea. This conclusion agrees with the fact that the Rev. 
W. Elhs found marine remains at a considerable height, which he 
believes were interstratified with volcanic matter; as is likewise de- 
scribed to be the case by Messrs. Tyerman and Bennett at Huaheine, an 
island in this same archipelago. Mr. Stutchbury also discovered near 
the summit of one of the loftiest mountains of Tahiti, at the height of 
several thousand feet, a stratum of semi-fossil coral. None of these 
remains have been specifically examined. On the coast, where masses 
of coral-rock would have afforded the clearest evidence, I looked in 
vain for any signs of recent elevation. For references to the above 
authorities, and for more detailed reasons for not believing that Tahiti 
has been recently elevated, I must refer to the " Structure and Dis- 
tribution of Coral-Reefs." 

Mauritius. — Approaching this island on the northern or north- 
western side, a curved chain of bold mountains, surmounted by rugged 
pinnacles, is seen to rise from a smooth border of cultivated land, 
which gently slopes down to the coast. At the first glance, one is 
tempted to believe that the sea lately reached the base of these moun- 
tains, and upon examination, this view, at least with respect to the 
inferior parts of the border, is found to be perfectly correct. Several 
authors f have described masses of upraised coral-rock round the 
greater part of the circumference of the island. Between Tamarin Bay 

* MacCulloch, however, has described and given a plate of (" Geolog. 
Trans.," ist series, vol. iv., p. 225) a trap rock, with cavities filled up hori- 
zontally with quartz and chalcedony. The upper halves of these cavities 
are often filled by layers, which follow each irregularity of the surface, and 
by little depending stalactites of the same siliceous substances. 

f Captain Carmichael, in Hooker's " Bot. Misc.," vol. ii., p. 301. Captain 
Lloyd has lately, in the "Proceedings of the Geological Society " (vol. iii., p. 
317% described carefully some of these masses. In the "Voyage a I'lsle 
de France, par un Officier du Roi," many interesting facts are given on this 
subject. Consult also " Voyage aux Quatre Igles d'Afrique, pgr M. Bory St. 
VinccnJ." 



MAURITIUS. I8S 

and the Great Black River I observed, in company with Captain Lloyd, 
two hillocks of coral-rock, formed in their lower part of hard calcareous 
sandstone, and in their upper of great blocks, slightly aggregated, of 
Astrsea and Madrepora, and of fragments of basalt ; they were divided 
into beds dipping seaward, in one case at an angle of 8°, and in the 
other at i8° ; they had a water-worn appearance, and they rose abruptly 
from a smooth surface, strewed with rolled debris of organic remains, 
to a height of about twenty feet. The Officier du Roi, in his most 
interesting tour in 1768 round the island, has described masses of 
upraised coral-rocks, still retaining that moat-like structure (see my 
" Coral Reefs ") which is characteristic of the living reefs. On the 
coast northward of Port Louis, I found the lava concealed for a con- 
siderable space inland by a conglomerate of corals and shells, like 
those on the beach, but in parts consolidated by red ferruginous matter. 
M. Bory St. Vincent has described similar calcareous beds over nearly 
the whole of the plain of Pamplemousses. Near Port Louis, when 
turning over some large stones, which lay in the bed of a stream at the 
head of a protected creek, and at the height of some yards above the 
level of spring tides, I fouird several shells of serpula still adhering to 
their under sides. 

The jagged mountainsnear Port Louis rise to a height of between 
two and three thousand feet ; they consist of strata of basalt, obscurely 
separated from each other by firmly aggregated beds of fragmentary 
matter ; and they are intersected by a few vertical dikes. The basalt 
in some parts abounds with large crystals of augite and olivine, and is 
generally compact. The interior of the island forms a plain, raised 
probably about a thousand feet above the level of the sea, and composed 
of streams of lava which have flowed round and between the rugged 
basaltic mountains. These more recent lavas are also basaltic, but less 
compact, and some of them abound with feldspar, so that they even fuse 
into a pale coloured glass. On the banks of the Great River, a section 
is exposed nearly five hundred feet deep, worn through numerous thin 
sheets of the lava of this series, which are separated from each other by 
beds of scoriae. They seem to have been of subaerial formation, and 
to have flowed from several points of eruption on the central platform, 
of which the Piton du Milieu is said to be the principal one. There 
are also several volcanic cones, apparently of this modern period, round 
the circumference of the island, especially at the northern end, where 
they form separate islets. 

The mountains composed of the more compact and crystalline basalt, 
form the main skeleton of the island. M. Bailly * states that they all 
" se developpent autour d'elle comme une ceinture d'immenses remparts, 
toutes affectant une pente plus ou moinsenclin^e vers le rivage de la mer ; 
tandis, au contraire, que vers le centre de I'ile elles presentent une coupe 
abrupte, et souvent taillee a pic. Toutes ces montagnes sont form^es de 
couches parallelcs inclinees du centre de Tile vers la mer." These state- 
ments have been disputed, though not in detail, by M. Quoy, in the voyage 
pf Frcycinet. As far as my limited means of observation went, I found 

* " Voyage aux Terrcs Austrsles," torn, i., p. 54. 



i86 MAURITIUS. 

them perfectly correct.* The mountains on the N.W. side of the island, 
which I examined, namely, La Pouce, Peter Botts, Corps de Garde, Les 
Mamelles, and apparently another farther southward, have precisely 
the external shape and stratification described by M. Bailly. They 
form about a quarter of his girdle of ramparts. Although these moun- 
tains now stand quite detached, being separated from each other by 
breaches, even several miles in width, through which deluges of lava 
have flowed from the interior of the island ; nevertheless, seeing their 
close general similarity, one must feel convinced that they originally 
formed parts of one continuous mass. Judging from the beautiful map 
of the Mauritius, published by the Admiralty from a French MS., there 
is a range of mountains (M. Bamboo) on the opposite side of the 
island, which correspond in height, relative position, and external form, 
with thosejust described. Whether the girdle was ever complete may 
well be doubted ; but from M. Bailly 's statements, and my own observa- 
tions, it may be safely concluded that mountains with precipitous 
inland flanks, and composed of strata dipping outwards, once extended 
round a considerable portion of the circumference of the island. The 
ring appears to have been oval and of vast size; its shorter axis, 
measured across from the inner sides of the mountains near Port Louis 
and those near Grand Port, being no less than thirteen geographical 
miles in length. M. Bailly boldly supposes that this enormous gulf, 
which has since been filled up to a great extent by streams of modern 
lava, was formed by the sinking in of the whole upper part of one great 
volcano. 

It is singular in how many respects those portions of St. Jago and of 
Mauritius which I visited agree in their geological history. At both 
islands, mountains of similar external form, stratification, and (at least 
in their upper beds) composition, follow in a curved chain the coast-line. 
These mountains in each case appear originally to have formed parts 
of one continuous mass. The basaltic strata of which they are 
composed, from their compact and crystalline structure, seem, when 
contrasted with the neighbouring basaltic streams of subaerial formation, 
to have flowed beneath the pressure of the sea, and to have been 
subsequently elevated. We may suppose that the vi-ide breaches 
between the mountains were in both cases worn by the waves, during 
their gradual elevation — of which process, within recent times, there is 
abundant evidence on the coast-land of both islands. At both, vast 
streams of more recent basaltic lavas have flowed from the interior of 
the island, round and between the ancient basaltic hills ; at both, more- 
over, recent cones of eruption are scattered around the circumference 
of the island ; but at neither have eruptions taken place within the 
period of history. As remarked in the last chapter, it is probable that 
these ancient basaltic mountains, which resemble (at least in many 
respects) the basal and disturbed remnants of two gigantic volcanoes, 
owe their present form, structure, and position, to the action of similar 
causes. 

* M. Lesson, in his account of this island, in the "Voyage of the Coquille,' 
seems to follow M. Bailly's views. 



ST. PAUL'S ROCKS. 187 

5/. Paul's Pocks. — This small island is situated in the Atlantic Ocean, 
nearly one degree north of the equator, and 540 miles distant from 
South America, in 29° 1 5' west longitude. Its highest point is scarcely 
fifty feet above the level of the sea ; its outline is irregular, and its 
entire circumference barely three-quarters of a mile. This little point 
of rock rises abruptly out of the ocean ; and, except on its western side, 
soundings were not obtained, even at the short distance of a quarter of 
a mile from its shore. It is not of volcanic origin ; and this circum- 
stance, which is the m.ost remarkable point in its history (as will 
hereafter be referred to), properly ought to exclude it from the present 
volume. It is composed of rocks, unlike any which I have met with, 
and which I cannot characterise by any name, and must therefore 
describe. 

The simplest, and one of the most abundant kinds, is a very compact, 
heavy, greenish-black rock, having an angular, irregular fracture, with 
some points just hard enough to scratch glass, and infusible. This 
variety passes into others of paler green tints, less hard, but with a 
more crystalline fracture, and translucent on their edges ; and these are 
fusible into a green enamel. Several other varieties are chiefly charac- 
terised by containing innumerable threads of dark-green serpentine, 
and by having calcareous matter in their interstices. These rocks have 
an obscure, concretionary structure, and are full of variously coloured 
angular pseudo fragments. These angular pseudo fragments consist of 
the first-described dark green rock, of a brown softer kind, of serpentine, 
and of a yellowish harsh stone, which, perhaps, is related to serpentine 
rock. There are other vesicular, calcareo-ferruginous, soft stones. 
There is no distinct stratification, but parts are imperfectly laminated ; 
and the whole abounds with innumerable veins, and vein-like masses, 
both small and large. Of these vein-like masses, some calcareous ones, 
which" contain minute fragments of shells, are clearly of subsequent 
origin to the others. 

A glossy iiicrustation. — Extensive portions of these rocks are coated 
by a layer of a glossy polished substance, with a pearly lustre and of a 
greyish white colour ; it follows all the inequalities of the surface, to 
which it is firmly attached. When examined with a lens, it is found to 
consist of numerous exceedingly thin layers, their aggregate thickness 
being about the tenth of an inch. It is considerably harder than cal- 
careous spar, but can be scratched with a knife ; under the blowpipe 
it scales off, decrepitates, slightly blackens, emits a fetid odour, and 
becomes strongly alkaline : it does not effervesce in acids.* I presume 
this substance has been deposited by water draining from the birds' 
dung, with which the rocks are covered. At Ascension, near a cavity 
in the rocks which was filled with a laminated mass of infiltrated birds' 
dung, I found some irregularly formed, stalactitical masses of apparently 
the same nature. These masses, when broken, had an earthy texture ; 
but on their outsides, and especially at their extremities, they were 
formed of a pearly substance, generally in little globules, like the 

* In my "Journal" I have described this substance; I then believed that 
it was an impure phosphate of lime. 



i88 BASALTIC ROCKS. 

enamel of teeth, but more translucent, and so hard as just to scratch 
plate-glass. This substance slightly blackens under the blowpipe, 
emits a bad smell, then becomes quite white, swelling a little, and fuses 
into a dull white enamel ; it does not become alkaline ; nor does it 
effervesce in acids. The whole mass had a collapsed appearance, as if 
in the formation of the hard glossy crust the whole had shrunk much. 
At the Abrolhos Islands on the coast of Brazil, where also there is 
much birds' dung, I found a great quantity of a brown, arborescent 
substance adhering to some trap-rock. In its arborescent form, this 
substance singularly resembles some of the branched species of 
Nullipora. Under the blowpipe, it behaves like the specimens from 
Ascension ; but it is less hard and glossy, and the surface has not the 
shrunk appearance. 



CHAPTER III. 

ASCENSION. 

Basaltic lavas. — Numerous craters truncated on the same side. — Singular 
structure of volcanic bombs. — Aeriform explosions. — Ejected granitic 
fragments. — Trachytic rocks. — Singular veins. — Jasper, its manner of 
formation. — Concretions in pumiceous tuff. — Calcareous deposits and 
frondescent incrustations on the coast. — Remarkable laminated beds, 
alternating with, and passing into, obsidian. — Oi'igin of obsidian. — 
Lamination of volcanic rocks. 

This island is situated in the Atlantic Ocean, in lat. 8° S., long. 14° W. 
It has the form of an irregular triangle (see accompanying map), each 
side being about six miles in length. Its highest point is 2,870 feet* 
above the level of the sea. The whole is volcanic, and, from the absence 
of proofs to the contrary, I believe of subaerial origin. The funda- 
mental rock is everywhere of a pale colour, generally compact, and of 
a feldspathic nature. In the S.E. portion of the island, where the 
highest land is situated, well characterised trachyte, and other con- 
generous rocks of that varying family, occur. Nearly the entire circum- 
ference is covered up by black and rugged streams of basaltic lava, 
with here and there a hill or single point of rock (one of which near the 
sea-coast, north of the Fort, is only two or three yards across) of the 
trachyte still remaining exposed. 

Basaltic rocks. — The overlying basaltic lava is in some parts extremely 
vesicular, in others little so ; it is of a black colour, but sometimes 
contains crystals of glassy feldspar, and seldom much olivine. These 
streams appear to have possessed singularly little fluidity; their side 
walls and lower ends being very steep, and even as much as between 
twenty and thirty feet in height. Their surface is extraordinarily rugged, 

♦ Geographical Journal^ vol. v., p. 243. 







1^ 



=^ 



ASCENSION. 189 

and from a short distance appears as if studded with small craters. 
These projections consist of broad, irregularly conical, hillocks, traversed 
by fissures, and composed of the same unequally scoriaceous basalt 
with the surrounding streams, but having an obscure tendency to a 
columnar structure ; they rise to a height between ten and thirty feet 
above the general surface, and have been formed, as I presume, by the 
heaping up of the viscid lava at points of greater resistance. At the 
base of several of these hillocks, and occasionally likewise on more 
level parts, solid ribs, composed of angulo-globular masses of basalt, 
resembling in size and outline arched sewers or gutters of brickwork, 
but not being hollow, project between two or three feet above the 
surface of the streams ; what their origin may have been, I do not 
know. Many of the superficial fragments from these basaltic streams 
present singularly convoluted forms ; and some specimens could hardly 
be distinguished from logs of dark-coloured wood without their bark. 

Many of the basaltic streams can be traced, either to points of 
eruption at the base of the great central mass of trachyte, or to separate, 
conical, red-coloured hills, which are scattered over the northern and 
western borders of the island. Standing on the central eminence, I 
counted between twenty and thirty of these cones of eruption. The 
greater number of them had their truncated summits cut off obliquely, 
and they all sloped towards the S.E., whence the trade-wind blows.* 
This structure no doubt has been caused by the ejected fragments and 
ashes being always blown, during eruptions, in greater quantity towards 
one side than towards the other. M. Moreau de Jonnes has made a 
similar observation with respect to the volcanic orifices in the West 
Indian Islands. 

Volcanic bo??ibs. — These occur in great numbers strewed on the 
ground, and some of them lie at considerable distances from any points 
of eruption. They vary in size from that of an apple to that of a man's 
body ; they are either spherical or pear-shaped, or with the hinder part 
(corresponding to the tail of a comet) irregular, studded with projecting 
jjoints, and even concave. Their surfaces are rough, and fissured with 
branching cracks ; their internal structure is either irregularly scoriaceous 
and compact, or it presents a symmetrical and very curious appearance. 
An irregular segment of a bomb of this latter kind, of which I found 
several, is accurately represented in the woodcut on the next page. Its 
size was about that of a man's head. The whole interior is coarsely 
cellular ; the cells averaging in diameter about the tenth of an inch ; 
but nearer the outside they gradually decrease in size. This part is 
succeeded by a well-defined shell of compact lava, having a nearly 
uniform thickness of about the third of an inch ; and the shell is over- 
laid by a somewhat thicker coating of finely cellular lava (the cells 
varying from the fiftieth to the hundredth of an inch in diameter), which 
forms the external surface : the line separating the shell of compact 

* M. Lesson, in the " Zoology of the Voyage of the Coquille " (p. 490), 
has observed this fact. Mr. Hennah (" Geolog. Proceedings," 1835, p. 189) 
further remarks that the most extensive beds of ashes at Ascension invariably 
occur on the leeward side of the island, 



190 ASCENSION. 

lava from the outer scoriaceous crust is distinctly defined. This struc- 
ture is very simply explained, if we suppose a mass of viscid, scoria- 
ceous matter, to be projected with a rapid, rotatory motion through the 
air; for whilst the external crust, from cooling, became solidified (in 
the state we now see it), the centrifugal force, by relieving the pressure 
in the interior parts of the bomb, would allow the heated vapours to 
expand their cells ; but these being driven by the same force against 
the already-hardened crust, would become, the nearer they were to this 
part, smaller and smaller or less expanded, until they became packed 
into a solid, concentric shell. As we know that chips from a grind- 
stone* can be flirted off, when made to revolve with sufficient velocity, 
we need not doubt that the centrifugal force would have power to 

No. 3. 




Fragment of a spherical volcanic bomb, with the interior parts coarsely cellular, 
coated by a concentric la3'er of compact lava, and this again by a crust of finely 
cellular rock. 

modify the structure of a softened bomb, in the manner here supposed. 
Geologists have remarked, that the external form of a bomb at once 
bespeaks the history of its aerial course, and few now see that the 
internal structure can speak, with almost equal plainness, of its rotatory 
movement. 

M. Bory St Vincent f has described some balls of lava from the Isle 
of Bourbon, which have a closely similar structure ; his explanation, 
however (if I understand it rightly), is very different from that which 

* Nichol's "Architecture of the Heavens." 

■j- "Voyage aux Quatre Isles dAfriquc," torn, i., p. 222. 




VOLCANIC BOMBS. I9^ 

1 have given ; for he supposes that they have rolled, like snowballs, 
down the sides of the crater. M. Beudant, * also, has described some 
singular little balls of obsidian, never more than six or eight inches in 
diameter, which he found strewed on the surface of the ground : their 
form is always oval ; sometimes they are much swollen in the middle, 
and even spindle-shaped : their surface is regularly marked with con- 
centric ridges and furrows, all of which on the same ball are at right 
angles to one axis : their interior is compact and glassy. M. Beudant 
supposes that masses of lava, when soft, were 
shot into the air, with a rotatory movement round ^°* 4* 

the same axis, and that the form and superficial 
ridges of the bombs were thus produced. Sir 
Thomas Mitchell has given me what at first ap- 
pears to be the half of a much flattened oval ball 
of obsidian ; it has a singular artificial-like ap- 
pearance, which is well represented (of the natural 
size) in the accompanying woodcut. It was founa 
in its present state, on a great sandy plain be- 
tween the rivers Darling and Murray, in Australia, 
and at the distance of several hundred miles 
from any known volcanic region. It seems to 
liave been embedded in some reddish tufaceous 
matter ; and may have been transported either by 

the aborigines or by natural means. The external ,. , . , u r , j- 
saucer consists of compact obsidian, of a bottle- ^jJom'^'AuTt^a'lL The 
green colour, and is filled with finely cellular upper figure gives a front 
black lava, much less transparent and glassy than ^^^^ ^ ^J^l ^°w^^ \?'^,« 

, T_ . T ^, ^ , c • "^ , J View of the same object, 

the obsidian. The external surface is marked 

with four or five not quite perfect ridges, which are represented rather 
too distinctly in the woodcut. Here, then, we have the external structure 
described by M. Beudant, and the internal cellular condition of the 
bombs from Ascension. The lip of the saucer is slightly concave, 
exactly like the margin of a soup-plate, and its inner edge overlaps a 
little the central cellular lava. This structure is so symmetrical round 
the entire circumference, that one is forced to suppose that the bomb 
burst during its rotatory course, before being quite solidified, and that 
the lip and edges were thus slightly modified and turned inwards. 
It may be remarked that the superficial ridges are in planes, at right 
angles to an axis, transverse to the longer axis of the flattened oval : 
to explain this circumstance, we may suppose that when the born'o 
burst, the axis of rotation changed. 

Aeriforin explosions. — The flanks of Green Mountain and the sur- 
rounding country are covered by a great mass, some hundred feet in 
thickness, of loose fragments. The lower beds generally consist ot 
fine-grained, slightly consolidated tuffs,t and the upper beds of great 

* 'Voyage en Hongrie," torn, ii., p. 214. 

f Some of this peperino, or tuff, is sufficiently hard not to be broken by 
the greatest force of the fingers. 




192 ASCENSION. 

loose fragments, with alternating finer beds.* One white ribbon-like 
layer of decomposed, pumiceous breccia, was curiously bent into deep 
unbroken curves, beneath each of the large fragments in the superin- 
cumbent stratum. From the relative position of these beds, I presume 
that a narrow-mouthed crater, standing nearly in the position of Green 
Mountain, like a great air-gun, shot forth, before its final extinction, 
this vast accumulation of loose matter. Subsequently to this event, 
considerable dislocations have taken place, and an oval circus has been 
formed by subsidence. This sunken space lies at the north-eastern 
foot of Green Mountain, and is well represented in the accompanying 
map. Its longer axis, which is connected with a N.E. and S.W. line of 
fissure, is three-fifths of a nautical mile in length ; its sides are nearly 
perpendicular, except in one spot, and about four hundred feet in 
height; they consist, in the lower part, of a pale basalt with feldspar, and 
in the upper part, of the tuff and loose ejected fragments ; the bottom 
is smooth and level, and under almost any other climate a deep lake 
would have been formed here. From the thickness of the bed of loose 
fragments, with which the surrounding country is covered, the amount 
of aeriform matter necessary for their projection must have been 
enormous ; hence we may suppose it probable that after the explosions 
vast subterranean caverns were left, and that the falling in of the roof 
of one of these produced the hollow here described. At the Galapagos 
Archipelago, pits of a similar character, but of a much smaller size, 
frequently occur at the bases of small cones of eruption. 

Ejected gra7iitic f7'agme7its. — In the neighbourhood of Green Moun- 
tain, fragments of extraneous rock are not unfrequently found embedded 
in the midst of masses of scoriae. Lieutenant Evans, to whose kindness 
I am indebted for much information, gave me several specimens, and 
I found others myself. They nearly all have a granitic structure, are 
brittle, harsh to the touch, and apparently of altered colours. First, a 
white syenite, streaked and mottled with red ; it consists of well- 
crystallised feldspar, numerous grains of quartz, and brilliant, though 
small, crystals of hornblende. The feldspar and hornblende in this 
and the succeeding cases have been determined by the refiecting gonio- 
meter, and the quartz by its action under the blowpipe. The feldspar 
in these ejected fragments, like the glassy kind in the trachyte, is from 
its cleavage a potash-feldspar. Secoiidly, a brick-red mass of feldspar, 
quartz, and small dark patches of a decayed mineral ; one minute 
particle of which I was able to ascertain, by its cleavage, to be horn- 

* On the northern side of the Green Mountain a thin seam, about an inch 
in thickness, of compact oxide of iron, extends over a considerable area ; it 
lies conformably in the lower part of the stratified mass of ashes and fragments. 
This substance is of a reddish-brown colour, with an almost metallic lustre; 
it is not magnetic, but becomes so after having been heated under the 
blowpipe, by which it is blackened and partly fused. This seam of compact 
stone, by intercepting the little rain-water which falls on the island, gives 
rise to a small dripping spring, first discovered by Dampier. It is the only 
fresh water on the island, so that the possibility of its being inhabited has 
entirely depended on the occurrence of this ferruginous layer. 



TRACHYTIC ROCKS. 193 

blende. Thirdly^ a mass of confusedly crystallised white feldspar, 
with little nests of a dark-coloured mineral, often carious, externally 
rounded, having a glossy fracture, but no distinct cleavage : from com- 
parison with the second specimen, I have no doubt that it is fused 
hornblende. Fourl/ily, a rock, which at first appears a simple aggrega- 
tion of distinct and large-sized crystals of dusty-coloured Labrador 
feldspar ; * but in their interstices there is some white granular feldspar, 
abundant scales of mica, a little altered hornblende, and, as I believe, 
no quartz. I have described these fragments in detail, because it is 
raret to find granitic rocks ejected from volcanoes with their mitierals 
unchanged, as is the case with the first specimen, and partially with the 
second. One other large fragment, found in another spot, is deserving 
of notice ; it is a conglomerate, containing small fragments of granitic, 
cellular, and jaspery rocks, and of hornstone porphyries, embedded in 
a base of wacke, threaded by numerous thin layers of a concretionary 
pitchstone passing into obsidian. These layers are parallel, slightly 
tortuous, and short ; they thin out at their ends, and resemble in form 
the layers of quartz in gneiss. It is probable that these small embedded 
fragments were not separately ejected, but were entangled in a fluid 
volcanic rock, allied to obsidian ; and we shall presently see that 
several varieties of this latter series of rock assume a laminated 
structure. 

Trachytic series of rocks. — Those occupy the more elevated and 
central, and likewise the south-eastern, parts of the island. The 
trachyte is generally of a pale brown colour, stained with small darker 
patches ; it contains broken and bent crystals of glassy feldspar, grains 
of specular iron, and black microscopical points, which latter, from 
being easily fused, and then becoming magnetic, I presume are horn- 
blende. The greater number of the hills, however, are composed of a 
quite white, friable stone, appearing like a trachytic tuff. Obsidian, 

* Professor Miller has been so kind as to examine this mineral. He 
obtained two good cleavages of 86° 30' and 86° 50'. The mean of several, 
which I made, was 86° 30'. Professor Miller states that these crystals, when 
reduced to a fine powder, are soluble in hydrochloric acid, leaving some 
undissolved silex behind ; the addition of oxalate of ammonia gives a copious 
precipitate of lime. He further remarks, that according to Von Kobell, 
anorthite (a mineral occurring in the ejected fragments at Mount Somma) is 
always white and transparent, so that if this be the case, these crystals from 
Ascension must be considered as Labrador feldspar. Professor Miller adds, 
that he has seen an account, in Erdmann's "Journal fur tecnische Chemie,' 
of a mineral ejected from a volcano, which had the external characters of 
Labrador feldspar, but differed in the analysis from that given by mineralo- 
gists of this mineral : the author attributed this difference to an error in the 
analysis of Labrador feldspar, which is very old. 

f Daubeny, in his work on Volcanoes (p. 386), remarks that this is the 
case ; and Humboldt, in his " Personal Narrative " (vol. i., p. 236), says, " In 
general, the masses of known primitive rocks, I mean those which perfectly 
resemble our granites, gneiss, and mica-slate, are very rare in lavas : the 
substances we generally denote by the name of granite, thrown out by 
Vesuvius, are mixtures of nepheline, mica, and pyroxene," 

13 



194 ASCENSION. 

hornstone, and several kinds of laminated feldspathic rocks, are associ- 
ated with the trachyte. There is no distinct stratification ; nor could I 
distinguish a crateriform structure in any of the hills of this series. 
Considerable dislocations have taken place ; and many fissures in these 
rocks are yet left open, or are only partially filled with loose fragments. 
Within the space,* mainly formed of trachyte, some basaltic streams 
have burst forth; and not far from the summit of Green Mountain, 
there is one stream of quite black, vesicular basalt, containing minute 
crystals of glassy feldspar, which have a rounded appearance. 

The soft white stone above mentioned is remarkable from its singular 
resemblance, when viewed in mass, to a sedimentary tuff: it was long 
before I could persuade myself that such was not its origin ; and other 
geologists have been perplexed by closely similar formations in trachytic 
regions. In two cases, this white earthy stone formed isolated hills ; 
in a third, it was associated with columnar and laminated trachyte ; but 
I was unable to trace an actual junction. It contains numerous crystals 
of glassy feldspar and black microscopical specks, and is marked with 
small darker patches, exactly as in the surrounding trachyte. Its basis, 
however, when viewed under the microscope, is generally quite earthy ; 
but sometimes it exhibits a decidedly crystalline structure. On the hill 
marked " Crater of an old volcano," it passes into a pale greenish-grey 
variety, differing only in its colour, and in not being so earthy ; the 
passage was in one case effected insensibly ; in another, it was formed 
by numerous, rounded and angular, masses of the greenish variety, 
being embedded in the white variety ; — in this latter case, the appear- 
ance was very much like that of a sedimentary deposit, torn up and 
abraded during the deposition of a subsequent stratum. Both these 
varieties are traversed by innumerable tortuous veins (presently to be 
described), which are totally unlike injected dikes, or indeed any other 
veins which I have ever seen. Both varieties include a few scattered 
fragments, large and small, of dark-coloured scoriaceous rocks, the cells 
of some of which are partially filled with the white earthy stone ; they 
likewise include some huge blocks of a cellular porphyry.t These 
fragments project from the weathered surface, and perfectly resemble 
fragments embedded in a true sedimentary tuff. But as it is known 
that extraneous fragments of cellular rock are sometimes included in 
columnar trachyte, in phonolite,:|: and in other compact lavas, this cir- 
cumstance is not any real argument for the sedimentary origin of the 
white earthy stone. § The insensible passage of the greenish variety 

* This space is nearly included by a line sweeping round Green Moun- 
tain, and joining the hills, called the Weather Port Signal, Holyhead, and 
that denominated (improperly in a geological sense) "the Crater of an old 
volcano." 

f The porph3a-y is dark coloured ; it contains numerous, often fractured, 
crystals of white opaque feldspar, also decomposing crystals of oxide of 
iron ; its vesicles include masses of delicate, hair-like, crystals, apparently of 
analcime. 

X D'Aubuisson, " Traits de G^ognosie," torn, ii., p. 548. 

§ Dr. Daubeny (on Volcanoes, p. 180) seems to have been led to believe that 



VEINS IN THE EARTHY TRACHYTIC MASSES. 195 

into the white one, and likewise the more abrupt passage by fragments 
of the former being embedded in the latter, might result from slight 
differences in the composition of the same mass of molten stone, and 
from the abrading action of one such part still fluid on another part 
already solidified. The curiously formed veins have, I believe, been 
formed by siliceous matter being subsequently segregated. But my 
chief reason for believing that these soft earthy stones, with their 
extraneous fragments, are not of sedimentary origin, is the extreme 
improbability of crystals of feldspar, black microscopical specks, and 
small stains of a darker colour occurring in the same proportional 
numbers in an aqueous deposit, and in masses of solid trachyte. More- 
over, as I have remarked, the microscope occasionally reveals a 
crystalline structure in the apparently earthy basis. On the other 
hand, the partial decomposition of such great masses of trachyte, form- 
ing whole mountains, is undoubtedly a circumstance of not easy 
explanation. 

Veins in the earthy trachytic masses. — These veins are extraordinarily 
numerous, intersecting in the most complicated manner both coloured 
varieties of the earthy trachyte : they are best seen on the flanks of the 
" Crater of the old volcano." They contain crystals of glassy feldspar, 
black microscopical specks and little dark stains, precisely as in the 
surrounding rock ; but the basis is very different, being exceedingly 
hard, compact, somewhat brittle, and of rather less easy fusibility. The 
veins vary much, and suddenly, from the tenth of an inch to one inch 
in thickness ; they often thin out, not only on their edges, but in their 
central parts, thus leaving round, irregular apertures ; their surfaces are 
rugged. They are inclined at every possible angle with the horizon, or 
are horizontal ; they are generally curviHnear, and often interbranch one 
with another. From their hardness they withstand weathering, and 
projecting two or three feet above the ground, they occasionally extend 
some yards in length ; these plate-like veins, when struck, emit a 
sound, almost like that of a drum, and they may be distinctly seen to 
vibrate ; their fragments, which are strewed on the ground, clatter like 
pieces of iron when knocked against each other. They often assume 
the most singular forms ; I saw a pedestal of the earthy trachyte, 
covered by a hemispherical portion of a vein, like a great umbrella, 
sufficiently large to shelter two persons. I have never met with, or 
seen described, any veins like these ; but in form they resemble the 
ferruginous seams, due to some process of segregation, occurring not 
uncommonly in sandstones, — for instance, in the New Red sandstone 
of England. Numerous veins of jasper and of siliceous sinter, occurring 
on the summit of this same hill, show that there has been some abun- 
dant source of silica, and as these plate-like veins differ from the trachyte 

certain trachytic formations of Ischia and of the Pay de D6me, which closely 
resemble these of Ascension, were of sedimentary origin, chiefly from the 
frequent presence in them "of scoriform portions, different in colour from 
the matrix." Dr. Daubeny adds, that on the other hand, Brocchi, and other 
eminent geologists, have considered these beds as earthy varieties of 
trachyte ; he considers the subject deserving of further attention. 



19^ AScM:§lOM, 

only in their greater hardness, brittleness, and less easy fusibility, it 
appears probable that their origin is due to the segregation or infiltra- 
tion of siliceous matter, in the same manner as happens with the oxides 
of iron in many sedimentary rocks. 

Siliceous sinter atid jasper. — The siliceous sinter is either quite white, 
of little specific gravity, and with a somewhat pearly fracture, passing 
into pinkish pearly quartz ; or it is yellowish white, with a harsh 
fracture, and it then contains an earthy powder in small cavities. Both 
varieties occur, either in large irregular masses in the altered trachyte, 
or in seams included in broad, vertical, tortuous, irregular veins of a 
compact, harsh stone of a dull red colour, appearing like a sandstone. 
This stone, however, is only altered trachyte ; and a nearly similar 
variety, but often honej'combed, sometimes adheres to the projecting 
plate-like veins, described in the last paragraph. The jasper is of an 
ochre yellow or red colour ; it occurs in large irregular masses, and 
sometimes in veins, both in the altered trachyte and in an associated 
mass of scoriaceous basalt. The cells of the scoriaceous basalt are 
lined or filled with fine, concentric layers of chalcedony, coated and 
studded with bright-red oxide of iron. In this rock, especially in the 
rather more compact piirts, irregular angular patches of the red jasper 
are included, the edges of which insensibly blend into the surrounding 
mass ; other patches occur having an intermediate character between 
perfect jasper and the ferruginous, decomposed, basaltic base. In these 
patches, and likewise in the large vein-like masses of jasper, there 
occur little rounded cavities, of exactly the same size and form with the 
air-cells, which in the scoriaceous basalt are filled and lined with layers 
of chalcedony. Small fragments of the jasper, examined under the 
microscope, seem to resemble the chalcedony with its colouring matter 
not separated into layers, but mingled in the siliceous paste, together 
with some impurities. I can understand these facts, — namely, the 
blending of the jasper into the semi-decomposed basalt, — its occurrence 
in angular patches, which clearly do not occupy pre-existing hollows in 
the rock, — and its containing little vesicles filled with chalcedony, like 
those in the scoriaceous lava, — only on the supposition that a fluid, 
probably the same fluid which deposited the chalcedony in the air-cells, 
removed in those parts where there were no cavities, the ingredients 
of the basaltic rock, and left in their place silica and iron, and thus 
produced the jasper. In some specimens of silicified wood, I have 
observed, that in the same manner as in the basalt, the solid parts 
were converted into a dark-coloured homogeneous stone, whereas the 
cavities formed by the larger sap-vessels (which may be compared with 
the air-vesicles in the basaltic lava) and other irregular hollows, appa- 
rently produced by decay, were filled with concentric layers of chalce- 
dony ; in this case, there can be little doubt that the same fluid deposited 
the homogeneous base and the chalcedonic layers. After these con- 
siderations, I cannot doubt but that the jasper of Ascension may be 
viewed as a volcanic rock silicified, in precisely the same sense as this 
term is applied to wood, when silicified; we are equally ignorant of 
the means by which every atom of wood, whilst in a perfect state, is 



SILICEOUS SINTER AND JASPER. 197 

removed and replaced by atoms of silica, as we are of the means by 
which the constituent parts of a volcanic rock could be thus acted on.* 
I was led to the careful examination of these rocks, and to the conclu- 
sion here given, from having heard the Rev. Professor Henslow express 
a similar opinion, regarding the origin in trap-rocks of many chalcedonies 
and agates. Siliceous deposits seem to be very general, if not of 
universal occurrence, in partially decomposed trachytic tuffs ; f and as 
these hills, according to the view above given, consist of trachyte 
softened and altered in situ, the presence of free silica in this case may 
be added as one more instance to the list. 

Concretions in pimiiceous iiiff. — The hill, marked in the map "Crater 
of an old volcano," has no claims to this appellation, which I could 
discover, except in being surmounted by a circular, very shallow, saucer- 
like summit, nearly half a mile in diameter. This hollow has been 
nearly filled up with many successive sheets of ashes and scoriae, of 
different colours, and slightly consolidated. Each successive saucer- 
shaped layer crops out all round the margin, forming so many rings of 
various colours, and giving to the hill a fantastic appearance. The 
outer ring is broad, and of a white colour ; hence it resembles a course 
round which horses have been exercised, and has received the name of 
the Devil's Riding School, by which it is most generally known. These 
successive layers of ashes must have fallen over the whole surrounding 
country, but they have all been blown away except in this one hollow, 
in which probably moisture accumulated, either during an extraordinary 
year when rain fell, or during the storms often accompanying volcanic 
eruptions. One of the layers of a pinkish colour, and chiefly derived 
from small, decomposed fragments of pumice, is remarkable, from con- 
taining numerous concretions. These are generally spherical, from 
half an inch to three inches in diameter ; but they are occasionally 
cylindrical, like those of iron-pyrites in the chalk of Europe. They 
consist of a very tough, compact, pale-brown stone, with a smooth and 
even fracture. They are divided into concentric layers by thin white 
partitions, resembling the external superficies ; six or eight of such 
layers are distinctly defined near the outside ; but those towards the 
inside generally become indistinct, and blend into a homogeneous 

* Beudant (" Voyage en Hongrie," torn, iii., pp. 502, 504) describes kidney- 
shaped masses of jasper-opal, which either blend into the surrounding 
trachytic conglomerate, or are embedded in it like chalk-flints; and he com- 
pares them with the fragments of opaliscd wood, which are abundant in this 
same formation. Beudant, however, appears to have viewed the process of 
their formation rather as one of simple infiltration than of molecular ex- 
change ; but the presence of a concretion, wholly different from the surround- 
ing matter, if not formed in a pre-existing hollow, clearly seems to me to 
require, either a molecular or mechanical oisplacemcnt of the atoms, which' 
occupied the space afterwards filled by it. The jasper-opal of Hungary 
passes into chalcedony, and therefore in this case, as in that of Ascension, 
jasper seems to be intimately related in origin with chalcedony. 

f Beudant ("Voyage Min.," torn, iii., p. 507) enumerates C£ises in Hungary, 
Germany, Central France, Italy, Greece, and Mexico. 



198 ASCENSION. 

mass. I presume that these concentric layers were formed by the 
shrinking of the concretion, as it became compact. The interior part 
is generally fissured by minute cracks or septaria, which are lined, both 
by black, metallic, and by other white and crystalline specks, the nature 
of which I was unable to ascertain. Some of the larger concretions 
consist of a mere spherical shell, filled with slightly consolidated ashes. 
The concretions contain a small proportion of carbonate of lime : a 
fragment placed under the blowpipe decrepitates, then whitens and 
fuses into a blebby enamel, but does not become caustic. The sur- 
rounding ashes do not contain any carbonate of lime ; hence the con- 
cretions have probably been formed, as is so often the case, by the 
aggregation of this substance. I have not met with any account of 
similar concretions ; and considering their great toughness and compact- 
ness, their occurrence in a bed, which probably has been subjected 
only to atmospheric moisture, is remarkable. 

Formation of calcareous rocks on the sea-coast. — On several of the 
sea-beaches, there are immense accumulations of small, well-rounded 
particles of shells and corals, of white, yellowish, and pink colours, in- 
terspersed with a few volcanic particles. At the depth of a few feet, 
these are found cemented together into stone, of which the softer 
varieties are used for building ; there are other varieties, both coarse 
and fine-grained, too hard for this purpose : and I saw one mass 
divided into even layers half an inch in thickness, which were so com- 
pact that when struck with a hammer they rang like flint. It is believed 
by the inhabitants, that the particles become united in the course of a 
single year. The union is effected by calcareous matter ; and in the 
most compact varieties, each rounded particle of shell and volcanic 
rock can be distinctly seen to be enveloped in a husk of pellucid 
carbonate of lime. Extremely few perfect shells are embedded in these 
agglutinated masses ; and I have examined even a large fragment under 
a microscope, without being able to discover the least vestige of striae 
or other marks of external form : this shows how long each particle 
must have been rolled about, before its turn came to be embedded and 
cemented.* One of the most compact varieties, when placed in acid, 
was entirely dissolved, with the exception of some flocculent animal 
matter ; its specific gravity was 2-63. The specific gravity of ordinary 
limestone varies from 2*6 to 275 ; pure Carrara marble was found by 
Sir H. De la Beche f to be 27. It is remarkable that these rocks of 
Ascension, formed close to the surface, should be nearly as compact as 
marble, which has undergone the action of heat and pressure in the 
plutonic regions. 

The great accumulation of loose calcareous particles, lying on the 
beach near the Settlement, commences in the month of October, 
moving towards the S.W., which, as I was informed by Lieutenant 

* The eggs of the turtle being buried by the parent, sometimes become 
enclosed in the solid rock. Mr. Lyell has given a figure (" Principles of 
Geology," book iii., ch. 17) of some eggs, containing the bones of young 
turtles, found thus entombed. 

t " Researches in Theoretical Geology," p. 12, 



CALCAREOUS INCRUSTATION. 199 

Evans, is caused by a change in the prevailing direction of the currents. 
At this period the tidal rocks, at the S.W. end of the beach, where the 
calcareous sand is accumulating, and round which the currents sweep, 
become gradually coated with a calcareous incrustation, half an inch in 
thickness. It is quite white, compact, with some parts slightly spathose, 
and is firmly attached to the rock. After a short time it gradually dis- 
appears, being either redissolved, when the water is less charged with 
lime, or more probably is mechanically abraded. Lieutenant Evans 
has observed these facts, during the six years he has resided at Ascen- 
sion. The incrustation varies in thickness in different years : in 183 1 it 
was unusually thick. When I was there in July, there was no remnant 
of the incrustation ; but on a point of basalt, from which the quarrymen 
had lately removed a mass of the calcareous freestone, the incrustation 
was perfectly preserved. Considering the position of the tidal-rocks, 
and the period at which they become coated, there can be no doubt that 
the movement and disturbance of the vast accumulation of calcareous 
particles, many of them being partially agglutinated together, cause the 
waves of the sea to be so highly charged with carbonate of lime, that 
they deposit it on the first objects against which they impinge. I have 
been informed by Lieutenant Holland, R.N., that this incrustation is 
formed on many parts of the coast, on most of which, I believe, there 
are likewise great masses of comminuted shells. 

A frondescent calcareous incrtcstaiion. — In many respects this is a 
singular deposit ; it coats throughout the year the tidal volcanic rocks, 
that project from the beaches composed of broken shells. Its general 
appearance is well represented in the woodcut on the next page ; but the 
fronds or discs, of which it is composed, are generally so closely crowded 
together as to touch. These fronds have their sinuous edges finely 
crenulated, and they project over their pedestals or supports ; their 
upper surfaces are either slightly concave, or slightly convex ; they are 
highly polished, and of a dark grey or jet black colour ; their form is 
irregular, generally circular, and from the tenth of an inch to one inch 
and a half in diameter ; their thickness, or amount of their projection 
from the rock on which they stand, varies much, about a quarter of an 
inch being perhaps most usual. The fronds occasionally become more 
and more convex, until they pass into botryoidal masses with their 
summits fissured ; when in this state, they are glossy and of an intense 
black, so as to resemble some fused metallic substance. I have shown 
the incrustation, both in this latter and in its ordinary state to several 
geologists, but not one could conjecture its origin, except that perhaps 
it was of volcanic nature 1 

The substance forming the fronds has a very compact and often 
almost crystalline fracture ; the edges being translucent, and hard 
enough easily to scratch calcareous spar. Under the blowpipe it imme- 
diately becomes white, and emits a strong animal odour, like that from 
fresh shells. It is chiefly composed of carbonate of lime ; when placed 
in muriatic acid it froths much, leaving a residue of sulphate of lime, 
and of an oxide of iron, together with a black powder, which is not 
soluble in heated acids. This latter substance seems to be carbonaceous, 



200 ASCENSION. 

and is evidently the colouring matter. The sulphate ot lime is ex- 
traneous, and occurs in distinct, excessively minute, lamellar plates, 
studded on the surface of the fronds, and embedded between the fine 
layers of which they are composed ; when a fragment is heated in the 
blowpipe, these lamellae are immediately rendered visible. The original 
outline of the fronds may often be traced, either to a minute particle of 
shell fixed in a crevice of the rock, or to several cemented together ; 
these first become deeply corroded, by the dissolving power of the 
waves, into sharp ridges, and then are coated with successive layers of 
the glossy, grey, calcareous incrustation. The inequalities of the 
primary support aiTect the outline of every successive layer, in the same 
manner as may often be seen in bezoar-stones, when an object like a 
nail forms the centre of aggregation. The crenulated edges, however, 
of the frond appear to be due to the corroding power of the surf on its 
own deposit, alternating with fresh depositions. On some smooth 

No. 5. 




