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CONTENTS. 



CHAPTER I. 

ST. J AGO, 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 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— Fcldspathic 
rocks 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 . . page I to 22 

CHAPTER II. 

Fernando Noronha — Precipitous hill of phonolite. — ^Terceira — 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 eleva- 
tion — Structure of its more ancient mountains ; similarity with St. Jago. — 
St. Paul's Rocks — Not of volcanic origin — their singular mineralogical 
composition ....... page 23 to 33 

CHAPTER III. 

ASCENSION. 

Basaltic lavas — Numerous craters truncated on the same side — Singular 
structure of volcanic bombs — ^Aeriform explosions — Ejected granitic frag- 
ments — ^Trachytic rocks — Singular veins — Jasper; its manner of formation — 
Concretions in pumiceous tuff — Calcareous deposits and frondescent in- 
crustations on the coast — Remarkable laminated beds, alternating with, 
and passing into obsidian — Origin of obsidian — Lamination of volcanic 
rocks ........ page 34 to 72 



VI CONTENTS. 



CHAPTER IV. 

8T. HELENA. 

Lavas of the feldspathic, basaltic, and submarine series — Section of Flagstaff 
Hill and of the Bam — 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 ....... page 73 to 96 

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; 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-strcams, 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 ....... page 97 to 116 

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 volcanos 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 . . . page 117 to 129 

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 — Tra- 
vertin with leaves of extinct plants — Elevation of the land. — New Zealand — 
King George's Sound — Superficial ferruginous beds — Superficial calcareous 



CONTENTS. VU 

deposit, with casts of branches ; its origin from drifted particles of shells 
and corals ; its extent. — Cape of Good Hope — Junction of the granite and 
clay-slate — Sandstone formation . . . . page 130 to 162 



APPENDIX. 

DESCRIPTION OF FOSSIL SHELLS, BY G. B. SOWERBY, ESQ., F.L.S. 

From a tertiary deposit at St. Jago, in the Cape de Verde group page 153 — 4 
Extinct land-shells from St. Helena .... page 155 to 158 
From the Palaeozoic formation of Van Diemen's Land . page 158 to 160 

DESCRIPTION OF FOSSIL CORALS FROM THE PALEOZOIC FORMATION 
OF VAN DIEMEN's LAND, BY W. LONSDALE, ESQ., F.G.S. 

page 161 to 169 

Ir^DEX .... ... page 171 to 175 







CHAPTER I. 



ST. JAGO, IN THE CAPE DE VERDE ARCH) 



Rocks of the lowest series. — A calcareous sedimeatary deposit, mth recent 
slieUa, altered by the contact of superincumbent lava, its horizonlalily 
and extent — Subsequent volcanic eruptions, associated mitk caleareows 
matter in an earthy and fibrous form, and often enclosed uiithin tlie 
separate cells of the scoria — Ancient and obliterated orifices of eruption 
of small siza — Difficulty of tracing over a bare plain recent streams of 
UoM — lidand kills of more ancient volcanic rock — Decomposed olivine in 
targe masses — Feldspathie rocks beneath the upper cryntaUine basaltic 
strata — Uniform structure and form of (In more ancient volcanic hills — 
Form of the valleys near the coast— Conglomerate now forming on the 

The island of St. Jago extends in a N.N.W. and S.S.E. 
direction, thirty mUea in length by about twelve 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, pre- 




ST. JAOO. [fSAP. It 

flciits a varii-d outline : smooth conical hills of a, reddish 
colour (like Red Hill in the accompanying vrood-cnt),* and 
others lesB regular, flat-topped, and of a blackish colour (like 
A, B, C) rise from successive, step-formed plains of la.T». 
At a distance, a chain of mountains, many thousand feet is 
height, traversea the interior of the island. There is do 
active volcano in St. Jago, and only one in the group, 
namely at Fogo. The island since being inhabited, lias not 
suffered from destructive earthquakes. 

The lowest rocks exposed on the coast near Porto Praya, 
are highly crystalhne and compact ; they appear to be 0/ 
ancient, submarine, volcanic origin ; they are uuconl'ormably 
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 suc- 
cessive streams from the interior of the island, between the 
square-topped hills marked A, B, C, &c. 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 intimately related in mineralog^cal 
composition, aud in other respects, with the lowest series of 
the coast-rocks, with which tliey seem to be continuous. 
Mineralogical description of the rocks of the lojcest series. — 

I These rocks possess an extremely varying character ; they 
consist of black, brown and gray, 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 pro- 
bably does tlie present ease offer a real exception, for the 
mica (at least in my best characterized specimen, in which 

I one nodule of this mineral is nearly half an inch in length,) 
is as perfectly rounded as a pebble in a conglomerate, and 

* The outline of the coast, the position of the villages, streamlets, 
and of most of the hills in this wood-cut, are copied from the char! made 
on hoard H. M. S. Leven. The square topped hills (A, B, C, S:c.) are 
put in merely by eye, to illustrate my description. 



evidently has not been crystallized in the base, in which it is 
now inclosed, but has proceeded from the fusion of some 
pre-existing; rock. These compact lavas alternate with tuffs, 
amygdaloids and wacke, and in some places with coarse 
conglomerate. Some of tlie argillaceons waekes 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, fiised into a jet black 
enamel, whilst some of the green varieties afforded only a 
pale gray bead. Numerous dikes, consisting cliiefly of highly 
compact augitic rocks, and of gray amygdaloidal varieties, 
intersect the strata, which have in several places been dislo- 
cated 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 dis- 
turbances took place before the deposition of the recent 
sedimentary bed ; and the surface, also, had previously been 
denuded to a great extent, as is shown by many truncated 
dikes. 

Description of the calcareous deposit overlying the fore- 
going volcanic 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 tlie 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 ; 



tliis is covered by a bed of pebblea, fmssiiig into eondi 
and mixed with fragments of echini, claws of crabs, and 
Bhells ; the oyster sheila stiU 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 NuUipone, retainii 
their proper forms, but with their surfaces slightly abradfl 
these bodies (plants as they are now generally considered 
be), exhibit under a microscope of ordinary power, ] 
traces of organization in their internal structure. 'H 
Cleoi^e R, Sowerby has been so good as to examine t 
shells which I collected : there are fourteen speeiee in, 
sufficiently perfect condition for their characters to be ma 
out with some degree of certainty, and four which can ' 
referred only to their genera. Of the fourteen shells, 
which a list is given in the Appendix, eleven are rece 
species; one, though undescribed, is perhaps identical wi 
a species, which I found living in the harbour of Pot 
Praya ; the two remaining species are unknown, and hai 
been described by Mr. Sowerby. Until the shells of ti 
Archipelago and of the neighbouring coasts are better know 
it would be rash to assert that even these two latter si 
are extinct. The number of species which certainly heloi 
to existing kinds, although few in number, are sufficie 
to show that the deposit belongs to a late tertiary perio 
Prom its mineralogical character, from the number and st 
of the embedded fragments, and from the abundance ■ 
Patellae, and other littoral. shells, it is evident that the who( 
was accumualted in a shallow sea, near an ancient coast-l 

Effects produced by the Jiowing of the superincumbent 
lava over the calcareous deposit. — These effects are very cnrioi 
The calcareous matter is altered to the depth of about a 
beneath the hne of junction; and a most perfect gradatii 
can be traced, from loosely aggregated, small, particles 
shells, corrallines, and Ntilliporsp, into a rock, in whid 



CALCAREOUS DEPOSIT 



BY HEAT. 



[ not a trace of mechBiiical origin can be discovered, even witli 
a microscope. Where the metamorphic change has heeii 
greatest, two varietiea occar. 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 crystallized 
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 
retiuning 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 without any crystalline structure. It consists of a white, 
opaque, compact, calcareous stone, thickly mottled with 
rounded, though irregular, 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 blow-pipe, 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, eartliy substance (earthy feldspar?) ; and 
these rocks, there was good reason to suppose, had undergone 
the action of heat, — a view which thus receives confirmation. 



) spotted structure may possibly afford some indicatioi) 
1 distinguishing those formationa 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 esperieuce, that moat geologista, when 
examining arenaceo-quartzose districts, must have experi- 
enced. 