An incrustation of calcareous and animal matter, coating the tidal-rocks at Ascension. 

basaltic rocks on the coast of St. Jago, I found an exceedingly thin layer 
of brown calcareous matter, which under a lens presented a miniature 
likeness of the crenulated and polished fronds of Ascension ; in this 
case a basis was not afforded by any projecting extraneous particles. 
Although the incrustation at Ascension is persistent throughout the year ; 
yet from the abraded appearance of some parts, and from the fresh 
appearance of other parts, the whole seems to undergo a round of decay 
and renovation, due probably to changes in the form of the shifting 
beach, and consequently in the action of the breakers : hence probably 
it is, that the incrustation never acquires a great thickness. Considering 
the position of the encrusted rocks in the midst of the calcareous beach, 
together with its composition, I think there can be no doubt that its 
origin is due to the dissolution and subsequent deposition of the matter 
composing the rounded particles of shells and corals.* From this source 

* The sclenite, as I 1 ave remarked js c?ctraneouSj ancj must have been 



CALCAREOUS INCRUSTATION. 201 

it derives its animal matter, which is evidently the colouring principle. 
The nature of the deposit, in its incipient stage, can often be well seen 
upon a fragment of white shell, when jammed between two of the 
fronds ; it then appears exactly like the thinnest wash of a pale grey 
varnish. Its darkness varies a little, but the jet blackness of some of 
the fronds and of the botryoidal masses seems due to the translucency 
of the successive grey layers. There is, however, this singular circum- 
stance, that when deposited on the under side of ledges of rock or in 
fissures, it appears always to be of a pale, pearly grey colour, even when 
of considerable thickness : hence one is led to suppose, that an abun- 
dance of light is necessary to the development of the dark colour, in the 
same manner as seems to be the case with the upper and exposed 
surfaces of the shells of living mollusca, which are always dark, com- 
pared with their under surfaces and with the parts habitually covered 
by the mantle of the animal. In this circumstance, — in the immediate 
loss of colour and in the odour emitted under the blowpipe, — in the 
degree of hardness and translucency of the edges, — and in the beautiful 
polish of the surface,* rivalhng when in a fresh state that of the finest 
Oliva, there is a striking analogy between this inorganic incrustation 
and the shells of living molluscous animals.f This appears to me to be 
an interesting physiological fact.f 

derived from the sea-water. It is an interesting circumstance thus to find 
the waves of the ocean, sufficiently charged with sulphate of lime, to deposit 
it on the rocks, against which they dash every tide. Dr. Webster has 
described ("Voyage of the Chanticleer" vol. ii., p. 319) beds of gypsum and 
salt, as much as two feet in thickness, left by the evaporation of the spray 
on the rocks on the windward coast. Beautiful stalactites of selenite, 
resembhng in form those of carbonate of lime, are formed near these beds. 
Amorphous masses of gypsum, also, occur in caverns in the interior of the 
island ; and at Cross Hill (an old crater) I saw a considerable quantity of 
salt oozing from a pile of scoriae. In these latter cases, the salt and gypsum 
appear to be volcanic products. 

* From the fact described in my "Journal of Researches," of a coating 
of oxide of iron, deposited by a streamlet on the rocks in its bed (like a 
nearly similar coating at the great cataracts of the Orinoco and Nile), 
becoming finely polished where the surf acts, I presume that the surf in this 
instance, also, is the polishing agent. 

t In the section descriptive of St. Paul's Rocks, I have described a glossy, 
pearly substance, which coats the rocks, and an allied stalactitical incrustation 
from Ascension, the crust of which resembles the enamel of teeth, but is hard 
enough to scratch plate-glass. Both these substances contain animal matter, 
and seem to have been derived from water in filtering through birds' dung. 

X Mr. Horner and Sir David Brewster have described ("Philosophical 
Transactions," 1836, p. 65) a singular "aitificial substance, resembling shell." 
It is deposited in fine, transparent, highly polished, brown-coloured laminae, 
possessing peculiar optical properties, on the inside of a vessel, in which 
cloth, first prepared with glue and then with lime, is made to revolve rapidly 
in water. It is much softer, more transparent, and contains more animal 
matter, than the natural incrustation at Ascension ; but we here again see 
the strong tendency which carbonate of lime jind animal matter evince to 
form £i solid substance ^lligd to shell, 



202 ASCENSION. 

Singular laminated beds alternating with and passing into obsi- 
dian. — These beds occur within the trachytic district, at the western 
base of Green Mountain, under which they dip at a high inclination. 
They are only partially exposed, being covered up by modern ejections ; 
from this cause, I was unable to trace their junction with the trachyte, 
or to discover whether they had flowed as a stream of lava, or had 
been injected amidst the overlying strata. There are three principal 
beds of obsidian, of which th^. thickest forms the base of the section. 
The alternating stony layers appear to me eminently curious, and 
shall be first described, and afterwards their passage into the obsidian. 
They have an extremely diversified appearance ; five principal varieties 
may be noticed, but these insensibly blend into each other by endless 
gradations. 

First, — A pale grey, irregularly and coarsely laminated,* harsh-feel- 
ing rock, resembling clay-slate which has been in contact with a trap- 
dike, and with a fracture of about the same degree of crystalline 
structure. This rock, as well as the following varieties, easily fuses into 
a pale glass. The greater part is honeycombed with irregular, angular, 
cavities, so that the whole has a curious appearance, and some frag- 
ments resemble in a remarkable manner silicified logs of decayed wood. 
This variety, especially where more compact, is often marked with thin 
whitish streaks, which are either straight or v^rrap round, one behind 
the other, the elongated carious hollows. 

Secondly, — A bluish grey or pale brown, compact, heavy, homo- 
geneous stone, with an angular, uneven, earthy fracture ; viewed, 
however, under a lens of high power, the fracture is seen to be 
distinctly crystalline, and even separate minerals can be distinguished. 

Thirdly, — A stone of the same kind with the last, but streaked with 
numerous, parallel, slightly tortuous, white lines of the thickness of 
hairs. These white lines are more crystalline than the parts between 
them ; and the stone splits along them : they frequently expand into 
exceedingly thin cavities, which are often only just perceptible with a 
lens. The matter forming the white lines becomes better crystallised in 
these cavities, and Professor Miller was fortunate enough, after several 
trials, to ascertain that the white crystals, which are the largest, were 
of quartz.t and that the minute green transparent needles were augite, 
or, as they would more generally be called, diopside : besides these 
crystals, there are some minute, dark specks without a trace of 

* This term is open to some misinterpretation, as it may be applied both 
to rocks divided into laminae of exactly the same composition, and to layers 
firmly attached to each other, with no fissile tendency, but composed of 
different minerals, or of different shades of colour. The term "laminated," in 
this chapter, is applied in these latter senses ; where a homogeneous rock 
splits, as in the former sense, in a given direction, like clay-slate, I have used 
the term " fissile." 

•f Professor Miller informs me that the crystals which he measured had the 
faces P, -s, m of the figure (147) given by Haidinger in his Translation of 
Mohs ; and he adds, that it is remarkable, that none of them had the slightest 
trace of faces r of the regular six-sided prism. 



OBSIDIAN FORMATION. 203 

crystallisation, and some fine, white, granular, crystalline matter 
which is probably feldspar. Minute fragments of this rock are easily 
fusible. 

Fourthly, — A compact crystalline rock, banded in straight lines with 
innumerable layers of white and grey shades of colour, varying in width 
from the thirtieth to the two-hundredth of an inch ; these layers seem 
to be composed chiefly of feldspar, and they contain numerous perfect 
crystals of glassy feldspar, which are placed lengthways ; they are also 
thickly studded with microscopically minute, amorphous, black specks, 
which are placed in rows, either standing separately, or more frequently 
united, two or three or several together, into black lines, thinner than a 
hair. When a small fragment is heated in the blowpipe, the black specks 
are easily fused into black brilliant beads, which become magnetic, — 
characters that apply to no common mineral except hornblende or 
augite. With the black specks there are mingled some others of a red 
colour, which are magnetic before being heated, and no doubt are oxide 
of iron. Round two little cavities, in a specimen of this variety, I 
found the black specks aggregated into minute crystals, appearing like 
those of augite or hornblende, but too dull and small to be measured 
by the goniometer ; in the specimen, also, I could distinguish amidst 
the crystalline feldspar, grains, which had the aspect of quartz. By 
trying with a parallel ruler, I found that the thin grey layers and the 
black hair-like lines were absolutely straight and parallel to each other. 
It is impossible to trace the gradation from the homogeneous grey 
rocks to these striped varieties, or indeed the character of the different 
layers in the same specimen, without feeling convinced that the more 
or less perfect whiteness of the crystalline feldspathic matter depends 
on the more or less perfect aggregation of diffused matter, into the 
black and red specks of hornblende and oxide of iron. 

Fifthly, — A compact heavy rock, not laminated, with an irregular, 
angular, highly crystalline, fracture ; it abounds with distinct crystals of 
glassy feldspar, and the crystalline feldspathic base is mottled with a 
black mineral, which on the weathered surface is seen to be aggregated 
into small crystals, some perfect, but the greater number imperfect. 
I showed this specimen to an experienced geologist, and asked him 
what it was ; he answered, as I think every one else would have done, 
that it was a primitive greenstone. The weathered surface, also, of the 
foregoing (No. 4) banded variety, strikingly resembles a worn fragment 
of finely laminated gneiss. 

These five varieties, with many intermediate ones, pass and repass 
into each other. As the compact varieties are quite subordinate to the 
others, the whole may be considered as laminated or striped. The 
laminae, to sum up their characteristics, are either quite straight, or 
slightly tortuous, or convoluted ; they are all parallel to each other, and 
to the intercalating strata of obsidian ; they are generally of extreme 
thinness; they consist either of an apparently homogeneous, compact 
rock, striped with different shades of grey and brown colours, or of 
crystalline feldspathic layers in a more or less perfect state of purity, 
and of different thicknesses, with distinct crystals of glassy feldspar 



204 ASCENSION. 

placed lengtluvaj'S, or of very thin laj-ers chiefly composed of minute 
crystals of quartz and augite, or composed of black and red specks of 
an augitic mineral and of an oxide of iron, either not crystallised or 
imperfectly so. After having fully described the obsidian, I shall return 
to the subject of the lamination of rocks of the trachytic series. 

The passage of the foregoing beds into the strata of glassy obsidian is 
effected in several ways : first, angulo-modular masses of obsidian, both 
large and small, abruptly appear disseminated in a slaty, or in an 
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly 
fracture. Secondly, small irregular nodules of the obsidian, either 
standing separately, or united into thin layers, seldom more than the 
tenth of an inch in thickness, alternate repeatedly with very thin layers 
of a feldspathic rock, which is striped with the finest parallel zones of 
colour, like an agate, and which sometimes passes into the nature of 
pitchstone ; the interstices between the nodules of obsidian are generally 
filled by soft white matter, resembling pumiceous ashes. Thirdly, the 
whole substance of the bounding rock suddenly passes into an angulo- 
concretionary mass of obsidian. Such masses (as well as the small 
nodules) of obsidian are of a pale green colour, and are generally 
streaked with different shades of colour, parallel to the laminae of the 
surrounding rock ; they likewise generally contain minute -white 
sphaerulites, of which half is sometimes embedded in a zone of one 
shade of colour, and half in a zone of another shade. The obsidian 
assumes its jet black colour and perfectly conchoidal fracture, only when 
in large masses; but even in these, on careful examination and on 
holding the specimens in different lights, I could generally distinguish 
parallel streaks of different shades of darkness. 

One of the commonest transitional rocks deserves in several respects 
a further description. It is of a very complicated nature, and consists 
of numerous thin, slightly tortuous layers of a pale-coloured feldspathic 
stone, often passing into an imperfect pitchstone, alternating with layers 
formed of numberless little globules of two varieties of obsidian, and of 
two kinds of sphaerulites, embedded in a soft or in a hard pearly base. 
The sphaerulites are either white and translucent, or dark brown and 
opaque ; the former are quite spherical, of small size, and distinctly 
radiated from their centre. The dark brown sphaerulites are less per- 
fectly round, and vary in diameter from the twentieth to the thirtieth of an 
inch ; when broken they exhibit towards their centres, which are whitish, 
an obscure radiating structure ; two of them when united sometimes 
have only one central point of radiation ; there is occasionally a trace of 
or a hollow crevice in their centres. They stand either separately, or are 
united two or three or many together into irregular groups, or more com- 
monly into layers, parallel to the stratification of the mass. This union 
in many cases is so perfect, that the two sides of the layer thus formed, 
are quite even ; and these layers, as they become less brown and 
opaque, cannot be distinguished from the alternating layers of the pale- 
coloured feldspathic stone. The sphaerulites, when not united, are 
generally compressed in the plane of the lamination of the mass ; and 
in this same plane, they are often marked internally, by zones of different 



OBSIDIAN FORMATION, 205 

shades of colour, and externally by small ridges and furrows. In the 
upper part of the accompanying woodiut, the sphaerulites with the parallel 
ridges and furrows are represented on an enlarged scale, but they arc 
not well executed ; and in the lower part, their usual manner of grouping 
is shown. In another specimen, a thin layer formed of the brown 




Opaque brown spheerulitcs, drawn on an enlarged scale. The upper ones are exter- 
nally marked with parallel ridges. The internal radiating structure of the lower 
ones, is much too plainly represented. 

sphaerulites closely united together, intersects, as represented in the 
woodcut, No. 7, a layer of similar composition ; and after running for 
a short space in a slightly curved line, again intersects it, and likewise 
a second layer lying a little way beneath that first intersected. The 
small nodules also of obsidian are sometimes externally marked with 
ridges and furrows, parallel to the lamination of the mass, but always 

No. 7. 




A la3-er ormcd by the union of minute brown sphjcrulitcs, intersecting two other 
similar layers : the whole represented of nearly the natural size. 

less plainly than the sphaerulites. These obsidian nodules are generally 
angular, with their edges blunted : they are often impressed with the 
form of the adjoining sphaerulites, than which they are always larger; 
the separate nodules seldom appear to have drawn each other out by 
exerting a mutually attractive force. Had I not found in some cases, 
a distinct centre of attraction in these nodules of obsidian, I should have 



2o6 ASCENSION. 

been led to have considered them as residuary matter, left during the 
formation of the pearlstone, in which they are embedded, and of the 
sphaerulitic globules. 

The sphaerulites and the little nodules of obsidian in these rocks so 
closely resemble, in general form and structure, concretions in sedimen- 
tary deposits, that one is at once tempted to attribute to them an 
analogous origin. They resemble ordinary concretions in the following 
respects : in their external form, — in the union of two or three, or of 
several, into an irregular mass, or into an even-sided layer, — in the 
occasional intersection of one such layer by another, as in the case of 
chalk-flints, — in the presence of two or three kinds of nodules, often 
close together, in the same basis, — in their fibrous, radiating structure, 
with occasional hollows in their centres, — in the co-existence of a 
laminary, concretionary, and radiating structure, as is so well developed 
in the concretions of magnesian limestone, described by Professor 
Sedgwick.* Concretions in sedimentary deposits, it is known, are due 
to the separation from the surrounding mass of the whole or part of 
some mineral substance, and its aggregation round certain points of 
attraction. Guided by this fact, I have endeavoured to discover whether 
obsidian and the sphaerulites (to which may be added marekanite and 
pearlstone, both of them occurring in nodular concretions in the trachytic 
series) differ in their constituent parts, from the minerals generally 
composing trachytic rocks. It appears from three analyses, that obsidian 
contains on an average 76 per cent, of silica ; from one analysis, that 
sphaerulites contain 79'i2; from two, that marekanite contains 79*25; 
and from two other analyses, that pearlstone contains 75 "62 of silica. t 
Now, the constituent parts of trachyte, as far as they can be distinguished 
consist of feldspar, containing 65-21 of sihca ; or of albite, containing 
6909; of hornblende, containing 55*27, J and of oxide of iron: so that 
the foregoing glassy concretionary substances all contain a larger pro- 
portion of silica than that occurring in ordinary feldspathic or trachytic 
rocks. D'Aubuisson,§ also, has remarked on the large proportion of 
silica compared with alumina, in six analyses of obsidian and pearlstone 
given in Brongniart's " Mineralogy." Hence I conclude, that the fore- 
going concretions have been formed by a process of aggregation, strictly 
analogous to that which takes place in aqueous deposits, acting chiefly 
on the silica, but likewise on some of the other elements of the sur- 
rounding mass, and thus producing the different concretionary varieties. 
From the well-known effects of rapid cooling || in giving glassiness of 

* "Geological Transactions," vol. iii., part i., p. 37. 

f The foregoing analyses are taken from Beudant, "Traite de Miner- 
alogie," torn, ii., p. 113; and one analysis of obsidian, from Phillips's 
" Mineralogy." 

\ These analyses are taken from Von Kobell's "Grundzuge der Miner- 
alogie," 1838. 

§ "Traite de Geogn.," torn, ii., p. 535. 

II This is seen in the manufacture of common glass, and in Gregory Watts's 
experiments on molten trap; also on the natural surfaces of lava-streams, 
and on the side-walls of dikes. 



OBSIDIAN FORMATION. 207 

texture, it is probably necessary that the entire mass, in cases like that 
of Ascension, should have cooled at a certain rate ; but considering the 
repeated and complicated alterations of nodules and thin layers of a 
glassy texture with other layers quite stony or crystalline, all within the 
space of a few feet or even inches, it is hardly possible that they could 
have cooled at different rates, and thus have acquired their different 
textures. 

The natural sphaerulites in these rocks * very closely resemble those 
produced in glass, when slowly cooled. In some fine specimens of 
partially devitrified glass, in the possession of Mr. Stokes, the sphaeru- 
lites are united into straight layers with even sides, parallel to each 
other, and to one of the outer surfaces, exactly as in the obsidian. 
These layers sometimes interbranch and form loops ; but I did not see 
any case of actual intersection. They form the passage from the 
perfectly glassy portions, to those nearly homogeneous and stony, with 
only an obscure concretionary structure. In the same specimen, also, 
sphaerulites differing slightly in colour and in structure, occur embedded 
close together. Considering these facts, it is some confirmation of the 
view above given of the concretionary origin of the obsidian and natural 
sphaeruMtes, to find that M. Dartigues,t in his curious paper on this 
subject, attributes the production of sphaerulites in glass, to the different 
ingredients obeying their own laws of attraction and becoming aggre- 
gated. He is led to believe that this takes place, from the difficulty in 
remelting sphaerulitic glass, without the whole be first thoroughly 
pounded and mixed together ; and likewise from the fact, that the 
change takes place most readily in glass composed of many ingredients. 
In confirmation of M. Dartigues' view, I may remark, that M. Fleuriau 
de Bellevue t found that the sphaerulitic portions of devitrified glass 
were acted on both by nitric acid and under the blowpipe, in a different 
manner from the compact paste in which they were embedded. 

Comparison of the obsidian beds and alternaiing strata of Ascension, 
with those of other countries. — I have been struck with much surprise, 
how closely the excellent description of the obsidian rocks of Hungary, 
given by Beudant,§ and that by Humboldt, of the same formation in 

* I do not know whether it is generally known, that bodies having 
exactly the same appearance as sphaerulites, sometimes occur in agates. Mr. 
Robert Brown showed me in an agate, formed within a cavity in a piece of 
silicified wood, some little specks, which were only just visible to the naked 
eye : these specks, when placed by him under a lens of high power, presented 
a beautiful appearance : they were perfectly circular, and consisted of the 
finest fibres of a brown colour, radiating with great exactness from a 
common centre. These little radiating stars are occasionally intersected, and 
portions are quite cut off by the fine, ribbon-hke zones of colour in the agate. 
In the obsidian of Ascension, the halves of a sphaerulite often lie in different 
zones of colour, but they are not cut off by them, as in the agate. 

t Journal de Physique, torn. 59 (1804), pp. 10, 12. 

\ Idem, torn. 60 (1805), p. 418. 

§ " Voyage en Hongrie," torn, i., p. 330 ; torn, ii., pp. 221 and 315 ; tom. iii., 
pp. 369, 371, 377, 381. 



2o8 ASCENSION. 

Mexico and Pefii,* and likewise the descriptions given by several 
authors! of the trachytic regions in the Italian islands, agree with my 
Observations at Ascension. Many passages might have been trans- 
ferred without alteration from the works of the above authors, and 
would have been applicable to this island. They all agree in the 
laminated and stratified character of the whole series ; and Humboldt 
speaks of some of the beds of obsidian being ribboned like jasper.J 
They all agree in the nodular or concretionary character of the obsidian, 
and of the passage of these nodules into layers. They all refer to the 
repeated alterations, often in undulatory planes, of glassy, pearly, stony, 
and crystalline layers : the crystalline layers, however, seem to be much 
more perfectly developed at Ascension, than in the above-named 
countries. Humboldt compares some of the stony beds, when viewed 
from a distance, to strata of a schistose sandstone. Sphaerulites are 
described as occurring abundantly in all cases ; and they everywhere 
seem to mark the passage, from the perfectly glassy to the stony and 
crystalline beds. Beudant's account § of his " perlite lithoide globulaire " 
in every, even the most trifling particular, might have been written for 
the little brown sphserulitic globules of the rocks of Ascension. 

From the close similarity in so many respects, between the obsidian 
formations of Hungary, Mexico, Peru, and of some of the Italian islands, 
with that of Ascension, I can hardly doubt that in all these cases, the 
obsidian and the sphaerulites owe their origin to a concretionary 
aggregation of the silica, and of some of the other constituent elements, 
taking place whilst the liquefied mass cooled at a certain required rate. 
It is, however, well known, that in several places, obsidian has flowed 
in streams like lava ; for instance, at Teneriffe, at the Lipari Islands, 
and at Iceland. 1| In these cases, the superficial parts are the most 

* "Essai Geognostique," pp. 176, 326, 328. 

f P. Scrope, in "Geological Transactions," vol. ii. (second series), p. 195. 
Consult, also, Dolomieu's "Voyage aux Isles Lipari," and D'Aubuisson, 
"Traitede Geogn.," torn, ii., p. 534. 

X In Mr. Stokes' fine collection of obsidians from Mexico, I observe that the 
sphaerulites are generally much larger than those of Ascension ; they are 
generally white, opaque, and are united into distinct layers : there are many 
singular varieties, different from any at Ascension. The obsidians are finely 
zoned, in quite straight or curved lines, with exceedingly slight differences of 
tint, of cellularity, and of more or less perfect degrees of glassiness. Tracing 
some of the less perfectly glassy zones, they are seen to become studded 
with minute white sphaerulites, which become more and more numerous, 
until at last they unite and form a distinct layer: on the other hand, at 
Ascension, only the brown sphaerulites unite and form layers ; the white 
ones always being irregularly disseminated. Some specimens at the Geo- 
logical Society, said to belong to an obsidian formation from Mexico, have an 
earthy fracture, and are divided in the finest parallel laminae, by specks of a 
black mineral, like the augitic or hornblendic specks in the rocks at Ascension. 

§ Beudant's " Voyage," torn, iii., p. 373. 

||- For Teneriffe, seeVon. Buch, " Descript. des Isles Canaries," pp. 184 and 
190 ; for the Lipari Islands, see Dolomieu's "Voyage," p. 34; for Iceland, see 
Mackenzie's " Travels," p. 369. 



LAMINATION OF TRACHYTIC ROCKS. 209 

perfectly glassy, the obsidian passing at the depth of a few feet into an 
opaque stone. In an analysis by Vauquelin of a specimen of obsidian 
from Hecla, which probably flowed as lava, the proportion of silica is 
nearly the same as in the nodular or concretionary obsidian from 
Mexico. It would be interesting to ascertain, whether the opaque 
interior portions and the superficial glassy coating contained the same 
proportional constituent parts : we know from M. Dufrenoy * that the 
exterior and interior parts of the same stream of lava sometimes differ 
considerably in their composition. Even should the whole body of the 
stream of obsidian turn out to be similarly composed with nodular 
obsidian, it would only be necessary, in accordance with the foregoing 
facts, to suppose that lava in these instances had been erupted with its 
ingredients mixed in the same proportion, as in the concretionary 
obsidian. 

Lamination of volcanic rocks of the trachytic series. 

We have seen that, in several and widely distant countries, the strata 
alternating with beds of obsidian, are highly laminated. The nodules, 
also, both large and small, of the obsidian, are zoned with different 
shades of colour; and I have seen a specimen from Mexico in Mr. 
Stokes' collection, with its external surface weathered t into ridges and 
furrows, corresponding with the zones of different degrees of glassiness : 
Humboldtjt moreover, found on the Peak of Teneriffe, a stream of 
obsidian divided by very thin, alternating, layers of pumice. Many 
other lavas of the feldspathic series are laminated ; thus, masses of 
common trachyte at Ascension are divided by fine earthy lines, along 
which the rock splits, separating thin layers of slightly different shades 
of colour; the greater number, also, of the embedded crystals of glassy 
feldspar are placed lengthways in the same direction. Mr. P. Scrope § 
has described a remarkable columnar trachyte in the Panza Islands, 
which seems to have been injected into an overlying mass of trachytic 
conglomerate : it is striped with zones, often of extreme tenuity, of 
different textures and colours ; the harder and darker zones appearing 
to contain a larger proportion of silica. In another part of the island, 
there are layers of pearlstone and pitchstone, which in many respects 
resemble those of Ascension. The zones in the columnar trachyte are 
generally contorted ; they extend uninterruptedly for a great length in 
a vertical direction, and apparently parallel to the walls of the dike- 
like mass. Von Buch |1 has described at Teneriffe, a stream of lava 

* " Memoircs pour servir a une Descript. Gdolog. de la France," torn, iv., 

P-37I. 

t MacCuUoch states ("Classification of Rocks," p. 531), that the exposed 
surfaces of the pitchstone dikes in Arran are furrowed, "with undulating 
lines, resembHng certain varieties of marbled paper, and which evidently 
result from some corresponding difference of laminar structure." 

X " Personal Narrative," vol. i., p. 222. 

§ "Geological Transactions," vol. ii. (second scries), p. 1^5. 

jl " Description dcs Ilcii Canaries," p. 1S4. _ 

14 



2IO LAMINATION OF VOLCANIC ROCKS 

containing innumerable, thin, plate-like crystals of feldspar, which are 
arranged like white threads, one behind the other, and which mostly 
follow the same direction. Dolomieu * also states, that the grey lavas 
of the modern cone of Vulcano, which have a vitreous texture, are 
streaked with parallel white lines : he further describes a solid pumice- 
stone which possesses a fissile structure, like that of certain micaceous 
schists. Phonolite, which I may observe is often, if not always, an 
injected rock, also, often has a fissile structure ; this is generally due to 
the parallel position of the embedded crystals of feldspar, but some- 
times, as at Fernando Noronha, seems to be nearly independent of their 
presence.f From these facts we see, that various rocks of the felds- 
pathic series have either a laminated or fissile structure, and that it 
occurs both in masses which have injected into overlying strata, and in 
others which have flowed as streams of lava. 

The laminae of the beds, alternating with the obsidian at Ascension, 
dip at a high angle under the mountain, at the base of which they, are 
situated ; and they do not appear as if they had been inclined by 
violence. A high inclination is common to these beds in Mexico, Peru, 
and m some of the Italian islands : J on the other hand, in Hungary, 
the layers are horizontal ; the laminae, also, of some of the lava-streams 
above referred to, as far as I can understand the descriptions given of 
them, appear to be highly inclined or vertical. I doubt whether in any 
of these cases, the laminae have been tilted into their present position ; 
and in some instances, as in that of the trachyte described by Mr. 
Scrope, it is almost certain that they have been originally formed with 
a high inclination. In many of these cases, there is evidence that the 
mass of liquefied rock has moved in the direction of the laminae. At 
Ascension, many of the air-cells have a drawn-out appearance, and are 
crossed by coarse semi-glassy fibres, in the direction of the laminae ; 
and some of the layers, separating the sphaerulitic globules, have a 
scored appearance, as if produced by the grating of the globules. I 
have seen a specimen of zoned obsidian from Mexico, in Mr. Stokes' 
collection, with the surfaces of the best-defined layers streaked or 
furrowed with parallel lines ; and these lines or streaks precisely re- 
sembled those, produced on the surface of a mass of artificial glass by 
its having been poured out of a vessel. Humboldt, also, has described 
little cavities, which he compares to the tails of comets, behind sphaeru- 
lites in laminated obsidian rocks from Mexico, and Mr. Scrope has 

* " Voyage aux Isles de Lipari," pp. 35 and 85. 

t In this case, and in that of the fissile pnmice-stone, the structure is very 
different from that in the foregoing cases, where the laminae consist of alter- 
nate layers of different composition or texture. In some sedimentary 
formations, however, which apparently are homogeneous and fissile, as in 
glossy clay-slate, there is reason to believe, according to D'Aubuisson, that 
the laminae are really due to excessively thin, alternating, layers of mica. 

X See Phillips' "Mineralogy," for the Italian Islands, p. 136. For Mexico 
and Peru, see Humboldt's "Essai Geognostique." Mr. Edwards, also de- 
scribes the high inclination of the obsidian rocks of the Cerro del Navaja in 
Mexico, in the Pyoc. of the Gcolog. Soc. for June, 1838. 



OF THE TRACHYTIC SERIES. 211 

described other cavities behind fragments embedded in his laminated 
trachyte, and which he supposes to have been produced during the 
movement of the mass.* From such facts, most authors have attributed 
the lamination of these volcanic rocks to their movement whilst liquefied. 
Although it is easy to perceive, why each separate air-cell, or each 
fibre in pumice-stone,t should be drawn out in the direction of the 
moving mass ; it is by no means at first obvious why such air-cells and 
fibres should be arranged by the movement, in the same planes, in 
laminae absolutely straight and parallel to each other, and often of 
extreme tenuity ; and still less obvious is it, why such layers should 
come to be of slightly different composition and of different textures. 

In endeavouring to make out the cause of the lamination of these 
igneous feldspathic rocks, let us return to the facts so minutely described 
at Ascension. We there see, that some of the thinnest layers are 
chiefly formed by numerous, exceedingly minute, though perfect, 
cry^als of different minerals ; that other layers are formed by the 
union of different kinds of concretionary globules, and that the layers 
thus formed, often cannot be distinguished from the ordinary feldspathic 
and pitchstone layers, composing a large portion of the entire mass. 
The fibrous radiating structure of the sphaerulites seems, judging from 
many analogous cases, to connect the concretionary and crystalline 
forces : the separate crystals, also, of feldspar all lie in the same 
parallel planes, t These allied forces, therefore, have played an im- 
portant part in the lamination of the mass, but they cannot be considered 
the primary force ; for the several kinds of nodules, both the small- 
est and largest, are internally zoned with excessively fine shades of 
colour, parallel to the lamination of the whole ; and many of them are, 
also, externally marked in the same direction with parallel ridges and 
furrows, which have not been produced by weathering. 

Some of the finest streaks of colour in the stony layers, alternating 
with the obsidian, can be distinctly seen to be due to an incipient 
crystallisation of the constituent minerals. The extent to which the 
minerals have crystallised can, also, be distinctly seen to be connected 

* "Geological Transactions," vol. ii. (second series), p. 200, etc. These 
embedded fragments, in some instances, consist of the laminated trach3rte 
broken off and "enveloped in those parts, which still remained liquid." 
Beudant, also, frequently refers, in his great work on " Hungary " (torn, iii., 
p. 386), to trachytic rocks, irregularly spotted with fragments of the same 
varieties, which in other parts form the parallel ribbons. In these cases, we 
must suppose, that after part of the molten mass had assumed a laminated 
structure, a fresh irruption of lava broke up the mass, and involved fragments, 
and that subsequently the whole became relaminated. 

t Dolomieu's "Voyage," p. 64. 

t The formation, indeed, of a large crystal of any mineral in a rock of 
mixed composition implies an aggregation of the requisite atoms, allied to 
concretionary action. The cause of the crystals of feldspar in these rocks 
of Ascension, being all placed lengthways, is probably the same with that 
which elongates and flattens all the brown sphserulitic globules (which 
behave like feldspar under the blowpipe) in this same direction. 



212 LAMINATION OF VOLCANIC ROCKS 

with the greater or less size, and with the number, of the minute, 
flattened, crenulated air-cavities or fissures. Numerous facts, as in the 
case of geodes, and of cavities in silicified wood, in primary rocks, and 
in veins, show that crystallisation is much favoured by space. Hence, 
I conclude, that, if in a mass of cooling volcanic rock, any cause pro- 
duced in parallel planes a number of minute fissures or zones of less 
tension (which from the pent-up vapours would often be expanded 
into crenulated air-cavities), the crystallisation of the constituent parts, 
and probably the formation of concretions, would be superinduced or 
much favoured in such planes ; and thus, a laminated structure of the 
kind we are here considering would be generated. 

That some cause does produce parallel zones of less tension in 
volcanic rocks, during their consolidation, we must admit in the case 
of the thin alternate layers oi obsidian and pumice described by 
Humboldt, and of the small, flattened, crenulated air-cells in the 
laminated rocks of Ascension ; for on no other principle can we conceive 
why the confined vapours should through their expansion form air-cells 
or fibres in separate, parallel planes, instead of irregularly throughout 
the mass. In Mr. Stokes' collection, I have seen a beautiful example 
of this structure, in a specimen of obsidian from Mexico, which is 
shaded and zoned, like the finest agate, with numerous, straight, parallel 
layers, more or less opaque and white, or almost perfectly glassy ; the 
degree of opacity and glassiness depending on the number of micro- 
scopically minute, flattened air-cells ; in this case, it is scarcely possible 
to doubt but that the mass, to which the fragment belonged, must have 
been subjected to some, probably prolonged, action, causing the tension 
slightly to vary in the successive planes. 

Several causes appear capable of producing zones of different tension, 
in masses semi-liquefied by heat. In a fragment of devitrified glass, I 
have observed layers of sphaerulites which appeared, from the manner 
in which they were abruptly bent, to have been produced by the simple 
contraction of the mass in the vessel, in which it cooled. In certain 
dikes on Mount Etna, described by M. EHe de Beaumont,* as bordered 
by alternating bands of scoriaceous and compact rock, one is led to 
suppose that the stretching movement of the surrounding strata, which 
originally produced the fissures, continued whilst the injected rock 
remained fluid. Guided, however, by Professor Forbes' + clear descrip- 
tion of the zoned structure of glacier-ice, far the most probable explana- 
tion of the laminated structure of these feldspathic rocks appears to be, 
that they have been stretched whilst slowly flowing onwards in a pasty 
condition, J in precisely the same manner as Professor Forbes believes, 
that the ice of moving glaciers is stretched and fissured. In both cases, 

* "Mem. pour servir," etc., torn, iv., p. 131. 

t Edinburgh New Phil. Journal, 1842, p. 350. 

X I presume that this is nearly the same explanation which Mr. Scrope 
had in his mind, when he speaks ("Geolog. Transact," vol. ii., second series, 
p. 228) of the ribboned structure of his trachytic rocks, having arisen, from 
"a linear extension of the mass, while in a state of imperfect liquidity, 
coupled with a concretionary process." 



OF THE TRACHYTIC SERIES. 213 

the zones may be compared to those in the finest agates ; in both, they 
extend in the direction in which the mass has flowed, and those exposed 
on the surface are generally vertical : in the ice, the porous laminae are 
rendered distinct by the subsequent congelation of infiltrated water, in 
the stony feldspathic lavas, by subsequent crystalline and concretionary 
action. The fragment of glassy obsidian in Mr. Stokes' collection, which 
is zoned with minute air-cells must strikingly resemble, judging from Pro- 
fessor Forbes' descriptions, a fragment of the zoned ice ; and if the rate of 
cooling and nature of the mass had been favourable to its crystallisation or 
to concretionary action, we should here have had the finest parallel zones 
of different composition and texture. In glaciers, the lines of porous ice 
and of minute crevices seem to be due to an incipient stretching, caused 
by the central parts of the frozen stream moving faster than the sides 
and bottom, which are retarded by friction : hence in glaciers of certain 
forms and towards the lower end of most glaciers, the zones become 
horizontal. May we venture to suppose that in the feldspathic lavas 
with horizontal laminae, we see an analogous case? All geologists, 
who have examined trachytic regions, have come to the conclusion, 
that the lavas of this series have possessed an exceedingly imperfect 
fluidity ; and as it is evident that only matter thus characterised would 
be subject to become fissured and to be formed into zones of different 
tensions, in the manner here supposed, we probably see the reason 
why augitic lavas, which appear generally to have possessed a high 
degree of fluidity, are not,* like the feldspathic lavas, divided into 
laminae of different composition and texture. Moreover, in the augitic 
series, there never appears to be any tendency to concretionary action, 
which we have seen plays an important part in the lamination of 
rocks, of the trachytic series, or at least in rendering that structure 
apparent. 

Whatever may be thought of the explanation here advanced of the 
laminated structure of the rocks of the trachytic series, I venture to call 
the attention of geologists to the simple fact, that in a body of rock at 
Ascension, undoubtedly of volcanic origin, layers often of extreme 
tenuity, quite straight, and parallel to each other, have been produced ; 
— some composed of distinct crystals of quartz and diopside, mingled 
with amorphous augitic specks and granular feldspar, — others entirely 
composed of these black augitic specks, with granules of oxide of iron, 
— and lastly, others formed of crystalline feldspar, in a more or less 
perfect state of purity, together with numerous crystals of feldspar, 
placed lengthways. At this island, there is reason to believe, and in 
some analogous cases, it is certainly known, that the laminae have 
originally been formed with their present high inclination. Facts of 
this nature are manifestly of importance, with relation to the structural 

* Basaltic lavas, and many other rocks, are not unfrequently divided into 
thick laminae or plates, of the same composition, which are either straight or 
curved ; these being crossed by vertical lines of fissure, sometimes become 
united into columns. This structure seems related, in its origin, to that by 
which many rocks, both igneous and sedimentary, become traversed by 
parallel systems of fissures. . 



214 ST. HELENA. 

origin of that grand series of plutonic rocks, which like the volcanic 
have undergone the action of heat, and which consist of alternate 
layers of quartz, feldspar, mica, and other minerals. 



CHAPTER IV. 

ST. HELENA. 

Lavas of the feldspathic, basaltic, and submarine series. — Section of Flagstaff 
Hill and of the Barn. — Dikes. — Turk's Cap and Prosperous Bays. — 
Basaltic ring. — Central crateriform ridge, with an internal ledge and a 
parapet. — Cones of phonolite. — Superficial beds of calcareous sandstone. 
— Extinct land-shells. — Beds of detritus. — Elevation of the land. — 
Denudation. — Craters of elevation. 

The whole island is of volcanic origin ; its circumference, according to 
Beatson,* is about twenty-eight miles. The central and largest part 
consists of rocks of a feldspathic nature, generally decomposed to an 
extraordinary degree ; and when in this state, presenting a singular 
assemblage of alternating, red, purple, brown, yellow, and white, soft, 
argillaceous beds. From the shortness of our visit, I did not examine 
these beds with care ; some of them, especially those of the white, 
yellow, and brown shades, originally existed as streams of lava, but the 
greater number were probably ejected in the form of scoriae and ashes : 
other beds of a purple tint, porphyritic with crystal-shaped patches of 
a white, soft substance, which are now unctuous, and yield, like wax, a 
polished streak to the nail, seem once to have existed as solid claystone- 
porphyries : the red argillaceous beds generally have a brecciated 
structure, and no doubt have been formed by the decomposition of 
scoriae. Several extensive streams, however, belonging to this series, 
retain their stony character ; these are either of a blackish-green colour, 
with minute acicular crystals of feldspar, or of a very pale tint, and 
almost composed of minute, often scaly, crystals of feldspar, abounding 
with microscopical black specks ; they are generally compact and 
laminated ; others, however, of similar composition, are cellular and 
somewhat decomposed. None of these rocks contain large crystals of 
feldspar, or have the harsh fracture peculiar to trachyte. These feld- 
spathic lavas and tuffs are the uppermost or those last erupted ; innumer- 
able dikes, however, and great masses of molten rock, have subsequently 
been injected into them. They converge, as they rise, towards the 
central curved ridge, of which one point attains the elevation of 2,700 
feet. This ridge is the highest land in the island ; and it once formed 
the northern rim of a great crater, whence the lavas of this series 
flowed : from its ruined condition, from the southern half having been 
removed, and from the violent dislocation which the whole island has 
undergone, its structure is rendered very obscure. 

* Governor Beatson 's "Account of St. Helena." 



BASAL SUBMARINE LAVAS. 215 

Basaltic series. — The margin of the island is formed by a rude circle 
of great, black, stratified, ramparts of basalt, dipping seaward, and worn 
into cliffs, which are often nearly perpendicular, and vary in height 
from a few hundred feet to two thousand. This circle, or rather horse- 
shoe shaped ring, is open to the south, and is breached by several 
other wide spaces. Its rim or summit generally projects little above 
the level of the adjoining inland country ; and the more recent feld- 
spathic lavas, sloping down from the central heights, generally abut 
against and overlap its inner margin ; on the north-western side of the 
island, however, they appear (judging from a distance) to have flowed 
over and concealed portions of it. In some parts, where the basaltic 
ring has been breached, and the black ramparts stand detached, the 
feldspathic lavas have passed between them, and now overhang the 
sea-coast in lofty cliffs. The basaltic rocks are of a black colour and 
thinly stratified ; they are generally highly vesicular, but occasionally 
compact ; some of them contain numerous crystals of glassy feldspar 
and octahedrons of titaniferous iron ; others abound with crystals of 
augite and grains of olivine. The vesicles are frequently lined with 
minute crystals (of chabasie ?) and even become amygdaloidal with 
them. The streams are separated from each other by cindery matter, 
or by a bright red, friable, saliferous tuff, which is marked by successive 
lines like those of aqueous deposition ; and sometimes it has an obscure, 
concretionary structure. The rocks of this basaltic series occur nowhere 
except near the coast. In most volcanic districts the trachytic lavas 
are of anterior origin to the basaltic ; but here we see, that a great pile 
of rock, closely related in composition to the trachytic family, has been 
erupted subsequently to the basaltic strata : the number, however, of 
dikes, abounding with large crystals of augite, with which the feldspathic 
lavas have been injected, shows perhaps some tendency to a return to 
the more usual order of superposition. 

Basal submarine lavas. — The lavas of this basal series lie immediately 
beneath both the basaltic and feldspathic rocks. According to Mr. 
Scale,* they may be seen at intervals on the sea-beach round the entire 
island. In the sections which I examined, their nature varied much ; 
some of the strata abound with crystals of augite ; others are of a brown 
colour, either laminated or in a rubbly condition ; and many parts are 
highly amygdaloidal with calcareous matter. The successive sheets are 
either closely united together, or are separated from each other by beds 
of scoriaceous rock and of laminated tuff, frequently containing well- 
rounded fragments. The interstices of these beds are filled with gypsum 
and salt ; the gypsum also sometimes occurring in thin layers. From 
the large quantity of these two substances, from the presence of rounded 
pebbles in the tuffs, and from the abundant amygdaloids, I cannot doubt 
that these basal volcanic strata flowed beneath the sea. This remark 
ought perhaps to be extended to a part of the superincumbent basaltic 
rocks ; but on this point, I was not able to obtain clear evidence. The 

* "Geognosy of the Island of St. Helena." Mr, Seale has constructed a 
gigantic model of St. Helena, well worth visiting, which is now deposited at 
Addiscombe College, in Surrey. 



2i6 ST. HELENA. 

strata of the basal series, whenever I examined them, were intersected 
by an extraordinary number of dikes. 

Flagstaff Hill and the Barn. — I will now describe some of the more 
remarkable sections, and will commence with these two hills, which 
form the principal external feature on the north-eastern side of the 
island. The square, angular outline, and black colour of the Barn, at 
once show that it belongs to the basaltic series ; whilst the smooth, 
conical figure, and the varied bright tints of Flagstaff Hill, render it 
equally clear, that it is composed of the softened, feldspathic rocks. 
These two lofty hills are connected (as is shown in the accompanying 
woodcut) by a sharp ridge, which is composed of the rubbly lavas of the 
basal series. The strata of this ridge dip westward, the inclination 
becoming less and less towards the Flagstaff; and the upper feldspathic 
strata of this hill can be seen, though with some difficulty, to dip con- 
formably to the W.S.W. Close to the Barn, the strata of the ridge are 
nearly vertical, but are much obscured by innumerable dikes ; under 
this hill, they probably change from being vertical into being inclined 
into an opposite direction ; for the upper or basaltic strata, which are 

No. 8. . 




^^^^■■' ■■■:::::::::'■■■ r^ l^^SoiSI^ East. 



Flagstaff Hill. The Barn. 

2,272 feet high. 2,015 feet high. 

The double lines represent the basaltic strata ; the single, the basal submarine strata; 
the dotted, the upper feldspathic strata ; the dikes are shaded transversely. 

about eight hundred or one thousand feet in thickness, are inclined 
nc»-th-eastward, at an angle between thirty and forty degrees. 

This ridge, and likewise the Barn and Flagstaff Hills, are interlaced 
by dikes, many of which preserve a remarkable parallelism in a N.N.W. 
and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic 
with larg« crystals of augite ; others are formed of a fine-grained and 
brown-coloured trap. Most of these dikes are coated by a glossy layer,* 
from one to two-tenths of an inch in thickness, which, unlike true pitch- 
stone, fuses into a black enamel ; this layer is evidently analogous to 
the glossy superficial coating of many lava streams. The dikes can 
often be followed for great lengths both horizontally and vertically, and 

* This circumstance has been observed (Lyell, " Principles of Geology," 
vol. iv., chap, x., p. 9) in the dikes of the Atrio del Cavallo, but apparently it 
is not of very common occurrence. Sir G. Mackenzie, however, states 
(p. 372, "Travels in Iceland") that all the veins in Iceland have a "black 
vitreous coating on their sides." Captain Carmichael, speaking of the dikes 
in Tristan d'Acunha, a volcanic island in the Southern Atlantic, says 
("Linnaean Transactions," vol. xii., p. 485) that their sides, "where they 
come in contact with the rocks, are invariably in a semi-vitrified state." 



FLAGSTAFF HILL AND THE BARN. 217 

they seem to preserve a nearly uniform thickness : * Mr. Seale states, 
that one near the Barn, in a height of 1,260 feet, decreases in width 
only four inches, — from nine feet at the bottom, to eight feet and eight 
inches at the top. On the ridge, the dikes appear to have been guided 
in their course, to a considerable degree, by the alternating soft and 
hard strata : they are often firmly united to the harder strata, and they 
preserve their parallelism for such great lengths, that in very many in- 
stances it was impossible to conjecture, which of the beds were dikes, 
and which streams of lava. The dikes, though so numerous on this 
ridge, are even more numerous in the valleys a little south of it, and to 
a degree I never saw equalled anywhere else: in these valleys they 
extend in less regular lines, covering the ground with a network, like a 
spider's web, and with some parts of the surface even appearing to 
consist wholly of dikes, interlaced by other dikes. 