The lowest and most scoriaoeous part of the lava, in rolliog 
over the sedimentary deiKJsit at the bottom of the sea, has 
caught up large quantities of calcareous matter, which now 
forms a snow-white, liighly crystalline, basis to a breccia, in- 
cluding small pieces of black, glossy seorix. A little above 
this, where the lime is less abundant, and the lava more com- 
pact, numerous little balls, composed of spicula of calcareous 
spar, radiating from common centres, occupy the interstices. 
In one part of Quail Island, the hme has thus been crystal- 
hzed by the heat of tlie superincumbent lava, where it ia 
only thirteen feet in thickness ; nor had the lava been 
originally thicker, and since reduced by degradation, as 
could l>e told from the degree of cellularity of its surface. 
I have already observed that the sea must have been shallow 
in which the calcareous deposit was accumulated. In tliis 
case, therefore, the carbonic acid gas has been retained 
under a pressure, insignificant compared with that (a column 
of water, 1708 feet in height) originally supposed by Sir 
James Hall to be requisite for this end : but since his 
periments, it has been discovered that pressure has less 
with the retention of carbonic acid gas, than the na1 
of the circumjacent atmosphere ; and hence, as is sta 
be the case by Mr. Faraday,* masses of limestone are 
times fused and crystallized even in common Inne- 
Carbonate of lime can he heated to almost any de; 

• I am much i^idebted to Mr. E. W. Braylej' in having given m 
following references to papers on this subject : FarRday, in the Edim 
burgh New Philosophical Journal, vol. xv, p. 398; Gay Lussae, 1 
AnnaleE de Chem. Pt Phys. torn. Ixiii. p. 219, translated in the Londod 
and Edinburgh Philosophical Magazine, vol. x. p. 496. 



CALCAREOUS HORIZONTAL DEPOSIT. 

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 decomposition, 
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 scoriffi, 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 scorite, embedded in the crystalline 
calcareous basis, are of a jet black colour, with a glossy 
fracture like pitchatooe. Tlieir 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 sub- 
stance: a change wluch 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 scorise, which have undergone 
.a closely similar change. 

The extent and horizontolity of the calcareous stratum. — TTie 
upper line of surface of the calcareous stratum, which is so 
conspicuous from being quite white and so nearly hori- 
zontal, 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 stra- 
tum 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 1 cannot doubt from its regularity, that it e 
much farther. In Bume ravines at right angles to the c 
it is seen gently dipping towards the sea, probably i 
same inclination as when deposited round the ancient e 
of the island. I found only one inland section, namely, ! 
base of the hill marked A, where, at the height of soil 
hundred feet, this bed was exposed ; it here rested • 
usual compact augitic rock associated with wacke, and waj 

, covered by the wide-spread sheet of modern basaltic lava. 
Some exceptions occur to the horizoutality of the white I 
stratum : at Quail Island, its upper surface is only forty tea 
above the level of the sea ; litre also the capping of lara i 
only between twelve and fifteen feet in thickness; on d 
other hand, at the N.E. side of Porto Praya harbour, tl 
calcareous stratum, as well as the rock on which it rest 
attain a height above the average level : the inequali^ ^ 
level in these two cases is not, as I believe, owing to uneqm 
elevation, but to original irregularities at the bottom of til 
sea. Of this fact, at Quail Island, there was clear erideot 
in the calcareous deposit being in one part of much ( 
than the average thickness, and in another part being entird 
absent ; in this latter case, the modem basaltic lavas rest 
directly on those of more ancient origin. 

Under Signal Post Hill, the white stratum dips into t 
sea in a remarkable manner. This hill is conical, 450 feet J 
height, and retains some traces of having had a craterifon 
structure; it is composed chiefly of matter erupted posteriori 
to the elevation of the great basaltic plain, but partly i 
lava of apparently submarine origin and of considerabi 
antiquity. The surrounding plain, as well as the eaaten 
flank of this hill, have been worn into steep precipices, ovev 
hanging the sea. In these precipices, the white calcareon 
stratum may be seen, at the height of about 70 feet aboil 
the beach, running for some miles both northward aQi 
southward of the hill, in a line appearing to be perfeetJ 
horizontal: but for a space of a quarter of a mile directi 
under the hill, it dips into the sea and disappears. On tl 




CALCABSOCS DBP06IT. 



south side the dip is gradual, on the north side it is more 
abrupt, as is shown in tlie woodcut. As neither the cal- 



BIOXAL FOOT HILL. 



C— Upiier basaltic Isia, 

eareouB stratum, nor the superincumbent basaltic lava (as 
far as the latter can be distinguished from the more modern 
ejections), appear to thicken as they dip, I infer that these 
strata were not originally accumulated in a trough, the centre 
of which afterwards became a point of eruption ; hut 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 subterranean 
motive power, from expending part of its force, in repeatedly 
erupting volcanic matter from beneath this point, would, it 
is likely, have less force to uplift it. Something of the same 
kind seems to have occurred near Red Hill, for when tracing 
upwards the naked 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 area. 

The basaltic lava, superincumbent on the calcareous deposit. — 
This lava is of a pale gray 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 ai'e 
compact, or at most crenulated with a few minute cavities, 
and are often columnar. At Quail Island this structure was 







P 



MMtmed in a striking m&nner ; tlie lava in one part 
dirided into horizontal laminie, which became in anotlier paff 
split bj Tertical tiseuresinto fivc-gidedpUtDs; and these again, 
being piled on each other, insensibly became soldered together, 
furming fine eymmetrical columns. Tlie lower surface of tlie 
lava is visJcular, but sometimes only to the thickness of a few 
inches; the npper surface, which is likewise vesicular, b 
divided into balls, frequently as much as three feet in 
diameter, made up of concentric layers. The mass is com- 
posed of more than one stream ; its total tliickness being, on 
an average, about eighty feet : the lower portion has cer- 
tainly flowed beneath the sea, and probably likewise the 
upper portion. The chief part of this lava lias flowed from 
the central districts, between the hlUa marked A, B, C, dec. 
the woodcut-map. The surface of the country, near 
coast, is level and barren ; towards the interior, the 
rises by successive terraces, of which four, when viewed from 
a distance, cotdd be distinctly counted. 

Voleanie erttpHons subsequent to the elevation of tlie coatt- 
land ; the ejected matter 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 600 feel ; it is composed of bright red, 
highly ecoriaceous 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 120 feet in height, into the sea: this line of 
cliff is continuous with that forming the coast, and bounding 
the pUiu on both sides of this hill ; these streams, therefore, 
emplcd, after the formation of the coast-clifis. from Red 
Hill, «h«i ii must have stood, as it now does, above the 



CALCABEOtIS MATTER ENTANQLEO IN LAVA. 11 

level of the sea. This conclusion accords with the highly 
Bcoriaceoua condition of all the rock on it, appearing to be 
of subaerial formation ; and this is important, as there are 
some beds of calcareous matter near its summit, which might, 
at a hasty glance, have been mistaken for a submarine de- 
posit. These beds consist of white, earthy, carbonate of 
lime, extremely friable, ao 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 aa 
quick-lime, 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, diffi- 
cult to find a particle of the lime which does not change 
colour under the blowpipe, most of tliem even becoming 
glazed. The scoriaceous fragments and the calcareous 
matter are associated in the most irregular manner, some- 
times in obscure beds, but more generally as a confiised 
breccia, the lime in some parts and the scoria in others 
being most abundant. Sir H. De la Beche has been so kind 
as to have some of the purest specimens analyzed, 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 hmestones. 

Fragments of the scorise embedded in the calcareous mass, 
when broken, exhibit many of their cells lined and partly 
filled with a white, delicate, excessively fragile, moss -like, or 
rather conferya-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 the -nj'g^ 
of an inch in diameter ; they are either simply branched, or 
more commonly united into an irregular mass of net-work, 
with the meshes of very unequal sizes and of unequal num- 
bers of sides. Some of the fibres are thickly covered with 
extremely minute spicula, occasionally aggregated into little 
tufla ; and hence they have a hairy appearance. These 
epicula are of the same diameter throughout their length ; 



f they are easily detached, bo tLiit tlie object-glasa of the 
microscope Boon becomes Bcattered over with tbem. Within 
the cells of many fragments of the acoris, tlie lime exhibits 
this fibrous structure, but generally in a less perfect degree. 
These cells do not appear to be connected with one another. 

r There can be no doubt, as will presently be shown, that the 

1 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 kno« 
nothing analogous. From the earthy condition of the 
fibres, this structure does not appear to be related to crys- 
tallization. 