From the complexity produced by the dikes, from the high inclination 
and anticlinal dip of the strata of the basal series, which are overlaid, at 
the opposite ends, of the short ridge, by two great masses of different 
ages and of different composition, I am not surprised that this singular 
section has been misunderstood. It has even been supposed to form 
part of a crater ; but so far is this from having been the case, that the 
summit of Flagstaff Hill once formed the lower extremity of a sheet of 
lava and ashes, which were erupted from the central, crateriform ridge. 
Judging from the slope of the contemporaneous streams in an adjoining 
and undisturbed part of the island, the strata of the Flagstaff Hill must 
have been upturned at least twelve hundred feet, and probably much 
more, for the great truncated dikes on its summit show that it has been 
largely denuded. The summit of this hill now nearly equals in height 
the crateriform ridge ; and before having been denuded, it was probably 
higher than this ridge, from which it is separated by a broad and much 
lower tract of country ; we here, therefore, see that the lower extremities 
of a set of lava-streams have been tilted up to as great a height as, or 
perhaps greater height than, the crater, down the flanks of which they 
originally flowed. I believe that dislocations on so grand a scale are 
extremely raref in volcanic districts. The formation of such numbers 
of dikes in this part of the island shows that the surface must here have 
been stretched to a quite extraordinary degree : this stretching, on the 
ridge between Flagstaff and Barn Hills, probably took place subse- 
quently (though perhaps immediately so) to the strata being tilted ; for 
had the strata at that time extended horizontally, they would in all pro- 
bability have been fissured and injected transversely, instead of in the 
planes of their stratification. Although the space between the Barn and 
Flagstaff Hill presents a distinct anticlinal line extending north and 
south, and though most of the dikes range with much regularity in the 
same line, nevertheless, at only a mile due south of the ridge the strata 
lie undisturbed. Hence the disturbing force seems to have acted under 

* " Geognosy of the Island of St. Helena," plate 5. 

f M. Constant Prevost (" Mem. de la Soc. Geolog.," torn, ii.) observes, that 
"les produits volcaniques n'ont que localement et rarement meme d^rang^ le 
sol, a travers lequel ils se sont fait jour." 



2i8 ST. HELENA. 

a point, rather than along a Hne. The manner in which it has acted, is 
probably explained by the structure of Little Stony-top, a mountain 
2,000 feet high, situated a few miles southward of the Barn ; we there 
see, even from a distance, a dark-coloured, sharp, wedge of compact 
columnar rock, with the bright-coloured feldspathic strata, sloping away 
on each side from its uncovered apex. This wedge, from which it derives 
its name of Stony-top, consists of a body of rock, which has been injected 
whilst liquefied into the overlying strata ; and if we may suppose that 
a similar body of rock lies injected, beneath the ridge connecting the 
Bam and Flagstaff, the structure there exhibited would be explained. 

Turk's Cap and Prosperous Bays. — Prosperous Hill is a great, black, 
precipitous mountain, situated two miles and a half south of the Barn, 
and composed, like it, of basaltic strata. These rest, in one part, on the 
brown-coloured, porphyritic beds of the basal series, and in another 
part, on a fissured mass of highly scoriaceous and amygdaloidal rock, 
which seems to have formed a small point of eruption beneath the sea, 

No. 9. 

Flagstaff Hill. 
Hold -fast-Tom. 




Prosperous Hill. The Barn. 

The double lines represent the basaltic strata ; the single, the basal submarine strata ; 
the dotted, the upper feldspat^iic strata. 

contemporaneously with the basal series. Prosperous Hill, like the 
Barn, is traversed by many dikes, of which the greater number range 
north and south, and its strata dip, at an angle of about 20°, rather 
obliquely from the island towards the sea. The space between Pros- 
perous Hill and the Barn, as represented in this woodcut, consists of 
lofty cHffs, composed of the lavas of the upper or feldspathic series, 
which rest, though unconformably, on the basal submarine strata, as 
we have seen that they do at Flagstaff Hill. Differently, however, from 
in that hill, these upper strata are nearly horizontal, gently rising 
towards the interior of the island ; and they are composed of greenish- 
black, or more commonly, pale brown, compact lavas, instead of softened 
and highly coloured matter. These brown-coloured, compact lavas, 
consist almost entirely of small glimmering scales, or of minute acicular 
crystals, of feldspar, placed close by the side of each other, and 
abounding with minute black specks, apparently of hornblende. The 
basaltic strata of Prosperous Hill project only a little above the level of 
the gently-sloping, feldspathic streams, which wind round and abut 
against their upturned edges. The inclination of the basaltic strata 
seems to be too great to have been caused by their having flowed down 
a slope, and they must have been tilted into their present position before 
the eruption of the feldspathic streams. 

Basaltic ring. — Proceeding round the Island, the lavas of the upper 
series, southward of Prosperous Hill, overhang the sea in lofty precipices. 
Further on, the headland, called Great Stony-top, is composed, as I 



RING OF BASALTIC CLIFFS. 219 

believe, of basalt ; as is Long Range Point, on the inland side of which 
the coloured beds abut. On the southern side of the island, we see 
the basaltic strata of the South Barn, dipping obliquely seaward at a 
considerable angle ; this headland, also, stands a little above the level 
of the more modern, feldspathic lavas. Further on, a large space of 
coast, on each side of Sandy Bay, has been much denuded, and there 
seems to be left only the basal wreck of the great, central crater. The 
basaltic strata reappear, with their seaward dip, at the foot of the hill, 
called Man-and-Horse ; and thence they are continued along the whole 
north-western coast to Sugar-Loaf Hill, situated near to the Flagstaff; 
and they everywhere have the same seaward inclination, and rest, in 
some parts at least, on the lavas of the basal series. We thus see that 
the circumference of the island is formed by a much-broken ring, or 
rather, a horse-shoe, of basalt, open to the south, and interrupted on 
the eastern side by many wide breaches. The breadth of this marginal 
fringe on the north-western side, where alone it is at all perfect, appears 
to vary from a mile to a mile and a half. The basaltic strata, as well 
as those of the subjacent basal series, dip, with a moderate inclination, 
where they have not been subsequently disturbed, towards the sea. 
The more broken state of the basaltic ring round the eastern half, 
compared with the western half of the island, is evidently due to the 
much greater denuding power of the waves on the eastern or windward 
side, as is shown by the greater height of the cliffs on that side, than to 
leeward. Whether the margin of basalt was breached, before or after 
the eruption of the lavas of the upper series, is doubtful ; but as 
separate portions of the basaltic ring appear to have been tilted before 
that event, and from other reasons, it is more probable, that some at least 
of the breaches were first formed. Reconstructing in imagination, as far 
as is possible, the ring of basalt, the internal space or hollow, which has 
since been filled up with the matter erupted from the great central crater, 
appears to have been of an oval figure, eight or nine miles in length by 
about four miles in breadth, and with its axis directed in a N.E. and 
S.W. hne, coincident with the present longest axis of the island. 

The central cuTved ridge. — This ridge consists, as before remarked, 
of grey feldspathic lavas, and of red, brecciated, argillaceous tuffs, like 
the beds of the upper coloured series. The grey lavas contain numerous, 
minute, black, easily fusible specks ; and but very few large crystals of 
feldspar. They are generally much softened ; with the exception of 
this character, and of being in many parts highly cellular, they are quite 
similar to those great sheets of lava which overhang the coast at 
Prosperous Bay. Considerable intervals of time appear to have elapsed, 
judging from the marks of denudation, between the formation of the 
successive beds, of which this ridge is composed. On the steep northern 
slope, I observed in several sections a much worn undulating surface of 
red tuff, covered by grey, decomposed, feldspathic lavas, with only a 
thin earthy layer interposed between them. In an adjoining part, I 
noticed a trap-dike, four feet wide, cut off and covered up by the feld- 
spathic lava, as is represented in the woodcut. The ridge ends on the 
eastern side in a hook, which is not represented clearly enough in any 



220 ST. HELENA. 

map which I have seen ; towards the western end, it gradually slopes 
down and divides into several subordinate ridges. The best defined 
portion between Diana's Peak 'and Nest Lodge, which supports the 
highest pinnacles in the island varying from 2,000 to 2,700 feet, is 
rather less than three miles long in a straight line. Throughout this 
space the ridge has a uniform appearance and structure ; its curvature 
resembles that of the coast-line of a great bay, being made up of many 
smaller curves, all open to the south. The northern and outer side is 
supported by narrow ridges or buttresses, which slope down to the 
adjoining country. The inside is much steeper, and is almost precipi- 
tous ; it is formed of the basset edges of the strata, which gently decline 
outwards. Along some parts of the inner side, a httle way beneath 
the summit, a flat ledge extends, which imitates in outline the smaller 
curvatures of the crest. Ledges of this 
^°' ^°* kind occur not un frequently within volcanic 

ti — ^ craters, and their formation seems to be 

^ ' i^^-:E^-:=--2=L^=-==L.r==L due to the sinking down of a level sheet 
^[•^^^^^^^7^^^^ of hardened lava, the edges of which remain 

8 Ym^/Z/^^SiO^^^^w?^ ^^^^^ ^^ ^^^ round a pool, from which the 

W///////4vAS^y>^^ water has been drained) adhering to the 

riKa: sides.* 

I r- c ^A \Z- 1 In some parts, the ridge is surmounted 

„— Grey feldspathic lava. , n ,. j- 1 

^-Alayer.one inch in thickness, by a wall or parapet, perpendicular on 
ofa reddish earthy matter, both sides. Near Diana's Peak this wall 
3-Brjciated, red, argillaceous jg extremely narrow. At the Galapagos 
Archipelago I observed parapets, having a 
quite similar structure and appearance, surmounting several of the 
craters; one, which I more particularly examined, was composed of 
glossy, red scoriae firmly cemented together ; being externally perpen- 
dicular, and extending round nearly the whole circumference 01 the 
crater, it rendered it almost inaccessible. The Peak of Teneriffe and 
Cotopaxi, according to Humboldt, are similarly constructed ; he states t 
that " at their summits a circular wall surrounds the crater, which wall, 
at a distance, has the appearance of a small cylinder placed on a 
truncated cone. On Cotopaxi | this peculiar structure is visible to the 
naked eye at more than two thousand toises' distance ; and no person 
has ever reached its crater. On the Peak of Teneriffe, the parapet is so 
high, that it would be impossible to reach the caldera, if on the eastern 
side there did not exist a breach." The origin of these circular parapets 
is probably due to the heat or vapours from the crater, penetrating and 
hardening the sides to a nearly equal depth, and afterwards to the 
mountain being slowly acted on by the weather, which would leave the 
hardened part, projecting in the form of a cylinder or circular parapet. 

* A most remarkable instance of this structure is described in Ellis's 
"Polynesian Researches" (second edit.), where an admirable drawing is 
given of the successive ledges or terraces, on the borders of the immense 
crater at Hawaii, in the Sandwich Islands. 

j- "Personal Narrative," vol. i., p. 17U 

\ Humboldt's " Picturesque Atlas, ' folio, pi. 10. 



CENTRAL CURVED RIDGE. 221 

From the points of structure in the central ridge, now enumerated, — 
namely, from the convergence towards it of the beds of the upper 
series, — from the lavas there becoming highly cellular, — from the flat 
ledge, extending along its inner and precipitous side, like that within 
some still active craters, — from the parapet-like wall on its summit, — 
and lastly, from its peculiar curvature, unlike that of any common line 
of elevation, I cannot doubt that this curved ridge forms the last 
remnant of a great crater. In endeavouring, however, to trace its 
former outline, one is soon baffled ; its western extremity gradually 
slopes down, and, branching into other ridges, extends to the sea-coast ; 
the eastern end is more curved, but it is only a little better defined. 
Some appearances lead me to suppose that the southern wall of the 
crater joined the present ridge near Nest Lodge ; in this case the crater 
must have been nearly three miles long, and about a mile and a half in 
breadth. Had the denudation of the ridge and the decomposition of 
its constituent rocks proceeded a few steps further, and had this ridge, 
like several other parts of the island, been broken up by great dikes 
and masses of injected matter, we should in vain have endeavoured to 
discover its true nature. Even now we have seen that at Flagstaff Hill 
the lower extremity and most distant portion of one sheet of the erupted 
matter has been upheaved to as great a height as the crater down 
which it flowed, and probably even to a greater height. It is interest- 
ing thus to trace the steps by which the structure of a volcanic district 
becomes obscured, and finally obliterated: so near to this last stage 
is St. Helena, that I believe no one has hitherto suspected that the 
central ridge or axis of the island is the last wreck of the crater, whence 
the most modern volcanic streams were poured forth. 

The great hollow space or valley southward of the central curved 
ridge, across which the half of the crater must once have extended, is 
formed of bare, water-worn hillocks and ridges of red, yellow, and 
brown rocks, mingled together in chaos-like confusion, interlaced by 
dikes, and without any regular stratification. The chief part consists 
of red decomposing scoriae, associated with various kinds of tuff and 
yellow argillaceous beds, full of broken crystals, those of augite being 
particularly large. Here and there masses of highly cellular and 
amygdaloidal lavas protrude. From one of the ridges in the midst of 
the valley, a conical precipitous hill, called Lot, boldly stands up, and 
forms a most singular and conspicuous object. It is composed of 
phonolite, divided in one part into great curved laminae, in another, 
into angular concretionary balls, and in a third part into outwardly 
radiating columns. At its base the strata of lava, tuff, and scoriae, dip 
away on all sides ; * the uncovered portion is 197 feet f in height, and 
its horizontal section gives an oval figure. The phonohte is of a greenish- 

* Abich, in his "Views of Vesuvius" (plate vi.), has shown the manner in 
which beds, under nearly similar circumstances, are tilted up. The upper 
beds are more turned up than the lower; and he accounts for this, by 
showing that the lava insinuates itself horizontally between the lower beds. 

f This height is given by Mr. Scale, in his Geognosy of the island. The 
height of the summit above the level of the sea, is said to be 1,444 feet. 



222 ST. HELENA. 

grey colour, and is full of minute acicular crystals of feldspar ; in most 
parts it has a conchoidal fracture, and is sonorous, yet it is crenulated 
with minute air-cavities. In a S.W. direction from Lot, there are some 
other remarkable columnar pinnacles, but of a less regular shape, namely, 
Lot's Wife, and the Asses' Ears, composed of allied kinds of rock. From 
their flattened shape, and their relative position to each other,- they are 
evidently connected on the same line of fissure. It is, moreover, remark- 
able that this same N.E. and S.W. line, joining Lot and Lot's Wife, if 
prolonged would intersect Flagstaff Hill, which, as before stated, is 
crossed by numerous dikes running in this direction, and which has a 
disturbed structure, rendering it probable that a great body of once 
fluid rock lies injected beneath it. 

In this same great valley there are several other conical masses of 
injected rock (one, I observed, was composed of compact greenstone), 
some of which are not connected, as far as is apparent, with any line 
of dike ; whilst others are obviously thus connected. Of these dikes, 
three or four great lines stretch across the valley in a N.E. and S.W. 
direction, parallel to that one connecting the Asses' Ears, Lot's Wife, 
and probably Lot. The number of these masses of injected rock is a 
remarkable feature in the geology of St. Helena. Besides those just 
mentioned, and the hypothetical one beneath Flagstaff Hill, there is 
Little Stony-top and others, as I have reason to believe, at the Man- 
and-Horse, and at High Hill. Most of these masses, if not all of them, 
have been injected subsequently to the last volcanic eruptions from the 
central crater. The formation of conical bosses of rock on lines of 
fissure, the walls of which are in most cases parallel, may probably be 
attributed to inequalities in the tension, causing small transverse 
fissures ; and at these points of intersection the edges of the strata 
would naturally yield, and be easily turned upwards. Finally, I may 
remark, that hills of phonolite everywhere are apt* to assume singular 
and even grotesque shapes, like that of Lot : the peak at Fernando 
Noronha offers an instance ; at St. J ago, however, the cones of phonolite, 
though tapering, have a regular form. Supposing, as seems probable, 
that all such hillocks or obelisks have originally been injected, whilst 
liquefied, into a mould formed by yielding strata, as certainly has been 
the case with Lot, how are we to account for the frequent abruptness 
and singularity of their outHnes, compared with similarly injected 
masses of greenstone and basalt? Can it be due to a less perfect 
degree of fluidity, which is generally supposed to be characteristic of 
the allied trachytic lavas ? 

Superficial deposits. — Soft calcareous sandstone occurs in extensive, 
though thin, superficial beds, both on the northern and southern shores 
of the island. It consists of very minute, equal-sized, rounded particles 
of shells, and other organic bodies, which partially retain their yellow, 
brown, and pink colours, and occasionally, though very rarely, present 
an obscure trace of their original external forms. I in vain endeavoured 
to find a single unrolled fragment of a shell. The colour of the particles 

* D'Aubuisson, in his "Traite deGeognosie" (torn, ii., p. 540), particularly 
remarks that this is the case. 



SUPERFICIAL CALCAREOUS BEDS. 223 

is the most obvious character by which their origin can be recognised, 
the tints being affected (and an odour produced) by a moderate heat, 
in the same manner as in fresh shells. The particles are cemented 
together, and are mingled with some earthy matter : the purest masses, 
according to Beatson, contain 70 per cent, of carbonate of lime. The 
beds, varying in thickness from two or three feet to fifteen feet, coat 
the surface of the ground ; they generally lie on that side of the valley 
which is. protected from the wind, and they occur at the height of 
several hundred feet above the level of the sea. Their position is the 
same which sand, if now drifted by the trade-wind, would occupy ; 
and no doubt they thus originated, which explains the equal size and 
minuteness of the particles, and likewise the entire absence of whole 
shells, or even of moderately-sized fragments. It is remarkable that 
at the present day there are no shelly beaches on any part of the coast, 
whence calcareous dust could be drifted and winnowed ; we must, 
therefore, look back to a former period when before the land was worn 
into the present great precipices, a shelving coast, like that of Ascension, 
was favourable to the accumulation of shelly detritus. Some of the beds 
of this limestone are between six hundred and seven hundred feet above 
the sea ; but part of this height may possibly be due to an elevation of 
the land, subsequent to the accumulation of the calcareous sand. 

The percolation of rain-water has consolidated parts of these beds 
into a solid rock, and has formed masses of dark brown, stalagmitic 
limestone. At the Sugar-Loaf quarry, fragments of rock on the adjoin- 
ing slopes * have been thickly coated by successive fine layers of cal- 
careous matter. It is singular, that many of these pebbles have their 
entire surfaces coated, without any point of contact having been left 
uncovered ; hence, these pebbles must have been lifted up by the slow 
deposition between them of the successive films of carbonate of lime. 
Masses of white, finely oolitic rock are attached to the outside of some 
of these coated pebbles. Von Buch has described a compact limestone 
at Lanzarote, which seems perfectly to resemble the stalagmitic deposi- 
tion just mentioned : it coats pebbles, and in parts is finely oolitic : it 
forms a far-extended layer, from one inch to two or three feet in thick- 
ness, and it occurs at the height of 800 feet above the sea, but only 
on that side of the island exposed to the violent north-western winds. 
Von Buch remarks, t that it is not found in hollows, but only on the 
unbroken and inclined surfaces of the mountain. He believes, that 
it has been deposited by the spray which is borne over the whole 
island by these violent winds. ' It appears, however, to me much more 
probable that it has been formed, as at St. Helena, by the percolation 
of water through finely comminuted shells : for when sand is blown on 

* In the earthy detritus on several parts of this hill, irregular masses of 
very impure, crystallised sulphate of lime occur. As this substance is now 
being abundantly deposited by the surf at Ascension, it is possible that 
these masses may thus have originated ; but if so, it must have been at a 
period when the land stood at a much lower level. This earthy selenite is 
now found at a height of between six hundred and seven hundred feet. 

t " Description des Isles Canaries," p. 293. 



224 ST. HELENA. 

a much-exposed coast, it always tends to accumulate on broad, even 
surfaces, which offer a uniform resistance to the winds. At the neigh- 
bouring island, moreover, of Feurteventura,* there is an earthy lime- 
stone, which, according to Von Buch, is quite similar to specimens 
which he has seen from St. Helena, and which he believes to have been 
formed by the drifting of shelly detritus. 

The upper beds of the limestone, at the above-mentioned quarry on 
the Sugar-Loaf Hill, are softer, finer-grained and less pure, than the 
lower beds. They abound with fragments of land-shells, and with 
some perfect ones ; they contain, also, the bones of birds, and the large 
eggs,t apparently of water-fowl. It is probable that these upper beds 
remained long in an unconsolidated form, during which time, these 
terrestrial productions were embedded. Mr. G. R. Sowerby has kindly 
examined three species of land-shells, which I procured from this bed, 
and has described them in detail. One of them is a Succinea, identical 
with a species now living abundantly on the island ; the two others, 
namely, Coc/i/ogena fossilis and Helix bipltcaia, are not known in a 
recent state : the latter species was also found in another and different 
locality, associated with a species of Cochlogena which is undoubtedly 
extinct. 

Beds of extinct land-shells. — Land-shells, all of which appear to be 
species now extinct, occur embedded in earth, in several parts of the 
island. The greater number have been found at a considerable height 
on Flagstaff Hill. On the N.W. side of this hill, a rain-channel ex- 
poses a section of about twenty feet in thickness, of which the upper 
part consists of black vegetable mould, evidently washed down from the 
heights above, and the lower part of less black earth, abounding with 
young and old shells, and with their fragments : part of this earth is 
slightly consolidated by calcareous matter, apparently due to the 
partial decomposition of some of the shells. Mr. Scale, an intelligent 
resident, who first called attention to these shells, gave me a large 
collection from another locality, where the shells appear to have 
been embedded in very black earth. Mr. G. R. Sowerby has examined 
these shells, and has described them. There are seven species, 
namely, one Cochlogena, two species of the genus Cochlicopa, and 
four of Helix ; none of these are known in a recent state, or have been 
found in any other country. The smaller species were picked out 
of the inside of the large shells of the Cochlogena aurisvulpina. This 
last-mentioned species is in many respects a very singular one ; it was 
classed, even by Lamarck, in a marine genus, and having thus been 
mistaken for a sea-shell, and the smaller accompanying species having 
been overlooked, the exact localities where it was found have been 
measured, and the elevation of this island thus deduced ! It is very 
remarkable that all the shells of this species found by me in one spot, 
form a distinct variety, as described by Mr. Sowerby, from those 

* Idemy pp. 314 and 374. 

f Colonel Wilkes, in a catalogue presented with some specimens to the 
Geological Society, states that as many as ten eggs were found by one person. 
Dr. Buckland has remarked (" Geolog. Trans.," vol. v., p. 474) on these eggs. 



ELEVATION OF THE LAND. 225 

procured from another locality by Mr. Scale. As this Cochlogena is a 
large and conspicuous shell, I particularly inquired from several intelli- 
gent countrymen whether they had ever seen it alive ; they all assured 
me that they had not, and they would not even believe that it was a 
land animal : Mr. Scale, moreover, who was a collector of shells all his 
life at St. Helena, never met with it alive. Possibly some of the smaller 
species may turn out to be yet living kinds ; but, on the other hand, the 
two land-shells which are now living on the island in great numbers, 
do not occur embedded, as far as is yet known, with the extinct 
species. - I have shown in my "Journal,"* that the extinction of these 
land-shells possibly may not be an ancient event ; as a great change took 
place in the state of the island about one hundred and twenty years ago, 
when the old trees died, and were not replaced by young ones, these being 
destroyed by the goats and hogs, which had run wild in numbers, from 
the year 1502. Mr. Scale states, that on Flagstaff Hill, where we have 
seen that the embedded land-shells are especially numerous, traces are 
everywhere discoverable, which plainly indicate that it was once thickly 
clothed with trees ; at present not even a bush grows there. The 
thick bed of black vegetable mould which covers the shell-bed, on the 
flanks of this hill, was probably washed down from the upper part, as 
soon as the trees perislied, and the shelter afforded by them was 
lost. 

Elevation of the latid. — Seeing that the lavas of the basal series, 
wliich are of submarine origin, are raised above the level of the sea, 
and at some places to the height of many hundred feet, I looked out 
for superficial signs of the elevation of the land. The bottoms of some 
of the gorges, which descend to the coast, are filled up to the depth of 
about a hundred feet, by rudely divided layers of sand, muddy clay, 
and fragmentary masses ; in these beds, Mr. Scale has found the bones 
of the tropic-bird and of the albatross ; the former now rarely, and the 
latter never visiting the island. From the difference between these 
layers, and the sloping piles of detritus which rest on them, I suspect 
that they were deposited, when the gorges stood beneath the sea. Mr. 
Scale, moreover, has shown that some of the fissure-like gorges t 
become, with a concave outline, gradually rather wider at the bottom 
than at the top ; and this peculiar structure was probably caused by the 
wearing action of the sea, when it entered the lower part of these 
gorges. At greater heights, the evidence of the rise of the land is even 
less clear : nevertheless, in a bay-like depression on the table-land 
behind Prosperous Bay, at the height of about a thousand feet, there are 
flat-topped masses of rock, which it is scarcely conceivable, could have 
been insulated from the surrounding and similar strata, by any other 
agency than the denuding action of a sea-beach. Much denudation, 
indeed, has been effected at great elevations, which it would not be 
easy to explain by any other means : thus, the flat summit of the Barn, 
which is 2,000 feet high, presents, according to Mr. Scale, a perfect net- 

* "Journal of Researches," p. 582. 

f A fissure-like gorge, near Stony-top, is said by Mr. Scale to be 840 feet 

deep, and only 115 feet in width. 

15 



226 ST. HELENA. 

work of truncated dikes ; on hills like the Flagstaff, formed of soft rock, 
we might suppose that the dikes had been worn down and cut off by 
meteoric agency, but we can hardly suppose this possible with the hard, 
basaltic strata of the Barn. 

Coast denudatio7i. — The enormous cliffs, in many parts between one and 
two thousand feet in height, with which this prison-like island is sur- 
rounded, with the exception of only a few places, where narrow valleys 
descend to the coast, is the most striking feature in its scenery. We have 
seen that portions of the basaltic ring, two or three miles in length by one 
or two miles in breadth, and from one to two thousand feet in height, 
have been wholly removed. There are, also, ledges and banks of rock, 
rising out of profoundly deep water, and distant from the present coast 
between three and four miles, which, according to Mr. Scale, can be 
traced to the shore, and are found to be the continuations of certain 
well-known great dikes. The swell of the Atlantic Ocean has obviously 
been the active power in forming these cliffs ; and it is interesting to 
observe that the lesser, though still great, height of the cliffs on the 
leeward and partially protected side of the island (extending from the 
Sugar-Loaf Hill to South West Point), corresponds with the lesser 
degree of exposure. When reflecting on the comparatively low coasts 
of many volcanic islands, which also stand exposed in the open ocean, 
and are apparently of considerable antiquity, the mind recoils from an 
attempt to grasp the number ot centuries of exposure, necessary to have 
ground into mud and to have dispersed the enormous cubic mass of 
hard rock which has been pared off the circumference of this island. 
The contrast in the superficial state of St. Helena, compared with the 
nearest island, namely, Ascension, is very striking. At Ascension, the 
surfaces of the lava-streams are glossy, as if just poured forth, their 
boundaries are well defined, and they can often be traced to perfect 
craters, whence they were erupted ; in the course of many long walks, 
I did not observe a single dike ; and the coast round nearly the entire 
circumference is low, and has been eaten back (though too much stress 
must not be placed on this fact, as the island may have been subsiding) 
into a little wall only from ten to thirty feet high. Yet during the 
340 years, since Ascension has been known, not even the feeblest 
signs of volcanic action have been recorded.* On the other hand, at 
St. Helena, the course of no one stream of lava can be traced, either 
by the state of its boundaries or of its superficies ; the mere wreck 
of one great crater is left ; not the valleys only, but the surfaces of some 
of the highest hills, are interlaced by worn-down dikes, and, in many 

* In the Nautical Magazine for 1835, p. 642, and for 1838, p. 361, and in 
the ** Comptes Rendus," April 1838, accounts are given of a series of volcanic 
phenomena — earthquakes — troubled water — floating scoriae and columns of 
smoke — which have been observed at intervals since the middle of the last 
century, in a space of open sea between longitudes 20° and 22° west, about 
half a degree south of the equator. These facts seem to show, that an 
island or an archipelago is in process of formation in the middle of the 
Atlantic : a line joining St. Helena and Ascension, prolonged, intersects this 
slowly nascent focus of volcanic action. 



CRATERS OF ELEVATION. 227 

places, the denuded summits of great cones of injected rock stand 
exposed and naked ; lastly, as we have seen, the entire circuit of the 
island has been deeply worn back into the grandest precipices. 

Craters of Elevation. 

- There is much resemblance in structure and in geological history 
between St. Helena, St, Jago, and Mauritius. All three islands are 
bounded (at least in the parts which I was able to examine) by a ring 
of basaltic mountains, now much broken, but evidently once continuous. 
These mountains have, or apparently once had, their escarpments 
steep towards the interior of the island, and their strata dip outwards. 
I was able to ascertain, only in a few cases, the inclination of the beds ; 
nor was this easy, for the stratification was generally obscure, except 
when viewed from a distance. I feel, however, little doubt that, 
according to the researches of M. Elie de Beaumont, their average 
inclination is greater than that which they could have acquired, con- 
sidering their thickness and compactness, by flowing down a sloping 
surface. At St. Helena, and at St. Jago, the basaltic strata rest on 
older and probably submarine beds of different composition. At all 
three islands, deluges of more recent lavas have flowed from the centre 
of the island, towards and between the basaltic mountains ; and at St. 
Helena the central platform has been filled up by them. All three 
islands have been raised in mass. At Mauritius the sea, within a late 
geological period, must have reached to the foot of the basaltic moun- 
tains, as it now does at St. Helena ; and at St. Jago it is cutting back 
the intermediate plain towards them. In these three islands, but 
especially at St. Jago and at Mauritius, when, standing on the summit 
of one of the old basaltic mountains, one looks in vain towards the 
centre of the island, — the point towards which the strata beneath one's 
feet, and of the mountains on each side, rudely converge, — for a source 
whence these strata could have been erupted ; but one sees only a vast 
hollow platform stretched beneath, or piles of matter of more recent 
origin. 

These basaltic mountains come, I presume, into the class of Craters 
of elevation : it is immaterial whether the rings were ever completely 
formed, for the portions which now exist have so uniform a structure, 
that, if they do riot form fragments of true craters, they cannot be 
classed with ordinary lines of elevation. With respect to their origin, 
after having read the works of Mr. Lyell,* and of MM. C. Prevost and 
Virlet, I cannot believe that the great central hollows have been formed 
by a simple dome-shaped elevation, and the consequent arching of the 
strata. On the other hand, I have very great difficulty in admitting 
that these basaltic mountains are merely the basal fragments of great 
volcanoes, of which the summits have either been blown off, or more 
probably swallowed up by subsidence. These rings are, in some 
instances, so immense, as at St. Jago and at Mauritius, and their 
occurrence is so frequent, that I can hardly persuade myself to adopt 
♦ •' Principles of Geology " (fifth edit.), vol. ii., p. 171. 



228 CRATERS OF ELEVATION. 

this explanation. Moreover, I suspect that the following circumstances, 
from their frequent concurrence, are someway connected together, — a 
connection not implied in either of the above views : namely, first, the 
broken state of the ring ; showing that the now detached portions have 
been exposed to great denudation, and in some cases, perhaps, render- 
ing it probable that the ring never vv^as entire ; secondly, the great 
amount of matter erupted from the central area after or during the for- 
mation of the ring ; and thirdly, the elevation of the district in mass. As 
far as relates to the inclination of the strata being greater than that which 
the basal fragments of ordinary volcanoes would naturally possess, I can 
readily believe that this inclination might have been slowly acquired by 
that amount of elevation, of which, according to M. Elie de Beaumont, the 
numerous upfilled fissures or dikes are the evidence and the measure, 
— a view equally novel and important, which we owe to the researches 
of that geologist on Mount Etna. 

A conjecture, including the above circumstances, occurred to me, 
when, — with my mind fully convinced, from the phenomena of 1835 in 
South America,* that the forces which eject matter from volcanic orifices 
and raise continents in mass are identical, — I viewed that part of the 
coast of St. Jago, where the horizontally upraised, calcareous stratum 
dips into the sea, directly beneath a cone of subsequently erupted lava. 
The conjecture is that, during the slow elevation of a volcanic district 
or island, in the centre of which one or more orifices continue open, 
and thus relieve the subterranean forces, the borders are elevated more 
than the central area ; and that the portions thus upraised do not slope 
gently into the central, less elevated area, as does the calcareous stratum 
under the cone at St. Jago, and as does a large part of the circumfer- 
ence of Iceland,! but that they are separated from it by curved faults. 

* I have given a detailed account of these phenomena, in a paper read 
before the Geological Society in March 1838. At the instant of time, when 
an. immense area was convulsed and a large tract elevated, the districts 
immediately surrounding several of the great vents in the Cordillera re- 
mained quiescent; the subterranean forces being apparently relieved by the 
eruptions, which then recommenced with great violence. An event of some- 
what the same kind, but on an infinitely smaller scale, appears to have 
taken place, according to Abich ("Views of Vesuvius," plates i. and ix,), 
within the great crater of Vesuvius, where a platform on one side of a 
fissure was raised in mass twenty feet, whilst on the other side, a train of 
small volcanoes burst forth in eruption. 

f It appears, from information communicated to me in the most obliging 
manner by M. E. Robert, that the circumferential parts of Iceland, which 
are composed of ancient basaltic strata alternating with tuff, dip inland, 
thus forming a gigantic saucer. M. Robert found that this was the case, 
with a few and quite local exceptions, for a space of coast several hundred 
miles in length. I find this statement corroborated, as far as regards one 
place, by Mackenzie, in his "Travels" (p. 377)> and in another place by 
some MS. notes kindly lent me by Dr. Holland, The coast is deeply 
indented by creeks, at the head of which the land is generally low. M. 
Robert informs me, that the inwardly dipping strata appear to extend as far 
as this line, and that their inclination usually corresponds with the slope of 



GALAPAGOS ARCHIPELAGO. 229 

We might expect, from what we see along ordinary faults, that the 
strata on the upraised side, already dipping outwards from their 
original formation as lava-streams, would be tilted from the line of 
fault, and thus have their inclination increased. According to this 
hypothesis, which I am tempted to extend only to some few cases, it is 
not probable that the ring would ever be formed quite perfect ; and 
from the elevation being slow, the upraised portions would generally 
be exposed to much denudation, and hence the ring become broken ; 
we might also expect to find occasional inequalities in the dip of the 
upraised masses, as is the case at St. Jago. By. this hypothesis the 
elevation of the districts in mass, and the flowing of deluges of lava 
from the central platforms, are likewise connecteci together. On this 
view the marginal basaltic mountains of the three foregoing islands 
might still be considered as forming " Craters of elevation ; " the kind of 
elevation implied having been slow, and the central hollow or platform 
having been formed, not by the arching of the surface, but simply by 
that part having been upraised to a less height. 



CHAPTER V. 

GALAPAGOS ARCHIPELAGO. 



Chatham Island. — Craters composed of a peculiar kind of tulT. —Small basaltic 
craters, with hollows at their bases. — Albemarle Island, fluid lavas, their 
composition. — Craters of tuff, inclination of their exterior diverging 
strata, and structure of their interior converging .strata. — James Island, 
segment of a small basaltic crater ; fluidity and composition of its lava- 
streams, and of its ejected fragments. — Concluding remarks on the 
craters of tuff", and on the breached condition of their southern sides. — 
Mineralogical composition of the rocks of the archipelago. — Elevation of 
the land. — Direction of the fissures of eruption. 

This archipelago is situated under the equator, at a distance of 
between five and six hundred miles from the west coast of South 

the surface, from the high coast-mountains to the low land at the head of 
these creeks. In the section described by Sir G. Mackenzie, the dip is 120. 
The interior parts of the island chiefly consist, as far as is known, of recently 
erupted matter. The great size, however, of Iceland, equalling the bulkiest 
part of England, ought perhaps to exclude it from the class of islands we 
have been considering; but I cannot avoid suspecting that if the coast- 
mountains, instead of gently sloping into the less elevated central area, had 
been separated from it by irregularly curved faults, the strata would have 
been tilted seaward, and a "Crater of elevation," lilcc that of St. Jago or that 
of Mauritius, but of much vaster dimensions, would have been formed. I will 
only further remark, that the frequent occurrence of extensive lakes at the 
foot of large volcanoes, and the frequent association of volcanic and fresh- 
water strata, seem to indicate that the areas around volcanoes are apt to be 
depressed beneath the general level of the adjoining country, either from 
having been less elevated, or from the effects of subsidence. 



230 GALAPAGOS ARCHIPELAGO. 

America. It consists of five principal islands, and of several small ones, 
which together are equal in area,* but not in extent of land, to Sicily, 
conjointly with the Ionian Islands. They are all volcanic : on two, 
craters have been seen in eruption, and on several of the other islands, 
streams of lava have a recent appearance. The larger islands are 
chiefly composed of solid rock, and they rise with a tame outline to a 
heiglit of between one and four thousand feet. They are sometimes, 
but not generally, surmounted by one principal orifice. The craters 
vary in size from mere spiracles to huge caldrons several miles in cir- 
cumference ; they are extraordinarily numerous, so that I should think, 

No. TI. 
_ 60 Miles 



^ Tower I. 




^TJamesl. 
^ 'f^ ^. Tn3UFatigahUl. 

J^Chatham I. 
B(urrLngtonl. ^'^ 



Oiarks /. Hoo£» U 

GALAPAGOS AKCHIPELAGO. 

if enumerated, they would be found to exceed two thousand ; they are 
formed either of scoriae and lava, or of a brown-coloured tuff ; and these 
latter craters are in several respects remarkable. The whole group was 
surveyed by the officers of the Beagle. I visited myself four of the 
principal islands, and received specimens from all the others. Under 
the head of the different islands I will describe only that which appears 
to me deserving of attention. 

* I exclude from this measurement, the small volcanic islands of Culpepper 
and "Wenman, lying seventy miles northward of the group. Craters were 
visible on all the islands of the group, except on Towers Island, which is one 
of the lowest ; this island is, however, formed of volcanic rocks. 



ALTERED TUFF. 231 

Chatham Island. Craters co7nposed of a singular kmd of tuff. — • 
Towards the eastern end of this island there occur two craters composed 
of two kinds of tuff; one kind being friable, like slightly consoHdated 
ashes ; and the other compact, and of a different nature from anything 
which 1 have met with described. This latter substance, where it is 
best characterised, is of a yellowish-brown colour, translucent, and with 
a lustre somewhat resembhng resin ; it is brittle, with an angular, 
rough, and very irregular fracture, sometimes, however, being slightly 
granular, and even obscurely crystalline: it can readily be scratched 
with a knife, yet some points are hard enough just to mark common 
glass ; it fuses with ease into a blackish-green glass. The mass 
contains numerous broken crystals of olivine and augite, and small 
particles of black and brown scoriae : it is often traversed by thin seams 
of calcareous matter. It generally affects a nodular or concretionary 
structure. In a hand specimen, this substance would certainly be 
mistaken for a pale and peculiar variety of pitchstone ; but when seen 
in mass its stratification, and the numerous layers of fragments of 
basalt, both angular and rounded, at once render its subaqueous origin 
evident. An examination of a series of specimens shows that this 
resin-like substance results from a chemical change on small particles 
of pale and dark-coloured scoriaceous rocks ; and this change could be 
distinctly traced in different stages round the edges of even the same 
particle. The position near the coast of all the craters composed of 
this kind of tuff or peperino, and their breached condition, renders it 
probable that they were all formed when standing immersed in the sea ; 
considering this circumstance, together with the remarkable absence of 
large beds of ashes in the whole archipelago, I think it highly probable 
that much the greater part of the tuff has originated from the trituration 
of fragments of the grey, basaltic lavas in the mouths of craters standing 
in the sea. It may be asked whether the heated water within these 
craters has produced this singular change in the small scoriaceous 
particles, and given to them their translucent, resin-like fracture. Or 
has the associated lime played any part in this change ? 1 ask these 
questions from having found at St. Jago, in the Cape de Verde Islands, 
that where a great stream of molten lava has flowed over a calcareous 
bottom into the sea, the outermost film, which in other parts resembles 
pitchstone, is changed, apparently by its contact with the carbonate of 
lime, mto a resin-like substance, precisely like the best characterised 
specimens of the tuff from this archipelago.* 

To return to the two craters : one of them stands at the distance of a 
league from the coast, the intervening tract consisting of a calcareous 
tuff, apparently of submarine origin. This crater consists of a circle of 
hills some of which stand quite detached, but all have a very regular, 

* The concretions containing lime, which I have described at Ascension, 
as formed in a bed of ashes, present some degree of resemblance to this 
substance, but they have not a resinous fracture. At St. Helena, also, I 
found veins of a somewhat similar, compact, but non-resinous substance, 
occurring in a bed of pumiceous ashes, apparently free from calcareous 
matter : in neither of these cases could heat have acted. 



233 



GALAPAGOS ARCHIPELAGO. 



qud-qua versal dip, at an inclination of between thirty and forty degrees. 
The lower beds, to the thickness of several hundred feet, consist of the 
resin-likc stone, with embedded fragments of lava. The upper beds, 
which are between thirty and forty feet in thickness, are composed of 
a thinly stratified, fine-grained, liarsh, friable, brown-coloured tuff, or 
peperino.* A central mass without any stratification, which must 
formerly have occupied the hollow of the crater, but is now attached 
only to a few of the circumferential hills, consists of a tuff, intermediate 
in character between that with a resin-like, and that with an earthy 
fracture. Tliis mass contains white calcareous matter in small 
patches. The second crater (520 feet in height) must have existed 
until the eruption of a recent, great stream of lava, as a separate islet ; 
a fine section, worn by the sea, shows a grand funnel-shaped mass of 
basalt, surrounded by steep, sloping flanks of tuff, having in parts an 
earthy, and in others a semi-resinous fracture. The tuff is traversed by 
several broad, vertical dikes, with smooth and parallel sides, which I 
did not doubt were formed of basalt, until I actually broke off fragments. 

These dikes, how- 
^°- ^^' ever, consist of tuff 

like that of the sur- 
rounding strata, but 
more compact, and 
with a smoother frac- 
ture ; hence we must 
conclude, that fissures 
were formed and filled 
up with the finer mud 
or tuff from the crater, 
before its interior was 
occupied, as it now is, 
by a solidified pool of 
basalt. Other fissures 
have been subsequently formed, parallel to these singular dikes, and 
are merely filled with loose rubbish. The change from ordinary scori- 
aceous particles to the substance with a semi-resinous fracture, could 
be clearly followed in portions of the compact tuff of these dikes. 

At the distance of a few miles from these two craters, stands the 
Kicker Rock, or islet, remarkable from its singular form. It is un- 
stratified, and is composed of compact tuff, in parts having the resin- 
like fracture. It is probable that this amorphous mass, like that similar 
mass in the case first described, once filled up the central hollovv of a 
crater, and that its flanks, or sloping walls, have since been worn quite 
away by the sea, in which it stands exposed. 

Sjnall basaltic craters. — A bare, undulating tract, at the eastern end of 
Chatham Island, is remarkable from the number, proximity, and form of 
the small basaltic craters with which it is studded. They consist, either 
* Those geologists who restrict the term of "tuff" to ashes of a white 
colour, resulting from the attrition of feldspathic lavas, would call these 
brown-coloured strata " peperino." 




The Kicker Rock, 400 feet high. 



SMALL BASALTIC CRATERS. 233 

of a mere conical pile, or, but less commonly, of a circle, of black and 
red, glossy scoriae, partially cemented together. They vary in diameter 
from thirty to one hundred and fifty yards, and rise from about fifty to one 
hundred feet above the level of the surrounding plain. From one small 
eminence, I counted sixty of these craters, all of which were within 
a third of a mile from each other, and many were much closer. I 
measured the distance between two very small craters, and found that 
it was only thirty yards from the summit-rim of one to the rim of the 
other. Small streams of black, basaltic lava, containing olivine and 
much glassy feldspar, have flowed from many, but not from all of 
these craters. The surfaces of the more recent streams were ex- 
ceedingly rugged, and were crossed by great fissures ; the older streams 
were only a little less rugged ; and they were all blended and mingled 
together in complete confusion. The different growth, however, of the 
trees on the streams, often plainly marked their different ages. Had 
it not been for this latter character, the streams could in few cases 
have been distinguished ; and, consequently, this wide undulatory tract 
might have (as probably many tracts have) been erroneously con- 
sidered as formed by one great deluge of lava, instead of by a multitude 
of small streams, erupted from many small orifices. 

In several parts of this tract, and especially at the base of the small 
craters, there are circular pits, with perpendicular sides, from twenty 
to forty feet deep. At the foot of one small crater, there were three of 
these pits. They have probably been formed, by the falling in of the 
roofs of small caverns.* In other parts, there are mammiform hillocks, 
which resemble great bubbles of lava, with their summits fissured by 
irregular cracks, which appeared, upon entering them, to be very deep ; 
lava has not flowed from these hillocks. There are, also, other very 
regular, mammiform hillocks, composed of stratified lava, and sur- 
mounted by circular, steep-sided hollows, which, I suppose have been 
formed by a body of gas, first, arching the strata into one of the bubble- 
like hiflocks, and then, blowing off its summit. These several kinds 
of hillocks and pits, as well as the numerous, small, scoriaceous craters, 
all show that this tract has been penetrated, almost like a sieve, by the 
passage of heated vapours. The more regular hillocks could only have 
been heaved up, whilst the lava was in a softened state.t 

* M. Elie de Beaumont has described ("Mem. pour servir," etc., torn, iv,, 
p. 113) many "petits cirques d'eboulement " on Etna, of some of which the 
origin is historically known. 

f Sir G. Mackenzie ("Travels in Iceland," pp. 389 to 392) has described 
a plain of lava at the foot of Hecla, everywhere heaved up into great bubbles 
or blisters. Sir George states that this cavernous lava coinposes the upper- 
most stratum ; and the same fact is affirmed by Von Buch ("Descript. des 
Isles Canaries," p. 1 59), with respect to the basaltic stream near Rialcjo, in 
TenerifTe. It appears singular that it should be thS upper streams that are 
chiefly cavernous, for one sees no reason why the upper and lower should 
not have been equally affected at different times ; — have the inferior streams 
flowed beneath the pressure of the sea, and thus been flattened, after the 
passage through them, of bodies of gas? 