Other fragments of the scoriaceous rock from this hiU, 
when broken, are often seen marked with short and irre- 
gular 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 ap- 
pearance in badly kneaded dough, of balls and drawn-oat 
streaks of flour, which have remained unmixed with the 
paste ; and I cannot doubt that small masses of the lime, 
in the same manner remaining unmixed with the fluid lava, 
have been drawn out, when the whole was iu motion. 
I carefully examined, by trituration and solution in acids, 
pieces of the scoriae, taken from within half-an-inch of those 
cells which were filled with the calcareous powder, anil 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 reti- 
culation, 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 150 yards in width, cutting through the basaltic 

I plain and underlying beds, but since filled up by a stream 



CALCAHE0TT8 MATTER ENTANGLED IN LAVA. 13 

more modem lava. This lava is dark gray, and in most 
parts compact and rudely columnar ; but at a little distance 
from the coaat, it includes in an irregular manner, a brec- 
ciated mass of red BcoriEe mingled with a considerable quan- 
tity of white, friable, and in some parts, nearly pure earthy 
lime, like that on the summit of Ked Hill. Tliis 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 he) 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 
waa 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 subsequently to the elevation of the great basaltic 
plain. It accorda with this view, that I found on Signal 
Post Hill, a mass of earthy, calcareous matter of the same 
nature, mingled with scorife. 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 de- 
rived from similar volcanic eruptions, as well as from tritu- 
rated organic remains : the underlying, ancient, crystalline 
rocks, also, are associated with much carbonate of lime, 
filling amygdaloidal cavities, and forming irregular masses, 
the nature of which latter I was unable to understand. 

Considering the abundance of earthy lime near the sum- 
mit of Red Hill, a volcanic cone 600 feet in height, of sub- 
aerial growth, — considering the intimate manner in which 
minute particles and large masses of scorise are embedded in 
the masses of nearly pure lime, and on the other Iiand, the 
manner in which small kernels and streaks of the calcareous 
powder are included in solid pieces of the scoriie, — consider- 
ing, also, the similar occurrence of lime and scoriae within a 
stream of lava, also supposed, with good reason, to have 



h 



14 BT. JAGO. [CBAP. I. 

been of modem subai^rial origin, and to liave flowed from t 
hill, where earthy lime also occiirs: I think, eonsidering 
these facts, there can be no doubt that the lime has been 
erupted, mingled with the molten lara, I am not aware 
that any eimdar case has been described : it appears to me sb 
interesting one, inasmuch as most geolc^sts must have spe- 
culated on the probable effects of a volcanic focus, bursting 
through deep-seated beds of different mineralogical compo- 
sition. The great abundance of free sdex in the trachytes of 
some countries (as descnbed by Beudant in Hungary, and 
by P. Scrope in the Panza Islands), perhaps solves the en- 
quiry with respect to deep-seated beds of quartz; and we 
probably here see it answered, where the volcanic action hu 
invaded subjacent masses of limestone. One is naturally led 
to conjecture, Id what state the now earthy carbonate of lime 
existed, when ejected with the intensely heated lava : from 
the extreme cellnlarity of the scoria on Red Hill, the pres- 
sure cannot have been great, and as most volcauic eruptions 
are accompanied by the emission of large quantities of 
and other gases, we here have the most favourable conditioiii^, 
according to the views at present entertained by chemists, fi» 
the expulsion of the carbonic acid.* Has the slow 
sorption of this gas, it may be asked, given to the lime in 
cells of the lava, that peculiar tibrous structure, like tha 
an efflorescing salt ? Finally, I may remark on the gi 
contrast in appearance between this earthy lime, which m 
have been heated in a free atmosphere of steam and oti 
gases, with the white, crystalline, calcareous spar, produi 

■ ISTiilst deep beneath the surface, the carbonate of lime \ 
piesame. in a fluid state. HutCon, it is known, thought that all amn 
daloids were produced hy drops of molten limestone floating ii 
trap, like oil in water : this no doubt is erroneous, but if the n 
forming the summit of Red Hilt bad been cooled under the pressure o 
a moderately deep sea, or within the walls of a dike, we should, i 
probability, have had a trap rock associated with large massed of i 
pact, ciystalline, calcareous spar, which, according to the views ei 
tained by many geologistB, would have been wrongly attributed to 
Bcqueat infiltration. 



over aiailM «ivifcy fins ami dbe debrs «if or^ti^mv nwiiitli»» 
at the bottoR of a dUkwr 9m. 

JBflL — TU» kill b» ainndhi V«im s^«fnil mnm 

tpedalir wick iiefeniu i c i i ^ tW nmnftrkiibtk^ 

inwUdktbe wUaecmkaMuo^scnciim^ ittodikfr ^hMjl 

it kfls a^kraai soBnic wick okrare tnw^t^ gt' a ccm ^ ^r tf^ fiM t 
stracCne, sad » coaipoeeii o£b«salKic n^ck^^ :$iMttir cvwt^^ 
o&en k%kir cdUsr* wick iatline^ b<^ o4r Wim^ ^vmnik 
of whaA flOHie aie aseodftied wick ^mrtkv KiHiew lik^ IW4 
Uiii it kM bccB d» sovrce of entpbonss^ sn W^iMa^y %^ ik^ 
deratkn of ^e ssmmndiBir basaltic fibuu; b«l ^aUki^ ikal 
bin, it kas andcfgOBe considenUe deaudatioM^ aad ka» W^fM 
the seat of Toleaaie actioa at a raaoie period^ wkea beiH*4ilk 
tbe sea. I j^dge of tkis htter cireamsiaiiee ttromi Ainiii^ \m 
its inland flaak, tbe last remnants of tbrte Mnall )huuI« ^wT 
eruption. These points are coupoi^ of {jrl^^a^AV mxMriws 
cemented bj crystalline calcareous spar^ exaclW Kki^ ibi^ 
great submarine calcareous deposit^ when^ lhf> ht^hn) 
laTa has rolled over it: their demoIiAh<Hl utatt^ oaUi I 
think, be explained only by the doiuulin)( aotion uf t\w 
wayes of the sea. I was glided to tlio tlmt oriHtH> hy oli- 
senring a she^ of lava, about 200 ynnU aquArts yfM\ «tfH^|i)iik 
sides, superimposed on the basaltic plain, witli no atyoiiiiiif 
hillock, whence it could havo been eruptf^i \ aiul tht) \\\\\y 
trace of a crater which I was able to diNCHwe^r, noiiMlMttMl of 
some inclined beds of scoriaa at one of itM coriictrM, At fliD 
distance of fifty yards from a second levol-top|MMl patch of 

• Of these, one common variety is remarkable for being ftill of imall 
fragments of a dark jasper-red earthy mineral, which, when examined 
carefully, shows an indistinct cleavage ; the little fragments are elon* 
gated in form, are soft, are magnetic before and after being heated, mi 
fuse with difficulty into a dull enamel. This mineral is evident)/ uUmJjf 
related to the oxides of iron, but I cannot ascertain what it smmCI^ jui- 
The rock containing this mineral, is crenulated with smsl) My^lliM 
cavities, which are lined and filled with yellowish oryuUU ^ mImmiA 
of lime. 



lava, but of mucli smaller size, I found an irregular c 
group of masses of pemented, scoriaceous breccia, about 
six feet in height, wliicli doubtless had once formed the point 
of eruption. The third orifice is now marked only by ai 
irregular circle of cemented scoria, about four yards ii 
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 hori- 
zontal basal section of a volcanic spiracle, which, togethtf 
with all its ejected matter, has been almost totally obliterated. 

The stream of lava, which fills the narrow gorge* 
ward of the town of Praya, judging from its course, » 
as before remarked, to have come from Signal Post HiD, 
and to have flowed over the plain, after its elevation : 
same observation applies to a stream, (possibly part of the 
same one,) capping the sea diffs, a httle eastward of the 
gorge. When I endeavoured to follow these streams over 
the stony level plain, which is almost destitute of soil dnd 
vegetation, I was much surprised to find, that although 
composed of hard basaltic matter, and not having beea 
exposed to marine denudation, all distinct traces of them sooa 
became utterly lost. But I have since observed at the Gal^ 
pagos Archipelago, thai it is often impossible to follow ev 
great deluges of quite recent lava across older streams, exc< 
by the size of the bushes growing on them, or by the coot- 
parative states of glossiness of their surfaces, — characten 
which a short lapse of time would be sufficient quite I 
obscure. I may remark, that in a level country, with a diy 
chmate, and with the vrind blowing always in one direction^ 
(as at the Cape de Verde Archipelago,) the effects of atmo 
spheric degradation is probably much greater than nonld a 
first be expected ; for sod in this case accumulates only in 

* The sides of this gorge, where the upper basaltic slratnin is intc 
aected, are almost perpendicnlar. The lava, which has since filled 
up, is attached (0 these sides, almost as firmly as a dike is to its 
In most cases, where a stream of lava has flowed down a valley, it 
bounded on each side by loose 



LLS. 1!) 