234 GALAPAGOS ARCHIPELAGO. 

Albemarle Island. — This island consists of five, great, flat-topped 
craters, which, together with the one on the adjoining island of Nar- 
borough, singularly resemble each other, in lorm and height. The 
southern one is 4,700 feet high, two others are 3,720 feet, a third onlj'' 
50 feet higher, and the remaining ones apparently of nearly the same 
height. Three of these are situated on one line, and their craters 
appear elongated in nearly the same direction. The northern crater, 
which is not the largest, was found by the triangulation to measure, 
externally, no less than three miles and one-eighth of a mile in diameter. 
Over the lips of these great, broad caldrons, and from little orifices 
near their summits, deluges of black lava have flowed down their 
naked sides. 

Fluidity of different lavas. — Near Tagus or Banks' Cove, I examined 
one of these great streams of lava, which is remarkable from the 
evidence of its former high degree of fluidity, especially when its 
composition is considered. Near the sea-coast this stream is several 
miles in width. It consists of a black, compact base, easily fusible 
into a black bead, with angular and not very numerous air-cells, and" 
•thickly studded with large, fractured crystals of glassy albite,* varying 
from the tenth of an inch to half an inch in diameter. This lava, 
although at first sight appearing eminently porphyritic, cannot properly 
be considered so, for the crystals have evidently been enveloped," 
rounded, and penetrated by the lava, like fragments of foreign rock in 
a trap-dike. This was very clear in some specimens of a similar lava,' 
from Abingdon Island, in which the only difference was, that the 
vesicles were spherical and more numerous. The albite in these lavas 
is in a similar condition with the leucite of Vesuvius, and with the 
olivine, described by Von Buch,t as projecting in great balls from the 
basalt of Lanzarote. Besides the albite, this lava contains scattered 
grains of a green mineral, with no distinct cleavage, and closely re- 
sembling olivine ; % but as it fuses easily into a green glass, it belongs 
probably to the augitic family: at James Island, however, a similar 
lava contained true olivine. I obtained specimens from the actual 

* In the Cordillera of Chile, I have seen lava very closely resembling this 
variety at the Galapagos Archipelago. It contained, however, besides the 
albite, well-formed crystals of augite, and the base (perhaps in consequence 
of the aggregation of the augitic particles) was a shade lighter in colour. I 
may here remark, that in all these cases, I call the feldspathic crystals, albite^ 
from their cleavage-planes (as measured by the reflecting goniometer) 
corresponding with those of that mineral. As, however, other species of 
this genus have lately been discovered to cleave in nearly the same planes' 
with albite, this determination must be considered as only provisional. I 
examined the crystals in the lavas of many different parts of the Galapagos 
group, and I found that none of them, with the exception of some crystals 
from one part of James fsland, cleaved in the direction of orthite or potash- 
feldspar. 

f "Description des Isles Canaries," p. 295. 

\ Humboldt mentions that he mistook a green augitic mineral, occurring 
in the volcanic rocks of the Cordillera of Quito, for olivine. 



CRATERS OF TUFF. 235 

surface, and from a depth of four feet, but they differed in no respect. 
The high degree of fluidity of this lava-stream was at once evident, 
from its smooth and gently sloping surface, from the manner in which 
the main stream was divided by small inequalities into little rills, and 
especially from the manner in which its edges, far below its source, 
and where it must have been in some degree cooled, thinned out to 
almost nothing ; the actual margin consisting of loose fragments, few 
of which were larger than a man's head. The contrast between this 
margin, and the steep walls, above twenty feet high, bounding many of 
the basaltic streams at Ascension, is very remarkable. It has generally 
been supposed that lavas abounding with large crystals, and including 
angular vesicles,* have possessed little fluidity; but we see that the 
case has been very different at Albemarle Island. The degree of 
fluidity in different lavas, does not seem to correspond with any ap- 
parent corresponding amount of difference in their composition: at 
Chatham Island, some streams, containing much glassy albite and some 
olivine, are so rugged, that they may be compared to a sea frozen 
during a storm ; whilst the great stream at Albermarle Island is almost 
as smooth as a lake when ruffled by a breeze. At James Island, black 
basaltic lava, abounding with small grains of olivine, presents an inter- 
mediate degree of roughness; its surface being glossy, and the de- 
tached fragments resembling, in a very singular manner, folds of drapery, 
cables, and pieces of the bark of trees.t 

Craters of tuff . — About a mile southward of Banks' Cove, there is a 
fine elliptic crater, about five hundred feet in depth, and three-quarters 
of a mile in diameter. Its bottom is occupied by a lake of brine, out of 
which some little crateriform hills of tuff rise. The lower beds are formed 
of compact tuff, appearing like a subaqueous deposit ; whilst the upper 
beds, round the entire circumference, consist of a harsh, friable tuff, of 
little specific gravity, but often containing fragments of rock in layers. 
This upper tuff contains numerous pisolitic balls, about the size of small 
bullets, which differ from the surrounding matter, only in being slightly 

* The irregular and angular form of the vesicles is probably caused by 
the unequal yielding of a mass composed, in almost equal proportion, of 
solid crystals and of a viscid base. It certainly seems a general circumstance, 
as might have been expected, that in lava, which has possessed a high degree 
of iluidity, as well as an even-sized gram, the vesicles are internally smooth 
and spherical. 

f A specimen of basaltic lava, with a few small broken crystals or albite, 
given me by one of the officers, is perhaps worthy of description. It consists 
of cylindrical ramifications, some of which are only the twentieth of an inch 
in diameter, and are drawn out into the sharpest points. The mass has not 
been formed like a stalactite, for the points terminate both upwards and 
downwards. Globules, only the fortieth of an inch in diameter, have dropped 
from some of the points, and adhere to the adjoining branches. The lava is 
vesicular, but the vesicles never reach the surface of the branches, which 
are smooth and glossy. As it is generally supposed that vesicles are always 
elongated in the direction of the movement of the fluid mass, I may observe, 
that in these cylindrical branches, which vary from a quarter to only the 
twentieth of an inch in diameter, every air-cell is spherical. 



236 GALAPAGOS ARCHIPELAGO. 

harder and finer grained. The beds dip away very regularly on all 
sides, at angles varying, as I found by measurement, from twenty-five 
to thirty degrees. The external surface of the crater slopes at a nearly 
similar inclination, and is formed by slightly convex ribs, like those on 
the shell of a pecten or scallop, which become broader as they extend 
from the mouth of the crater to its base. These ribs are generally 
from eight to twenty feet in breadth, but sometimes they are as much 
as forty feet broad ; and they resemble old, plastered, much flattened 
vaults, with the plaster scaling off in plates : they are separated from 
each other by gullies, deepened by alluvial action. At their upper and 
narrow ends, near the- mouth of the crater, these ribs often consist of 
real hollow passages, like, but rather smaller than, those often formed 
by the cooling of the crust of a lava-stream, whilst the inner parts have 
flowed onward ; — of which structure I saw many examples at Chatham 
Island. There can be no doubt but that tliese hollow ribs or vaults 
have been formed in a similar manner, namely, by the setting or harden- 
ing of a superficial crust on streams of mud, which have flowed down 
from the upper part of the crater. In another part of this same crater, 
I saw open concave gutters between one and two feet wide, which 
appear to have been formed by the hardening of the lower surface of a 
mud-stream, instead of, as in the former case, of the upper surface. 
From these facts I think it is certain that the tuff must have flowed as 
mud.* This mud may have been formed either within the crater, or 
from ashes deposited on its upper parts, and afterwards washed down 
by torrents of rain. The former method, in most of the cases, appears 
the more probable one ; at James Island, however, some beds of the 
friable kind of tuff extend so continuously over an uneven surface, that 
probably they were formed by the falling of showers of ashes. 

Within this same crater, strata of coarse tuff, chiefly composed of 
fragments of lava, abut, like a consolidated talus, against the inside 
walls. They rise to a height of between one hundred and one hundred 
and fifty feet above the surface of the internal brine-lake ; they dip 
inwards, and are inclined at an angle varying from thirty to thirty-six 
degrees. They appear to have been formed beneath water, probably 
at a period when the sea occupied the hollow of the crater. I was 
surprised to observe that beds having this great inclination did not, as 
far as they could be followed, thicken towards their lower extremities. 

Batiks' Cove, — This harbour occupies part of the interior of a 
shattered crater of tuff larger than that last described. All the tuff is 

* This conclusion is of some interest, because M. Dufrenoy (" Mem. pour 
servir," torn, iv., p. 274) has argued from strata of tuff, apparently of similar 
composition with that here described, being inclined at angles between 18° 
and 20°, that Monte Nuevo and some other craters of Southern Italy have 
been formed by upheaval. From the facts given above, of the vaulted cha- 
racter of the separate rills, and from the tuff not extending in horizontal 
sheets round these crateriform hills, no one will suppose that the strata have 
here been produced by elevation ; and yet we see that their inclination is 
above 20°, and often as much as 30°. The consolidated strata, also, of the 
internal talus, as will be immediately seen, dips at an angle of above 30°. 



BANKS' COVE. 



237 



compact, and includes numerous fragments of lava ; it appears like a 
subaqueous deposit. The most remarkable feature in this crater is the 
great development of strata converging inwards, as in the last case, at 
a considerable inclination, and often deposited in irregular curved layers. 
These interior converging beds, as well as the proper, diverging crateri- 
form strata, are represented in the accompanying rude, sectional sketch 
of the headlands, forming this Cove. The internal and external strata 
differ little in composition, and the former have evidently resulted from 
the wear and tear, and redeposition of the matter forming the external 
crateriform strata. From the great development of these inner beds, a 
person walking round the rim of this crater might fancy himself on a 
circular anticlinal ridge of stratified sandstone and conglomerate. The 
sea is wearing away the inner and outer strata, and especially the 
latter ; so that the inwardly converging strata will, perhaps, in some 
future age, be left standing alone — a case which might at first perplex 
a geologist.* 



No. 13. 







A sectional sketch of the headlands forming Banks' Cove, showing the diverging 
crateriform strata, and the converging stratified talus. The highest point of these 
hills is 817 feet above the sea. 



James Island, — Two craters of tuff on this island are the only re- 
maining ones which require any notice. One of them lies a mile and a 
half inland from Puerto Grande : it is circular, about the third of a mile 
in diameter, and 400 feet in depth. It differs from all the other tui^- 
craters which I examined, in having the lower part of its cavity, to the 
height of between one hundred and one hundred and fifty feet, formed by 
a precipitous wall of basalt, giving to the crater the appearance of having 
burst through a solid sheet of rock. The upper part of this crater consists 
of strata of the altered tuff, with a semi-resinous fracture. Its bottom is 

* I believe that this case actually occurs in the Azores, where Dr. 
Webster ("Description," p. 185) has described a basin-formed, little island, 
composed oi strata of tuff, dipping inwards and bounded externally by steep 
sea-worn clifTs. Dr. Daubeny supposes (on Volcanoes, p. 266), that this 
cavity must have been formed by a circular subsidence. It appears to me 
far more probable, that we here have strata which were originally deposited 
within the hollow of a crater, of which the exterior walls have since been 
removed by the sea. 



238 



GA LA PA COS ARCIIIPEL A GO. 



occupied by a shallow lake of brine, covering layers of salt, which rest on 
deep black mud. The other crater lies at the distance of a few miles, 
and is only remarkable from its size and perfect condition. Its summit 
is 1,200 feet above the level of the sea, and the interior hollow is 600 
feet deep. Its external sloping surface presented a curious appearance 
from the smoothness of the wide layers of tuff, which resembled a vast 
plastered floor. Brattle Island is, I believe, the largest crater in the 
Archipelago composed of tuff; its interior diameter is nearly a nautical 
mile. At present it is in a ruined condition, consisting of little more 
than half a circle open to the south ; its great size is probably due, in 
part, to internal degradation, from the action of the sea. 

Segment of a small basaltic crater. — One side of Fresh-water Bay, in 
James Island, is bounded by a promontory, which forms the last wreck 
of a great crater. On the beach of this promontory, a quadrant-shaped 
segment of a small subordinate point of eruption stands exposed. It 
consists of nine separate little streams of lava piled upon each other ; 

No. 14. 




Segment of a very small orifice of eruption, on the beach of Fresh-water Bay. 

and of an irregular pinnacle, about fifteen feet high, of reddish-brown, 
vesicular basalt, abounding with large crystals of glassy albite, and 
with fused augite. This pinnacle, and some adjoining paps of rock on 
the beach, represent the axis of the crater. The streams of lava can 
be followed up a little ravine, at right angles to the coast, for between 
ten and fifteen yards, where they are hidden by detritus : along the 
beach they are visible for nearly eighty yards, and I do not believe that 
they extend much further. The three lower streams are united to 
the pinnacle ; and at the point of junction (as shown in the above 
rude sketch made on the spot), they are slightly arched, as if in the 
act of flowing over the lip of the crater. The six upper streams no 
doubt were originally united to this same column before it was worn 
down by the sea. The lava of these streams is of similar composition 
with that of the pinnacle, excepting that the crystals of albite appear 
to be more comminuted, and the grains of fused augite are absent. 
Each stream is separated from the one above it by a few inches, or at 
most by one or two feet in thickness, of loose fragmentary scoriae. 



EJECTED FRAGMENTS, 239 

apparently derived from the abrasion of the streams in passing over 
each other. All these streams are very remarkable from their thinness. 
I carefully measured several of them ; one was eight inches thick, but 
was firmly coated with three inches above, and three inches below, of 
red scoriaceous rock (which is the case with all the streams), making 
altogether a thickness of fourteen inches : this thickness was preserved 
quite uniformly along the entire length of the section. A second stream 
was only eight inches thick, including both the upper and lower scoria- 
ceous surfaces. Until examining this section, I had not thought it pos- 
sible that lava could have flowed in such uniformly thin sheets over a 
surface far from smooth. These little streams closely resemble in 
composition that great deluge of lava at Albemarle Island, which 
likewise must have possessed a high degree of fluidity. 

Pseudo-extraneous, ejected fragments. — In the lava and in the scoriae 
of this little crater, 1 found several fragments, which, from their angular 
form, their granular structure, their freedom frorh air-cells, their brittle 
and burnt condition, closely resembled those fragments of primary 
rocks which are occasionally ejected, as at Ascension, from volcanoes. 
These fragments consist of glassy albite, much mackled, and with very 
imperfect cleavages, mingled with semi-rounded grains, having tarnished, 
glossy surfaces, of a steel-blue mineral. The crystals of albite are 
coated by a red oxide of iron, appearing like a residual substance ; and 
their cleavage-planes also are sometimes separated by excessively fine 
layers of this oxide, giving to the crystals the appearance of being ruled 
like a glass micrometer. There was no quartz. The steel-blue mineral, 
which is abundant in the pinnacle, but which disappears in the streams 
derived from the pinnacle, has a fused appearance, and rarely presents 
even a trace of cleavage ; I obtained, however, one measurement, which 
proved that it was augite ; and in one other fragment, which differed 
from the others, in being slightly cellular, and in gradually blending 
into the surrounding matrix the small grains of this mineral were 
tolerably well crystallised. Although there is so wide a difference in 
appearance between the lava of the little streams, and especially of 
their red scoriaceous crusts, and one of these angular ejected fragments, 
which at first sight might readily be mistaken for syenite, yet I believe 
that the lava has originated from the melting and movement of a mass 
of rock of absolutely similar composition with the fragments. Besides 
the specimen above alluded to, in which we see a fragment becoming 
slightly cellular, and blending into the surrounding matrix, some of 
the grains of the steel-blue augite also have their surfaces becoming 
very finely vesicular, and passing into the nature of the surrounding 
paste ; other grains arc throughout, in an intermediate condition. The 
paste seems to consist of the augite more perfectly fused, or, more 
probably, merely disturbed in its softened state by the movement of 
the mass, and mingled with the oxide of iron and with finely com- 
minuted, glassy albite. Hence probably it is that the fused albite, 
which is abundant in the pinnacle, disappears in the streams. The 
albite is in exactly the same state, with the exception of most of the 
crystals being smaller in the lava and in the embedded fragments ; but 



240 GALAPAGOS ARCHIPELAGO. 

in the fragments they appear to be less abundant : this, however, would 
naturally happen from the intumescence of the augitic base, and its 
consequent apparent increase in bulk. It is interesting thus to trace 
the steps by which a compact granular rock becomes converted into a 
vesicular, pseudo-porphyritic lava, and finally into red scoriae. The 
structure and composition of the embedded fragments show that they 
are parts either of a mass of primary rock which has undergone con- 
siderable change from volcanic action, or more probably of the crust 
of a body of cooled and crystallised lava, which has afterwards been 
broken up and re-liquefied; the crust being less acted on by the 
renewed heat and movement. 

Co7icluding re??iarks on the tuff-craters. — These craters, from the 
pecuharity of the resin-like substance which enters largely into their 
composition, from their structure, their size and number, present the 
most striking feature in the geology of this Archipelago. The majority 
of them form either separate islets, or promontories attached to the 
larger islands ; and those which now stand at some little distance from 
the coast are worn and breached, as if by the action of the sea. From 
this general circumstance of their position, and from the small quantity 
of ejected ashes in any part of the Archipelago, I am led to conclude, 
that the tuff has been chiefly produced, by the grinding together of frag- 
ments of lava within active craters, communicating with the sea. In the 
origin and composition of the tuff, and in the frequent presence of a 
central lake of brine and of layers of salt, these craters resemble, though 
on a gigantic scale, the " salses," or hillocks of mud, which are common 
in some parts of Italy and in other countries.* Their closer connection, 
however, in this Archipelago, with ordinary volcanic action, is shown by 
the pools of solidified basalt, with which they are sometimes filled up. 

It at first appears very singular, that all the craters formed of tuff 
have their southern sides, either quite broken down and wholly removed, 
or much lower than the other sides. I saw and received accounts of 
twenty-eight of these craters ; of these, twelve form separate islets,t 
and now exist as mere crescents quite open to the south, with occasion- 
ally a few points of rock marking their former circumference : of the 
remaining sixteen, some form promontories, and others stand at a little 
distance inland from the shore ; but alHhave their southern sides either 
the lowest, or quite broken down. Two, however, of the sixteen had 

* D'Aubuisson's "Traite de Geognosie," torn, i., p. 189. I may remark, 
that I saw at Terceira, in the Azores, a crater of tuff or peperino, very similar 
to these of the Galapagos Archipelago. From the description given in Frey- 
cinet's "Voyage," similar ones occur at the Sandwich Islands; and probably 
they are present in many other places. 

f These consist of the three Grossman Islets, the largest of which is 600 
feet in height; Enchanted Island; Gardner Island (760 feet high) ; Champion 
Island (331 feet high); Enderby Island ; Brattle Island; two islets near In- 
defatigable Island ; and one near James Island. A second crater near James 
Island (with a salt lake in its centre) has its southern side only about twenty 
feet high, whilst the other parts of the circumference are about three hundred 
feet in height. 



ELEVATION OF THE LAND. 24I 

their northern sides also low, whilst their eastern and western sides 
were perfect. I did not see, or hear of, a single exception to the rule, 
of these craters being broken down or low on the side, which faces a 
point of the horizon between S.E. and S.W. This rule does not apply 
to craters composed of lava and scoriae. The explanation is simple : at 
this Archipelago, the waves from the trade-wind, and the swell propa- 
gated from the distant parts of the open ocean, coincide in direction 
(which is not the case in many parts of the Pacific), and with their 
united forces attack the southern sides of all the islands ; and conse- 
quently the southern slope, even when entirely formed of hard basaltic 
rock, is invariably steeper than the northern slope. As the tuff-craters 
are composed of a soft material, and as probably all, or nearly all, have at 
some period stood immersed in the sea, we need not wonder that they 
should invariably exhibit on their exposed sides the effects of this great 
denuding power. Judging from the worn condition of many of these 
craters, it is probable that some have been entirely washed away. As 
there is no reason to suppose, that the craters formed of scoriae and lava 
were erupted whilst standing in the sea, we can see why the rule does 
not apply to them. At Ascension, it was shown that the mouths of the 
craters, which are there all of terrestrial origin, have been affected by 
the trade-wind ; and this same power might here, also, aid in making 
the windward and exposed sides of some of the craters originally the 
lowest. 

Mineralogical composition of the rocks. — In the northern islands, the 
basaltic lavas seem generally to contain more albite than they do in the 
southern half of the Archipelago ; but almost all the streams contain 
some. The albite is not unfrequently associated with olivine. I did 
not observe in any specimen distinguishable crystals of hornblende or 
augite ; I except the fused grains in the ejected fragments, and in the 
pinnacle of the little crater, above described. I did not meet with a 
single specimen of true trachyte ; though some of the paler lavas, when 
abounding with large crystals of the harsh and glassy albite, resemble 
in some degree this rock ; but in every case the basis fuses into a black 
enamel. Beds of ashes and far-ejected scoriae, as previously stated, are 
almost absent ; nor did I see a fragment of obsidian or of pumice. Von 
Buch * believes that the absence of pumice on Mount Etna is conse- 
quent on the feldspar being of the Labrador variety ; if the presence of 
pumice depends on the constitution of the feldspar, it is remarkable, 
that it should be absent in this archipelago, and abundant in the Cor- 
dillera of South America, in both of which regions the feldspar is of the 
albitic variety. Owing to the absence of ashes, and the general inde- 
composable character of the lava in this Archipelago, the islands are 
slowly clothed with a poor vegetation, and the scenery has a desolate 
and frightful aspect. 

Elevation of the land. — Proofs of the rising of the land are scanty and 

imperfect. At Chatham Island, I noticed some great blocks of lava, 

cemented by calcareous matter, containing recent shells ; but they 

occurred at the height of only a few feet above high-water mark. One 

* " Description des Isles Canaries," p. 328. 

16 



242 GALAPAGOS ARCHIPELAGO. 

of the officers gave me some fragments of shells, which he found em- 
bedded several hundred feet above the sea, in the tuff of two craters, 
distant from each other. It is possible, that these fragments may have 
been carried up to their present height in an eruption of mud ; but as, in 
one instance, they were associated with broken oyster-shells, almost 
forming a layer, it is more probable that the tuff was uplifted with the 
shells in mass. The specimens are so imperfect that they can be recog- 
nised only as belonging to recent marine genera. On Charles Island, 
I observed a line of great rounded blocks, piled on the summit of a 
vertical cliff, at the height of fifteen feet above the line, where the sea 
now acts during the heaviest gales. This appeared, at first, good evi- 
dence in favour of the elevation of the land ; but it was quite deceptive, 
for I afterwards saw on an adjoining part of this same coast, and heard 
from eye-witnesses, that wherever a recent stream of lava forms a 
smooth inclined plane, entering the sea, the waves during gales have the 
power of rolling tip rounded blocks to a great height, above the line of 
their ordinary action. As the little cliff in the foregoing case is formed 
by a stream of lava, which, before being worn back, must have entered 
the sea with a gently sloping surface, it is possible or rather it is pro- 
bable, that the rounded boulders, now lying on its summit, are merely 
the remnants of those which had been rolled up during storms to their 
present height. 

Direction of the fissures of eruption. — The volcanic orifices in this 
group cannot be considered as indiscriminately scattered. Three great 
craters on Albemarle Island form a well-marked line, extending N.W. 
by N. and S.E. by S. Narborough Island, and the great crater on the 
rectangular projection of Albemarle Island, form a second parallel line. 
To the east, Hood's Island, and the islands and rocks between it and 
James Island, form another nearly parallel line, which, when prolonged, 
includes Culpepper and Wenman Islands, lying seventy miles to the 
north. The other islands lying further eastward, form a less regular 
fourth line. Several of these islands, and the vents on Albemarle 
Island, are so placed, that they likewise fall on a set of rudely parallel 
lines, intersecting the former lines at right angles ; so that the principal 
craters appear to lie on the points where two sets of fissures cross each 
other. The islands themselves, with the exception of Albemarle Island, 
are not elongated in the same direction with the Hues on which they 
stand. The direction of these islands is nearly the same with that 
which prevails in so remarkable a manner in the numerous archipelagoes 
of the great Pacific Ocean. Finally, I may remark, that amongst the 
Galapagos Islands there is no one dominant vent much higher than all 
the others, as may be observed in many volcanic archipelagoes : the 
highest is the great mound on the south-western extremitv af Albemarle 
Island, which exceeds by barely a thousand feet seveial other neigh- 
bouring craters. 



TRACHYTE AND BASALT, 243 

CHAPTER VI. 

TRACHYTE AND BASALT. — DISTRIBUTION OF VOLCANIC ISLES. 

The sinking of crystals in fluid lava. — Specific gravity of the constituent 
parts of trachyte and of basalt, and their consequent separation. — 
Obsidian. — Apparent non-separation of the elements of plutonic rocks. — 
Origin of trap-dikes in the plutonic series. — Distribution of volcanic 
islands; their prevalence in the great oceans. — ^They are generally arranged 
in lines. — The central volcanoes of Von Buch doubtful. — Volcanic islands 
bordering continents. — Antiquity of volcanic islands, and their elevation 
in maj^s. — Eruptions on parallel lines of fissure within the same geological 
period. 

On the separation of the constituent minerals of lava, according to their 
specific gravities. — One side of Fresh-water Bay, in James Island, is 
formed by the wreck of a large crater, mentioned in the last chapter, of 
which the interior has been filled up by a pool of basalt, about two 
hundred feet in thickness. This basalt is of a grey colour, and contains 
many crystals of glassy albite, which become much more numerous in 
the lower, scoriaceous part. This is contrary to what might have been 
expected, for if the crystals had been originally disseminated in equal 
numbers, the greater intumescence of this lower scoriaceous part would 
have made them appear fewer in number. Von Buch * has described 
a stream of obsidian on the Peak of Teneriffe, in which the crystals of 
feldspar become more and more numerous, as the depth or thickness 
increases, so that near the lower surface of the stream the lava even 
resembles a primary rock. Von Buch further states, that M. Dr6e, in 
his experiments in melting lava, found that the crystals of feldspar 
always tended to precipitate themselves to the bottom of the crucible. 
In these cases, I presume there can be no doubt f that the crystals sink 
from their weight. The specific gravity of feldspar varies | from 2*4 to 
2-58, whilst obsidian seems commonly to be from 23 to 2*4; and in a 
fluidified state its specific gravity would probably be less, which would 

* "Description des Isles Canarie?," pp. 190 and 191. 

t In a mass of molten. iron, it is found {Edinburgh New Philosophical 
Journal, vol. xxiv., p. 66) that the substances, which have a closer affinity for 
oxygen than iron has, rise from the interior of the mass to the surface. But 
a similar cause can hardly apply to the separation of the crystals of these 
lava-streams. The cooling of the surface of lava seems, in some cases, to 
have affected its composition ; for Dufrenoy (" Mem. pour servir," tom. iv., 
p. 271) found that the interior parts of a stream near Naples contained two- 
thirds of a mineral which was acted on by acids, whilst the surface consisted 
chiefly of a mineral unattackable by acids. 

+ I have taken the specific gravities of the simple minerals from Von 
Kobell, one of the latest and best authorities, and of the rocks from various 
authorities. Obsidian, according to Phillips, is 2*35 ; and Jameson says it 
never exceeds 2-4 ; but a specimen from Ascension, weighed by myself, was 
242. 



244 TRACHYTE AND BASALT. 

facilitate the sinking of the crystals of feldspar. At James Island, the 
crystals of albite, although no doubt of less weight than the grey 
basalt, in the parts where compact, might easily be of greater specific 
gravity than the scoriaceous mass, formed of melted lava and bubbles of 
heated gas. 

The sinking of crystals through a viscid substance like molten rock, as 
is unequivocally shown to have been the case in the experiments of 
M. Dree, is worthy of further consideration, as throwing light on the 
separation of the trachytic and basaltic series of lavas. Mr. P. Scrope 
has speculated on this subject ; but he does not seem to have been 
aware of any positive facts, such as those above given ; and he has 
overlooked one v^ery necessary element, as it appears to me, in the 
phenomenon — namely, the existence of either the lighter or heavier 
mineral in globules or in crystals. In a substance of imperfect fluidity, 
like molten rock, it is hardly credible, that the separate, infinitely small 
atoms, whether of feldspar, augite, or of any other mineral, would have 
power from their slightly different gravities to overcome the friction 
caused by their movement ; but if the atoms of any one of these 
minerals became, whilst the others remained fluid, united into crystals 
or granules, it is easy to perceive that from the lessened friction, their 
sinking or floating power would be greatly increased. On the other 
hand, if all the minerals became granulated at the same time, it is 
scarcely possible, from their mutual resistance, that any separation 
could take place. A valuable, practical discovery, illustrating the effect 
of the granulation of one element in a fluid mass, in aiding its separa- 
tion, has lately been made : when lead containing a small proportion of 
silver, is constantly stirred whilst cooling, it becomes granulated, and 
the grains of imperfect crystals of nearly pure lead sink to the bottom, 
leaving a residue of melted metal much richer in silver ; whereas if the 
mixture be left undisturbed, although kept fluid for a length of time, the 
two metals show no signs ot separating.* The sole use of the stirring 
seems to be, the formation of detached granules. The specific gravity 
of silver is 10-4, and of lead ii'35 : the granulated lead, which sinks, is 
never absolutely pure, and the residual fluid metal contains, when 
richest, only ^ttt P^rt of silver. As the difference in specific gravity, 
caused by the different proportions of the two metals, is so exceedingly 
small, the separation is probably aided in a great degree by the differ- ^ 
ence in gravity between the lead, when granular though still hot, and 
when fluid. 

In a body of liquefied volcanic rock, left for some time without any 
violent disturbance, we might expect, in accordance with the above 
facts, that if one of the constituent minerals became aggregated into 

* A full and interesting account of this discovery, by Mr. Pattinson, was 
read before the British Association in September 1 838. In some alloys, 
according to Turner ("Chemistry," p. 210), the heaviest metal sinks, and it 
appears that this takes place whilst both metals are fluid. Where there is 
a considerable difference in gravity, as between iron and the slag formed 
' during the fusion of the ore, we need not be surprised at the atoms separat- 
ing, without either substance being granulated. 



TRACHYTE AND BASALT. 245 

crystals or granules, or had been enveloped in this state from some 
previously existing mass, such crystals or granules would rise or sink, 
according to their specific gravity. Now we have plain evidence of 
crystals being embedded in many lavas, whilst the paste or basis has 
continued fluid. I need only refer, as instances, to the several, great, 
pseudo-porphyritic streams at the Galapagos Islands, and to the trachy- 
tic streams in many parts of the world, in which we find crystals of 
feldspar bent and broken by the movement of the surrounding, semi- 
fluid matter. Lavas are chiefly composed of three varieties of feldspar, 
varying in specific gravity from 2-4 to 274 ; of hornblende and augite, 
varying from 3-0 to 3*4; of olivine, varying from 3*3 to 3-4; and lastly, 
of oxides of iron, with specific gravities from 4*8 to 5 '2. Hence crystals 
of feldspar, enveloped in a mass of liquefied, but not highly vesicular 
lava, would tend to rise to the upper parts ; and crystals or granules 
of the other minerals, thus enveloped, would tend to sink. We ought 
not, however, to expect any perfect degree of separation in such viscid 
materials. Trachyte, which consists chiefly of feldspar, with some 
hornblende and oxide of iron, has a specific gravity of about 2*45 ; * 
whilst basalt, composed chiefly of augite and feldspar, often with much 
iron and olivine, has a gravity of about 3*0. Accordingly we find, that 
where both trachytic and basaltic streams have proceeded from the 
same orifice, the trachytic streams have generally been first erupted 
owing, as we must suppose, to the molten lava of this series having 
accumulated in the upper parts of the volcanic focus. This order of 
eruption has been observed by Beudant, Scrope, and by other authors ; 
three instances, also, have been given in this volume. As the later 
eruptions, however, from most volcanic mountains, burst through their 
basal parts, owing to the increased height and weight of the internal 
column of molten rock, we see why, in most cases, only the lower 
flanks of the central, trachytic masses, are enveloped by basaltic 
streams. The separation of the ingredients of a mass of lava, would, 
perhaps, sometimes take place within the body of a volcanic mountain, 
if lofty and of great dimensions, instead of within the underground 
focus ; in which case, trachytic streams might be poured forth, almost 
contemporaneously, or at short recurrent intervals, from its summit, 
and basaltic streams from its base : this seems to have taken place 
at Teneriff'e.t I need only further remark, that from violent disturb- 
ances the separation of the two series, even under otherwise favour- 
able conditions, would naturally often be prevented, and likewise their 
usual order of eruption be inverted. From the high degree of fluidity 
of most basaltic lavas, these perhaps, alone, would in many cases reach 
the surface. 

* Trachyte from Java, was found by Von Buch to be 2*47 ; from Auvergne, 
by De la Beche, it was 2*42 ; from Ascension, by myself, it was 2-42. Jame- 
son and other authors give to basalt a specific gravity of 3-0 ; but specimens 
from Auvergne were found, by De la Beche, to be only 278; and from the 
Giant's Causeway, to be 2*91. 

t Consult Von Buch's well-known and admirable " Description Physique" 
pf this island, which might s^rve as a model of descriptive geology, 



246 TRACHYTE AND BASALT. 

As we have seen that crystals of feldspar, in the instance described 
by Von Buch, sink in obsidian, in accordance with their known greater 
specific gravity, we might expect to find in every trachytic district, 
where obsidian has flowed as lava, that it had proceeded from the upper 
or highest orifices. This, according to Von Buch, holds good in a 
remarkable manner both at the Lipari Islands and on the Peak of 
Teneriffe; at this latter place obsidian has never flowed from a less 
height than 9,200 feet. Obsidian, also, appears to have been erupted 
from the loftiest peaks of the Peruvian Cordillera. I will only further 
observe, that the specific gravity of quartz varies from 2-6 to 2-8 ; and 
therefore, that when present in a volcanic focus, it would not tend 
to sink with the basaltic bases ; and this, perhaps, explains the frequent 
presence, and the abundance of this mineral, in the lavas of the 
trachytic series, as observed in previous parts of this volume. 

An objection to the foregoing theory will, perhaps, be drawn from 
the plutonic rocks not being separated into two evidently distinct 
series, of different specific gravities ; although, like the volcanic, they 
have been liquefied. In answer, it may first be remarked, that we have 
no evidence of the atoms of any one of the constituent minerals in the 
plutonic series having been aggregated, whilst the others remained 
fluid, which we have endeavoured to show is an almost necessary con- 
dition of their separation ; on the contrary, the crystals have generally 
impressed each other with their forms.* 

In the second place, the perfect tranquillity, under which it is probable 
that the plutonic masses, buried at profound depths, have cooled, 
would, most likely, be higlily unfavourable to the separation of their 
constituent minerals ; for, if the attractive force, which during the 
progressive cooling draws together the molecules of the different 
minerals, has power sufficient to keep them together, the friction 
between such half-formed crystals or pasty globules would effectually 
prevent the heavier ones from sinking, or the lighter ones from rising. 
On the other hand, a small amount of disturbance, which would 
probably occur in most volcanic foci, and which we have seen does not 
prevent the separation of granules of lead from a mixture of molten 
lead and silver, or crystals of feldspar from streams of lava, by breaking 

* The crystalline paste of phonolite is frequently penetrated by long 
needles of hornblende ; from which it appears that the hornblende, though 
the more fusible mineral, has crystallised before, or at the same time with a 
more refractory substance. Phonolite, as far as my observations serve, in 
every instance appears to be an injected rock, like those of the plutonic 
series ; hence probably, like these latter, it has generally been cooled with- 
out repeated and violent disturbances. Those geologists who have doubted 
whether granite could have been formed by igneous liquefaction, because 
minerals of different degrees of fusibility impress each other with their forms, 
could not have been aware of the fact of crystallised hornblende penetrating 
phonolite, a rock undoubtedly of igneous origin. The viscidity, which it 
is now known, that both feldspar and quartz retain at a temperature much 
below their points of fusion, easily explains their mutual impressment. 
Consult on this subject Mr. Horner's paper on Bonn, "Geolog. Transact.," 
vol. iv., p. 439; and " L'Institut," with respect to quartz, 1839, p. 161. 



TRACHYTE AND BASALT. 2^7 

and dissolving the less perfectly formed globules, would permit the 
more perfect and therefore unbroken crystals, to sink or rise, according 
to their specific gravity. 

Although in plutonic rocks two distinct species, corresponding to 
the trachytic and basaltic series, do not exist, I much suspect that a 
certain amount of separation of their constituent parts has often taken 
place. I suspect this from having observed how frequently dikes of 
greenstone and basalt intersect widely extended formations of granite 
and the allied metamorphic rocks. I have never examined a district in 
an extensive granitic region without discovering dikes ; I may instance 
the numerous trap-dikes, in several districts of Brazil, Chile, and 
Australia, and at the Cape of Good Hope : many dikes likewise occur 
in the great granitic tracts of India, in the north of Europe, and in 
other countries. Whence, then, has the greenstone and basalt, forming 
these dikes, come? Are we to suppose, like some of the elder 
geologists, that a zone of trap is uniformly spread out beneath the 
granitic series, which composes, as far as we know, the foundations of 
the earth's crust ? Is it not more probable, that these dikes have been 
formed by fissures penetrating into partially cooled rocks of the granitic 
and metamorphic series, and by their more fluid parts, consisting chiefly 
of hornblende, oozing out, and being sucked into such fissures? At 
Bahia, in Brazil, in a district composed of gneiss and primitive greenstone, 
I saw many dikes, of a dark augitic (for one crystal certainly was of 
this mineral) or hornblendic rock, which, as several appearances clearly 
proved, either had been formed before the surrounding mass had 
become solid, or had together with it been afterwards thoroughly 
softened.* On both sides of one of these dikes, the gneiss was 
penetrated, to the distance of several yards, by numerous, curvilinear 
threads or streaks of dark matter, which resembled in form clouds of 
the class called cirrhi-comae ; some few of these threads could be 
traced to their junction with the dike. When examining them, I 
doubted whether such hair-like and curvilinear veins could have been 
injected, and I now suspect, that instead of having been injected from 
the dike, they were its feeders. If the foregoing views of the origin of 
trap-dikes in widely extended granitic regions far from rocks of any 
other formation, be admitted as probable, we may further admit, in the 
case of a great body of plutonic rock, being impelled by repeated move- 
ments into the axis of a mountain-chain, that its more liquid constituent 
parts might drain into deep and unseen abysses ; afterwards, perhaps, 
to be brought to the surface under the form, either of injected masses 
of greenstone and augitic porphyry,t or of basaltic eruptions. Much of 

* Portions of these dikes have been broken off, and are now surrounded 
by the primary rocks, with their laminae conformably winding round them. 
Dr. Hubbard, also (Si/Uman's Journal, vol. xxxiv., p. 119), has described an 
interlacement of trap-veins in the granite of the White Mountains, which he 
thinks must have been formed when both rocks were soft. 

t Mr. Phillips ("Lardner's Encyclop.," vol. ii., p. 115) quotes Von Buch's 
statement, that augitic porphyry ranges parallel to, and is found constantly 
at the base of, great chains of mountains. Humboldt, also, has remarked 



248 THE DISTRIBUTION OF 

the difficulty which geologists have experienced when they have com- 
pared the composition of volcanic with plutonic formations, will, I 
think, be removed, if we may believe that most plutonic masses have 
been, to a certain extent, drained of those comparatively weighty and 
easily liquefied elements, which compose the trappean and basaltic 
series of rocks. 

On the distribtition of volcanic islaiids. — During my investigations on 
coral-reefs, I had occasion to consult the works of many voyagers, and 
I was invariably struck with the fact, that with rare exceptions, the 
innumerable islands scattered throughout the Pacific, Indian, and 
Atlantic Oceans, were composed either of volcanic, or of modern coral- 
rocks. It would be tedious to give a long catalogue of all the volcanic 
islands ; but the exceptions which I have found are easily enumerated : 
in the Atlantic, we have St. Paul's Rock, described in this volume, and 
the Falkland Islands, composed of quartz and clay-slate ; but these 
latter islands are of considerable size, and lie not very far from the 
South American coast:* in the Indian Ocean, the Seychelles (situated 
in a line prolonged from Madagascar) consist of granite and quartz : in 
the Pacific Ocean, New Caledonia, an island of large size, belongs (as 
far as is known) to the primary class. New Zealand, which contains 
much volcanic rock and some active volcanoes, from its size cannot be 
classed with the small islands, which we are now considering. The 
presence of a small quantity of non-volcanic rock, as of clay-slate on 
three of the Azores, t or of tertiary limestone at Madeira, or of clay-slate 
at Chatham Island in the Pacific, or of lignite at Kerguelen Land, 
ought not to exclude such islands or archipelagoes, if formed chiefly 
of erupted matter, from the volcanic class. 

The composition of the numerous islands scattered through the great 
oceans being with such rare exceptions volcanic, is evidently an exten- 
sion of that law, and tlie effect of those same causes, whether chemical 
or mechanical, from which it results, that a vast majority of the volcanoes 
now in action stand either as islands in the sea, or near its shores. 
This fact of the ocean-islands being so generally volcanic is also 
interesting in relation to the nature of the mountain-chains on our con- 

the frequent occurrence of trap-rock, in a similar position ; of which fact 
I have observed many examples at the foot of the Chilian Cordillera. The 
existence of granite in the axes of great mountain chains is always probable, 
and I am tempted to suppose, that the laterally injected masses of augitic 
porphyry and of trap, bear nearly the same relation to the granitic axes 
which basaltic lavas bear to the central trachytic masses, round the flanks of 
which they have so frequently been erupted. 

* Judging from Forster's imperfect observation, perhaps Georgia is not 
volcanic. Dr. Allan is my informant with regard to the Seychelles. I do 
not know of what formation Rodriguez, in the Indian Ocean, is composed. 

•}• This is stated on the authority of Count V. de Bedemar, with respect to 
Flores and Graciosa (Charlsworth, "Magazine of Nat. Hist.," vol. i., p. 557). 
St. Maria has no volcanic rock, according to Captain Boyd (Von Buch's 
" Descript.," p. 365). Chatham Island has been described by Dr. Dieffenbach, 
in the " Geographical Journal," 1841, p. 201. As yet we have received only 
imperfect notices on Kerguelen Land, from the Antarctic Expedition, 



VOLCANIC ISLANDS. 249 

tiiients, which are comparatively seldom volcanic; and yet we are led 
to suppose that where our continents now stand an ocean once extended. 
Do volcanic eruptions, we may ask, reach the surface more readily 
through fissures formed during the first stages of the conversion of the 
bed of the ocean into a tract of land ? 

Looking at the charts of the numerous volcanic archipelagoes, we see 
that the islands are generally arranged either in single, double, or 
triple rows, in lines which are frequently curved in a slight degree.* 
Each separate island is either rounded, or more generally elongated in 
the same direction with the group in which it stands, but sometimes 
transversely to it. Some of the groups which are not much elongated 
present little symmetry in their formes; M. Virletf states that this is 
the case with the Grecian Archipelago : in such groups I suspect (for 
I am aware how easy it is to deceive oneself on these points), that the 
vents are generally arranged on one line, or on a set of short parallel 
lines, intersecting at nearly right angles another line, or set of lines. 
The Galapagos Archipelago offers an example of this structure, for 
most of the islands and the chief orifices on the largest island are so 
grouped as to fall on a set of lines ranging about N.W. by N., and on 
another set ranging about W.S.W. : in the Canary Archipelago we have 
a simpler structure of the same kind: in the Cape de Verde group, 
which appears to be the least symmetrical of any oceanic volcanic 
archipelago, a N.W. and S.E. line formed by several islands, if prolonged, 
would intersect at right angles a curved line, on which the remaining 
islands are placed. 

Von Buch X has classed all volcanoes under two heads, namely, 
cefilral volcanoes, round which numerous eruptions have taken place 
on all sides, in a manner almost regular, and volcanic chains. In the 
examples given of the first class, as far as position is concerned, I can 
see no grounds for their being called "central;" and the evidence of 
any difference in mineralogical nature between central volcanoes and 
volca7iic chaiiis appears sliglit. No doubt some one island in most 
small volcanic archipelagoes is apt to be considerably higher than the 
others ; and in a similar manner, whatever the cause may be, that on 
the same island one vent is generally higher than all the others. Von 
Buch does not include in his class of volcanic chains small archipelagoes, 
in which the islands are admitted by him, as at the Azores, to be 
arranged in lines ; but when viewing on a map of the world how perfect 
a series exists from a few volcanic islands placed in a row to a train 
of linear archipelagoes following each other in a straight line, and so on 
to a great wall like the Cordillera of America, it is difficult to believe 
that there exists any essential difference between short and long 
volcanic chains. Von Buch § states that his volcanic chains surmount, 

* Professors William and Henry Darwin Rogers have lately insisted 
much, in a memoir read before the American Association, on the regularly 
curved lines of elevation in parts of the Appalachian range, 

f "Bulletin de la Soc. Geolog.," tom. iii., p. Iio, 

X "Description dcs Isles Canaries," p. ^24, 

§ Idem, p. 393. 



250 THE DISTRIBUTION OF VOLCANIC ISLANDS. 

or are closely connected with, mountain-ranges of primary formation : 
but if trains of linear archipelagoes are, in the course of time, by the 
long-continued action of the elevatory and volcanic forces, converted 
into mountain-ranges, it would naturally result that the inferior primary 
rocks would often be uplifted and brought into view. 

Some authors have remarked that volcanic islands occur scattered, 
though at very unequal distances, along the shores of the great 
continents, as if in some measure connected with them. In the case 
of Juan Fernandez, situated 330 miles from the coast of Chile, there 
was undoubtedly a connection between the volcanic forces acting under 
this island and imder the continent, as was shown during the earth- 
quake of 1835. The islands, moreover, of some of the small volcanic 
groups which thus border continents, are placed in lines, related to 
those along which the adjoining shores of the continents trend ; I may 
instance the lines of intersection at the Galapagos, and at the Cape de 
Verde Archipelagoes, and the best marked line of the Canary Islands. 
If these facts be not merely accidental, we see that many scattered 
volcanic islands and small groups are related not only by proximity, but 
in the direction of the fissures of eruption to the neighbouring continents 
— a relation, which Von Buch considers, characteristic of his great 
volcanic chains. 