Tlie basal strata of tliese hills, as well as some neighbour- 
ing;, separate, bare, rounded hillocks, coaaist of compact, fine- 
grained, non-crystalline (or so slightly as scarcely to be 
perceptible,) ferruginous feldspatliic rocks, and generally in 
a state of serai-decomposition. Their fracture is exceedingly 
irregular, and splintery ; yet small fragments are often very 
tough, Tliey contain much ferruginous matter, either in the 
form of minute grains with a metallic lustre, or of brown 
hair-like threads ; the rock in this latter case assuming a 
pseudo-brceciated structure. These rocks sometimes contain 
mica and veins of 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 hke 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 characterized streams (as will be 
described in an ensuing chapter) of nearly similar com- 
position. Amidst the hillocks composed of these rocks, 
I found in three places, smooth conical hilla of phonolite, 

pdllier, which are suppoaed to be due to the decompoBition of oli 
I do not, however, find, that tlie action of this mineral under the blow- 
pipe being entirely altered, as it becomes decompoeed, has been 
noticed ; and the knowledge of this fact is important, as at first it ap- 
pears highly improbable, that a hard, transparent, refractory mineral 
should be changed into a soft, eaBily-fufied, clay, like this of St. Jago. 
1 shall hereafter describe a green substance, forming threads within the 
ceils of some vesicular basaltic rocks in Van Diemen's Land, which be- 
have under the blowpipe like the green wacke of St, Jago j but its 
occurrence in cylindrical threads, shows it can not have resulted 
from the decomposition of olivine, a mineral always existing in the 
fbnn of grains or crystals. 



20 



ST. JAOO. 



[CHiP.L 



abounding with fine cryRtals of glassy feldspar, and iri& 
needles of hornblende. These cones of phonolite, I befieie^ 
bear the same relation to the surrounding feldspathie stnti, 
which some masses of coarsely crystallized angitie roek,ii 
another part of the island, bear to the sarroanding banh, 
namely, that both have been injected. The rocks <tf t 
feldspathie nature being anterior in ori^n to the basiltie 
strata, which cap them, as well as to the basaltic streanu of 
the coast-plains, accords with the usual order of succesBUi 
of these two grand divisions of the volcanic series. 

The strata of most of these hills in the upper part, where 
alone the planes of division are distinguishable, are incfined 
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 strati 
are scarcely deflected from a horizontal plane, and in F (ai 
far as I could judge without ascending it) they are slight^ 
inclined in a reverse direction, that is, inwards and towards 
the centre of the island. Notwithstanding these differences 
of inclination, their correspondence in external form, and ib 
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 originaDj 
formed parts of one platform; which platform, as befine 
remarked, probably extended round a considerable portkm 
of the circumference of the island. The upper strata cct- 
tainly flowed as lava, and probably beneath the sea, as pe^ 
haps did the lower feldspathie masses : how then come 
these strata to hold their present position, and whence were 
they erupted ? 

In the centre of the island * there are lofly mountains, 

♦ I saw very little of the inland parts of the island. Near the village 
of St. Domingo, there are magnificent clifis of rather coarsely crystal- 
lized 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 com- 
pact fine-grained basalt, conformably covered by a bed of pebbles. 
Near Fuentes, I met with pap-formed hills of the compact feldspathie 
series of rocks. 



VALLEYS NEAH THE COAST. 