In volcanic archipelagoes, the orifices are seldom in activity on more 
than one island at a time ; and the greater eruptions usually recur only 
after long intervals. Observing the number of craters, that are usually 
found on each island of a group, and the vast amount of matter which 
has been erupted from them, one is led to attribute a high antiquity 
even to those groups, which appear, like the Galapagos, to be of com- 
paratively recent origin. This conclusion accords with the prodigious 
amount of degradation, by the slow action of the sea, which their 
originally sloping coasts must have suffered, when they are worn back, 
as is so often the case, into grand precipices. We ought not, however, 
to suppose, in hardly any instance, that the whole body of matter, 
forming a volcanic island, has been erupted at the level, on which it 
now stands : the number of dikes, which seem invariably to intersect 
the interior parts of every volcano, show, on the principles explained by 
M. Elie de Beaumont, that the whole mass has been uplifted and 
fissured. A connection, moreover, between volcanic eruptions and con- 
temporaneous elevations in mass * has, I think, been shown to exist in 
my work on Coral-Reefs, both from the frequent presence of upraised 
organic remains, and from the structure of the accompanying coral-reefs. 
Finally, I may remark, that in the same Archipelago, eruptions have 
taken place within the historical period on more than one of the parallel 
lines of fissure : thus, at the Galapagos Archipelago, eruptions have 
taken place from a vent on Narborough Island, and from one on Albe- 
marle Island, which vents do not fall on the same line ; at the Canary 
Islands, eruptions have taken place in Teneriffe and Lanzarote ; and at 

* A similar conclusion is forced on us, by the phenomena, which accom- 
panied the earthquake of 1835, at Conception, and which are detailed in my 
paper (vol. v., p. 601) in the "Geological Transactions." 



SANDSTONE PLATFORMS. 251 

the Azores, on the three parallel lines of Pico, St. Jorge, and Terceira. 
Believing that a mountain-axis differs essentially from a volcano, only 
in plutonic rocks having been injected, instead of volcanic matter having 
been ejected, this appears to me an interesting circumstance ; for we 
may infer from it as probable, that in the elevation of a mountain-chain, 
two or more of the parallel lines forming it may be upraised and 
injected within the same geological period. 



CHAPTER VII. 

New South Wales. — Sandstone formation. — Embedded pseudo-fragments of 
shale. — Stratification. — Current-cleavage. — Great valleys. — Van Diemen's 
Land. — Palaeozoic formation. — Newer formation with volcanic rocks. — 
Travertin with leaves of extinct plants. — Elevation of the land. — New 
Zealand. — King George's Sound. — Superficial ferruginous beds. — Super- 
ficial calcareous deposits, with casts of branches. — Their origin from 
drifted particles of shells and corals. — Their extent. — Cape of Good Hope. 
— Junction of the granite and clay-slate. — Sandstone formation. 

The Beagle, in her homeward voyage, touched at New Zealand, 
Australia, Van Diemen's Land, and the Cape of Good Hope. In order 
to confine the Third Part of these Geological Observations to South 
America, I will here briefly describe all that I observed at these places 
worthy of the attention of geologists. 

New South Wales. — My opportunities of observation consisted of a 
ride of ninety geographical miles to Bathurst, in a W.N.W. direction 
from Sydney. The first thirty miles from the coast passes over a sand- 
stone country, broken up in many places by trap-rocks, and separated 
by a bold escarpment overhanging the river Nepean, from the great 
sandstone platform of the Blue Mountains. This upper platform is 
I, coo feet high at the edge of the escarpment, and rises in a distance of 
twenty-five miles to between three and four thousand feet above the level 
of the sea. At this distance the road descends to a country rather less 
elevated, and composed in chief part of primary rocks. There is much 
granite, in one part passing into a red porphyry with octagonal crystals 
of quartz, and intersected in some places by trap-dikes. Near the 
Downs of Bathurst I passed over much pale-brown, glossy clay-slate, 
with the shattered laminae running north and south ; I mention this 
fact, because Captain King informs me that, in the country a hundred 
miles southward, near Lake George, the mica-slate ranges so invariably 
north and south that the inhabitants take advantage of it in finding their 
way through the forests. 

The sandstone of the Blue Mountains is at least 1,200 feet thick, 
and in some parts is apparently of greater thickness ; it consists of 



252 NEW SOUTH WALES, 

small grains of quartz, cemented by white earthy matter, and it abounds 
with ferruginous veins. The lower beds sometimes alternate with 
shales and coal : at Wolgan I found in carbonaceous shale leaves of 
the Glossopieris Broivnii^ a fern which so frequently accompanies the 
coal of Australia. The sandstone contains pebbles of quartz; and 
these generally increase in number and size (seldom, however, exceeding 
an inch or two in diameter) in the upper beds : I observed a similar 
circumstance in the grand sandstone formation at the Cape of Good 
Hope. On the South American coast, where tertiary and supra-tertiary 
beds have been extensively elevated, I repeatedly noticed that the 
uppermost beds were formed of coarser materials than the lower : this 
appears to indicate that, as the sea became shallower, the force of the 
waves or currents increased. On the lower platform, however, between 
the Blue Mountains and the coast, I observed that the upper beds of 
the sandstone frequently passed into argillaceous shale, — the effect, 
probably, of this lower space having been protected from strong currents 
during its elevation. The sandstone of the Blue Mountains evidently 
having been of mechanical origin, and not having suffered any meta- 
morphic action, I was surprised at observing that, in some specimens, 
nearly all the grains of quartz were so perfectly crystallised with bril- 
liant facets that they evidently had not in their present form been 
aggregated in any previously existing rock.* It is difficult to imagine 
how these crystals could have been formed ; one can hardly believe 
that they were separately precipitated in their present crystallised state. 
Is it possible that rounded grains of quartz may have been acted on by 
a fluid corroding their surfaces, and depositing on them fresh silica ? I 
may remark -that, in the sandstone formation of the Cape of Good 
Hope, it is evident that silica has been profusely deposited from aqueous 
solution. 

In several parts of the sandstone I noticed patches of shale which 
might at the first glance have been mistaken for extraneous fragments ; 
their horizontal laminae, however, being parallel with those of the sand- 
stone, showed that they were the remnants of thin, continuous beds. 
One such fragment (probably the section of a long narrow strip) seen in 
the face of a cliff, was of greater vertical thickness than breadth, which 
proves that this bed of shale must have been in some slight degree con- 
solidated, after having been deposited, and before being worn away by 
the currents. Each patch of the shale shows, also, how slowly many 
of the successive layers of sandstone were deposited. These pseudo- 
fragments of shale will perhaps explain, in some cases, the origin or 
apparently extraneous fragments in crystalline metamorphic rocks. I 
mention this, because I found near Rio de Janeiro a well-defined 
angular fragment, seven yards long by two yards in breadth, of gneiss 

* I have latel}' seen, in a paper by Smith (the father of English geologists), 
in the Magazme of Natural History, that the grains of quartz in the millstone, 
grit of England are often crystalHsed. Sir David Brewster, in a paper read 
before the British Association, 1840, states, that in old decomposed glass, the 
silex and metals separate into concentric rings, and that the silex regains its 
crystalline structure, as is shown by its action on light, 



» 



SANDSTONE PLATFORMS, 253 

containing garnets and mica in laj^ers, enclosed in the ordinary, strati- 
fied, porphyritic gneiss of the country. The laminae of the fragment 
and of the surrounding matrix ran in exactly the same direction, but 
they dipped at different angles. I do not wish to affirm that this 
singular fragment (a solitary case, as far as I know) was originally 
deposited in a layer, like the shale in the Blue Mountains, between the 
strata of the porphyritic gneiss, before they were metamorphosed ; but 
there is sufficient analogy between the two cases to render such an 
explanation possible. 

Stratification of the escarp??ie?tt. — The strata of the Blue Mountains 
appear to the eye horizontal ; but they probably have a similar inclina- 
tion with the surface of the platform, which slopes from the west 
towards the escarpment over the Nepean, at an angle of one degree, or 
of one hundred feet in a mile.* The strata of the escarpment dip 
almost conformably with its steeply inclined face, and with so much 
regularity, that they appear as if thrown into their present position ; but 
on a more careful examination, they are seen to thicken and to thin out, 
and in the upper part to be succeeded and almost capped by horizontal 
beds. These appearances render it probable, that we here see an 
original escarpment, not formed by the sea having eaten back into the 
strata, but by the strata having originally extended only thus far. 
Those who have been in the habit of examining accurate charts of 
sea-coasts, where sediment is accumulating, will be aware, that the 
surfaces of the banks thus formed, generally slope from the coast very 
gently towards a certain line in the offing, beyond vi^hich the depth in 
most cases suddenly becomes great. I may instance the great banks 
of sediment within the West Indian Archipelago,t which terminate in 
submarine slopes, inclined at angles of between thirty and forty degrees, 
and sometimes even at more than forty degrees : every one knows how 
steep such a slope would appear on the land. Banks of this nature, if 
uplifted, would probably have nearly the same external form as the 
platform of the Blue Mountains, where it abruptly terminates over the 
Nepean. 

Currettt-cleavage. — The strata of sandstone in the low coast country, 
and likewise on the Blue Mountains, are often divided by cross or 
current laminae, which dip in different directions, and frequently at an 
angle of forty-five degrees. Most authors have attributed these cross 
layers to successive small accumulations on an inclined surface ; but 
from a careful examination in some parts of the New Red Sandstone of 
England, I beUeve that such layers generally form parts of a series of 

* This is stated on the authority of Sir T. Mitchell, in his "Travels," 
vol. ii., p. 357. 

f I have described these very curious banks in the Appendix to my 
volume on the structure of Coral-Reefs. I have ascertained the inclination 
of the edges of the banks, from information given me by Captain B. Allen, 
one of the surveyors, and by carefully measuring the horizontal distances 
between the last sounding on the bank and the first in the deep water. 
Widely extended banks in all parts of the West Indies have the same general 
form 01 surface. 



254 NEW SOUTH WALES. 

curves, like gigantic tidal ripples, the tops of which have since been cut 
off, either by nearly horizontal layers, or by another set of great ripples, 
the folds of which do not exactly coincide with those below them. It is 
well known to surveyors that mud and sand are disturbed during storms 
at considerable depths, at least from three hundred to four hundred and 
fifty feet,* so that the nature of the bottom even becomes temporarily 
changed ; the bottom, also, at a depth between sixty and seventy feet, 
has been observed t to be broadly rippled. One may, therefore, be 
allowed to suspect, from the appearance just mentioned in the New 
Red Sandstone, that at greater depths, the bed of the ocean is heaped 
up during gales into great ripple-like furrows and depressions, which are 
afterwards cut off by the currents during more tranquil weather, and 
again furrowed during gales. 

Valleys in the sandstone platforms. — The grand valleys, by which the 
Blue Mountains and the other sandstone platforms of this part of 
Australia are penetrated, and which long offered an insuperable obstacle 
to the attempts of the most enterprising colonist to reach the interior 
country, form the most striking feature in the geology of New South 
Wales. They are of grand dimensions, and are bordered by continuous 
links of lofty cliffs. It is not easy to conceive a more magnificent 
spectacle, than is presented to a person walking on the summit-plains, 
when without any notice he arrives at the brink of one of these cliffs, 
which are so perpendicular, that he can strike with a stone (as I have 
tried) the trees growing, at the depth of between one thousand and one 
thousand five hundred feet below him ; on both hands he sees headland 
beyond headland of the receding line of cliff, and on the opposite side of 
the valley, often at the distance of several miles, he beholds another fine 
rising up to the same height with that on which he stands, and formed of 
the same horizontal strata of pale sandstone. The bottoms of these 
valleys are moderately level, and the fall of the rivers flowing in them, 
according to Sir T. Mitchell, is gentle. The main valleys often send into 
the platform great baylike arms, which expand at their upper ends ; and 
on the other hand, the platform often sends promontories into the valley, 
and even leaves in them great, almost insulated, masses. So continuous 
are the bounding lines of cliff, that to descend into some of these valleys, 
it is necessary to go round twenty miles ; and into others, the surveyors 
have only lately penetrated, and the colonists have not yet been able to 
drive in their cattle. But the most remarkable point of structure in these 
valleys, is, that although several miles wide in their upper parts, they 
generally contract towards their mouths to such a degree as to become 
impassable. The Surveyor-General, Sir T. Mitchell, t in vain endea- 
voured, first on foot and then by crawling between the great fallen 

* See Martin White, on " Soundings in the British Channel," pp. 4 and 
166. 

f M. Siau on the " Action of Waves,'' Edin. New Phil, Journ., vol. 
xxxi., p. 245. 

X "Travels in Australia," vol. i,, p. 154. — I must express my obligation to 
Sir T. Mitchell, for several interesting personal communications on the 
subject of these great valleys of New South Wales. 



GREAT VALLEYS. 255 

fragments of sandstone, to ascend through the gorge by which the 
river Grose joins the Nepean ; yet the valley of the Grose in its upper 
part, as I saw, forms a magnificent basin some miles in width, and is on 
all sides surrounded by cliffs, the summits of which are believed to be 
nowhere less than 3,000 feet above the level of the sea. When cattle 
are driven into the valley of the Wolgan by a path (which I descended) 
partly cut by the colonists, they cannot escape ; for this valley is in 
every other part surrounded by perpendicular cliffs, and eight miles 
lower down, it contracts, from an average width of half a mile, to a 
mere chasm impassable to man or beast. Sir T. Mitchell * states, that 
the great valley of the Cox river with all its branches contracts, where 
it unites with the Nepean, into a gorge 2,200 yards wide, and about 
one thousand feet in depth. Other similar cases might have been 
added. 

The first impression, from seeing the correspondence of the horizontal 
strata, on each side of these valleys and great amphitheatre-like 
depressions, is that they have been in chief part hollowed out, like 
other valleys, by aqueous erosion ; but when one reflects on the enor- 
mous amount of stone, which on this view must have been removed, in 
most of the above cases through mere gorges or chasms, one is led to 
ask whether these spaces may not have subsided. But considering the 
form of the irregularly branching valleys, and of the narrow promontories, 
projecting into them from the platforms, we are compelled to abandon 
this notion. To attribute these hollows to alluvial action, would be 
preposterous ; nor does the drainage from the summit-level always 
fall, as I remarked near the Weatherboard, into the head of these 
valleys, but into one side of their bay-like recesses. Some of the 
inhabitants remarked to me, that they never viewed one of these bay- 
like recesses, with the headlands receding on both hands, without being 
struck with their resemblance to a bold sea-coast. This is certainly 
the case ; moreover, the numerous fine harbours, with their widely 
branching arms, on the present coast of New South Wales, which are 
generally connected with the sea by a narrow mouth, from one mile to 
a quarter of a mile in width, passing through the sandstone coast-cliffs, 
present a likeness, though on a miniature scale, to the great valleys of 
the interior. But then immediately occurs the startling difficulty, why 
has the sea worn out these great, though circumscribed, depressions on 
a wide platform, and left mere gorges, through which the whole vast 
amount of triturated matter must have been carried away? The only 
light I can throw on this enigma, is by showing that banks appear to 
be forming in some seas of the most irregular forms, and that the sides 
of such banks are so steep (as before stated) that a comparatively small 
amount of subsequent erosion would form them into cliffs : that the 
waves have power to form high and precipitous cliffs, even in land- 
locked harbours, I have observed in many parts ot South America. In 
the Red Sea, banks with an extremely irregular outline and composed 
of sediment, are penetrated by the most singularly shaped creeks with 
narrow mouths : this is likewise the case, though on a larger scale, 

* Idenif vol. ii., p. 358. 



256 VAN DIEMEN'S LAND. 

with the Bahama Banks. Such banks, I have been led to suppose,* 
have been formed by currents heaping sediment on an irregular bottom. 
That in some cases, the sea, instead of spreading out sediment in a 
uniform sheet, heaps it round submarine rocks and islands, it is hardly 
possible to doubt, after having examined the charts of the West Indies. 
To apply these ideas to the sandstone platforms of New South Wales, 
I imagine that the strata might have been heaped on an irregular 
bottom by the action of strong currents, and of the undulations of an 
open sea ; and that the valley-like spaces thus left unfilled might, during 
a slow elevation of the land, have had their steeply sloping flanks worn 
into cliffs ; the worn-down sandstone being removed, either at the time 
when the narrow gorges were cut by the retreating sea, or subsequently 
by alluvial action. 

Va7t Dieme7i's Land, 

The southern part of this island is mainly formed of mountains of 
greenstone, which often assumes a syenitic character, and contains 
much hypersthene. These mountains, in their lower half, are generally 
encased by strata containing numerous small corals and some shells. 
These shells have been examined by Mr. G. B. Sowerby, and have 
been described by him : they consist of two species of Producta, and 
of six of Spirifera ; two of these, namely, P, rugata and 6". rotundata, 
resemble, as far as their imperfect condition allows of comparison, 
British mountain-limestone shells. Mr. Lonsdale has had the kindness 
to examine the corals ; they consist of six undescribed species, belong- 
ing to three genera. Species of these genera occur in the Silurian, 
Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks, 
Jhat all these fossils have undoubtedly a Palaeozoic character, and that 
probably they correspond in age to a division of the system above the 
Silurian formations. 

The strata containing these remains are singular from the extreme 
variability of their mineralogical composition. Every intermediate form 
is present, between flinty-slate, clay-slate passing into grey wacke, pure 
limestone, sandstone, and porcellanic rock ; and some of the beds can 
only be described as composed of a siliceo-calcareo-clay-slate. The 
formation, as far as I could judge, is at least a thousand feet in thick- 
ness : the upper few hundred feet usually consist of a siliceous sand- 
stone, containing pebbles and no organic remains ; the inferior strata, 
of which a pale flinty slate is perhaps the most abundant, are the most 
variable ; and these chiefly abound with the remains. Between two 
beds of hard crystalline limestone, near Newtown, a layer of white soft 
calcareous matter is quarried, and is used for whitewashing houses. 
From information given to me by Mr. Frankland, the Surveyor-General, 

* See the "Appendix" to the Part on Coral-Reefs. The fact of the 
sea heaping up mud round a submarine nucleus, is worthy of the notice 
of geologists : for outlyers of the same composition with the coast banks 
are thus formed ; and these, if upheaved and worn into clifis, would 
naturally be thought to have been once connected together. 



VOLCANIC ROCKS. 257 

it appears that this Palaeozoic formation is found in different parts of 
the whole island ; from the same authority, I may add, that on the 
north-eastern coast and in Bass' Straits primary rocks extensively 
occur. 

The shores of Storm Bay are skirted, to the height of a few hundred 
feet, by strata of sandstone, containing pebbles of the formation just 
described, with its characteristic fossils, and therefore belonging to a 
subsequent age. These strata of sandstone often pass into shale, and 
alternate with layers of impure coal ; they have in many places been 
violently disturbed. Near Hobart Town, I observed one dike, nearly a 
hundred yards in width, on one side of which the strata were tilted at 
an angle of 60° and on the other they were in some parts vertical, and 
had been altered by the effects of the heat. On the west side of Storm 
Bay, I found these strata capped by streams of basaltic lava with 
olivine ; and close by there vs^as a mass of brecciated scoriae, containing 
pebbles of lava, which probably marks the place of an ancient submarine 
crater. Two of these streams of basalt were separated from each other 
by a layer of argillaceous wacke, which could be traced passing into 
partially altered scoriae. The wacke contained numerous rounded 
grains of a soft, grass-green mineral, with a waxy lustre, and translucent 
on its edges : under the blowpipe it instantly blackened, and the points 
fused into a strongly magnetic, black enamel. In these characters, it 
resembles those masses of decomposed olivine, described at St. Jago in 
the Cape de Verde group ; and 1 should have thought that it had thus 
originated, had I not found a similar substance, in cylindrical threads, 
within the cells of the vesicular basalt, — a state under which olivine 
never appears ; this substance,* I believe, would be classed as bole by 
mineralogists. 

T?'ave?iin with extinct //<2;?/^.— Behind Hobart Town there is a 
small quarry of a hard travertin, the lower strata of which abound 
with distinct impressions of leaves. Mr. Robert Brown has had the kind- 
ness to look at my specimens, and he informed me that there are four 
or five kinds, none of which he recognises as belonging to existing 
species. The most remarkable leaf is palmate, like that of a fan-palm, 
and no plant having leaves of this structure has hitherto been dis- 
covered in Van Diemen's Land. The other leaves do not resemble the 
most usual form of the Eucalyptus (of which tribe the existing forests 
are chiefly composed), nor do they resemble that class of exceptions 
to the common form of the leaves of the Eucalyptus, which occur in 
this island. The travertin containing this remnant of a lost vegetation, 
is of a pale yellow colour, hard, and in parts even crystalline ; but not 
compact, and is everywhere penetrated by minute, tortuous, cylindrical 
pores. It contains a very few pebbles of quartz, and occasionally layers 
of chalcedonic nodules, like those of chert in our Greensand. From 

* Chlorophaeite, described by Dr. MacCiilloch ("Western Islands," vol. i., 
p. 504) as occurring in a basaltic amygdaloid, differs from this substance, in 
remaining unchanged before the blowpipe, and in blackening from exposure 
to the air. May we suppose that olivine, in undergoing the remarkable 
change described at St. Jago, passes through several states ? 

17 



258 VAN DIEMEN'S LAND. 

the pureness of this calcareous rock, it has been searched for in other 
places, but has never been found. From this circumstance, and from 
the character of the deposit, it was probably formed by a calcareous 
spring entering a small pool or narrow creek. The strata have subse- 
quently been tilted and fissured ; and the surface has been covered by 
a singular mass, with which, also, a large fissure has been filled up, 
formed of balls of trap embedded in a mixture of vvacke and a white, 
earthy, alumino-calcareous substance. Hence it would appear, as if a 
volcanic eruption had taken place on the borders of the pool, in which 
the calcareous matter was depositing, and had broken it up and drained 
it. 

Elevation of the land. — Both the eastern and western shores of the 
bay, in the neighbourhood of Hobart Town, are in most parts covered 
to the height of thirty feet above the level of high-water mark, with 
broken shells, mingled with pebbles. The colonists attribute these 
shells to the aborigines having carried them up for food : undoubtedly, 
there are many large mounds, as was pointed out to me by Mr. Frank- 
land, which have been thus formed ; but I think from the numbers of 
the shells, from their frequent small size, from the manner in which 
they are thinly scattered, and from some appearances in the form of 
the land, that we must attribute the presence of the greater number to 
a small elevation of the land. On the shore of Ralph Bay (opening 
into Storm Bay) I observed a continuous beach about fifteen feet above 
high-water mark, clothed with vegetation, and by digging into it, pebbles 
encrusted with Serpulce were found : along the banks, also, of the river 
Derwent, I found a bed of broken sea-shells above the surface of the 
river, and at a point where the water is now much too fresh for sea- 
shells to live ; but in both these cases, it is just possible, that before 
certain spits of sand and banks of mud in Storm Bay were accumulated, 
the tides might have risen to the height where we now find the shells.* 

Evidence more or less distinct of a change of level between the land 
and water, has been detected on almost all the land on this side of the 
globe. Captain Grey, and other travellers, have found in Southern 
Australia upraised shells, belonging either to the recent, or to a late 
tertiary period. The French naturalists in Baudin's expedition, found 
shells similarly circumstanced on the S.W. coast of Australia. The 
Rev. W. B. Clarke t finds proofs of the elevation of the land, to the 
amount of 400 feet, at the Cape of Good Hope. In the neighbour- 

* It would appear that some changes are now in progress in Ralph Bay, 
for I was assured by an intelligent farmer, that oysters were formerly 
abundant in it, but that about the year 1 834 they had, without any apparent 
cause, disappeared. In the "Transactions of the Maryland Academy" 
(vol. i., part i., p. 28), there is an account by Mr. Ducatel, of vast beds of 
oysters and clams having been destroyed by the gradual filling up of the 
shallow lagoons and channels, on the shores of the southern United States. 
At Chiloe, in South America, I heard of a similar loss, sustained by the 
inhabitants, in the disappearance from one part of the coast of an edible 
species of Ascidia. 

f " Proceedings of the Geological Society," vol. iii., p. 420. 



SUPERFICIAL FERRUGINOUS BEDS. 259 

hood of the Bay of Islands in New Zealand,* I observed that the 
shores were scattered to some height, as at Van Diemen's Land, with 
sea-shells, which the colonists attribute to the natives. Whatever may 
have been the origin of these shells, I cannot doubt, after having seen a 
section of the valley of the Thames River (37° S.), drawn by the Rev 
W. Williams, that the land has been there elevated : on the opposite 
sides of this great valley, three step-like terraces, composed of an 
enormous accumulation of rounded pebbles, exactly correspond with 
each other : the escarpment of each terrace is about fifty feet in height. 
No one after having examined the terraces in the valleys on the western 
shores of South America, which are strewed with sea-shells, and have 
been formed during intervals of rest in the slow elevation of the land, 
could doubt that the New Zealand terraces have been similarly formed. 
I may add, that Dr. Dieffenbach, in his description of the Chatham 
Islands t (S.W. of New Zealand), states that it is manifest "that the 
sea has left many places bare which were once covered by its waters." 

King Georges Sound. 

This settlement is situated at the south-western angle of the Austra 
lian continent! the whole country is granitic, with the constituent 
minerals sometimes obscurely arranged in straight or curved laminae. 
In these cases, the rock would be called by Humboldt, gneiss-granite, 
and it is remarkable that the form of the bare conical hills, appearing 
to be composed of great folding layers, strikingly resembles, on a small 
scale, those composed of gneiss-granite at Rio de Janeiro, and those 
described by Humboldt at Venezuela. These plutonic rocks are, in 
many places, intersected by trappean-dikes ; in one place, I found ten 
parallel dikes ranging in an E. and W. line ; and not far off another 
set of eight dikes, composed of a different variety of trap, ranging at 
right angles to the former ones. I have observed in several primary 
districts, the occurrence of systems of dikes parallel and close to each 
other. 

Superficial fe? rtiginotis beds. — The lower parts of the country are 
everywhere covered by a bed, following the inequalities of the surface, 

* I will here give a catalogue of the rocks which I met with near the Bay 
of Islands, in New Zealand : — 1st, Much basaltic lava, and scoriform rocks, 
forming distinct craters ; — 2nd, A castellated hill of horizontal strata of fle^h- 
coloured limestone, showing when fractured distinct crystalline facets : the 
rain has acted on this rock in a remarkable manner, corroding its surface 
into a miniature model of an Alpine country : I observed here layers of chert 
and clay ironstone ; and in the bed of a stream, pebbles of clay-slate ; — 
3rd, The shores of the Bay of Islands are formed of a feldspathic rock, of a 
bluish-grey colour, often much decomposed, with an angular fracture, and 
crossed by numerous ferruginous seams, but without any distinct stratifica- 
tion or cleavage. Some varieties arc highly crystalline, and would at once 
be pronounced to be trap; others strikingly resembled clay-slate, slightly 
altered by heat : I was unable to form any decided opinion on this formation, 

t Geographical Journal, vol. xi., pp. 202, 205. 



26o KING GEORGES SOUND. 

of a honeycombed sandstone, abounding with oxides of iron. Beds of 
nearly similar composition are common, I beheve, along the whole 
western coast of Australia, and on many of the East Indian islands. At 
the Cape of Good Hope, at the base of the mountains formed of granite 
and capped with sandstone, the ground is everywhere coated either by 
a fine-grained, rubbly, ochraceous mass, like that at King George's 
Sound, or by a coarser sandstone with fragments of quartz, and rendered 
hard and heavy by an abundance of the hydrate of iron, which presents, 
when freshly broken, a metallic lustre. Both these varieties have a 
very irregular texture, including spaces either rounded or angular, full 
of loose sand ; from this cause the surface is always honeycombed. 
The oxide of iron is most abundant on the edges of the cavities, where 
alone it affords a metallic fracture. In these formations, as well as in 
many true sedimentary deposits, it is evident that iron tends to become 
aggregated, either in the form of a shell, or of a network. The origin 
of these superficial beds, though sufficiently obscure, seems to be due 
to alluvial action on detritus abounding with iron. 

Superficial calcarco7cs deposit — A calcareous deposit on the summit 
of Bald Head, containing branched bodies, supposed by some authors 
to have been corals, has been celebrated by the descriptions of many 
distinguished voyagers.* It folds round and conceals irregular hum- 
mocks of granite, at the height of 600 feet above the level of the sea. It 
varies much in thickness ; where stratified, the beds are often inclined 
at high angles, even as much as at thirty degrees, and they dip in 
all directions. These beds are sometimes crossed by oblique and 
even-sided lamince. The deposit consists either of a fine, white cal- 
careous powder, in which not a trace of structure can be discovered, or 
of exceedingly munute, rounded grains, of brown, yellowish, and purplish 
colours ; both varieties being generally, but not always, mixed with 
small particles of quartz, and being cemented into a more or less 
perfect stone. The rounded calcareous grains, when heated in a slight 
degree, instantly lose their colours ; in this and in every other respect, 
closely resembling those minute, equal-sized particles of shells and 
corals, which at St. Helena have been drifted up the side of the 
mountains, and have thus been winnowed of all coarser fragments. I 
cannot doubt that the coloured calcareous particles here have had a 
similar origin. The impalpable powder has probably been derived 
from the decay of the rounded particles ; this certainly is possible, for 
on the coast of Peru, I have traced large tmhroken shells gradually 
falling into a substance as fine as powdered chalk. Both of the above- 
mentioned varieties of calcareous sandstone frequently alternate with, 
and blend into, thin layers of a hard substalagmitic t rock, which, even 

* I visited this hill, in company with Captain Fitzroy, and we came to a 
similar conclusion regarding these branching bodies. 

t I adopt this term from Lieutenant Nelson's excellent paper on the Bermuda 
Islands ("Geolog. Trans.,". vol. v., p. 106), for the hard, compact, cream- or 
brown-coloured stone, without any crystalline structure, which so often ac- 
companies superficial calcareous accumulations. I have observed such super- 
ficial beds, coated with substalagmitic rock, at the Cape of Good Hope, in 



SUPERFICIAL CALCAREOUS DEPOSIT. 261 

when the stone on each side contains particles of quartz, is entirely 
free from them : hence we must suppose that these layers, as well as 
certain vein-like masses, have been formed by rain dissolving the cal- 
careous matter and re-precipitating it, as has happened at St. Helena. 
Each layer probably marks a fresh surface, when the, now firmly 
cemented, particles existed as loose sand. These layers are sometimes 
brecciated and re-cemented, as if they had been broken by the slipping 
of the sand when soft. I did not find a single fragment of a sea-shell ; 
but bleached shells of the Helix melo, an existing land species, abound 
in all the strata ; and I likewise found another Helix, and the case of 
an Oniscus. 

The branches are absolutely undistinguishable in shape from the 
broken and upright stumps of a thicket ; their roots are often uncovered, 
and are seen to diverge on all sides ; here and there a branch lies 
prostrate. The branches generally consist of the sandstone, rather 
firmer than the surrounding matter, with the central parts filled, either 
with friable, calcareous matter, or with a substalagmitic variety ; this 
central part is also frequently penetrated by linear crevices, sometimes, 
though rarely, containing a trace of woody matter. These ^alcareous, 
branching bodies, appear to have been formed by fine calcareous matter 
being washed into the casts or cavities, left by the decay of branches 
and roots of thickets, buried under drifted sand. The whole surface 
of the hill is now undergoing disintegration, and hence the casts, which 
are compact and hard, are left projecting. In calcareous sand at the 
Cape of Good Hope, 1 found the casts, described by Abel, quite similar 
to these at Bald Head ; but their centres are often filled with black 
carbonaceous matter not yet removed. It is not surprising, that the 
woody matter should have been almost entirely removed from the casts 
on Bald Head ; for it is certain, that many centuries must have elapsed 
since the thickets were buried ; at present, owing to the form and 
height of the narrow promontory, no sand is drifted up, and the whole 
surface, as I have remarked, is wearing away. We must, therefore, 

several parts of Chile, and over wide spaces in La Plata and Patagonia. 
Some of these beds have been formed from decayed shells, but the origin 
of the greater number is sufficiently obscure. The causes which determine 
water to dissolve lime, and then soon to redcposit it, are not, I think, known. 
The surface of the substalagmitic layers appears always to be corroded by 
the rain-water. As all the above-mentioned countries have a long dry season, 
compared with the rainy one, I should have thought that the presence of the 
substalagmitic was connected with the climate, had not Lieutenant Nelson 
found this substance forming under sea-water. Disintegrated shell seems 
to be extremely soluble ; of which I found good evidence, in a curious rock 
at Coquimbo in Chile, which consisted of small, pellucid, empty husks, 
cemented together. A series of specimens clearly showed that these husks 
had originally contained small rounded particles of shells, which had been 
enveloped and cemented together by calcareous matter (as often happens on 
sea-beaches), and which subsequently had decayed, and been dissolved by 
water, that must have penetrated through the calcareous husks, without 
corroding them, — of which processes every stage could be seen. 



262 KING GEORGES SOUND. 

look back to a period when the land stood lower, of which the French 
naturalists * found evidence in upraised shells of recent species, for 
the drifting on Bald Head of the calcareous and quartzose sand, and 
the consequent embedment of the vegetable remains. There was only 
one appearance which at first made me doubt concerning the origin of 
the cast, — namely, that the finer roots from different stems sometimes be- 
came united together into upright plates or veins ; but when the manner 
is borne in mind in which fine roots often fill up cracks in hard earth, and 
that these roots would decay and leave hollows, as well as the stems, 
there is no real difficulty in this case. Besides the calcareous branches 
from the Cape of Good Hope, I have seen casts, of exactly the same 
forms, from Madeira f and from Bermuda ; at this latter place, the 
surrounding calcareous rocks, judging from the specimens collected by 
Lieutenant Nelson, are likewise similar, as is their subaerial formation. 
Reflecting on the stratification of the deposit on Bald Head, — on the 
irregularly alternating layers of substalagmitic rock, — on the uniformly 
sized, and rounded particles, apparently of sea-shells and corals, — on 
the abundance of land-shells throughout the mass, — and finally, on the 
absolute resemblance of the calcareous casts, to the stumps, roots, and 
branches of that kind of vegetation, which would grow on sand-hillocks, 
I think there can be no reasonable doubt, notwithstanding the different 
opinion of some authors, that a true view of their origin has been here 
given. 

Calcareous deposits, like these of King George's Sound, are of vast 
extent on the Australian shores. Dr. Fitton remarks, that " recent cal- 
careous breccia (by which term all these deposits are included) was found 
during Baudin's voyage, over a space of no less than twenty-five degrees 

* See M. P^ron's "Voyage," torn, i., p. 204. 

t Dr. J. Macaulay has fully described {Edinh. New Phil. Joiirn., vol. xxix., 
p. 350) the casts from Madeira. He considers (differently from Mr. Smith 
of Jordan Hill) these bodies to be corals, and the calcareous deposit to be 
of subaqueous origin. His arguments chiefly rest (for his remarks on their 
structure are vague) on the great quantity of the calcareous matter, and on 
the casts containing animal matter, as shown by their evolving ammonia. 
Had Dr. Macaulay seen the enormous masses of rolled particles of shells and 
corals on the beach of Ascension, and especially on coral-reefs ; and had he 
reflected on the effects of long-continued, gentle winds, in drifting up the 
finer particles, he would hardly have advanced the argument of quantity, 
which is seldom trustworthy in geology. If the calcareous matter has 
originated from disintegrated shells and corals, the presence of animal 
matter is what might have been expected. Mr. Anderson analysed for Dr. 
Macaulay part of a cast, and he found it composed of — 

Carbonate of lime 73*15 

Silica 1 1 -90 

Phosphate of lime 8-8 1 

Animal matter 4-25 

Sulphate of lime a trace 

981 1 



CAPE OF GOOD HOPE. 263 

of latitude and an equal extent of longitude, on the southern, western, 
and north-western coasts."* It appears also from M. P6ron, with whose 
observations and opinions on the origin of the calcareous matter and 
branching casts mine entirely accord, that the deposit is generally much 
more continuous than near King George's Sound. At Swan River, 
Archdeacon Scottf states that in one part it extends ten miles inland. 
Captain Wickham, moreover, informs me that during his late survey 
of the western coast, the bottom of the sea, wherever the vessel 
anchored, was ascertained, by crowbars being let down, to consist of 
white calcareous matter. Hence it seems that along this coast, as at 
Bermuda and at Keeling Atoll, submarine and subaerial deposits are 
contemporaneously in process of formation, from the disintegration of 
marine organic bodies. The extent of these deposits, considering their 
origin, is very striking ; and they can be compared in this respect only 
with the great coral-reefs of the Indian and Pacific Oceans. In other 
parts of the world, for instance in South America, there are superficial 
calcareous deposits of great extent, in which not a trace of organic 
structure is discoverable ; these observations would lead to the inquiry, 
whether such deposits may not, also, have been formed from disin- 
tegrated shells and corals. 



Cape of Good Hope, 

After the accounts given by Barrow, Carmichael, Basil Hall, and 
W. B. Clarke of the geology of this district, I shall confine myself to a 
few observations on the junction of the three principal formations. The 
fundamental rock is granite, J overlaid by clay-slate : the latter is 
generally hard, and glossy from containing minute scales of mica ; it 
alternates with, and passes into, beds of slightly crystalline, feldspathic, 
slaty rock. This clay-slate is remarkable from being in some places 
(as on the Lion's Rump) decomposed, even to the depth of twenty feet, 
into a pale-coloured, sandstone-like rock, which has been mistaken, I 
believe, by some observers, for a separate formation. I was guided 
by Dr. Andrew Smith to a fine junction at Green Point between the 
granite and clay-slate : the latter at the distance of a quarter of a mile 

* For ample details on this formation, consult Dr. Fitton's "Appendix 
to Captain King's Voyage." Dr. Fitton is indined to attribute a concre- 
tionary origin to the branching bodies : I may remark, that I have seen in 
beds of sand in La Plata cylindrical stems which no doubt thus originated ; 
but they differed much in appearance from these at Bald Head, and the 
other places above specified. 

t " Proceedings of the Geolog. Soc," vol. i., p. 320. 

X In several places I observed in the granite, small dark-coloured balls, 
composed of minute scales of black mica in a tough basis. In another place, 
I found crystals of black schorl radiating from a common centre. Dr. Andrew 
Smith found, in the interior parts of the country, some beautiful specimens 
of granite, with silvery mica radiating or rather branching, like moss, from 
central points. At the Geological Society, there are specimens of granite 
with crystallised feldspar branching and radiating in like manner. 



264 CAPE OF GOOD HOPE. 

from the spot, where the granite appears on the beach (though, probably, 
the granite is much nearer underground), becomes shghtly more com- 
pact and crystalline. At a less distance, some of the beds of clay-slate 
are of a homogeneous texture, and obscurely striped with different zones 
of colour, whilst others are obscurely spotted. Within a hundred yards 
of the first vein of granite, the clay-slate consists of several varieties ; 
some compact with a tinge of purple, others glistening with numerous 
minute scales of mica and imperfectly crystallised feldspar ; some 
obscurely granular, others porphyritic with small, elongated spots of a 
soft white mineral, which being easily corroded, gives to this variety a 
vesicular appearance. Close to the granite, the clay-slate is changed 
into a dark-coloured, laminated rock, having a granular fracture, which 
is due to imperfect crystals of feldspar, coated by minute, brilliant 
scales of mica. 

The actual junction between the granitic and clay-slate districts 
extends over a width of about two hundred yards, and consists of 
irregular masses and of numerous dikes of granite, entangled and sur- 
rounded by the clay-slate : most of the dikes range in a N.W. and S.E. 
line, parallel to the cleavage of the slate. As we leave the junction, 
thin beds, and lastly, mere films of the altered clay-slate are seen, quite 
isolated, as if floating, in the coarsely crystallised granite ; but although 
completely detached, they all retain traces of the uniform N.W. and 
S.E. cleavage. This fact has been observed in other similar cases, and 
has been advanced by some eminent geologists,* as a great difficulty 
on the ordinary theor}", of granite having been injected whilst liquefied ; 
but if we reflect on the probable state of the lower surface of a laminated 
mass, like clay-slate, after having been violently arched by a body of 
molten granite, we may conclude that it would be full of fissures parallel 
to the planes of cleavage ; and that these would be filled with granite, 
so that wherever the fissures were close to each other, mere parting 
layers or wedges of the slate would depend into the granite. Should, 
therefore, the whole body of rock afterwards become worn down and 
denuded, the lower ends of these dependent masses or wedges of slate 
would be left quite isolated in the granite ; yet they would retain their 
proper lines of cleavage, from having been united, whilst the granite 
was fluid, with a continuous covering of clay-slate. 

Following, in company with Dr. A. Smith, the line of junction between 
the granite and the slate, as it stretched inland, in a S.E. direction, 
we came to a place, where the slate was converted into a fine-grained, 
perfectly characterised gneiss, composed of yellowish-brown granular 
feldspar, of abundant black brilliant mica, and of few and thin laminae 
of quartz. From the abundance of the mica in this gneiss, compared 
with the small quantity and excessively minute scales, in which it exists 
in the glossy clay-slate, we must conclude, that it has been here formed 
by the metamorphic action— a circumstance doubted, under nearly 
similar circumstances, by some authors. The laminse of the clay-slate 
are straight ; and it was interesting to observe, that as they assumed 

* See M. Keilhau's "Theory on Granite," translated in the Edinburgh 
New Philosophical Journal, vol. xxiv., p. 402. 



SANDSTONE FORMATION. 265 

the character of gneiss, they became undulatory with some of the 
smaller flexures angular, like the laminae of many true metamorphic 
schists. 

Sandsto7te formation. — This formation makes the most imposing 
feature in the geology of Southern Africa. The strata are in many parts 
horizontal, and attain a thickness of about two thousand feet. The sand- 
stone varies in character ; it contains little earthy matter, but is often 
stained with iron ; some of the beds are very fine-grained and quite 
white ; others are as compact and homogeneous as quartz rock. In 
some places I observed a breccia of quartz, with the fragments almost 
dissolved in a siliceous paste. Broad veins of quartz, often including 
large and perfect crystals, are very numerous ; and it is evident in 
nearly all the strata, that silica has been deposited from solution in 
remarkable quantity. Many of the varieties of quartzite appeared 
quite like metamorphic rocks; but from the upper strata being as 
siliceous as the lower, and from the undisturbed junctions with the 
granite, which in many places can be examined, I can hardly believe 
that these sandstone-strata have been exposed to heat.* On the lines 
of junction between these two great formations, I found in several places 
the granite decayed to the depth of a few inches, and succeeded, either 
by a thin layer of ferruginous shale, or by four or five inches in thick- 
ness of the re-cemented crystals of the granite, on which the great pile 
of sandstone immediately rested. 

Mr. Schomburgk has described t a great sandstone formation in 
Northern Brazil, resting on granite, and resembling to a remarkable 
degree, in composition and in the external form of the land, this 
formation of the Cape of Good Hope. The sandstones of the great 
platforms of Eastern Australia, which also rest on granite, differ in con- 
taining more earthy and less siliceous matter. No fossil remains have 
been discovered in these three vast deposits. Finally, I may add that 
I did not see any boulders of far-transported rocks at tlie Cape of Good 
Hope, or on the eastern and western shores of Australia, or at Van 
Diemen's Land. In the northern island of New Zealand, I noticed 
some large blocks of greenstone, but whether their parent rock was 
far distant, I had no opportunity of determining. 

* The Rev. W. B. Clarke, however, states, to my surprise ("Geolog. 
Proceedings," vol. iii., p. 422), that the sandstone in some parts is pene- 
trated by granitic dikes : such dikes must belong to an epoch altogether 
subsequent to that when the molten granite acted on the clay-slate, 

f Geographical Journal^ vol. x., p. 246. 



GEOLOGICAL OBSERVATIONS 



SOUTH AMERICA. 



CRITICAL INTRODUCTION. 



OF the remarkable " trilogy " constituted by Darwin's writings 
which deal with the geology of the Beagle, the member 
which has perhaps attracted least attention, up to the present 
time is that which treats of the geology of South America. The 
actual writing of this book appears to have occupied Darwin 
a shorter period than either of the other volumes of the series ; 
his diary records that the work was accomplished within ten 
months, namely, between July 1844 and April 1845 \ but the 
book was not actually issued till late in the year following, 
the preface bearing the date "September 1846." Altogether, as 
Darwin informs us in his "Autobiography," the geological books 
" consumed four and a half years' steady work," most of the 
remainder of the ten years that elapsed between the return of the 
Beagle, and the completion of his geological books being, it is sad 
to relate, " lost through illness ! " 

Concerning the " Geological Observations on South America," 
Darwin wrote to his friend Lyell, as follows : — "My volume will 
be about 240 pages, dreadfully dull, yet much condensed. I 
think whenever you have time to look through it, you will think 
the collection of facts on the elevation of the land and on the 
formation of terraces pretty good." 

" Much condensed " is the verdict that everyone must endorse, 
on rising from the perusal of this remarkable book ; but by no 
means "dull." The three and a half years from April 1832 to 
September 1835, were spent by Darwin in South America, and 
were devoted to continuous scientific work ; the problems he 
dealt with were either purely geological or those which con- 
stitute the borderland between the geological and biological 
sciences. It is impossible to read the journal which he kept 
during this time without being impressed by the conviction 



270 CRITICAL INTRODUCTION. 

that it contains all the germs of thought which afterwards de- 
veloped into the " Origin of Species." But it is equally evident 
that after his return to England, biological speculations gradu- 
ally began to exercise a more exclusive sway over Darwin's 
mind, and tended to dispossess geology, which during the actual 
period of the voyage certainly engrossed most of his time 
and attention. The wonderful series of observations made 
during those three and a half years in South America could 
scarcely be done justice to, in the 240 pages devoted to their 
exposition. That he executed the work of preparing the book on 
South America in somewhat the manner of a task, is shown by 
many references in his letters. Writing to Sir Joseph Hooker in 
1845, ^^ says, "I hope this next summer to finish my South 
American Geology, then to get out a little Zoology, and hurrah 
for my species work ! " 

It would seem that the feeling of disappointment, which 
Darwin so often experienced in comparing a book when com- 
pleted, with the observations and speculations which had inspired 
it, was more keenly felt in the case of his volume on South 
America than any other. To one friend he writes, " I have of 
late been slaving extra hard, to the great discomfiture of wretched 
digestive organs, at South America, and thank all the fates, I have 
done three-fourths of it. Writing plain English grows with me 
more and more difficult, and never attainable. As for your pre- 
tending that you will read anything so dull as my pure geological 
descriptions, lay not such a flattering unction on my soul, for it 
is incredible." To another friend he writes, " You do not know 
what you threaten when you propose to read it — it is purely 
geological. I said to my brother, ' You will of course read it,' 
and his answer was, ' Upon my life, I would sooner even buy it.' " 

In spite of these disparaging remarks, however, we are strongly 
inclined to believe that this book, despised by its author, and 
neglected by his contemporaries, will in the end be admitted to 
be one of Darwin's chief titles to fame. It is, perhaps, an unfor- 
tunate circumstance that the great success which he attained in 
biology by the publication of the " Origin of Species " has, to 
some extent, overshadowed the fact that Darwin's claims as a 
geologist, are of the very highest order. It is not too much to 
say that, had Darwin not been a geologist, the "Origin of Species" 
could never have been written by him. But apart from those 
geological questions, which have an important bearing on bio- 
logical thought and speculation, such as the proofs of imper- 



CRITICAL INTRODUCTION. 271 

fection in the geological record, the relations of the later tertiary 
faunas to the recent ones in the same areas, and the apparent 
intermingling of types belonging to distant geological epochs, 
when we study the palaeontology of remote districts, — there are 
other purely geological problems, upon which the contributions 
made by Darwin are of the very highest value. I believe that 
the verdict of the historians of science will be that if Darwin had 
not taken a foremost place among the biologists of this century, 
his position as a geologist would have been an almost equally 
commanding one. 

But in the case of Darwin's principal geological work — that 
relating to the origin of the crystalline schists, — geologists 
were not at the time prepared to receive his revolutionary teach- 
ings. The influence of powerful authority was long exercised, 
indeed, to stifle his teaching, and only now, when this unfortunate 
opposition has disappeared, is the true nature and importance of 
Darwin's purely geological work beginning to be recognised. 

The two first chapters of the "Geological Observations on 
South America," deal with the proofs which exist of great, but 
frequently interrupted, movements of elevation during very recent 
geological times. In connection with this subject, Darwin's par- 
ticular attention was directed to the relations between the great 
earthquakes of South America — of some of which he had impres- 
sive experience — and the permanent changes of elevation which 
were taking place. He was much struck by the rapidity with 
which the evidence of such great earth movements is frequently 
obliterated ; and especially with the remarkable way in which the 
action of rain-water, percolating through deposits on the earth's 
surface, removes all traces of shells and other calcareous organisms. 
It was these considerations which were the parents of the general- 
isation that a palseontological record can only be preserved during 
those periods in which long-continued slow subsidence is going 
on. This in turn, led to the still wider and more suggestive con- 
clusion that the geological record as a whole is, and never can 
be more than, a series of more or less isolated fragments. The 
recognition of this important fact constitutes the keystone to any 
theory of evolution which seeks to find a basis in the actual study 
of the types of life that have formerly inhabited our globe. 

In his third chapter, Darwin gives a number of interesting facts, 
collected during his visits to the plains and valleys of Chili, which 
bear on the question of the origin of saliferous deposits — the 
accumulation of salt, gypsum, and nitrate of soda. This is a 



272 CRITICAL INTRODUCTION. 

problem that has excited much discussion among geologists, and 
which, in spite of many valuable observations, still remains to a 
great extent very obscure. Among the important considerations 
insisted upon by Darwin is that relating to the absence of marine 
shells in beds associated with such deposits. He justly argues 
that if the strata were formed in shallow waters, and then exposed 
by upheaval to subaerial action, all shells and other calcareous 
organisms would be removed by solution. 

Following Lyell's method, Darwin proceeds from the study of 
deposits now being accumulated on the earth's surface, to those 
which have been formed during the more recent periods of the 
geological history. 

His account of the great Pampean formation, with its wonderful 
mammalian remains — Mastodon, Toxodon, Scelidotherium, Mac- 
ratichenia, Megat/ien'iwi, Megaloityx, Mylodon, and Glyptodon — is 
full of interest. His discovery of the remains of a true Eqiiiis 
afforded a remarkable confirmation of the fact — already made out 
in North America — that species of horse had existed and become 
extinct in the New World, before their introduction by the 
Spaniards in the. sixteenth century. Fully perceiving the impor- 
tance of the microscope in studying the nature and origin of such 
deposits as those of the Pampas, Darwin submitted many of his 
specimens both to Dr. Carpenter in this country, and to Professor 
Ehrenberg in Berlin. Many very important notes on the micro- 
scopic organisms contained in the formation will be found 
scattered through the chapter. 

Darwin's study of the older tertiary formations, with their 
abundant shells, and their rehcs of vegetable life buried under 
great sheets of basalt, led him to consider carefully the question 
of climate during these earlier periods. In opposition to pre- 
valent views on this subject, Darwin points out that his observa- 
tions are opposed to the conclusion that a higher temperature 
prevailed universally over the globe during early geological periods. 
He argues that "the causes which gave to the older tertiary pro- 
ductions of the quite temperate zones of Europe a tropical 
character, tvere of a local character and did not affect the whole 
globe" In this, as in many similar instances, we see the beneficial 
influence of extensive travel in freeing Darwin's mind from pre- 
vailing prejudices. It was this widening of experience which 
rendered him so especially qualified to deal with the great problem 
of the origin of species, and in doing so to emancipate himself 
from ideas which were received with unquestioning faith by 



CRITICAL INTRODlICTIOM. i^ 

geologists whose studies had been circumscribed within the Hmits 
of Western Europe. 

In the Cordilleras of Northern and Central Chili, Darwin, when 
studying still older formations, clearly recognised that they contain 
an admixture of the forms of life, which in Europe are distinctive 
of the Cretaceous and Jurassic periods respectively. He was thus 
led to conclude that the classification of geological periods, which 
fairly well expresses the facts that had been discovered in the 
areas where the science was first studied, is no longer capable 
of being applied when we come to the study of widely distant 
regions. This important conclusion led up to the further genera- 
lisation that each great geological period has exhibited a geo- 
graphical distribution of the forms of animal and vegetable life, 
comparable to that which prevails in the existing fauna and flora. 
To those who are familiar with the extent to which the doctrine 
of universal formations has affected geological thought and specu- 
lation, both long before and since the time that Darwin wrote, 
the importance of this new standpoint to which he was able to 
attain will be sufficiently apparent. Like the idea of the extreme 
imperfection of the Geological Record, the doctrine of local geo- 
logical formations is found permeating and moulding all the 
palaeontological reasonings of his great work. 

In one of Darwin's letters, written while he was in South 
America, there is a passage we have already quoted, in which 
he expresses his inability to decide between the rival claims 
upon his attention of " the old crystalline group of rocks," and 
" the softer fossiliferous beds " respectively. The sixth chapter 
of the work before us, entitled " Plutonic and Metamorphic 
Rocks — Cleavage and Foliation," contains a brief summary of a 
series of observations and reasonings upon these crystalline 
rocks, which are, we believe, calculated to effect a revolution in 
geological science, and — though their value and importance have 
long been overlooked — are likely to entitle ]^arwin in the future 
to a position among geologists, scarcely, if at all, inferior to that 
which he already occupies among biologists. 

T)arwin's studies of the great rock- masses of the Andes con- 
vinced him of the close relations between the granitic or Plutonic 
rocks, and those which were undoubtedly poured forth as lavas. 
Upon his return, he set to work, with the aid of Professor Miller, 
to make a careful study of the minerals composing the granites 
and those which occur in the lavas, and he was able to show that 
in all essential respects they are identical. He was further able to 

i8 



274 CRITICAL INTRODUCTION. 

prove that there is a complete gradation between the highly 
crystalline or granitic rock-masses, and those containing more or 
less glassy matter between their crystals, which constitute ordinary 
lavas. The importance of this conclusion will be realised when 
we remember that it was then the common creed of geologists — 
and still continues to be so on the Continent — that all highly 
crystalline rocks are of great geological antiquity, and that the 
igneous ejections which have taken place since the beginning of 
the tertiary periods differ essentially, in their composition, their 
structure, and their mode of occurrence, from those which have 
made their appearance at earlier periods of the world's history. 

Very completely have the conclusions of Darwin upon these 
subjects been justified by recent researches. In England, the 
United States, and Italy, examples of the gradual passage of rocks 
of truly granitic structure into ordinary lavas have been described, 
and the reality of the transition has been demonstrated by the 
most careful studies with the microscope. Recent researches 
carried on in South America by Professor Stelzner, have also 
shown the existence of a class of highly crystalline rocks — the 
" Andengranites " — which combine in themselves many of the 
characteristics which were once thought to be distinctive of the so- 
called Plutonic and volcanic rocks. No one familiar with recent 
geological literature — even in Germany and France, where the 
old views concerning the distinction of igneous products of 
different ages have been most stoutly maintained — can fail to 
recognise the fact that the principles contended for by Darwin 
bid /air at no distant period to win universal acceptance among 
geologists all over the globe. 

Still more important are the conclusions at which Darwin 
arrived with respect to the origin of the schists and gneisses 
which cover so large an area in South America. 

Carefully noting, by the aid of his compass and clinometer, 
at every point which he visited, the direction and amount of in- 
clination of the parallel divisions in these rocks, he was led to a 
very important generalisation — namely, that over very wide areas 
the direction (strike) of the planes of cleavage in slates, and of 
foliation in schists and gneisses, remained constant, though the 
amount of their inclination (dip) often varied within wide limits. 
Further than this it appeared that there was always a close cor- 
respondence between the strike of the cleavage and foliation and 
the direction of the great axes along which elevation had taken 
place in the district. 



CRITICAL INTRODUCTION. 275 

In Tierra del Fuego, Darwin found striking evidence that the 
cleavage intersecting great masses of slate-rocks was quite 
independent of their original stratification, and could often, 
indeed, be seen cutting across it at right angles. He was also 
able to verify Sedgwick's observation that, in some slates, glossy 
surfaces on the planes of cleavage arise from the development of 
new minerals, chlorite, epidote or mica, and that in this way a 
complete graduation from slates to true schists may be traced. 

Darwin further showed that in highly schistose rocks, the folia 
bend around and encircle any foreign bodies in the mass, and 
that in some cases they exhibit the most tortuous forms and 
complicated puckerings. He clearly saw that in all cases the 
forces by which these striking phenomena must have been pro- 
duced were persistent over wide areas, and were connected with 
the great movements by which the rocks had been upheaved 
and folded. 

That the distinct folia of quartz, felspar, mica, and other 
minerals composing the metamorphic schists could not have been 
separately deposited as sediment was strongly insisted upon by 
Darwin ; and in doing so he opposed the view generally prevalent 
among geologists at that time. He was thus driven to the con- 
clusion that foliation, like cleavage, is not an original, but a 
superinduced structure in rock-masses, and that it is the result 
of re-crystallisation, under the controlling influence of great 
pressure, of the materials of which the rock was composed. 

In studying the lavas of Ascension, as we have already seen, 
Darwin was led to recognise the circumstance that, when igneous 
rocks are subjected to great differential movements during the 
period of their consolidation, they acquire a foliated structure, 
closely analogous to that of the crystalline schists. Like his 
predecessor in this field of inquiry, Mr. Poulett -Scrope, Charles 
Darwin seems to have been greatly impressed by these facts, and 
he argued from them that the rocks exhibiting the foliated 
structure must have been in a state of plasticity, like that of a 
cooling mass of lava. At that time the suggestive experiments 
of Tresca, Daubree, and others, showing that solid masses under 
the influence of enormous pressure become actually plastic, had 
not been published. Had Darwin been aware of these facts he 
would have seen that it was not necessary to assume a state of 
imperfect solidity in rock-masses in order to account for their 
having yielded to pressure and tension, and, in doing so, ac- 
quiring the new characters which distinguish the crystalline schists. 



276 CRITICAL INTRODUCTION. 

The views put forward by Darwin on the origin of the crys- 
talhne schists found an able advocate in Mr. Daniel Sharpe, who 
in 1852 and 1854 published two papers, dealing with the geology 
of the Scottish Highlands and of the Alps respectively, in which 
he showed that the principles arrived at by Darwin when studying 
the South American rocks afford a complete explanation of the 
structure of the two districts in question. 

But, on the other hand, the conclusions of Darwin and Sharpe 
were met with the strongest opposition by Sir Roderick Murchison 
and Dr. A. Geikie, who in 1861 read a paper before the Geo- 
logical Society " On the Coincidence between Stratification and 
Foliation in the Crystalline Rocks of the Scottish Highlands," in 
which they insisted that their observations in Scotland tended to 
entirely disprove the conclusions of Darwin that foliation in rocks 
is a secondary structure, and entirely independent of the original 
stratification of the rock-masses. 

Now it is a most significant circumstance that, no sooner did 
the officers of the Geological Survey commence the careful and 
detailed study of the Scottish Highlands than they found them- 
selves compelled to make a formal retraction of the views which 
had been put forward by Murchison and Geikie in opposition 
to the conclusions of Darwin. The officers of the Geological 
Survey have completely abandoned the view that the foliation of 
the Highland rocks has been determined by their original strati- 
fication, and admit that the structure is the result of the profound 
movements to which the rocks have been subjected. The same 
conclusions have recently been supported by observations made 
in many different districts — among which we may especially refer 
to those of Dr. H. Reusch in Norway, and those of Dr. J. 
Lehmann in Saxony. At the present time the arguments so 
clearly stated by Darwin in the work before us, have, after enduring 
opposition or neglect for a whole generation, begun to " triumph 
all along the line," and we may look forward confidently to the 
near future, when his claim to be regarded as one of the greatest 
of geological discoverers shall be fully vindicated. 

John W. Judd. 



CHAPTER I. 

ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. 

Upraised Shells of La Plata. — Bahia Blanca, Sand-dunes and Pumice-pebbles. 
— Step-formed Plains of Patagonia, with upraised Shells. — Terrace- 
bounded Valley of Santa Cruz, formerly a Sea-strait. — Upraised shells 
of Tierra del Fuego. — Length and breadth of the elevated area. — Equa- 
bility of the movements, as shovvrn by the similar heights of the plains. 
— Slowness of the elevatory process. — Mode of formation of the step- 
formed plains. — Summary. — Great Shingle Formation of Patagonia; its 
extent, origin, and distribution. — Formation of sea-cliffs. 

In the following Volume, which treats of the geology of South America, 
and almost exclusively of the parts southward of the Tropic of Capri- 
corn, I have arranged the chapters according to the age of the deposits, 
occasionally departing from this order, for the sake of geographical 
simplicity. 

The elevation of the land within the recent period, and the modifica- 
tions of its surface through the action of the sea (to which subjects I 
paid particular attention) will be first discussed ; I will then pass on 
to the tertiary deposits, and afterwards to the older rocks. Only those 
districts and sections will be described in detail which appear to me 
to deserve some particular attention ; and I will, at the end of each 
chapter, give a summary of the results. We will commence with the 
proofs of the upheaval of the eastern coast of the continent, from the 
Rio Plata southward ; and, in the Second Chapter, follow up the same 
subject along the shores of Chile and Peru. 

On the northern bank of the great estuary of the Rio Plata, near 
Maldonado, I found at the head of a lake, sometimes brackish but 
generally containing fresh water, a bed of muddy clay, six feet in 
thickness, with numerous shells of species still exisiting in the Plata, 
namely, the Azara labiaia, d'Orbig., fragments of Mytilus edulifonnis^ 
d'Orbig., Paludesitina Isahellei, d'Orbig., and the Sole7i Caribaus, Lam., 
which last was embedded vertically in the position in which it had 
lived. These shells lie at the height of only two feet above the lake, 
nor would they have been worth mentioning, except in connection with 
analogous facts. 



278 ELEVATION OF LA PLATA. 

At Monte Video, I noticed near the town, and along the base of the 
mount, beds of a living Mytilus, raised some feet above the surface of 
the Plata : in a similar bed, at a height from thirteen to sixteen feet, 
M. Isabelle collected eight species, which,* according to M. d'Orbigny, 
now live at the mouth of the estuary. At Colonia del Sacramiento, 
further westward, I observed at the height of about fifteen feet above 
the river, there of quite fresh water, a small bed of the same Mytilus, 
which lives in brackish water at Monte Video. Near the mouth of 
Uruguay, and for at least thirty-five miles northward, there are at 
intervals large sandy tracts, extending several miles from the banks of 
the river, but not raised much above its level, abounding with small 
bivalves, which occur in such numbers that at the Agraciado they are 
sifted and burnt for lime. Those which I examined near the A. S. 
Juan were much worn : they consisted of Mactra Isabellei, d'Orbig., 
mingled with few of Vemis si?iuosa, Lam., both inhabiting, as I am 
informed by M. d'Orbigny, brackish water at the mouth of the Plata, 
nearly or quite as salt as the open sea. The loose sand, in which these 
shells are packed, is heaped into low, straight, long lines of dunes, like 
those left by the sea at the head of many bays. M. d'Orbigny has de- 
scribed t an analogous phenomenon on a greater scale, near San Pedro 
on the river Parana, where he found widely extended beds and hillocks 
of sand, with vast numbers of the Azara labiata, at the height of 
nearly loo feet (English) above the surface of that river. The Azara 
inhabits brackish water, and is not known to be found nearer to San 
Pedro than Buenos Ayres, distant above a hundred miles in a 
straight line. Nearer Buenos Ayres, on the road from that place to 
San Isidro, there are extensive beds, as I am informed by Sir Wood- 
bine Parish,! of the Azara labiata, lying at about forty feet above the level 
of the river, and distant between two and three miles from it. These 
shells are always found on the highest banks in the district : they are 
embedded in a stratified earthy mass, precisely like that of the great 
Pampean deposit hereafter to be described. In one collection of these 
shells, there were some valves of the Venus si?tuosa, Lam., the same 
species found with the Mactra on the banks of the Uruguay. South of 
Buenos Ayres, near Ensenada, there are other beds of the Azara, some 
of which seem to have been embedded in yellowish, calcareous, semi- 
crystalline matter ; and Sir W. Parish has given me from the banks of 
the Arroyo del Tristan, situated in this same neighbourhood, at the 
distance of about a league from the Plata, a specimen of a pale-reddish, 
calcareo-argillaceous stone (precisely like parts of the Pampean deposit 
the importance of which fact will be referred to in a succeeding 
chapter), abounding with shells of an Azara, much worn, but which in 
general form and appearance closely resemble, and are probably 
identical with, the A. labiata. Besides these shells, cellular, highly 
crystalline rock, formed of the casts of small bivalves, is found near 
Ensenada ; and likewise beds of sea-shells, which from their appear- 

* "Voyage dans rAmerique Merid. : Part. Geolog., p. 21. 

t Ibid., p. 43. 

X "Buenos Ayres," etc., by Sir Woodbine Parish, p. 168. 



ELEVATION OF LA PLATA. 279 

ance appear to have lain on the surface. Sir W. Parish has given me 
some of these shells, and M. d'Orbigny pronounces them to be — 



1. Buccinanops globulosum, 

d'Orbig. 

2. Olivancillaria auricularia, do. 

3. Venus flexuosa, Lam. 



4. Cytheraea (imperfect). 

5. Mactra Isabellei, d'Orbig. 

6. Ostrca pulchella, do. 



Besides these, Sir W. Parish procured * (as named by Mr. G. B. 
Sowerby) the following shells : — 

7. Voluta colocynthis. I 9. Buccinum (not spec. ?). 

8. V. angulata. | 

All these species (with, perhaps, the exception of the last) are 
recent, and live on the South American coast. These shell-beds 
extend from one league to six leagues from the Plata, and must lie 
many feet above its level. I heard, also, of beds of shells on the 
Somborombon, and on the Rio Salado, at which latter place, as M, 
d'Orbigny informs me, the Mactra Isabellei and Venus si?iuosa are 
found. 

During the elevation of the Provinces of La Plata, the waters 
of the ancient estuary have but little affected (with the exception of 
the sand-hills on the banks of the Parana and Uruguay) the outline 
of the land. M, Parchappe,t however, has described groups of sand 
dunes scattered over the wide extent of the Pampas southward of 
Buenos Ayres, which M. d'Orbigny attributes with much probability to 
the action of the sea, before the plains were raised above its level. | 

* " Buenos Ayres," etc, by Sir W. Parish, p. 168. 

f D'Orbigny's "Voyage, Geolog.," p. 44. 

\ Before proceeding to the districts southward of La Plata, it may be worth 
while just to state, that there is some evidence that the coast of Brazil has 
participated in a small amount of elevation. Mr. Burchell informs me, that 
he collected at Santos (lat. 24° S.) oyster-shells, apparently recent, some 
miles from the shore, and quite above the tidal action. Westward of Rio 
de Janeiro, Captain Elliot is asserted (seeHarlan, " Med. and Phys. Res.," 
p. 35, and Dr. Meigs, in "Trans. Amer. Phil. Soc"), to have found human 
bones, encrusted with sea-shells, between fifteen and twenty feet above the 
level of the sea. Between Rio de Janeiro and Cape Frio I crossed sandy 
tracts abounding with sea-shells, at a distance of a league from the coast ; 
but whether these tracts have been formed by upheaval, or through the mere 
accumulation of drift sand, I am not prepared to assert. At Bahia (lat. 13° S.), 
in some parts near the coast, there are traces of sea-action at the height of 
about twenty. feet above its present level; there are also, in many parts, 
remnants of beds of sandstone and conglomerate with numerous recent shells, 
raised a little above the sea-level. I may add, that at the head of Bahia Bay 
there is a formation, about forty feet in thickness, containing tertiary shells 
apparently of fresh-water origin, now washed by the sea and encrusted with 
Balini ; this appears to indicate a small amount of subsidence subsequent 
to its deposition. At Pernambuco (lat. 8°S.), in the alluvial or tertiary cliffs, 
surrounding the low land on which the city stands, I looked in vain for 
organic remains, or other evidence of changes in level. 



28o ELEVATION OF BAHIA BLANCA. 

Southward of the Plata.— The coast as far as Bahia Blanca (in 
lat. 39° S.) is formed either of a horizontal range of cliffs, or of immense 
accumulations of sand-dunes. Within Bahia Blanca, a small piece of 
tableland, about twenty feet above high-water mark, called Piinta Alta, 
is formed of strata of cemented gravel and of red earthy mud, abounding 
with shells (with others lying loose on the surface), and the bones of 
extinct mammifers. These shells, twenty in number, together with a 
Balanus and two corals, are all recent species, still inhabiting the 
neighbouring seas. They will be enumerated in the Fourth Chapter, 
when describing the Pampean formation ; five of them are identical with 
the upraised ones from near Buenos Ayres. The northern shore of 
Bahia Blanca is, in main pait, formed of immense sand-dunes, resting 
on gravel with recent shells, and ranging in lines parallel to the shore. 
These ranges are separated from each other by flat spaces, composed 
of stiff impure red clay, in which, at the distance of about two miles 
from the coast, I found by digging a few minute fragments of sea-shells. 
The sand-dunes extend several miles inland, and stand on a plain, 
which slopes up to a height of between one hundred and two hundred 
feet. Numerous, small, well-rounded pebbles of pumice lie scattered 
both on the plain and sand-hillocks: at Monte Hermoso, on the flat 
summit of a cliff, I found many of them at a height of 120 feet (angular 
measurement) above the level of the sea. These pumice pebbles, no 
doubt, were originally brought down from the Cordillera by the rivers 
which cross the continent, in the same way as the river Negro anciently 
brought down, and still brings down, pumice, and as the river Chupat 
brings down scoriae : when once delivered at the mouth of a river, they 
would naturally have travelled along the coasts, and been cast up, 
during the elevation of the land, at different heights. The origin of the 
argillaceous flats, which separate the parallel ranges of sand-dunes, 
seems due to the tides here having a tendency (as 1 believe they have 
on most shoal, protected coasts) to throw up a bar parallel to the shore, 
and at some distance from it ; this bar gradually becomes larger, afford- 
ing a base for the accumulation of sand-dunes, and the shallow space 
within then becomes silted up with mud. The repetition of this pro- 
cess, without any elevation of the land, would form a level plain 
traversed by parallel lines of sand-hillocks ; during a slow elevation of 
the land, the hillocks would rest on a gently inclined surface, like that 
on the northern shore of Bahia Blanca. I did not observe any shells in 
this neighbourhood at a greater height than twenty feet ; and therefore 
the age of the sea-drifted pebbles of pumice, now standing at the 
height of 120 feet, must remain uncertain. 

The main plain surrounding Bahia Blanca I estimated at from two 
hundred to three hundred feet ; it insensibly rises towards the distant 
Sierra Ventana. There are in this neighbourhood some other and lower 
plains, but they do not abut one at the foot of the other, in the manner 
hereafter to be described, so characteristic of Patagonia. The plain 
on which the settlement stands is crossed by many low sand-dunes, 
abounding with the minute shells of the Paludestrina aiistralis, d'Orbig., 
which now lives in the bay. This low plain is bounded to the south, at 



ELEVATION OF PATAGONIA. 281 

the Cabeza del Buey, by the cliff-formed margin of a wide plain of the 
Pampean formation, which I estimated at sixty feet in height. On the 
summit of this cliff there is a range of high sand-dunes extending 
several miles in an east and west line. 

Southward of Bahia Blanca, the river Colorado flows between two 
plains, apparently from thirty to forty feet in height. Of these plains, 
the southern one slopes up to the foot of the great sandstone plateau of 
the Rio Negro ; and the northern one against an escarpment of the 
Pampean deposit ; so that the Colorado flows in a valley fifty miles in 
width, between the upper escarpments. I state this, because on the 
low plain at the foot of the northern escarpment, I crossed an immense 
accumulation of high sand-dunes, estimated by the Gauchos at no less 
than eight miles in breadth. These dunes range westward from the 
coast, which is twenty miles distant, to far inland, in lines parallel to 
the vafley ; they are separated from each other by argillaceous flats, 
precisely like those on the northern shore of Bahia Blanca. At present 
there is no source whence this immense accumulation of sand could 
proceed ; but if, as I believe, the upper escarpments once formed the 
shores of an estuary, in that case the sandstone formation of the river 
Negro would have afforded an inexhaustible supply of sand, which 
would naturally have accumulated on the northern shore, as on every 
part of the coast open to the south winds between Bahia Blanca and 
Buenos Ayres. 

At San Bias (40° 40/ S.), a little south of the mouth of the Colorado, 
M. d'Orbigny* found fourteen species of existing shells (six of them 
identical with those from Bahia Blanca), embedded in their natural 
positions. From the zone of depth which these shells are known to 
inhabit, they must have been uplifted thirty-two feet. He also found, 
at from fifteen to twenty feet above this bed, the remains of an ancient 
beach. 

Ten miles southward, but 120 miles to the west, at Port S. Antonio, 
the Officers employed on the Survey assured me that they saw 
many old sea-shells strewed on the surface of the ground, similar 
to those found on other parts of the coast of Patagonia. At San 
Josef, ninety miles south in nearly the same longitude, I found, above 
the gravel, which caps an old tertiary formation, an irregular bed and 
hillock of sand, several feet in thickness, abounding with shells of 
Patella deaurita, Mytilus Magellanicus, the latter retaining much of its 
colour; Fusus'Magellanicus (and a variety of the same), and a large 
Balanus (probably B. TuUpa), all now found on this coast : I estimated 
this bed at from eighty to one hundred feet above the level of the sea. 
To the westward of this bay, there is a plain estimated at between two 
hundred and three hundred feet in height : this plain seems, from many 
measurements, to be a continuation of the sandstone platform of the 
river Negro. The next place southward, where I landed, was at Port 
Desire, 340 miles distant ; but from the intermediate districts I received, 
through the kindness of the Officers of the Survey, especially from 
Lieutenant Stokes and Mr. King, many specimens and sketches, quite 
* " Voyage," etc., p. 54. 



282 ELEVATION OF PATAGONIA. 

sufficient to show the general uniformity of the whole line of coast. 
I may here state, that the whole of Patagonia consists of a tertiary for- 
mation, resting on and sometimes surrounding hills of porphyry and 
quartz : the surface is worn into many wide valleys and into level step- 
formed plains, rising one above another, all capped by irregular beds of 
gravel, chiefly composed of porphyritic rocks. This gravel formation 
will be separately described at the end of the chapter. 

My object in giving the following measurements of the plains, as 

No. I. 

SECTION OF STEP- FORMED PLAINS SOUTH OF NUEVO GULF. 

350 ft. An. M. 200-220 An. M. So Est. 



West. I , ----^ East. 

Level of sea. Scale tjV ^^ inch to 100 feet vertical. 

N.B. — An. M. always stands for anovular or trigronometrical measurement. 
Pa. M. ,, barometrical measurement. 

Est. ;, estimation by the Officers of the Survey. 

taken by the Officers of the Survey, is, as will hereafter be seen, to 
show the remarkable equability of the recent elevatory movements. 
Round the southern parts of Nnevo Gulf, as far as the River Chupat, 
(seventy miles southward of San Josef) there appear to be several 
plains, of which the best defined are here represented. 

The upper plain is here well defined (called Table Hills) ; its edge 
forms a cliff or line of escarpment many miles in length, projecting 
over a lower plain. The lowest plain corresponds with that at San 
Josef with the recent shells on its surface. Between this lowest and 
the uppermost plain, there is probably more than one step-formed 
terrace : several measurements show the existence of the intermediate 
one of the height given in the diagram. 

Near the north headland of the great Bay of St. George (100 miles 

No. 2. 

SECTION OF PL.AIXS IN THE BAY OF ST. GEORGE. 

1,200 feet Est. Not measured. 

\ , 

•» 530 An. M. 320 An. M. 250 An. M. 



Level of sea. Scale -^ j of inch to 100 feet vertical. 

south of the Chupat), two well-marked plains of 250 and 330 feet were 
measured : these are said to sweep round a great part of the Bay. At 
its south headland, 120 miles distant from the north headland, the 250 feet 
plain was again measured. In the middle of the bay, a higher plain was 



ELEVATION OF PATAGONIA. 283 

found at tvv^o neighbouring places (Tilli Roads and C. Marques) to be 580 
feet in height. Above this plain, towards the interior, Mr. Stokes informs 
me that there were several other step-formed plains, the highest of w^hich 
was estimated at 1,200 feet, and was seen ranging at apparently the 
same height for 150 miles northward. All these plains have been 
worn into great valleys and much denuded. The following section is 
illustrative of the general structure of the great Bay of St. George. At 
the south headland of the Bay of St. George (near C. Three Points) the 
250 plain is very extensive. At Port Desire (forty miles southward) I 
made several measurements with the barometer of a plain, which 
extends along the north side of the port and along the open coast, 
and which varies from 245 to 255 feet in height: this plain abuts 
against the foot of a higher plain of 330 feet, which extends also far 
northward along the coast, and likewise into the interior. In the 
distance a higher inland platform was seen, of which I do not know 
the height. In three separate places, I observed the cliff of the 245 — 255 
feet plain, fringed by a terrace or narrow plain estimated at about one 
hundred feet in height. These plains are represented in the following 
section : — 

No. 3. 

SECTION OF PLAINS AT PORT DESIRE. 
Not measured. ♦j^ « 



"^ 2ija ^3 



S§3 



^ III 
\ ^ 100 ft. Est. 



Level of sea. Scale -^nt of i^ich to 100 feet vertical. 

In many places, even at the distance of three and four miles from the 
coast, I found on the gravel-capped surface of the 245 — 255 feet, and of 
the 330 feet plain, shells of Mytilus Magellaniacs, M. edulis, Patella 
deaurila, and another Patella, too much worn to be identified, but 
apparently similar to one found abundantly adhering to the leaves of 
the kelp. These species are the commonest now living on this coast. 
The shells all appeared very old ; the blue of the mussels was much 
faded ; and only traces of colour could be perceived in the Patellas, 
of which the outer surfaces were scaling off. They lay scattered on 
the smooth surface of the gravel, but abounded most in certain 
patches, especially at the heads of the smaller valleys : they generally 
contained sand in their insides ; and I presume that they have been 
washed by alluvial action out of thin sandy layers, traces of which may 
sometimes be seen covering the gravel. The several plains have very 
level surfaces; but all are scooped out by numerous, broad, winding, 
flat-bottomed valleys, in which, judging from the bushes, streams 
never flow. These remarks on the state of the shells, and on the 



284 ELEVATION OF PATAGONIA. 

nature of the plains, apply to the following cases, so need not be 
repeated. 

Southward of Port Desire, the plains have been greatly denuded, 
with only small pieces of tableland marking their former extension. 
But opposite Bird Island, two considerable step-formed plains were 
measured, and found respectively to be 350 and 590 feet in height. 
This latter plain extends along the coast close to Port St. Julian (no 
miles south of Port Desire) ; where we have the following section :-— 

No. 4. 

SECTION OF PLAINS AT PORT S. JULIAN. 



— ^ ~* 





9^0 ft. An. M. 




560 An. M. 


430 An. M. 


2 


° V, 


1 






■r 
-^ 3 



Level of sea. Scale -^^ of inch to looleet vertical. 

The lowest plain was estimated at ninety feet : it is remarkable 
from the usual gravel-bed being deeply worn into hollows, which are 
filled up with, as well as the general surface covered by, sandy and 
reddish earthy matter : - in one of the hollows thus filled up, the 
skeleton of the Macrauchcnia Patachonica, as will hereafter be de- 
scribed, was embedded. On the surface and in the upper parts of this 
earthy mass, there were numerous shells of Myiihis MagellaiiicMS and 
M. edulis, Patella deaurita, and fragments of other species. This 
plain is tolerably level, but not extensive ; it forms a promontory seven 
or eight miles long, and three or four wide. The upper plains in the 
above diagram were measured by the Officers of the Survey; they 
were all capped by thick beds of gravel, and were all more or less 
denuded ; the 950 plain consists merely of separate, truncated, gravel- 
capped hills, two of which, by measurement, were found to differ only 
three feet. The 430 feet plain extends, apparently with hardly a break, 
to near the northern entrance of the Rio Santa Cruz (fifty miles to the 
south) ; but it was there found to be only 330 feet in height. 

On the southern side of the mouth of the Santa Cruz we have the 
following section, which I am able to give with more detail than in 
the foregoing cases : — 

No. 5. 

SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ. 

840 ft. An. M. TIO An. M. ^^ „u ^j^ 

m 5 .3 ca , 



.Q. 3J 



Level of sea. Scale ^-^^ of inch to 100 feet vertical. 



The plain marked 355 feet (as ascertained by the barometer and by 
angular measurement) is a continuation of the above-mentioned 330 



VALLEY OF" SANTA CRUZ. 285 

feet plain : it extends in a N.W. direclion along the southern shores of 
the estuary. It is capped by gravel, which in most parts is covered by 
a thin bed of sandy earth, and is scooped out by many flat-bottomed 
valleys. It appears to the eye quite level, but in proceeding in a 
S.S.W. course, towards an escarpment distant about six miles, and 
likewise ranging across the country in a N.W. line, it was found to 
rise at first insensibly, and then for the last half-mile, sensibly, close up 
to the base of the escarpment : at this point it was 463 feet in height, 
showing a rise of 108 feet in the six miles. On this 355 — 463 feet 
plain, 1 found several shells of Mytihis Magellaiiicus and of a Mytilus, 
which Mr. Sowerby informs me is yet unnamed, though well known as 
recent on this coast ; Patella deaurita; Fusus, I believe, Magellaiiicus, 
but the specimen has been lost ; and at the distance of four miles from 
the coast, at the height of about four hundred feet, there were frag- 
ments of the same Patella and of a Voluta (apparently V. aficilla) 
partially embedded in the superficial sandy earth. All these shells had 
the same ancient appearance with those from the foregoing localities. 
As the tides along this part of the coast rise at the Syzygal period 
forty feet, and therefore form a well-marked beach-line, I particularly 
looked out for ridges in crossing this plain, which, as we have seen, 
rises 108 feet in about six miles, but I could not see any traces of 
such. The next highest plain is 710 feet above the sea; it is very 
narrow, but level, and is capped with gravel ; it abuts to the foot of 
the 840 feet plain. This summit-plain extends as far as the eye can 
range, both inland along the southern side of the valley of the Santa 
Cruz, and southward along the Atlantic. 

The Valley of the R. Sa?tta Cms. — This valley runs in an east and 
west direction to the Cordillera, a distance of about one hundred 
and sixty miles. It cuts through the great Patagonian tertiary forma- 
tion, including, in the upper half of the valley, immense streams of 
basaltic lava, which, as well as the softer beds, are capped by gravel ; 
and this gravel, high up the river, is associated with a vast boulder 
formation.* In ascending the valley, the plain which at the mouth on 
the southern side is 355 feet high, is seen to trend towards the corre- 
sponding plain on the northern side, so that their escarpments appear 
like the shores of a former estuary, larger than the existing one : the 
escarpments, also, of the 840 feet summit- plain (with a corresponding 
northern one, which is met with some way up the valley), appear like 
the shores of a still larger estuary. Farther up the valley, the sides 
are bounded throughout its entire length by level, gravel-capped ter- 
races, rising above each other in steps. The width between the upper 
escarpments is on an average between seven and ten miles ; in one 
spot, however, where cutting through the basaltic lava, it was only one 
mile and a half. Between the escarpments of the second highest 
terrace the average width is about four or five miles. The bottom of 
the valley, at the distance of 1 10 miles from its mouth, begins sensibly 

* I have described this formation in a paper in the " Geological Transac- 
tions," vol. vi., p. 415. 



286 VALLEY OF SANTA CRUZ. 

to expand, and soon forms a considerable plain, 440 feet above the 
level of the sea, through which the river flows in a gut from twenty 
to forty feet in depth. I here found, at a point 140 miles from the 
Atlantic, and seventy miles from the nearest creek of the Pacific, at 
the height of 410 feet, a very old and worn shell of Patella deaurita. 
Lower down the valley, 105 miles from the Atlantic (long. 71° W.), and 
at an elevation of about 300 feet, I also found, in the bed of the river, 
two much worn and broken shells of the Voluia ancilla, still retaining 
traces of their colours; and one of the Palella deaioita. It appeared 
that these shells had been washed from the banks into the river; con- 
sidering the distance from the sea, the desert and absolutely unfre- 
quented character of the countr}^, and the very ancient appearance of 
the shells (exactly like those found on the plains nearer the coast), there 
is, I think, no cause to suspect that they could have been brought here 
by Indians. 

The plain at the head of the valley is tolerably level, but water-worn, 
and with many sand-dunes on it like those on a sea-coast. At the 
highest point to which we ascended, it was sixteen miles wide in a north 
and south line ; and forty-five miles in length in an east and west line. 
It is bordered by the escarpments, one above the other, of two plains, 
which diverge as they approach the Cordillera, and consequently re- 
semble, at two levels, the shores of great bays facing the mountains ; 
and these mountains are breached in front of the lower plain by a 
remarkable gap. The valley, therefore, of the Santa Cruz consists of a 
straight broad cut, about ninety miles in length, bordered by gravel- 
capped terraces and plains, the escarpments of which at both ends 
diverge or expand, one over the other, after the manner of the shores 
of great bays. Bearing in mind this peculiar form of the land — the 
sand-dunes on the plain at the head of the valley — the gap in the 
Cordillera, in front of it — the presence in two places of very ancient 
shells of existing species — and lastly, the circumstance of the 355 — 
453 feet plain, with the numerous marine remains on its surface, sweep- 
ing from the Atlantic coast, far up the valley, I think we must admit, 
that within the recent period, the course of the Santa Cruz formed a 
sea-strait intersecting the continent. At this period, the southern part 
of South America consisted of an archipelago of islands 360 miles in 
a north and south line. We shall presently see, that two other straits 
also, since closed, then cut through Tierra del Fuego ; I may add, that 
one of them must at that time have expanded at the foot of the Cordillera 
into a great bay (now Otway Water) like that which formerly covered 
the 440 feet plain at the head of the Santa Cruz. 

I have said that the valley in its whole course is bordered by gravel- 
capped plains. The following section, supposed to be drawn in a north 
and south line across the valley, can scarcely be considered as more 
than illustrative ; for during our hurried ascent it was impossible to 
measure all the plains at any one place. At a point nearly midway 
between the Cordillera and the Atlantic, I found the plain (A north) 
1,122 feet above the river ; all the lower plains on this side were here 
united into one great broken cliff : at a point sixteen miles lower down 



k 



VALLEY OF SANTA CRUZ. 287 

the stream, I found by ireasiircrxient and estimation that B {n) was 
869 above the river : very near to where A (n) was measured, C {n) was 
639 above the same level: the terrace D {n) was nowhere measured: 
the lowest E («) was in many places about twenty feet above the river. 
These plains or terraces were best developed where the valley was 
widest ; the whole five, like gigantic steps, occurred together only at a 
few points. The lower terraces are less continuous than the higher 
ones, and appear to be entirely lost in the upper third of the valley. 
Terrace C {s), however, was traced continuously for a great distance. 
The terrace B (??),. at a point fifty-five miles from the mouth of the 
river, was four miles in width ; higher up the valley this terrace (or at 
least the second highest one, for I could not always trace it continuously) 
was about eight miles wide. This second plain was generally wider 
than the lower ones — as indeed follows from the valley from A {n) to 
A {s) being generally nearly double the width of from B {n) to B {s). 

No. 6. 

NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE VALLEY 
OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE. 

A south. A north. 

V Bs. Co. y ■; 

: \- . />- Dg^ g Dn. Cn. 



1,122 It. S09 It. 639 ft. Bed of 639 ft. 869 ft. 1,122 ft. 

River. 

The height of each terrace above the level of the river, is shown by the number under 
it. Vertical scale -5^ of inch to 100 feet ; but terrace E, being only twenty feet 
above the river, has necessarily been raised. The horizontal distances much con- 
tracted ; the distance from the edge of A n to A s being on an average from seven 
to ten miles. 

Low down the valley, the summit-plain A {s) is continuous with the 
840 feet plain on the coast, but it is soon lost or unites with the escarp- 
ment of B {s). The corresponding plain A («), on the north side of the 
valley, appears to range continuously from the Cordillera to the head of 
the present estuary of the Santa Cruz, where it trends northward towards 
Port St. Julian. Near the Cordillera the summit-plain on both sides of 
the valley is between 3,200 and 3,300 feet in height ; at 100 miles from 
the Atlantic, it is 1,416 feet, and on the coast 840 feet, all above the sea- 
beach ; so that in a distance of 100 miles the plain rises 576 feet, and 
much more rapidly near to the Cordillera. The lower terraces B and C 
also appear to rise as they run up the valley ; thus D {n), measured at 
two points twenty-four miles apart, was found to have risen 185 feet. 
From several reasons I suspect, that this gradual inclination of the plains 
up the valley, has been chiefly caused by the elevation of the continent 
in mass, having been the greater the nearer to the Cordillera. 

All the terraces are capped with well-rounded gravel, which rests 
either on the denuded and sometimes furrowed surface of the soft 



288 VALLEY OF SANTA CVWZ. 

tertiary deposits, or on the basaltic lava. The difference in height 
between some of the lower steps or terraces seems to be entirely owing 
to a difference in the thickness of the capping gravel. Furrows and 
inequalities in the gravel, where such occur, are filled up and smoothed 
over wulh sandy earth. The pebbles, especially on the higher plains, 
are often whitewashed, and even cemented together by a white alumi- 
nous substance ; and I occasionally found this to be the case with the 
gravel on the terrace D. I could not perceive any trace of a similar 
deposition on the pebbles now thrown up by the river, and therefore I 
do not think that terrace D was river-formed. As the terrace E gene- 
rally stands about twenty feet above the bed of the river, my first im- 
pression was to doubt whether even this lowest one could have been so 
formed ; but it should always be borne in mind, that the horizontal 
upheaval of a district, by increasing the total descent of the streams, 
will always tend to increase, first near the sea-coast and tiien further 
and further up the valley, their corroding and deepening powers : so that 
an alluvial plain, formed almost on a level with a stream, will, after an 
elevation of this kind, in time be cut througli, and left standing at a 
height never again to be reached by the water. With respect to the 
three upper terraces of the Santa Cruz, I think there can be no doubt, 
that they w^ere modelled by the sea, when the valley was occupied by 
a strait, in the same manner (hereafter to be discussed) as the greater 
step-formed, shell-strewed plains along the coast of Patagonia. 

To return to the shores of the Atlantic : the 840 feet plain, at the 
mouth of the Santa Cruz, is seen extending horizontally far to the 
south ; and I am informed by the Officers of the Survey, that bending 
round the head of Coy Inlet (sixty-five miles southward), it trends 
inland. Outliers of apparently the same height are seen forty miles 
farther south, inland of the river Gallegos ; and a plain comes down 
to Cape Gregory (thirty-five miles southward), in the Strait of Magellan, 
which was estimated at between eight hundred and one thousand feet 
in height, and which, rising towards the interior, is capped by the 
boulder formation. South of the Strait of Magellan, there are large 
outlying masses of apparently the same great tableland, extending at 
intervals along the eastern coast of Tierra del Fuego : at two places 
here, no miles apart, this plain was found to be 950 and 970 feet in 
height. 

From Coy Inlet, where the high summit-plain trends inland, a plain 
estimated at 350 feet in height, extends for forty miles to the river 
Gallegos. From this point to the Strait of Magellan, and on each 
side of that Strait, the country has been much denuded and is less 
level. It consists chiefly of the boulder formation, which rises to a 
height of between one hundred and fifty and two hundred and fifty 
feet, and is often capped by beds of gravel. At N.S. Gracia, on the 
north side of the Inner Narrows of the Strait of Magellan, I found on 
the summit of a cliff, 160 feet in height, shells of existing Patellae and 
Mytili, scattered on the surface and partially embedded in earth. On 
the eastern coast, also, of Tierra del Fuego, in latitude 53° 20' south, I 



k 



ar±:a of Recent Elevation. ^§9 

found msiny Mytili on some level land, estimated at 200 feet in height. 
Anterior to the elevation attested by these shells, it is evident by the 
present form of the land, and by the distribution of the great erratic 
boulders * en the surface, that two sea-channels connected the Strait 
of Magellan both with Sebastian Bay and with Otway Water. 

Concluding re?narks on the recent eccvation of the south-eastern 
coasts of A77ierica, and on the action of the sea on the land. — Upraised 
shells of species, still existing as the commonest kinds in the adjoining 
sea, occur, as we have seen, at heights of between a few feet and 410 
feet, at intervals from latitude 33° 40' to 53° 20' south. This is a 
distance of 1,180 geographical miles — about equal from London to the 
North Cape of Sweden. As the boulder formation extends with 
nearly the same height 150 miles south of 53° 20', the most southern 
point where I landed and found upraised shells ; and as the level 
Pampas ranges many hundred miles northward of the point, where M. 
d'Orbigny found at the height of 100 feet beds of the Azara, the space 
in a north and south line, which has been uplifted within the recent 
period, must have been much above the 1,180 miles. By the term 
"recent," I refer only to that period within which the now living 
mollusca were called into existence ; for it will be seen in the Fourth 
Chapter, that both at Bahia Blanca and P. S. Julian, the mammiferous 
quadrupeds which co-existed with these shells belong to extinct species. 
I have said that the upraised shells were found only at intervals on 
this line of coast, but this in all probability maybe attributed to my 
not having landed at the intermediate points ; for wherever I did land, 
with the exception of the river Negro, shells were found : moreover, 
the shells are strewed on plains or terraces, which, as we shall immedi- 
ately see, extend for great distances with a uniform height. I ascended 
the higher plains only in a few places, owing to the distance at which 
their' escarpments generally range from the coast, so that I am far from 
knowing that 410 feet is the maximum of elevation of these upraised 
remains. The shells are those now most abundant in a living state in 
the adjoining sea.f All of them have an ancient appearance ; but 
some, especially the mussels, although lying fully exposed to the 
weather, retain to a considerable extent their colours : this circum- 
stance appears at first surprising, but it is now known that the colouring 
principle of the Mytilus is so enduring, that it is preserved when the 
shell itself is completely disintegrated.^ Most of the shells are 
broken; I nowhere found two valves united; the fragments are not 
rounded, at least in none of the specimens which I brought home. 

With respect to the breadth of the upraised area in an east and 
west line, we know from the shells found at the Inner Narrows of the 

♦ " Geolog. Transactions," vol. vi., p. 419. 

+ Captain King, "Voyages of Adventure and Beagle,'^ vol. i., pp. 6 and 133. 

I See Mr. Lyell's " Proofs of a Gradual Rising in Sweden," in the 
"Philosoph. Transact," 1835, P- ^- See also Mr. Smith of Jordan Hill, in 
the Edin. New Phil. Journal, vol. xxv., p. 393. 

19 



290 AREA OF RECENT ELEVATION. 

Strait of Magellan, that the entire width of the plain, although there 
very narrow, has been elevated. It is probable that in this southern- 
most part of the continent, the movement has extended under the 
sea far eastward ; for at the Falkland Islands, though I could not find 
any shells, the bones of whales have been noticed by several competent 
observers, lying on the land at a considerable distance from the sea, 
and at the height of some hundred feet above it.* Moreover, we know 
that in Tierra del Fuego the boulder formation has been uplifted within 
the recent period, and a similar formation occurs t on the north- 
western shores (Byron Sound) of these islands. The distance from 
this point to the Cordillera of Tierra del Fuego, is 360 miles, which we 
may take as the probable width of the recently upraised area. In the 
latitude of the R. Santa Cruz, we know from the shells found at the 
mouth and head, and in the middle of the valley, that the entire width 
(about 160 miles) of the surface eastward of the Cordillera has been 
upraised. From the slope of the plains, as shown by the course of 
the rivers, for several degrees northward of the Santa Cruz, it is probable 
that the elevation attested by the shells on the coast has likewise ex- 
tended to the Cordillera. When, however, we look as far northward 
as the provinces of La Plata, this conclusion would be very hazardous ; 
not only is the distance from Maldonado (where I found upraised 
shells) to the Cordillera great, namely, 760 miles, but at the head of 
the estuary of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic 
rocks has been observed,! which may well indicate an axis of elevation 
quite distinct from that of the Andes. Moreover, in the centre of the 
Pampas in the chain of Cordova, severe earthquakes have been felt ; § 
whereas at Mendoza, at the eastern foot of the Cordillera, only gentle 
oscillations, transmitted from the shores of the Pacific, have ever been 
experienced. Hence the elevation of the Pampas may be due to 
several distinct axes of movement ; and we cannot judge, from the 
upraised shells round the estuary of the Plata, of the breadth of the 
area uplifted within the recent period. 

* "Voyages of the Advmtture and Beagle,'''' vol. ii., p. 227. And Bougain- 
ville's "Voyage," tome i., p. 112. 

f I owe this fact to the kindness of Captain Sulivan, R.N,, a highly com- 
petent observer. I mention it more especially, as in my Paper (p. 427) on 
the Boulder Formation, I have, after having examined the northern and 
middle parts of the eastern island, said that the formation was here wholly 
absent. 

\ This volcanic formation will be described in Chapter IV. It is not im- 
probable that the height of the upraised shells at the head of the estuary of 
the Plata, being greater than at Bahia Blanca or at San Bias, may be owing 
to the upheaval of these latter places having been connected with the distant 
line of the Cordillera, whilst that of the provinces of La Plata was in con- 
nection with the adjoining tertiary volcanic axis. 

§ See Sir W, Parish's work on " La Plata," p. 242. For a notice of an 
earthquake which drained a lake near Cordova, see also Temple's " Travels 
in Peru." Sir W. Parish informs me, that a town between Salta and Tucu- 
man (north of Cordova) was formerly utterly overthrown by an earthquake. 



I 



UNIFORM HEIGHT OF TERRACES. 291 

Not only has the above specified long range of coast been elevated 
within the recent period, but I think it may be safely inferred from the 
similarity in height of the gravel-capped plains at distant points, that 
there has been a remarkable degree of equability in the elevatory pro- 
cess. I may premise, that when I measured the plains, it was simply 
to ascertain the heights at which shells occurred ; afterwards, comparing 
these measurements with some of those made during the Survey, I was 
struck with their uniformity, and accordingly tabulated all those which 
represented the summit-edges of plains. The extension of the 330 to 
355 feet plain is very striking, being found over a space of 500 geogra- 
phical miles in a north and south line. A table of the measurements is 
here given. The angular measurements and all the estimations are by 
the Officers of the Survey ; the barometrical ones by myself : — 

Feet. 
Gallegos River to Coy Inlet (partly angular meas. and partly estim.) 350 
South Side of Santa Cruz (ang. and barom. meas.) , . , . 355 
North Side of do. (juig- ni.) ....... 330 

Bird Island, plain opposite to (ang. m.) ...... 35^ 

Port Desire, plain extending far along coast (barom. m.) . . . 330 

St. George's Bay, north promontory (ang. m.) 330 

Table Land, south of New Bay (ang. m.) ...... 35*^ 

A plain, varying from 245 to 255 feet, seems to extend with much 
uniformity from Port Desire to the north of St. George's Bay, a distance 
of 170 miles ; and some approximate measurements, also given in the 
following table, indicate the much greater extension of 780 miles : — 

Feet. 
Coy Inlet, south of (partly ang. m. and partly estim.) . . 200 to 300 

Port Desire (barom. m.) 245 to 255 

C. Blanco (ang. m.) 250 

North Promontory of St. George's Bay (ang. m.) . , . 250 

South of New Bay (ang. m.) 200 to 220 

North of S.Josef (estim.) 20010300 

Plain of Rio Negro (ang. m.) 200 to 220 

Bahia Blanca (estim.) 200 to 300 

The extension, moreover, of the 560 to 580, and of the 80 to 100 feet, 
plains is remarkable, though somewhat less obvious than in the former 
cases. Bearing in mind that I have not picked these measurements out 
of a series, but have used all those which represented the edges of 
plains, I think it scarcely possible that these coincidences in height 
should be accidental. We must therefore conclude that the action, 
whatever it may have been, by which these plains have been modelled 
into their present forms, has been singularly uniform. 

These plains or great terraces, of which three and four often rise like 
steps one behind the other, are formed by the denudation of the old 
Patagonian tertiary beds, and by the deposition on their surfaces of a 
mass of well-rounded gravel, varj'ing, near the coast, from ten to thirty- 



292 VNIPORM HEIGHT OP TPRRACE^. 

five feet iil thickness, but increasing in thickness towards the interiof. 
The gravel is often capped by a thin irregular bed of sandy earth. The 
plains slope up, though seldom sensibly to the eye, from the summit- 
edge of one escarpment to the foot of the next highest one. Within a 
distance of 150 miles, between Santa Cruz to Port Desire, where the 
plains are particularly well developed, there are at least seven stages or 
steps, one above the other. On the three lower ones, namely, those of 
100 feet, 250 feet, and 350 feet in height, existing littoral shells are 
abundantly strewed, either on the surface, or partially embedded in the 
superficial sandy earth. By whatever action these three lower plains 
have been modelled, so undoubtedly have all the higher ones, up to a 
height of 950 feet at S. Julian, and of 1,200 feet (by estimation) along 
St. George's Bay. I think it will not be disputed, considering the 
presence of the upraised marine shells, that the sea has been the active 
power during stages of some kind in the elevatory process. 

We will now briefly consider this subject: if we look at the existing 
coast-line, the evidence of the great denuding power of the sea is very 
distinct ; for, from Cape St. Diego, in lat. 54° 30' to the mouth of the 
Rio Negro, in lat. 31° (a length of more than eight hundred miles), the 
shore is formed, with singularly few exceptions, of bold and naked 
cliffs : in many places the cliffs are high ; thus, south of the Santa Cruz, 
they are between eight and nine hundred feet in height, with their 
horizontal strata abruptly cut off, showing the immense mass of matter 
which has been removed. Nearly this whole line of coast consists of a 
series of greater or lesser curves, the horns of which, and likewise 
certain straight projecting portions, are formed of hard rocks ; hence the 
concave parts are evidently the effect and the measure of the denuding 
action on the softer strata. At the foot of all the cliffs, the sea shoals 
very gradually far outwards ; and the bottom, for a space of some miles, 
everywhere consists of gravel. I carefully examined the bed of the 
sea off the Santa Cruz, and found that its inclination was exactly the 
same, both in amount and in its peculiar curvature, with that of the 
355 feet plain at this same place. If, therefore, the coast, with the bed 
of the adjoining sea, were now suddenly elevated one or two hundred 
feet, an inland line of cliffs, that is an escarpment, would be formed, 
with a gravel-capped plain at its foot gently sloping to the sea, and 
having an inclination like that of the existing 355 feet plain. From the 
denuding tendency of the sea, this newly formed plain would in time 
be eaten back into a cliff : and repetitions of this elevatory and denuding 
process would produce a series of gravel-capped, sloping terraces, 
rising one above another, like those fronting the shores of Pata- 
gonia. 

^ The chief difficulty (for there are other inconsiderable ones) on this 
view, is the fact, — as far as I can trust two continuous lines of sound- 
ings carefully taken between Santa Cruz and the Falkland Islands, and 
several scattered observations on this and other coasts, — that the peb- 
bles at the bottom of the sea quickly and regularly decrease in size 
with the increasing depth and distance from the shore, whereas in the 
gravel on the sloping plains, no such decrease in size was perceptible. 



ELEVATION GRADUAL. 



293 



The following table gives the average result of many soundings off the 
Santa Cruz : — 

Under tvi^o miles from the shore, many of the pebbles were of large size, 
mingled with some small ones. 



Distance. 


Depth. 


Size of Pebbles. 


3 to 4 miles from the shore. 


II to 12 fathoms. 


As large as walnuts ; mingled 
in every case with some 
smaller ones. 


6 „ 7 M M 


17 M 19 » 


As large as hazel-nuts. 


10 ,, II )i », 


23 » 25 „ 


From three- to four-tenths of 
an inch in diameter. 


12 ,, „ 


iO „ 40 „ 


Two-tenths of an inch. 


22 to 150 „ „ 


45 M 65 „ 


One-tenth of an inch, to the 
finest sand. 

1 



I particularly attended to the size of the pebbles on the 355 feet 
Santa Cruz plain, and I noticed that on the summit-edge of the present 
sea cliffs many were as large as half a man's head ; and in crossing 
from these cliffs to the foot of the next highest escarpment, a distance of 
six miles, I could not observe any increase in their size. We shall 
presently see that the theory of a slow and almost insensible rise of the 
land, will explain all the facts connected with the gravel-capped 
terraces, better than the theory of sudden elevations of from one to two 
hundred feet. 

M. d'Orbigny has argued, from the upraised shells at San Bias being 
embedded in the positions in which they lived, and from the valves of 
the Azalea labiata high on the banks of the Parana being united and 
unrolled, that the elevation of Northern Patagonia and of La Plata must 
have been sudden ; for he thinks, if it had been gradual, these shells 
would all have been rolled on successive beach-lines. But in protected 
bays, such as in that of Bahia Blanca, wherever the sea is accumulating 
extensive mud-banks, or where the winds quietly heap up sand-dunes, 
beds of shells might assuredly be preserved buried in the positions in 
which they had lived, even whilst the land retained the same level ; 
any, the smallest, amount of elevation would directly aid in their 
preservation. I saw a multitude of spots in Bahia Blanca where this 
might have been effected ; and at Maldonado it almost certainly has 
been effected. In speaking of the elevation ol the land having been 
slow, I do not wish to exclude the small starts which accompany eartl)- 
quakes, as on the coast of Chile ; and by such movements beds c-f 
shells might easily be uplifted, even in positions exposed to a heavy 
surf, without undergoing any attrition: for instance, in 1835, a rocky 
flat off the island of Santa Maria was at one blow upheaved above 
high-water mark, and was left covered with gaping and putrefying 
mussel-shells, still attached to the bed on which they had lived. If 
M. d'Orbigny had been aware of the many long parallel lines of sand- 
hillocks, with infinitely numerous shells of the Mactra and Venus, at 



291 ELEVATION GRADUAL. 

a low level near the Uruguay ; if he had seen at Bahia Blanca the 
immense sand-dunes, with water-worn pebbles of pumice, ranging in 
parallel lines, one behind the other, up a height of at least 120 
feet ; if he had seen the sand-dunes, with the countless Paludestrinas, 
on the low plain near the Fort at this place, and that long line on the 
edge of the cliff, sixty feet higher up ; if he had crossed that long and 
great belt of parallel sand-dunes, eight miles in width, standing at the 
height of from forty to fifty feet above the Colorado, where sand could 
not now collect, — I cannot believe he would have thought that the 
elevation of this great district had been sudden. Certainly the sand- 
dunes (especially when abounding with shells), which stand in ranges 
at so many different levels, must all have required long time for 
their accumulation ; and hence I do not doubt that the last 100 feet of 
elevation of La Plata and Northern Patagonia has been exceedingly 
slow. 

If we extend this conclusion to Central and Southern Patagonia, the 
inclination of the successively rising gravel-capped plains can be ex- 
plained quite as well, as by the more obvious view already given of a 
few comparatively great and sudden elevations ; in either case we must 
admit long periods of rest, during which the sea ate deeply into the 
land. Let us suppose the present coast to rise at a nearly equable, 
slow rate, yet sufficiently quick to prevent the waves quite removing 
each part as soon as brought up ; in this case every portion of the 
present bed of the sea will successively form a beach-line, and from 
being exposed to a like action will be similarly affected. It cannot 
matter to what height the tides rise, even if to forty 'feet as at Santa 
Cruz, for they will act with equal force and in like manner on each 
successive line. Hence there is no difficulty in the fact of the 355 
feet plain at Santa Cruz sloping up 108 feet to the foot of the next 
highest escarpment, and yet having no marks of any one particular 
beach-line on it ; for the whole surface on this view has been a beach. 
I cannot pretend to follow out the precise action of the tidal-waves 
during a rise of the land, slow, yet sufficiently quick to prevent or check 
denudation : but if it be analogous to what takes place on protected 
parts of the present coast, where gravel is now accumulating in large 
quantities,* an inclined surface, thickly capped by well-rounded pebbles 
of about the same size, would be ultimately left. On the gravel now 
accumulating, the waves, aided by the wind, sometimes throw up a 
thin covering of sand, together with the common coast-shells. Shells 
thus cast up by gales, would, during an elevatory period, never again 
be touched by the sea. Hence, on this view of a slow and gradual 
rising of the land, interrupted by periods of rest and denudation, we 
can understand the pebbles being of about the same size over the 
entire width of the step-like plains, — the occasional thin covering of 
sandy earth, — and the presence of broken, unrolled fragments of those 
shells, which now live exclusively near the coast. 

* On the eastern side'.of Chiloe, which island we shall see in the next 
chapter is now rising, I observed that all the beaches and extensive tidal- 
flats were formed of shingle. 



GRAVEL FORMATION OF PATAGONIA. 295 

Suminary of Results. — It may be concluded that the coast on this side 
the continent, for a space of at least 1,180 miles, has been elevated to a 
height of 100 feet in La Plata, and of 400 feet in Southern Patagonia, 
within the period of existing shells, but not of existing mammifers. 
That in La Plata the elevation has been very slowly effected : that in 
Patagonia the movement may have been by considerable starts, but 
much more probably slow and quiet. In either case, there have been 
long intervening periods of comparative rest,* during which the sea 
corroded deeply, as it is still corroding, into the land. That the periods 
of denudation and elevation were contemporaneous and equable over 
great spaces of coast, as shown by the equable heights of the plains ; 
that there have been at least eight periods of denudation, and that the 
land, up to a height of from 950 to 1,200 feet, has been similarly 
modelled and affected : that the area elevated, in the southernmost 
part of the continent, extended in breadth to the Cordillera, and pro- 
bably seaward to the Falkland Islands ; that northward, in La Plata, 
the breadth is unknown, there having been probably more than one axis 
of elevation ; and finally, that, anterior to the elevation attested by these 
upraised shells, the land was divided by a Strait where the River Santa 
Cruz now flows, and that further southward there were other sea-straits, 
since closed. I may add, that at Santa Cruz, in lat. 50° S., the plains 
have been uplifted at least 1,400 feet, since the period when gigantic 
boulders were transported between sixty and seventy miles from their 
parent rock, on floating icebergs. 

Lastly, considering the great upward movements which this long line 
of coast has undergone, and the proximity of its southern half to the 
volcanic axis of the Cordillera, it is highly remarkable that in the many 
fine sections exposed in the Pampean, Patagonian tertiary, and Boulder 
formations, I nowhere observed the smallest fault or abrupt curvature 
m the strata. 



Gravel Formation of Patagonia. 

I will here describe in more detail than has been as yet incidentally 
done, the nature, origin, and extent of the great shingle covering of 
Patagonia: but I do not mean to affirm that all of this shingle, 
especially that on the higher plains, belongs to the recent period. A 
thin bed of sandy earth, with small pebbles of various porphyries and 
of quartz, covering a low plain on the north side of the Rio Colorado, 
is the extreme northern limit of this formation. These little pebbles 
have probably been derived from the denudation of a more regular bed 
of gravel, capping the old tertiary sandstone plateau of the Rio Negro. 
The gravel-bed near the Rio Negro is, on an average, about ten or 

* I say comparative and not absolute rest, because the sea acts, as we have 
seen, with great denuding power on this whole line of coast ; and therefore, 
during an elevation of the land, if excessively slow (and of course during a 
subsidence of the land), it is quite possible that lines of cliff might be 
formed. 



296 GRAVEL FORMATION OF PATAGONIA, 

twelve feet in thickness ; and the pebbles are larger than on the northern 
side of the Colorado, being from one to two inches in diameter, and 
composed chiefly of rather dark-tinted porphyries. Amongst them I 
here first noticed a variety often to be referred to, namely, a peculiar 
gallstone-yellow siliceous porphyry, frequently, but not invariably, 
containing grains of quartz. The pebbles are embedded in a white, 
gritty, calcareous matrix, yery like mortar, sometimes merely coating 
with a whitewash the separate stones, and somietimes forming the 
greater part of the mass. In one place I saw in the gravel concretionary 
nodules (not rounded) of crys^tallised gypsum, some as large as a man's 
head. I traced this bed for forty-five miles inland, and was assured 
that it extended far into the interior. As the surface of the calcareo- 
argillaceous plain of Pampean formation, on the northern side of the 
wide valley of the Colorado, stands at about the same height with the 
mortar-like cemented gravel capping the sandstone on the southern 
side, it is probable, considering the apparent equability of the subter- 
ranean movements along this side of America, that this gravel of the 
Rio Negro and the upper beds of the Pampean formation northward 
of the Colorado, are of nearly contemporaneous origin, and that the 
calcareous matter has been derived from the same source. 

Southward of the Rio Negro, the cliffs along the great bay of S. 
Antonio are capped with gravel : at San Josef, I found that the pebbles 
closely resembled those on the plain of the Rio Negro, but that they 
were not cemented by calcareous matter. Between San Josef and 
Port Desire, I was assured by the Officers of the Survey that the whole 
face of the country is coated with gravel. At Port Desire and over a 
space of twenty-five miles inland, on the three step-formed plains and 
in the valleys, I everywhere passed over gravel which, where thickest, 
was between thirty and forty feet. Here, as in other parts of Pata- 
gonia, the gravel, or its sandy covering, was, as we have seen, often 
strewed with recent marine shells. The sandy covering sometimes 
fills up furrows in the gravel, as does the gravel in the underlying 
tertiary formations. The pebbles are frequently whitewashed and even 
cemented together by a peculiar, white, friable, aluminous, fusible 
substance, which I believe is decomposed feldspar. At Port Desire, 
the gravel rested sometimes on the basal formation of porphyry, and 
sometimes on the upper or the lower denuded tertiary strata. It is 
remarkable that most of the porphyritic pebbles differ from those 
varieties of porphyry which occur here abundantly in situ. The peculiar 
gallstone-yellow variety was common, but less numerous than at Port 
S. Julian, where it formed nearly one-third of the mass of gravel ; the 
remaining part there consisting of pale grey and greenish porphyries 
with many crystals of feldspar. At Port S. Julian, I ascended one of 
the flat-topped hills, the denuded remnant of the highest plain, and 
found it, at the height of 950 feet, capped with the usual bed of gravel. 

Near the mouth of the Santa Cruz, the bed of gravel on the 355 
feet plain is from twenty to about thirty-five feet in thickness. The 
pebbles vary from minute ones to the size of a hen's egg, and even to 
that of half a man's head ; they consist of paler vs^ricties pf porphyry 



GRAVEL FORMATION OF PATAGONIA. 297 

than those found further northward, and there are fewer of the gallstone- 
yellow kind; pebbles of compact black clay-slate were here first ob- 
served. The gravel, as we have seen, covers the step-formed plains at 
the mouth, head, and on the sides of the great valley of the Santa 
Cruz. At a distance of no miles from the coast, the plain has risen 
to the height of 1,416 feet above the sea; and the gravel, with the 
associated great boulder formation, has attained a thickness of 212 feet. 
The plain, apparently with its usual gravel covering, slopes up to the 
foot of the Cordillera to the height of between 3,200 and 3,300 feet. 
In ascending the valley, the gravel gradually becomes entirely altered 
in character : high up, we have pebbles of crystalline feldspathijc rocks, 
compact clay-slate, quartzose schists, and pale-coloured porphyries; 
these rocks, judging both from the gigantic boulders in the surface and 
from some small pebbles embedded beneath 700 feet in thickness of 
the old tertiary strata, are the prevailing kinds in this part of the 
Cordillera ; pebbles of basalt from the neighbouring streams of basaltic 
lava are also numerous ; there are few or none of the reddish or of the 
gallstone-yellow porphyries so common near the coast. Hence the 
pebbles on the 350 feet plain at the mouth of the Santa Cruz cannot 
have been derived (with the exception of those of compact clay-slate, 
which, however, may equally well have come from the south) from the 
Cordillera in this latitude; but probably, in chief part, from farther 
north. 

Southward of the Santa Cruz, the gravel may be seen continuously 
capping the great 840 feet plain : at the Rio Gallegos, where this plain 
is succeeded by a lower one, there is, as I am informed by Captain 
Sulivan, an irregular covering of gravel from ten to twelve feet in 
thickness over the whole country. The district on each side of the 
Strait of Magellan is covered up either with gravel or the boulder 
formation : it was interesting to observe the marked difference between 
the perfectly rounded state of the pebbles in the great shingle formation 
of Patagonia, and the more or less angular fragments in the boulder 
formation. The pebbles and fragments near the Strait of Magellan 
nearly all belong to rocks known to occur in Fuegia. I was therefore 
much surprised in dredging south of the Strait to find, in lat. 54° 10' 
south, many pebbles of the gallstone-yellow siliceous porphyry ; I pro- 
cured others from a great depth off Staten Island, and others were 
brought me from the western extremity of the Falkland Islands.* 
The distribution of the pebbles of this peculiar porphyry, wliich I 
venture to affirm is not found in situ either in Fuegia, the Falkland 
Islands, or on the coast of Patagonia, is very remarkable, for they are 

* At my request, Mr. Kent collected for me a bag of pebbles from the 
beach of White Rock harbour, in the northern part of the sound, between 
the two Falkland Islands. Out of these well-rounded pebbles, varying in 
size from a walnut to a hen's egg, with some larger, thirty-eight evidently 
belonged to the rocks of these islands; twenty-six were similar to the 
pebbles of porphyry found on the Patagonian plains, which rocks do not 
exist in situ in the Falklands ; one pebble belonged to the peculiar yellow 
pjliqeous porphyry; thirty were of doubtful origin. 



298 GRAVEL FORMATION OF PATAGONIA. 

found over a space of 840 miles in a north and south line, and at the 
Falklands, 300 miles eastward of the coast of Patagonia, Their 
occurrence in Fuegia and the Falklands may, however, perhaps be 
due to the same ice-agency by which the boulders have been there 
transported. 

We have seen that porphyritic pebbles of a small size are first met 
with on the northern side of the Rio Colorado, the bed becoming well 
developed near the Rio Negro : from this latter point I have every 
reason to believe that the gravel extends uninterruptedly over the plains 
and valleys of Patagonia for at least 630 nautical miles southward to the 
Rio Gallegos. From the slope of the plains, from the nature of the 
pebbles, from their extension at the Rio Negro far into the interior, and 
at the Santa Cruz close up to the Cordillera, I think it highly prolDable 
that the whole breadth of Patagonia is thus covered. If so, the average 
width of the bed must be about two hundred miles. Near the coast the 
gravel is generally from ten to thirty feet in thickness ; and as in the 
valley of Santa Cruz it attains, at some distance from the Cordillera, 
a thickness of 214 feet, we may, I think, safely assume its average 
thickness over the whole area of 630 by 200 miles, at fifty feet ! 

The transportal and origin of this vast bed of pebbles is an interesting 
problem. From the manner in which they cap the step-formed plains, 
worn by the sea within the period of existing shells, their deposition, at 
least on the plains up to a height of 400 feet, must have been a recent 
geological event. From the form of the continent, we may feel sure 
that they have come from the westward, probably, in chief part from 
the Cordillera, but, perhaps, partly from unknown rocky ridges in the 
central districts of Patagonia. That the pebbles have not been trans- 
ported by rivers, from the interior towards the coast, we may conclude 
from the fewness and smallness of the streams of Patagonia : moreover, 
in the case of the one great and rapid river of Santa Cruz, we have good 
evidence that its transporting power is very trifling. This river is from 
two to three hundred yards in width, about seventeen feet deep in its 
middle, and runs with a singular degree of uniformity five knots an hour, 
with no lakes and scarcely any still reaches : nevertheless, to give one 
instance of its small transporting power, upon careful examination, 
pebbles of compact basalt could not be found in the bed of the river at 
a greater distance than ten miles below the point where the stream 
rushes over the debris of the great basaltic cHffs forming its shore : frag- 
ments of the cellular varieties have been washed down twice or thrice 
as far. That the pebbles in Central and Northern Patagonia have not 
been transported by ice-agency, as seems to have been the case to a 
considerable extent farther south, and likewise in the northern hemi- 
sphere, we may conclude, from the absence of all angular fragments in 
the gravel, and from the complete contrast in many other respects between 
the shingle and neighbouring boulder formation. 

Looking to the gravel on any one of the step-formed plains, I cannot 
doubt, from the several reasons assigned in this chapter, that it has been 
spread out and levelled by the long-continued action of the sea, probably 
during the slow rise of the land. The smooth and perfectly rounded 



DISTRIBUTION OF GRAVEL. 299 

condition of the innumerable pebbles alone would prove long-continued 
action. But how the whole mass of shingle on the coast-plains has 
been transported from the mountains of the interior, is another and 
more difficult question. The following considerations, however, show 
that the sea by its ordinary action has considerable power in distributing 
pebbles. A table has already been given, showing how very uniformly 
and gradually * the pebbles decrease in size with the gradually seaward 
increasing depth and distance. A series of this kind irresistibly leads to 
the conclusion, that the sea has the power of sifting and distributing the 
loose matter on its bottom. According to Martin White,t the bed of 
the British Channel is disturbed during gales at depths of sixty-three 
and sixty-seven fathoms, and at thirty fathoms, shingle and fragments of 
shells are often deposited, afterwards to be carried away again. Ground- 
swells, which are believed to be caused by distant gales, seem especially 
to affect the bottom : at such times, according to Sir R. Schomburgk,| 
the sea to a great distance round the West Indian Islands, at depths 
from five to fifteen fathoms, becomes discoloured, and even the anchors 
of vessels have been moved. There are, however, some difficulties in 
understanding how the sea can transport pebbles lying at the bottom, 
for, from experiments instituted on the power of running water, it would 
appear that the currents of the sea have not sufficient velocity to move 
stones of even moderate size : moreover, I have repeatedly found in the 
most exposed situations that the pebbles which lie at the bottom are 
encrusted with full-grown living corallines, furnished with the most 
delicate, yet unbroken spines : for instance, in ten fathoms water off the 
mouth of the Santa Cruz, many pebbles, under half an inch in diameter, 
were thus coated with Flustracean zoophytes. § Hence we must conclude 

* I may mention, that at the distance of 150 miles from the Patagonian 
shore I carefully examined the minute rounded particles in the sand, and 
found them to be fusible like the porphyries of the great shingle bed. 
I could even distinguish particles of the gallstone-yellow porphyry. It was 
interesting to notice how gradually the particles of white quartz increased, 
as we approached the Falkland Islands, which are thus constituted. In the 
whole line of soundings between these islands and the coast of Patagonia 
dead or living organic remains were most rare. On the relations between 
the depth of water and the nature of the bottom, see Martin White on 
"Soundings in the Channel," pp. 4, 6, 175; also Captain Beechey's "Voyage 
to the Pacific," chap, xviii. 

f "Soundings in the Channel," pp. 4, 166. M. Siau states {Edin. New 
Phil. Jour., vol. xxxi., p. 246), that he found the sediment, at a depth of 188 
metres, arranged in ripples of different degrees of fineness. There are some 
excellent discussions on this and allied subjects in Sir H. De la Beche's 
"Theoretical Researches." 

X Journal of Royal Geograph. Soc, vol. v., p. 25. It appears from Mr. 
Scott Russell's investigations (see Mr. Murchison's "Anniver. Address 
Geolog. Soc," 1843, p. 40), that in waves of translation the motion of the 
particles of water is nearly as great at the bottom as at the top. 

§ A pebble, one and a half inch square and half an inch thick, was given 
me, dredged up from twenty-seven fathoms depth off the western end of the 



300 DISTRIBUTION OF GRAVEL. 

that these pebbles are not often violently disturbed : it should, however, 
be borne in mind that the growth of corallines is rapid. The view, 
propounded by Professor Playfair, will, I believe, explain this apparent 
difficulty, — namely, that from the undulations of the sea tending to lift 
up and down pebbles or other loose bodies at the bottom, such are liable, 
when thus quite or partially raised, to be moved even by a very small 
force, a little onwards. We can thus understand how oceanic or tidal 
currents of no great strength, or that recoil movement of the bottom- 
water near the land, called by sailors the "undertow" (which I presume 
must extend out seaward as far as the breaking waves impel the surface- 
water towards the beach), may gain the power during storms of sifting 
and distributing pebbles even of considerable size, and yet without so 
violently disturbing them as to injure the encrusting corallines.* 

The sea acts in another and distinct manner in the distribution of 
pebbles, namely by the waves on the beach. Mr. Palmer,t in his 
excellent memoir on this subject, has shown that vast masses of shingle 
travel with surprising quickness along lines of coast, according to the 
direction with which the waves break on the beach and that this is 
determined by the prevailing direction of the winds. This agency must 
be powerful in mingling together and disseminating pebbles derived 
from different sources : we may, perhaps, thus understand the wide 
distribution of the gallstone-yellow porphyry ; and likewise, perhaps, 
the great difference in the nature of the pebbles at the mouth of the 
Santa Cruz from those in the same latitude at the head of the valley. 

I will not pretend to assign to these several and complicated agencies 
their shares in the distribution of the Patagonian shingle : but from the 
several considerations given in this chapter, and I may add, from the 
frequency of a capping of gravel on tertiary deposits in all parts of the 
world, as I have myself observed and seen stated in the works of 

Falkland Islands, where the sea is remarkably stormy, and subject to violent 
tides. This pebble was encrusted on all sides by a delicate living coralline. 
I have seen many pebbles from depths between forty and seventy fathoms 
thus encrusted ; one from the latter depth off Cape Horn. 

* I may take this opportunity of remarking on a singular, but very common 
character in the form of the bottom, in the creeks which deeply penetrate the 
western shores of Tierra del Fuego; namely, that they are almost invariably 
much shallower close to the open sea at their mouths than inland. Thus, 
Cook, in entering Christmas Sound, first had soundings in thirty-seven 
fathoms, then in fifty, then in sixty, and a little farther in no bottom with 
170 fathoms. The sealers are so familiar with this fact, that they always look 
out for anchorage near the entrances of the creeks. See, also, on this 
subject, the "Voyages of the Adventure and Beagle" vol. i., p. 375, and 
"Appendix," p. 313. This shoalness of the sea-channels near their entrances 
probably results from the quantity of sediment formed by the wear and tear 
of the outer rocks exposed to the full force of the open sea. I have no doubt 
that many lakes, for instance in Scotland, which are very deep within, and 
are separated from the sea apparently only by a tract of detritus, were 
originally sea-channels with banks of this nature near their mouths, which 
have since been upheaved. 

t "Philosophical Transactions," 1834, p. 576, 



romiATtON OF SEA-CLIFFS. 



301 



various authors, i cannot doubt that the power of widely dispersing 
gravel is an ordinary contingent on the action of the sea ; and that even 
in the case of the great Patagonian shingle-bed we have no occasion to 
call in the aid of debacles. I at one time imagined that perhaps an 
immense accumulation of shingle had originally been collected at the 
foot of the Cordillera ; and that this accumulation, when upraised 
above the level of the sea, had been eaten into and partially spread out 
(as off the present line of coast) ; and that the newly-spread out bed 
had in its turn been upraised, eaten into, and re-spread out ; and so 
onwards, until the shingle, which was first accumulated in great thick- 
ness at the foot of the Cordillera, had reached in thinner beds its 
present extension. By whatever means the gravel formation of Pata- 
gonia may have been distributed, the vastness of its area, its thickness, 
its superficial position, its recent origin, and the great degree of simi- 
larity in the nature of its pebbles, all appear to me well deserving the 
attention of geologists, m relation to the origin of the widely-spread 
beds of conglomerate belonging to past epochs. 

Formatio?i of Cliffs. — When viewing the sea-worn cliffs of Patagonia, 

No. 7. 

SECTION OF COAST-CUFFS AND BOTTOM OF SEA, OFF TIIe" ISLAND OF ST. HELENA. 




' Bcitom rocky 
Only to a depth of 
five or six fathoms. 



Mud and sand. 



Vertical and horizontal scale, two inches to a nautical mile. The point marked 
1,600 feet is at the foot of Hi^h Knoll; point marked 510 feet is on the edge of 
Ladder Hill. The strata consist of basaltic streams. 



in some parts between eight hundred and nine hundred feet in height, 
and formed of horizontal tertiary strata, which must once have extended 
far seaward — or again, when viewing the lofty cliffs round many 
volcanic islands, in which the gentle inclination of the lava-streams 
indicates the former extension of the land, a difficulty often occurred 
to me, namely, how the strata could possibly have been removed by 
the action of the sea at a considerable depth beneath its surface. The 
above section, which represents the general form of the land on 
the northern and leeward side of St. Helena (taken from Mr. Seale's 
large model and various measurements), and of the bottom of the 
adjoining sea (taken chiefly from Captain Austin's survey and some 
old charts), will show the nature of this difficulty. 

If, as seems probable, the basaltic streams were originally prolonged 
\yith nearly their present inclination, they must, as shown by the dotted 
line in the section, once have extended at least to a point, now covered 



302 FORMATION OF SEA-CLIFFS. 

by the sea to a depth of nearly thirty fathoms : but I have every reason 
to bcHeve they extended considerably further, for the inclination of the 
streams is less near the coast than further inland. It should also be 
observed, that other sections on the coast of this island would have 
given far more striking results, but I had not the exact measurements ; 
thus, on the windward side, the cliffs are about two thousand feet in 
height and the cut-off lava streams very gently inclined, and the bottom 
of the sea has nearly a similar slope all round the island. How, then, 
has all the hard basaltic rock, which once extended beneath the surface 
of the sea, been worn away ? According to Captain Austin, the bottom 
is uneven and rocky only to that very small distance from the beach 
within which the depth is from five to six fathoms ; outside this line, 
to a depth .of about one hundred fathoms, the bottom is smooth, gently 
inclined, and formed of mud and sand ; outside the one hundred fathoms, 
it plunges suddenly into unfathomable depths, as is so very commonly 
the case on all coasts where sediment is accumulating. At greater 
depths than the five or six fathoms, it seems impossible, under existing 
circumstances, that the sea can both have worn away hard rock, in parts 
to a thickness of at least 150 feet, and have deposited a smooth bed of 
fine sediment. Now, if we had any reason to suppose that St. Helena 
had, during a long period, gone on slowly subsiding, every difficulty 
would be removed : for looking at the diagram, and imagining a fresh 
amount of subsidence, we can see that the waves would then act on the 
coast-cliffs with fresh and unimpaired vigour, whilst the rocky ledge near 
the beach would be carried down to that depth, at which sand and mud 
would be deposited on its bare and uneven surface : after the formation 
near the shore of a new rocky shoal, fresh subsidence would carry it 
down and allow it to be smoothly covered up. But in the case of the 
many cliff-bounded islands, for instance in some of the Canary Islands 
and of Madeira, round which the inclination of the strata shows that the 
land once extended far into the depths of the sea, where there is no 
apparent means of hard rock being worn away — are we to suppose 
that all these islands have slowly subsided ? Madeira, I may remark, 
has, according to Mr. Smith of Jordan Hill, subsided. Are we to extend 
this conclusion to the high, cliff-bound, horizontally stratified shores of 
Patagonia, off which, though the water is not deep even at the distance 
of several miles, yet the smooth bottom of pebbles gradually decreasing 
in size wdth the increasing depth, and derived from a foreign source, 
seem to declare that the sea is now a depositing and not a corroding 
agent ? I am much inclined to suspect, that we shall hereafter find in 
all such cases, that the land with the adjoining- bed of the sea has in 
truth subsided: the time will, I believe, come, when geologists will 
consider it as improbable, that the land should have retained the same 
level during a whole geological period, as that the atmosphere sliould 
liave remained absolutely calm during an entire season. 



ELEVATION OF CHILOE. 303 

CHAPTER II. 

ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. 

Chonos Archipelago. — Chiloe, recent and gradual elev'ation of, traditions of 
the inhabitants on this subject. — Concepcion, earthquake and elevation 
of. — Valparaiso, great elevation of, upraised shells, earth of marine 
origin, gradual rise of the land within the historical period. — Coquimbo, 
elevation of, in recent times, terraces of marine origin, their inclination, 
their escarpments not horizontal. — Guasco, gravel terraces of. — Copiapo. 
— Peru. — Upraised shells of Cobija, Iquique, and Arica. — Lima, shell- 
beds and sea-beach on San Lorenzo, human remains, fossil earthen- 
ware, earthquake debacle, recent subsidence. — On the decay of upraised 
shells. — General summary. 

'OMMENCING at the south and proceeding northward, the first place at 
[which I landed, was at Cape Tres Montes, in lat. 46° 35'. Here, on 
the shores of Christmas Cove, I observed in several places a beach of 
)ebbles with recent shells, about twenty feet above high-water mark, 
southward of Tres Montes (between lat. 47° and 48°), Byron* remarks, 
' We thought it very strange, that upon the summits of the highest hills 
^ere found beds of shells, a foot or two thick." In the Chonos Archi- 
pelago, the island of Lemus (lat. 44° 30') was, according to M. Coste.f 
suddenly elevated eight feet, during the earthquake of 1829 : he adds, 
" Des roches jadis toujours couvertes par la mer, restant aujourd'hui 
constamment decouvertes." In other parts of this archipelago, I 
observed two terraces of gravel, abutting to the foot of each other : at 
Lowe's Harbour (43° 48'), under a great mass of the boulder formation, 
about three hundred feet in thickness, I found a layer of sand, with 
numerous comminuted fragments of sea-shells, having a fresh aspect, 
but too small to be identified. 

The Island of Chiloe. — The evidence of recent elevation is here 
more satisfactory. The bay of San Carlos is in most parts bounded by 
precipitous cliffs from about ten to forty feet in height, their bases being 
separated from the present line of tidal action by a talus, a few feet in 
height, covered with vegetation. In one sheltered creek (west of P. 
Arena), instead of a loose talus, there was a bare sloping bank of 
tertiary mudstone, perforated, above the line of the highest tides, by 
numerous shells of a Pholas now common in the harbour. The upper 
extremities of these shells, standing upright in their holes with grass 
growing out of them, were abraded about a quarter of an inch, to the 
same level with the surrounding worn strata. In other parts, I observed 
(as at Pudeto) a great beach, formed of comminuted shells, twenty feet 
above the present shore. In other parts again, there were small caves 
worn into the foot of the low cliffs, and protected from the waves by 
the talus with its vegetation : one such cave, which I examined, had its 
mouth about twenty feet, and its bottom, which was filled with sand 

* " Narrative of the Loss of the Wager.'''' 
■f- " Comptcs Rendus," October 1838, p. 706. 



304 Elevation of chiloe. 

containing fragments of shells and legs of crabs, from eiglit to ten feet 
above high-water mark. From these several facts, and from the appear- 
ance of the upraised shells, I inferred that the elevation had been quite 
recent ; and on inquiring from Mr, Williams, the Portmastcr, he told 
me he was convinced that the land had risen, or the sea fallen, four 
feet within the last four years. During this period, there had been one 
severe earthquake, but no particular change of level was then observed ; 
from the habits of the people who all keep boats in the protected 
creeks, it is absolutely impossible that a rise of four feet could have 
taken place suddenly and been unperceived. Mr. Williams believes 
that the change has been quite gradual. Without the elevatory move- 
ment continues at a quick rate, there can be no doubt that the sea will 
soon destroy the talus of earth at the foot of the cliffs round the bay, 
and will then reach its former lateral extension, but not of course its 
former level : some of the inhabitants assured me that one such talus, 
with a footpath on it, was even already sensibly decreasing in width. 

I received several accounts of beds of shells, existing at considerable 
heights in the inland parts of Chiloe ; and to one of these, near 
Catiman, I was guided by a countryman. Here, on the south side of 
the peninsula of Lacuy, there was an immense bed of the Ve7ius costel- 
lata and of an oyster, lying on the summit-edge of a piece of 
tableland, 350 feet (by the barometer) above the level of the sea. 
The shells were closely packed together, embedded in and covered 
by a very black, damp, peaty mould, two or three feet in thick- 
ness, out of which a forest of great trees was growing. Considering 
the nature and dampness of this peaty soil, it is surprising that the fine 
ridges on the outside of the Venus are perfectly preserved, though all 
the shells have a blackened appearance. I did not doubt that the 
black soil, which, when dry, cakes hard, was entirely of terrestrial origin, 
but on examining it under the microscope, I found many very minute 
rounded fragments of shells, amongst which I could distinguish bits of 
serpulae and mussels. The Venus costellata, and the ostrea {O. edulis, 
according to Captain King) are now the commonest shells in the 
adjoining bays. In a bed of shells, a few feet below the 350 feet bed, 
I found a horn of the little Cerviis hiunilis, which now inhabits Chiloe. 

The eastern or inland side of Chiloe, with its many adjacent islets, 
consists of tertiary and boulder deposits, worn into irregular plains 
capped by gravel. Near Castro, and for ten miles southward, and on 
the islet of Lemuy, I found the surface of the ground to a height of 
between twenty and thirty feet above high-water mark, and in several 
places apparently up to fifty feet, thickly coated by much comminuted 
shells, chiefly of the Vettus costellata and Mytilus Chiloensis ; the species 
now most abundant on this line of coast. As the inhabitants carry 
immense numbers of these shells inland, the continuity of the bed at 
the same height was often the only means of recognising its natural 
origin. Near Castro, on each side of the creek and rivulet of the 
Gamboa, three distinct terraces are seen : the lowest was estimated at 
about one hundred and fifty feet in height, and the highest at about five 
hundred feet, with the country irregularly rising behind it; obscure 



ELEVATION OF CONCEPCION. 305 

traces, also, of these same terraces could be seen along other parts of 
the coast. There can be no doubt that their three escarpments record 
pauses in the elevation of the island. I may remark that several pro- 
montories have the word Huapi, which signifies in the Indian tongue, 
island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi, 
etc. ; and these, according to Indian traditions, once existed as islands. 
In the same manner the term Pulo in Sumatra is appended * to the 
names of promontories, traditionally said to have been islands ; in 
Sumatra, as in Chiloe, there are upraised recent shells. The Bay of 
Carelmapu, on the mainland north of Chiloe, according to Aguerros,t 
was in 1643 a good harbour; it is now quite useless, except for 
boats. 

Valdivia. — I did not observe here any distinct proofs of recent 
elevation ; but in a bed of very soft sandstone, forming a fringe-like 
plain, about sixty feet in height, round the hills of mica-slate, there are 
shells of Mytilus, Crepidula, Solen, Novacuhna, and Cytheraea, too 
imperfect to be specifically recognised. At Imperial, seventy miles north 
of Valdivia, Agiierros % states that there are large beds of shells, at a 
considerable distance from the coast, which are burnt for lime. The 
island of Mocha, lying a little north of Imperial, was uplifted two feet,§ 
during the earthquake of 1835. 

Co7icepcion. — I cannot add anything to the excellent account by 
Captain Fitzroylj of the elevation of the land at this place, which accom- 
panied the earthquake of 1835. I will only recall to the recollection of 
geologists, that the southern end of the island of St. Mary was uplifted 
eight feet, the central part nine, and the northern end ten feet ; and 
the whole island more than the surrounding districts. Great beds of 
mussels, patellae, and chitons still adhering to the rocks were upraised 
above high-water mark ; and some acres of a rocky flat, which was 
formerly always covered by the sea, was left standing dry, and exhaled 
an offensive smell, from the many attached and putrefying shells. It 
appears from the researches of Captain Fitzroy that both the island of 
St. Mary and Concepcion (which was uplifted only four or five feet) in 
the course of some weeks subsided, and lost part of their first elevation. 
I will only add as a lesson of caution, that round the sandy shores of 
the great Bay of Concepcion, it was most difficult, owing to the oblite- 
rating effects of the great accompanying wave, to recognise any distinct 

* Marsden's "Sumatra," p. 31. 

f " Descripcion Hist, de la Provincia de Chiloe," p, 78. From the account 
given by the old Spanish writers, it would appear that several other harbours, 
between this point and Concepcion, were formerly much deeper than they 
now are. 

X Ibid., p. 25. 

§ "Voyages oi Adventure and Beagle^'' vol. ii., p. 415. 

II Ibid., vol. ii., p. 412, et seq. In vol. v. (p. 601) of the " Geological Trans- 
actions," I have given an account of the remarkable volcanic phenomena, 
which accompanied this earthquake. These phenomena appear to me to 
prove that the action, by which large tracts of land are uplifted, and by 
which volcanic eruptions are produced, is in every respect identical. 

20 



3o6 ELEVATION OF CONCEPCION. 

evidence of this considerable upheaval ; one spot must be excepted, 
where there was a detached rock which before the earthquake had 
always been covered by the sea, but afterwards was left uncovered. 

On the island of Quiriquina (in the Bay of Concepcion), I found, at 
an estimated height of four hundred feet, extensive layers of shells, 
mostly comminuted, but some perfectly preserved and closely packed 
in black vegetable mould ; they consisted of Concholepas, Fissurella, 
Mytilus, Trochus, and Balanus. Some of these layers of shells rested 
on a thick bed of bright-red, dry, friable earth, capping the surface 
of the tertiary sandstone, and extending, as I observed whilst sailing 
along the coast, for 150 miles southward: at Valparaiso, we shall 
presently see that a similar red earthy mass, though quite like 
terrestrial mould, is really in cliief part of recent marine origin. On the 
flanks of this island of Quiriquina, at a less height than the 400 
feet, there were spaces several feet square, thickly strewed with frag- 
ments of similar shells. During a subsequent visit of the Beagle to 
Concepcion, Mr. Kent, the assistant-surgeon, was so kind as to make 
for me some measurements with the barometer : he found many marine 
remains along the shores of the whole bay, at a height of about twenty 
feet ; and from the hill of Sentinella behind Talcahuano, at the height 
of 160 feet, he collected numerous shells, packed together close 
beneath the surface in black earth, consisting of two species of 
Mytilus, two of Crepidula, one of Concholepas, of Fissurella, Venus, 
Mactra, Turbo, Monoceros, and the Balafius psittactis. These shells 
were bleached, and within some of the Balani other Balani were 
growing, showing that they must have long lain dead in the sea. The 
above species I compared with living ones from the bay, and found 
them identical ; but having since lost the specimens, I cannot give their 
names : this is of little importance, as Mr. Broderip has examined a 
similar collection, made during Captain Beechey's expedition, and 
ascertained that they consisted of ten recent species, associated with 
fragments of Echini, crabs, and Flustrae ; some of these remains were 
estimated by Lieutenant Belcher to lie at the height of nearly a thousand 
feet above the level of the sea.* In some places round the bay, Mr. 
Kent observed that there were beds formed exclusively of the Mytilus 
Chiloensis : this species now Hves in parts never uncovered by the tides. 
At considerable heights, Mr. Kent found only a few shells ; but from 
the summit of one hill, 625 feet high, he brought me specimens of the 
Concholepas, Mytilus Chiloensis, and a Turbo. These shells were 
softer and more brittle than those from the height of 164 feet; and these 
latter had obviously a much more ancient appearance than the same 
species from the height of only twenty feet. 

Coast north of Concepcion. — The first point examined was at the 
mouth of the Rapel (160 miles N. of Concepcion and sixty miles S. of 
Valparaiso), where I observed a few shells at the height of 100 feet, 
and some barnacles adhering to the rocks three or four feet above 
the highest tides : M. Gay f found here recent shells at the distance 

* "Zoology of Captain Beechey's Voyage," p. 162, 
•}• " Annales des Scienc. Nat.," Avril 1S33. 



ELEVATION OF VALPARAISO. 307 

of two leagues from the shore. Inland there are some wide, gravel- 
capped plains, intersected by many broad, flat-bottomed valleys (now 
carrying insignificant streamlets), with their sides cut into successive 
wall-like escarpments, rising one above another, and in many places, 
according to M. Gay, worn into caves. The one cave (C. del Obispo) 
which I examined, resembled those formed on many sea-coasts, with 
its bottom filled with shingle. These inland plains, instead of 
sloping towards the coast, are inclined in an opposite direction 
towards the Cordillera, like the successively rising terraces on the 
inland or eastern side of Chiloe : some points of granite, which project 
through the plains near the coast, no doubt once formed a chain of 
outlying islands, on the inland shores of which the plains were accumu- 
lated. At Bucalemu, a few miles northward of the Rapel, I observed 
at the foot, and on the summit-edge of a plain, ten miles from the coast, 
many recent shells, mostly comminuted, but some perfect. There were, 
also, many at the bottom of the great valley of the Maypu. At San 
Antonio, shells are said to be collected and burnt for hme. At the 
bottom of a great ravine (Quebrada Onda, on the road to Casa Blanca), 
at the distance of several miles from the coast, I noticed a considerable 
bed, composed exclusively of Mesodcsma donaciforme, Desh., lying on a 
bed of muddy sand : this shell now lives associated together in great 
numbers, on tidal-flats on the coast of Chile. 

Valparaiso, 

During two successive years I carefully examined, part of the time 
in company with Mr. Alison, into all the facts connected with the recent 
elevation of this neighbourhood. In very many parts a beach of broken 
shells, about fourteen or fifteen feet above high-water mark, may be ob- 
served ; and at this level the coast-rocks, where precipitous, are corroded 
in a band. At one spot, Mr. Alison, by removing some birds' dung, 
found at this same level barnacles adhering to the rocks. For several 
miles southward of the bay, almost every flat Httle headland, between 
the heights of 60 and 230 feet (measured by the barometer), is smoothly 
coated by a thick mass of comminuted shells, of the same species, and 
apparently in the same proportional numbers with those existing in the 
adjoining sea. The Concholepas is much the most abundant, and the 
best preserved shell ; but I extracted perfectly preserved specimens of 
the Fissurella biradiata, a Trochus and Balanus (both well known, but 
according to Mr. Sowerby yet unnamed) and parts of the MyUliis 
Chiloeiisis. Most of these shells, as well as an encrusting Nullipora, 
partially retain their colour ; but they are brittle, and often stained red 
from the underlying brecciated mass of primary rocks ; some are packed 
together, either in black or reddish moulds; some lie loose on the 
bare rocky surfaces. The total number of these shells is immense ; 
they are less numerous, though still far from rare, up a height of 
1,000 feet above the sea. On the summit of a hill, measured 557 feet, 
there was a small horizontal band of comminuted shells, of which many 
consisted (and likewise from lesser heights) of very young and small 



3o8 ELEVATION OF VALPARAISO. 

specimens of the still living Concholepas, Trochiis, Patelhe, Crcpiduloe, 
and of Mytilus Magellaniais (?) : * several of these shells were under 
a quarter of an inch in their greatest diameter. My attention was called 
to this circumstance by a native fisherman, whom I took to look at these 
shell-beds ; and he ridiculed the notion of such small shells having been 
brought up for food ; nor could some of the species have adhered when 
alive to other larger shells. On another hill, some miles distant, and 
648 feet high, I found shells of the Concholepas and Trochus, perfect, 
though very old, with fragments of Mytilus Chilocitsis, all embedded 
in reddish-brown mould : I also found these same species, with frag- 
ments of an Echinus and of Balanus psittacus, on a hill 1,000 feet high. 
Above this height, shells became very rare, though on a hill 1,300 
feet high,t I collected the Concholepas, Trochus, Fissurella, and a 
Patella. At these greater heights the shells are almost invariably 
embedded in mould, and sometimes are exposed only by tearing up 
bushes. These shells obviously had a very much more ancient appear- 
ance than those from the lesser heights ; the apices of the Trochi were 
often worn down ; the little holes made by burrowing animals were 
greatly enlarged ; and the Concholepas was often perforated quite 
through, owing to the inner plates of shell having scaled off. 

Many of these shells, as I have said, were packed in, and were quite 
filled with, blackish or reddish-brown earth, resting on the granitic 
detritus. I did not doubt until lately that this mould was of purely 
terrestrial origin, when with a microscope examining some of it from 
the inside of a Concholepas from the height of about one hundred feet, 
I found that it was in considerable part composed of minute fragments 
of the|spines, mouth-bones, and shells of Ecliinr, and of minute fragments, 
of chiefly very young Patellae, Mytili, and other species. I found similar 
microscopical fragments in earth filling up the central orifices of some 
large Fissurellse. This earth when crushed emits a sickly smell, pre- 
cisely like that from garden-mould mixed with guano. The earth 
accidentally preserved within the shells, from the greater heights, has 
the same general appearance, but it is a little redder ; it emits the same 
smell when rubbed, but I was unable to detect with certainty any 
marine remains in it. This earth resembles in general appearance, as 
before remarked, that capping the rocks of Quiriquina in the Bay of 
Concepcion, on which beds of sea-shells lay. I have, also, shown that 
the black, peaty soil, in which the shells at the height of 350 feet at 
Chiloe were packed, contained many minute fragments of marine animals. 
These facts appear to me interesting, as they show that soils, which 
would naturally be considered of purely terrestrial nature, may owe 
their origin in chief part to the sea. 

Being well aware from what I have seen at Chiloe and in Tierra del 

* Mr. Cuming informs me that he does not think this species identical 
with, though closely resembling, the true M. Magellanicus of the southern 
and eastern coast of South America; it lives abundantly on the coast of 
Chile. 

f Measured by the baroirjeter: the highest point in the range bchin(J 
Valparaiso I found to be 1,626 feet above the level of the sea. 



ELEVATION OF VALPARAISO. 309 

Fuego, that vast quantities of shells are carried, during successive ages, 
far inland, vv^here the inhabitants chiefly subsist on these productions, 
I am bound to state that at greater heights than 557 feet, where the 
number of very young and small shells proved that they had not been 
carried up for food, the only evidence of the shells having been naturally 
left by the sea, consists in their invariable and uniforrri appearance 
of extreme antiquity — in the distance of some of the places from the 
coast, in others being inaccessible from the nearest part of the beach, 
and in the absence of fresh water for men to drink — in the shells not 
lying in heaps, — and, lastly, in the close similarity of the soil in which 
they are embedded, to that which lower down can be unequivocally 
shown to be in great part formed from the debris of the sea animals.* 

With respect to the position in which the shells lie, I was repeatedly 
struck here, at Concepcion, and at other places, with the frequency of 
their occurrence on the summits and edges either of separate hills, or 
of little flat headlands often terminating precipitously over the sea. 
The several above-enumerated species of mollusca, which are found 
strewed on the surface of the land from a few feet above the level of 
the sea up to the height of 1,300 feet, all now live either on the beach, 
or at only a few fathoms' depth : Mr. Edmondston, in a letter to Professor 
E. Forbes, states that in dredging in the Bay of Valparaiso, he found 
the common species of Concholepas, Fissurella, Trochus, Monoceros, 
Chitons, etc., living in abundance from the beach to a depth of seven 
fathoms ; and dead shells occurred only a few fathoms deeper. The 
common Tu7'ritella cingulata was dredged up living at even from ten 
to fifteen fathoms ; but this is a species which I did not find here 
amongst the upraised shells. Considering this fact of the species being 
all littoral or sub-littoral, considering their occurrence at various heights, 
their vast numbers, and their generally comminuted state, there can be 
little doubt that they were left on successive beach-lines during a 
gradual elevation of the land. The presence, however, of so many 
whole and perfectly preserved shells appears at first a difficulty on this 
view, considering that the coast is exposed to the full force of an open 
ocean : but we may suppose, either that these shells were thrown 
during gales on flat ledges of rock just above the level of high-water 
mark, and that during the elevation of the land they are never again 
touched by the waves, or, that during earthquakes, such as those of 

* In the "Proceedings of the Geolog. Soc," vol. ii., p. 446, I have given 
a brief account of the upraised shells on the coast of Chile, and have there 
stated that the proofs of elevation arc not satisfactory above the height of 
230 feet. I had at that time unfortunately overlooked a separate page 
written during my second visit to Valparaiso, describing the shells now in 
my possession from the 557 feet hill ; I had not then unpacked my collec- 
tions, and had not reconsidered the obvious appearance of greater antiquity 
of the shells from the greater heights, nor had I at that time discovered the 
marine origin of the earth in which many of the shells are packed. Con- 
sidering these facts, I do not now feel a shadow of doubt that the shells, at 
the height of 1,300 feet, have been upraised by natural causes into their 
present position. 



3IO ELEVATION OF VALPARAISO. 

1822, 1835, and 1837, rocky reefs covered with marine-animals were 
at one blow uplifted above the future reach of the sea. This latter 
explanation is, perhaps, the most probable one with respect to the beds 
at Concepcion entirely composed of the Mytilus Chiloensis, a species 
which lives below the lowest tides ; and likewise with respect to the 
great beds occurring both north and south of Valparaiso, of the Meso- 
desma donaciforme, — a shell which, as I am informed by Mr. Cuming, 
inhabits sandbanks at the level of the lowest tides. But even in the 
case of shells having the habits of this Mytilus and Mesodesma, beds 
of them, wherever the sea gently throws up sand or mud, and thus 
protects its own accumulations, might be upraised by the slowest 
movement, and yet remain undisturbed by the waves of each new 
beach-line. 

It is worthy of remark, that nowhere near Valparaiso above the 
height of twenty feet, or rarely of fifty feet, I saw any lines of erosion 
on the solid rocks, or any beds of pebbles ; this, I believe, may be 
accounted for by the disintegrating tendency of most of the rocks in 
this neighbourhood. Nor is the land here modelled into terraces : Mr. 
Alison, however, informs me, that on both sides of one narrow ravine, 
at the height of 300 feet above the sea, he found a succession of rather 
indistinct step-formed beaches, composed of broken shells, which 
together covered a space of about eighty feet vertical. 

I can add nothing to the accounts already published of the elevation 
of the land at Valparaiso,* which accompanied the earthquake of 1822: 
but I heard it confidently asserted, that a sentinel on duty, immediately 
after the shock, saw a part of a fort, which previously was not within 
the line of his vision, and this would indicate that the uplifting was not 
horizontal: it would even appear from some facts collected by Mr. 
Alison, that only the eastern half of the bay was then elevated. Through 
the kindness of this same gentleman, I am able to give an interesting 
account of the changes of level, which have supervened here within 
historical periods : about the year 1680 a long sea-wall (or Prefil) was 
built, of which only a few fragments now remain ; up to the year 18 17, 
the sea often broke over it, and washed the houses on the opposite side 
of the road (where the prison now stands); and even in 1819, Mr. J. 
Martin remembers walking at the foot of this wall, and being often 
obliged to climb over it to escape the waves. There now stands (1834) 
on the seaward side of this wall, and between it and the beach, in one 
part a single row of houses, and in another part two rows with a street 
between them. This great extension of the beach in so short a time 
cannot be attributed simply to the accumulation of detritus ; for a 
resident engineer measured for me the height between the lowest part 
of the wall visible, and the present beach-line at spring-tides, and the 
difference was eleven feet six inches. The church of S. Augustin is 
believed to have been built in 1614, and there is a tradition that the sea 
formerly flowed very near it ; by levelling, its foundations were found 

* Dr. Meyen ("Reise um Erde,"Th. I., s. 221) found in 1831 seaweed and 
other bodies still adhering to some rocks which during the shock of 1822 
were lifted above the sea. 



ELEVATION OF COQUIMBO. 311 

to stand nineteen feet six inches above the highest beach-line ; so that 
we see in a period of 220 years, the elevation cannot have been as 
much as nineteen feet six inches. From the facts given with respect 
to the sea-wall, and from the testimony of the elder inhabitants, it 
appears certain that the change in level began to be manifest about the 
year 18 17. The only sudden elevation of which there is any record 
occurred in 1822, and this seems to have been less than three feet. 
Since that year, I was assured by several competent observers, that 
part of an old wreck, which is firmly embedded near the beach, has 
sensibly emerged ; hence here, as at Chiloe, a slow rise of the land 
appears to be now in progress. It seems highly probable that the 
rocks which are corroded in a band at the height of fourteen feet above 
the sea were acted on during the period, when by tradition the base of 
S. Augustin church, now nineteen feet six inches above the highest 
water-mark, was occasionally washed by the waves. 

Valparaiso to Coquimbo. — For the first seventy-five miles north of 
Valparaiso I followed the coast-road, and throughout this space I 
observed innumerable masses of upraised shells. About Quintero 
there are immense accumulations (worked for lime) of the Mesodesma 
donaciforme, packed in sandy earth ; they abound chiefly about fifteen 
feet above higli-water, but shells are here found, according to Mr. 
Miers,* to a height of 500 feet, and at a distance of three leagues 
from the coast: I here noticed barnacles adhering to the rocks 
three or four feet above the highest tides. In the neighbourhood of 
Plazilla and Catapilco, at heights of between two hundred and three 
hundred feet, the number of comminuted shells, with some perfect ones, 
especially of the Mesodesma, packed in layers, was truly immense : the 
land at Plazilla had evidently existed as a bay, with abrupt rocky 
masses rising out of it, precisely like the islets in , the broken bays now 
indenting this coast. On both sides of the rivers Ligua, Longotomo, 
Guachen, and Quilimari, there are plains of gravel about two hundred 
feet in height, in many parts absolutely covered with shells. Close to 
Conchalee, a gravel-plain is fronted by a lower and similar plain about 
sixty feet in height, and this again is separated from the beach by a 
wide tract of low land : the surfaces of all three plains or terraces were 
strewed with vast numbers of the Concholepas, Mesodesma, an existing 
Venus, and other still existing littoral shells. The two upper terraces 
closely resemble in miniature the plains of Patagonia ; and like them 
are furrowed by dry, flat-bottomed, winding valleys. Northward of this 
place I turned inward ; and therefore found no more shells : but the 
valleys of Chuapa, Illapcl, and Limari, are bounded by gravel-capped 
plains, often including a lower terrace within. These plains send bay- 
like arms between and into the surrounding hills ; and they are con- 
tinuously united with other extensive gravel-capped plains, separating 
the coast mountain-ranges from the Cordillera. 

* "Travels in Chile," vol. i., pp. 395, 458. I received .several similar 
accounts from the inhabitants, and was assured that there are many shells 
on the plain of Casa Blanca, between Valparaiso and Santiago, at the height 
of 800 feet. 



312 ELEVATION OF COQUIMBO. 

I 
Cogui?fibo, 

A narrow fringe-like plain, gently inclined towards the Sea, here 
extends for eleven miles along the coast, with arms stretching np 
between the coast-mountains, and likewise up the valley of Coquimbo : 
at its southern extremity it is directly connected with the plain of 
Limari, out of which hills abruptly rise like islets, and other hills pro- 
ject like headlands on a coast. The surface of the fringe-like plain 
appears level, but differs insensibly in height, and greatly in composi- 
tion, in different parts. 

At the mouth of the valley of Coquimbo, the surface consists wholly 
of gravel, and stands from 300 to 350 feet above the level of the sea, 
being about one hundred feet higher than in other parts. In these other 
and lower parts the superficial beds consist of calcareous matter, and rest 
on ancient tertiary deposits hereafter to be described. The uppermost 
calcareous layer is cream-coloured, compact, smooth-fractured, sub- 
stalactiform, and contains some sand, earthy matter, and recent shells. 
It lies on, and sends wedge-like veins into,* a much more friable, 
calcareous, tuff-like variety ; and both rest on a mass about twenty feet 
in thickness, formed of fragments of recent shells, with a few whole 
ones, and with small pebbles firmly cemented together. This latter 
rock is called by the inhabitants losa, and is used for building : in many 
parts it is divided into strata, which dip at an angle of ten degrees sea- 
ward, and appear as if they had originally been heaped in successive 
layers (as may be seen on coral-reefs) on a steep beach. This stone is 
remarkable from being in parts entirely formed of empty, pellucid 
capsules or cells of calcareous matter, of the size of small seeds : a 
series of specimens unequivocally showed that all these capsules once 
contained minute rounded fragments of shells which have since been 
gradually dissolved by water percolating through the mass.t 

The shells embedded in the calcareous beds forming the surface of 
this fringe-like plain, at the height of from 200 to 250 feet above the 
sea. consist of — 



1. Venus opaca. 

2. Mulinia Byronensis. 

3. Pecten purpuratus. 

4. Mesodesma donaciforme. 

5. Turritella cingulata. 



6. Monoceros costatum. 

7. Concholepas Peruviana. 

8. Trochus (common Valpa- 

raiso species). 

9. Calyptraea Byronensis. 



Although these species are all recent, and are all found in the neigh- 
bouring sea, yet I was particularly struck with the difference in the 

* In many respects this upper hard, and the underlying more friable, 
varieties, resemble the great superficial beds at King George's Sound in 
Australia, which I have described in my " Geological Observations on 
Volcanic Islands." There could be little doubt that the upper layers there 
have been hardened by the action of rain on the friable, calcareous matter, 
and that the whole mass has originated in the decay of minutely comminuted 
sea-shells and corals. 

f I have incidentally described this rock in the above work on Volcanic 
Islands. 



ELEVATION OF COQUIMBO, 313 

proportional numbers of the several species, and of those now cast up 
on the present beach. I found only one specimen of the Concholepas, 
and the Pecten was very rare, though both these shells are now the 
commonest kinds, with the exception, perhaps, of the Calyptrcea 
7'adians, of which I did not find one in the calcareous beds. I wiil not 
pretend to determine how far this difference in the proportional numbers 
depends on the age of the deposit, and how far on the difference in 
nature between the present sandy beaches and the calcareous bottom, 
on which the embedded shells must have lived. 

On the bare surface of the calcareous plain, or in a thin covering oi 
sand, there were lying, at a height from 200 to 252 feet, many recent 
shells, which had a much fresher appearance than the embedded ones : 
fragments of the Concholepas, and of the common Mytilus, still 
retaining a tinge of its colour, were numerous, and altogether there was 
manifestly a closer approach in proportional numbers to those now 
lying on the beach. In a mass of stratified, slightly agglutinated sand, 

No. 8. 

SECTION OF PLAIN "OF COQUIMBO. 

Surface of plain 252 feet above sea. 

B D 






C 
D 
E 

F _ _ _^____ _____ 

Level of ^ea. 

A. Stratified sand, with recent shells in same proportions as on the beach, half filling 
up a ravine. 

B. Surface of plain, with scattered shells in nearly same proportions as on the beach. 

C. Upper calcareous bed, \ with recent shells, but not in same propor- 
1), Lower calcareous sandy bed (Losa), / tions as on the beach. 

E. Upper ferrugino-sandy old tertiary stratum, \ with all, or nearly all, extinct 

F. Lower old tertiary stratum, / shells. 

which in some places covers up the lower half of the seaward escarp- 
ment of the plain, the included shells appeared to be in exactly the 
same proportional numbers with those on the beach. On one side of a 
steep-sided ravine, cutting through the plain behind Herradura Bay, I 
observed a narrow strip of stratified sand, containing similar shells in 
similar proportional numbers ; a section of the ravine is represented in 
the above diagram, which serves also to show the general composi- 
tion of the plain. I mention this case of the ravine chiefly because 
without the evidence of the marine shells in the sand, any one would 
have supposed that it had been hollowed out by simple alluvial action. 
The escarpment of the fringe-like plain, which stretches for eleven 
miles along the coast, is in some parts fronted by two or three narrow, 
step-formed terraces, one of which at Herradura Bay expands into a 
small plain. Its surface was there formed of gravel, cemented together 
by calcareous matter ; and out of it I extracted the following recent 



314 



ELEVATION OF COQUIMBO. 



shells, which are in a more perfect condition than those from the upper 
plain: — 



Calyptraea radians. 

Turritella cingulata. 

Oliva Peruviana. 

Murex labiosus, var. 

Nassa (identical with 
species). 
6. Solen Dombeiana. 
7 Pecten purpuratus, 
8. Venus Chilensis. 



a 1 



iving 



9. Amphidesma rugulosum. The 

small irregular wrinkles of the 
posterior part of this shell are 
rather stronger than in the recent 
specimens of this species from 
Coquimbo. (G. B. Sowerby.) 

10. Balanus (identical with living 

species). 



On the syenitic ridge, which forms the southern boundary of Herra- 
dura Bay and Plain, I found the Concholepas and Tu7'ntella cingulata 
(mostly in fragments), at the height of 242 feet above the sea. I 
could not have told that these shells had not formerly been brought 
up by man, if I had not found one very small mass of them cemented 
together in a friable calcareous tuff. I mention this fact more par- 
ticularly, because I carefully looked, in many apparently favourable 
spots, at lesser heights on the side of this ridge, and could not find 
even the smallest fragment of a shell. This is only one instance out 
of many, proving that the absence of sea-shells on the surface, though 
in many respects inexplicable, is an argument of very little weight in 
opposition to other evidence on the recent elevation of the land. The 
highest point in this neighbourhood at which I found upraised shells 
of existing species was on an inland calcareous plain, at the height of 
252 feet above the sea. 

It would appear from Mr. Caldcleugh's researches,* that a rise has 
taken place here within the last century and a half; and as no sudden 
change of level has been observed during the not very severe earth- 
quakes, which have occasionally occurred here, the rising has probably 
been slow, like that now, or quite lately, in progress at Chiloe and at 
Valparaiso : there are three well-known rocks, called the Pelicans, 
which in 17 10, according to Feuill6e, were a fieur d'eau, but now are 
said to stand twelve feet above low-water mark : the spring-tides rise 
here only five feet. There is another rock, now nine feet above high- 
water mark, which in the time of Frezier and of Feuillee rose only five 
or six feet out of water. Mr. Caldcleugh, I may add, also shows (and 
I received similar accounts) that there has been a considerable decrease 
in the soundings during the last twelve years in the Bays of Coquimbo, 
Concepcion, Valparaiso, and Guasco ; but as in these cases it is nearly 
impossible to distinguish between the accumulation of sediment and 
the upheavement of the bottom, I have not entered into any details. 

Valley of Coquimbo. — The narrow coast-plain sends, as before stated, 
an arm, or more correctly a fringe, on both sides, but chiefly on the 
southern side, several miles up the valley. These fringes are worn 
into steps or terraces, which present a most remarkable appearance, 
and have been compared (though not very correctly) by Capt. Basil 
* " Proceedings of the Geological Society," vol. ii., p. 446. 



GRAVEL-TERRACES OF COQUIMBO. 



31s 



Hall, to the parallel roads of Glen Roy in Scotland : their origin has 
been ably discussed by Mr. Lyell.* The first section which I will give, 
is not drawn across the valley, but in an east and west line at its 
mouth, where the step-formed terraces debouch and present their very 
gently inclined surfaces towards the Pacific. 

The bottom plain (A) is about a mile in width, and rises quite 
insensibly from the beach to a height of twenty-five feet at the foot 
of the next plain ; it is sandy, and abundantly strewed with shells. 

Plain or terrace (B) is of small extent, and is almost concealed by 
the houses of the town, as is likewise the escarpment of terrace (C). 
[On both sides of a ravine, two miles south of the town, there are two 
little terraces, one above the other, evidently corresponding with (B) 
md (C) ; and on them marine remains of the species already enume- 
rated were plentiful. Terrace (E) is very narrow, but quite distinct 
md level ; a little southward of the town there were traces of a terrace 
[D) intermediate between (E) and (C). Terrace (F) is part of the 

No. 9. 

EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO, WHERE 
THEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA. 

Mi feet. (B.) 
— — V 302. 



• (C.) 

120 feet. (B/* 

70 fee*. 



(A.) 

JO feet 



Level of sea. Town of Coquiinbo. 

Vertical scale -^ of inch to 100 feet : horizontal scale much contracted. 

fringe-like plain, which stretches for the eleven miles along the coast ; 
it is here composed of shingle, and is 100 feet higher than where 
composed of calcareous matter. This greater height is obviously due 
to the quantity of shingle, which at some former period has been 
brought down the great valley of Coquimbo. 

Considering the many shells strewed over the terraces (A) (B) and 
(C), and a few miles southward on the calcareous plain, which is 
continuously united with the upper step-like plain (F), there cannot, 
I apprehend, be any doubt, that these six terraces have been formed 
by the action of the sea ; and that their five escarpments mark so 
many periods of comparative rest in the elevatory movement, during 
which the sea wore into the land. The elevation between these 
periods may have been sudden and on an average not more than 
seventy-two feet each time, or it may have been gradual and insensibly 
slow. From the shells on the three lower terraces, and on the upper 
one, and I may add on the three gravel-capped terraces at Conchalee, 
being all littoral and sub-littoral species, and from the analogical facts 
given at Valparaiso, and lastly from the evidence of a slow rising lately 
or still in progess here, it appears to me far more probable that the 
* "Principles of Geology " (ist edit.), vol. iii., p. 131. 



3i6 



GRAVEL-TERRACES OF COQUIMBO 



movement has been slow. The existence of these successive escarp- 
ments, or old cliff-lines, is in another respect highly instructive, for they 
show periods of comparative rest in the elevatory movement, and of 
denudation, which would never even have been suspected from a close 
examination of many miles of coast southward of Coquimbo. 

We come now to the terraces on the opposite sides of the east and 
west valley of Coquimbo : the following section is taken in a north and 
south line across the valley at a point about three miles from the sea. 
The valley measured from the edges of the escarpments of the upper 
plain (F) (F) is about a mile in width ; but from the bases of the 
bounding mountains it is from three to four miles wide. The terraces 
marked with an interrogative do not exist on that side of the valley^ 
but are introduced merely to render the diagram more intelligible. 

No. lo. 

NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO. 
Korth. South. 

F * B? E F ; 

D C B A B? G D? 

i 



Level of eaa. 
Vertical scale -j^^ of inch to lOo feet: horizontal scale much contracted: terraces 
marked with ? do not occur on that side of the valley, and are introduced only to 
make the diagram more intelligible. A river and bottom-plain of valley C, E, and F, 
on the south side of valley, are respectively, 197, 377, and 420 feet above the level of 
the sea. 

A A. The bottom of the valley, believed to be 100 feet above the sea : it is continuously 
united with the lowest plain (A) of the former section. 

B. This terrace higher up the valley expands considerably ; seaward it is soon lost, its 
escarpment being united with that of (C) : it is not developed at all on the south 
side of the valley. 

C. This terrace, like the last, is considerably expanded higher up the valley. These 
two terraces apparently correspond with (B) and (C) of the former section. 

D is not well developed in the line of this section; but seaward it expands into a 
plain : it is not present on the south side of the valley ; but it is met with, as stated 
under the former section, a little south of the town. 

E is well developed on the south side, but absent on the north side of the valley : 
though not continuously united with (E) of the former section, it apparently cor- 
responds with it. 

F. This is the surface-plain, and is continuously united with that which stretches like 
a fringe along the coast. In ascending the valley it gradually becomes narrower, 
and is at last, at the distance of about ten miles from the sea, reduced to a row of 
flat-topped patches on the sides of the mountains. None of the lower terraces 
extend so far up the valley. 

These five terraces are formed ot shingle and sand ; three of them, 
as marked by Captain B. Hall (namely, B, C, and F), are much more 
conspicuous than the others. From the marine remains copiously 
strewed at the mouth of the valley on the lower terraces, and south- 
ward of the town on the upper one, they are, as before remarked, 
undoubtedly of marine origin ; but within the valley, and this fact well 
deserves notice, at a distance of from only a mile and a half to three or 
four miles from the sea, I could not find even a fragment of a shell. 



NOT HORIZONTAL. 317 

On the inclinaHoii of the terraces of Coquinibo^ and 07t the upper 
and basal edges of their escarpments not bemg ho?i2o?ital. — The surfaces 
of these terraces slope in a shght degree, as shown by the two last 
sections taken conjointly, both towards the centre of the valley, and 
seawards towards its mouth. This double or diagonal inclination, 
which is not the same in the several terraces, is, as we shall immediately 
see, of simple explanation. There are, however, some other points 
which at first appear by no means obvious, — namely, first, that each 
terrace, taken in its whole breadth from the summit-edge of one escarp- 
ment to the base of that above it, and followed up the valley, is not 
horizontal ; nor have the several terraces, when followed up the valley, 
all the same inclination ; thus I found the terraces C, E, and F, 
measured at a point about two miles from the mouth of the valley, 
stood severally between fifty-six to seventy-seven feet higher than at 
the mouth. Again, if we look to any one line of cliff or escarpment, 
neither its summJt-edge nor its base is horizontal. On the theory of 
the terraces having been formed during a slow and equable rise of the 
land, with as many intervals of rest as there are escarpments, it appears 
at first very surprising that horizontal lines of some kind should not have 
been left on the land. 

The direction of the diagonal inclination in the different terraces 
being different, — in some being directed more towards the middle of the 
valley, in others more towards its mouth, — naturally follows on the view 
of each terrace, being an accumulation of successive beach-lines round 
bays, which must have been of different forms and sizes when the land 
stood at different levels : for if we look to the actual beach of a narrow 
creek, its slope is directed towards the middle ; whereas, in an open bay, 
or slight concavity on a coast, the slope is towards the mouth, that is, 
almost directly seaward ; hence as a bay alters in form and size, so will 
the direction of the inclination of its successive beaches become 
changed. 

If it were possible to trace any one of the many beach-lines, com- 
posing each sloping terrace, it would of course be horizontal ; but the 
only lines of demarcation are the summit and basal edges of the 
escarpments. Now the summit-edge of one of these escarpments marks 
the furthest line or point to which the sea has cut into a mass of gravel 
sloping seaward ; and as the sea will generally have greater power at 
the mouth than at the protected head of the bay, so will the escarpment 
at the mouth be cut deeper into the land, and its summit-edge be 
higher; consequently it will not be horizontal. With respect to the 
basal or lower edges of the escarpments, from picturing in one's mind 
ancient bays entirely surrounded at successive periods by cliff-formed 
shores, one's first impression is that they at least necessarily must be 
horizontal, if the elevation has been horizontal. But here is a fallacy : 
for after the sea has, during a cessation of the elevation, worn cliff's all 
round the siiores of a bay, when the movement recommences, and 
especially if it recommences slowly, it might well happen that, at 
the exposed mouth of the bay, the waves might continue for some 
time wearing into the land, whilst in the protected and upper parts 



318 



GRAVEL-TERRACES OF COQUIMBO 



No. ir. 



successive beach-lines might be accumulating in a sloping surface or 
terrace at the foot of the cliffs which had been lately reached : hence, 
supposing the whole line of escarpment to be finally uplifted above 
the reach of the sea, its basal line or foot near the mouth will run at a 
lower level than in the upper and protected parts of the bay ; conse- 
quently this basal line will not be horizontal. And it has already 
been shown that the summit-edges of each escarpment will generally 
be higher near the mouth (from the seaward sloping land being 
there most exposed and cut into) than near the head of the bay; 
therefore the total height of the escarpments will be greatest near the 
mouth ; and further up the old bay or valley they will on both sides 
generally thin out and die away : I have observed this thinning out of 
the successive escarpment at other places besides Coquimbo ; and for 
a long time I was quite unable to understand its meaning. The follow- 
ing rude diagram will perhaps render what I mean more intelligible ; it 

represents a bay in a 
district which has begun 
slowly rising. Before the 
movement commenced, it 
is supposed that the waves 
had been enabled to eat 
into the land and form 
cliffs, as far up, but with 
gradually diminishing 
power, as the points A 
A : after the movement 
had commenced and gone 
on for a little time, the 
sea is supposed still to 
have retained the power, 
at the exposed mouth of 
the bay, of cutting down 
and into the land as it 
slowly emerged; but in 
the upper parts of the 
bay it is supposed soon 
to have lost this power, owing to the more protected situation and 
to the quantity of detritus brought down by the river; consequently 
low land was there accumulated As this low land was formed during 
a slow elevatory movement, its surface will gently slope upwards from 
the beach on all sides. Now, let us imagine the bay, not to make 
the diagram more complicated, suddenly converted into a valley : the 
basal line of the cliffs will of course be horizontal, as fair as the 
beach is now seen extending in the diagram ; but in the upper part 
of the valley, this line will be higher, the level of the district having 
been raised whilst the low land was accumulating at the foot of the 
inland cliffs. If, instead of the bay in the diagram being suddenly 
converted into a valley, we suppose with much more probability it to 
be upraised slowly, then the waves in the upper parts of the bay will 




NOT HORIZONTAL. 319 

continue very gradually to fail to reach the cliffs, which are now in the 
diagram represented as washed by the sea, and which, consequently, 
will be left standing higher and higher above its level ; whilst at the 
still exposed mouth, it might well happen that the waves might be 
enabled to cut deeper and deeper, both down and into the cliffs, as 
the land slowly rose. 

The greater or lesser destroying power of the waves at the mouths 
ot successive bays, comparatively with this same power in their upper 
and protected parts, will vary as the bays become changed in form and 
size, and therefore at different levels, at their mouths and heads, more 
or less of the surfaces between the escarpments (that is, the accumu- 
lated beach-lines or terraces) will be left undestroyed : from what has 
gone before we can see that,, according as the elevatory movements 
after each cessation recommence more or less slowly, according to the 
amount of detritus delivered by the river at the heads of the successive 
bays, and according to the degree of protection afforded by their altered 
forms, so will a greater or less extent of terrace be accumulated in the 
upper part, to which there will be no surface at a corresponding level 
at the mouth : hence we can perceive why no one terrace, taken in its 
whole breadth and followed up the valley, is horizontal, though each 
separate beach-line must have been so ; and why the inclination of the 
several terraces, both transversely, and longitudinally up the valley, is 
not alike. 

I have entered into this case in some detail, for I was long perplexed 
(and others have felt the same difficulty) in understanding how, on the 
idea of an equable elevation with the sea at intervals eating into the 
land, it came that neither the terraces nor the upper nor lower edges 
of the escarpments were horizontal. Along lines of coast, even of great 
lengths, such as that of Patagonia, if they are nearly uniformly exposed, 
the corroding power of the waves will be checked and conquered by the 
elevatory movement, as often as it recommences, at about the same 
period ; and hence the terraces, or accumulated beach-lines, will com- 
mence being formed at nearly the same levels : at each succeeding 
period of rest, they will, also, be eaten into at nearly the same rate, 
and consequently there will be a much closer coincidence in their levels 
and inclinations, than in the terraces and escarpments formed round 
bays with their different parts very differently exposed to the action of 
the sea. It is only where the waves are enabled, after a long lapse of 
time, slowly to corrode hard rocks, or to throw up, owing to the supply 
of sediment being small and to the surface being steeply inclined, a 
narrow beach or mound, that we can expect, as at Glen Roy in 
Scotland,* a distinct line marking an old sea-level, and which will be 
strictly horizontal, if the subsequent elevatory movements have been 
so : for in these cases no discernible effects will be produced, except 
during the long intervening periods of rest ; whereas in the case of 
step-iormed coasts, such as those described in this and the preceding 
chapter, the terraces themselves are accumulated during the slow 
elevatory process, the accumulation commencing sooner in protected 

* "Philosophical Transactions," 1839, p. 39. 



320 GRAVEL TERRACES OF GUASCO. 

than in exposed situations, and sooner where there is copious supply 
of detritus than where there is little ; on the other hand, the steps or 
escarpments are formed during the stationary periods, and are more 
deeply cut down and into the coast-land in exposed than in protected 
situations ; — the cutting action, moreover, being prolonged in the most 
exposed parts, both during the beginning and ending, if slow, of the 
upward movement. 

Although in the foregoing discussion I have assumed the elevation 
to have been horizontal, it may be suspected, from the considerable 
seaward slope of the terraces, both up the valley of S. Cruz and up 
that of Coquimbo, that the rising has been greater inland than nearer 
the coast. There is reason to believe,* from the effects produced on 
the water-course of a mill during the earthquake of 1822 in Chile, that 
the upheaval one mile inland was nearly double, namely, between five 
and seven feet, to what it was on the Pacific. We know, also, from the 
admirable researches of M. Bravais.t that in Scandinavia the ancient 
sea-beaches gently slope from the interior mountain-ranges towards the 
coast, and that they are not parallel one to the other, showing that the 
proportional difference in the amount of elevation on the coast and in 
the interior, varied at different periods. 

Coquimbo to Gimsco. — In this distance of ninety miles, I found in 
almost every part marine shells up to a height of apparently from two 
hundred to three hundred feet. The desert plain near Choros is thus 
covered ; it is bounded by the escarpment of a higher plain, consisting 
of pale-coloured, earthy, calcareous stone, like that of Coquimbo, with 
the same recent shells embedded in it. In the valley of Chafieral, a 
similar bed occurs in which, differently from that of Coquimbo, I 
observed many shells of the Concholepas : near Guasco the same 
calcareous bed is likewise met with. 

In the valley of Guasco, the step-formed terraces of gravel are dis- 
placed in a more striking manner than at any other point. I followed 
the valley for thirty-seven miles (as reckoned by the inhabitants) from- 
the coast to Ballenar ; in nearly the whole of this distance, five grand 
terraces, running at corresponding heights on both sides of the broad 
valley, are more conspicuous than the three best-developed ones at 
Coquimbo. They give to the landscape the most singular and formal 
aspect ; and when the clouds hung low, hiding the neighbouring 
mountains, the valley resembled in the most striking manner that of 
Santa Cruz. The whole tliickness of these terraces or plains seems 
composed of gravel, rather firmly aggregated together, with occasional 
parting seams of clay : the pebbles on the upper plain are often white- 
washed with an aluminous substance, as in Patagonia. Near the 
coast I observed many sea-shells on the lower plains. At Freyrina 
(twelve miles up the valley), there are six terraces beside the bottom- 
surface of the valley: the two lower ones are here only from two 

* Mr. Place, in the Quarterly Journal of Science, 1824, vol. xvii., p. 42. 
f "Voyages de la Comm. du Noid," etc.; also, "Comptes Rendus," 
Oct. 1842. 



GRAVEL-TERRACES OF COPIAPO. 321 

hundred to three hundred yards m width, but higher up the valley they 
expand into plains ; the third terrace is generally narrow ; the fourth I 
saw only in one place, but there it was distinct for the length of a mile ; 
the fifth is very broad ; the sixth is the summit-plain, which expands 
inland into a great basin. Not having a barometer with me, I did not 
ascertain the height of these plains, but they appeared considerably 
higher than those at Coquimbo. Their width varies much, sometimes 
being very broad, and sometimes contracting into mere fringes of 
separate llat-topped projections, and then quite disappearing: at the 
one spot, where the fourth terrace was visible, the whole six terraces 
were cut off for a short space by one single bold escarpment. Near 
Ballenar (thirty-seven miles from the mouth of the river), the valley 
between the summit-edges of the highest escarpments is several miles 
in width, and the five terraces on both sides are broadly developed : the 
highest cannot be less than six hundred feet above the bed of the 
river, which itself must, I conceive, be some hundred feet above the 
sea. A north and south section across the valley in this part is here 
represented. 

No. 12, 

NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO, AND 

OF A PLAIN NORTH OF IT. 

North. South. 




Town of Eallenar, 



On the northern side of the valley the summit-plain of gravel (A) has 
two escarpments, one facing the valley, and the other a great basin-like 
plain (B), which stretches for several leagues northward. This narrow 
plain (A) with the double escarpment, evidently once formed a spit or 
promontory of gravel, projecting into and dividing two great bays, and 
subsequently was worn on both sides into steep cliffs. Whether 
the several escarpmicnts in this valley were formed during the 
same stationary periods with those of Coquimbo, I will not pretend 
to conjecture ; but if so the intervening and subsequent elevatory 
movements must have been here much more energetic, for these 
plains certainly stand at a much higher level than do those of 
Coquimbo. 

Copiapo. — From Guasco to Copiapo, I followed the road near the 
foot of the Cordillera, and therefore saw no upraised remains. At the 
mouth, however, of the valley of Copiapo there is a plain, estimated by 
Meyen * between fifty and seventy feet in heii^ht, of which the upper 
part consists chiefly of gravel, abounding with recent shells, chiefly of 
the Concholepas, Venus Doinbeyi, and Calyptrcca trochiformis. A little 

♦ *' Reise um die Erde," Th. I., s. 372, ei segf. 



322 GRAVEL-TERRACES OF COPIAPO. 

inland, on a plain estimated by myself at nearly three hundred feet, the 
upper stratum was formed of broken shells and sand cemented by white 
calcareous matter, and abounding with embedded recent shells, ol 
which the Mulinia Byronensis and Pecten pu7puraUcs were the most 
numerous. The lower plain stretches for some miles southward, and 
for an unknown distance northward, but not far up the valley; its 
seaward face, according to Meyen, is worn into caves above the level 
of the present beach. The valley of Copiapo is much less steeply 
inclined and less direct in its course than any other valley which I saw 
in Chile ; and its bottom does not generally consist of gravel : there are 
no step-formed terraces in it, except at one spot near the mouth of the 
great lateral valley