21 



but they are separated from the steep inland flanks of theae 
hills, by a wide space of lower counti-y: the interior mountaius, 
moreover, seem to have been the source of those great 
streams of basaltic lava, which, contracting as they pass be- 
tween the bases of the hills in question, expand into the 
coast-plains. Round the sfiores 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 im- 
mediately occurs, how come these masses to hold their pre- 
sent position, and whence were they erupted ? The same 
answer, whatever it may be, probably applies in these three 
ceases; 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 cal- 
careous deposit, with its existing species of marine remains, 
gives evidence. Considering bow well shells have been pre- 
served 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 intersected by a second valley joining it as & 
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 conglomerate. — On the shores of Quail Island, I 
found fragments of brick, bolts of iron, pebbles, and large 



22 ST. JAGO. [chap. I. 

firagments of basalt, united by a scanty base of impure cal- 
careous matter into a firm conglomerate. 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. 

FEKtiMiDoHioRoyBA.—PrecipiiouthiUo/phimolile. — TfiftCEiRA— rrocAy- 
iie Tockn; their singular decomposiiian by steam of high temperature.— 
Tabiti — Passage from wacke into trap ; singular voleania rock tmth 
Che vesicles half filed with mesoti/pe. — Mauritius — Proofs of Us recent 
elevation — Strtwture of its more atwient mountains ; similarity with St, 
Jago, — St. Paul's Boces — Not of voUimic origin — their singular 
mneralogical composition. 

Fernando Noronha, — During our short visit at this and 
the four following 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 hy 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 1000 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 distin- 
guish 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 1000 feet in height, which have been formed by the 
injection of fluid feldspathic 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 
hy numerous dikes, some of amygdaloidal basalt and others 



24 TBRCBIRA. [chap. IL 

of trachyte ; and beds of slaty phonolite with the plaiia 
of cleavage directed N. W. and S. E. Parts of this rodE, 
where the crystals were scanty, closely resembled oommfli 
clay-slate, altered by the contact of a trap-dike. The lami- 
nation of rocks, which undoubtedly have once been fluid, ap- 
pears to me a subject well deserving attention. On Ae 
beach there were numerous fragments of compact basdt, 
of which rock, a distant facade of colunms seemed to be 
formed. 

Terceita in the Azores. — ^The central parts of this ishal 
consist of irregularly rounded mountains of no g^reat dera- 
tion, composed of trachyte, which closely resembles in 
general character the trachyte of Ascension, presently to l)e 
described. This formation is in many parts overlaid, in the 
usual order of superposition, by streams of basaltic lays, 
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 k 
overlooked by a crateriform hill, (Mount Brazil) entirely 
built of thin strata of fine-grained, harsh, brown-colonred 
tuff. The upper beds are seen to overlap the basaltie 
streams, on which the town stands. This hill is almost iden- 
tical in structure and composition with numerous crater- 
formed 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 issu- 
ing 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 a 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 dissolving agent. 
After the mass is converted into clay, the oxide of iron seems 



TS&CKIEA. 25 

to be entirely removed from ^^me parts, which are left per- 
fectly white, whikt in i>tlier nei^hlMiiring parts, which are of 
the brightest re*l coI«)Qr. it •ieums to be deposited in greater 
qoantitT ; some other Tna.**es are marbled with the two dis- 
tinct colours. Portions ot the white clav, now that thev are 
dry, cannot be distingnisiieii by the eye from the finest pre- 
pared chalk; and when placed between the teeth they feel 
equally soft-grained : the inhabitants use this substance for 
white- washing their hoa:se«. The cause of the iron beinp: dis- 
solved in one part, and cl<>»e by, being again di*|)08itix], ib 
obscure; but the fact has been obser^'ed in M*verul utlj«^r 
places.* In some half-decayed specinieuH, I foiiii<l hiiiali. 
globular, affcrre^rations of yellow hyalite, rrM*iiil)liii^ ^mii- 
arabic, which no doubt bad been dopositiHl by llir hU.-hhi. 

As there is no escape for the niin-wuiiT, wliirJi liirkii;^ 
down the sides of the ravine-like hoHow, wlu'iirc iln- hUsul 
issues, it must all percolate downwanlH lli rough ilu' U^mrr 
at its bottom. Some of the inhabitaiil-H inloi'iiic<i mr, iam' : 
was on record that flames (some hiiiiiaouH iipj^'ajiUMse' Mt 
ori^nally proceeded from these cruckn, and flint, im- 
had been succeeded bv the steam : Uui 1 waj» 
ascertain how long this was ago, or anything 
subject. Wlien viewing the spot, 1 iiiiugiubU 
tion of a large mass of rock, like thr cumt 4 
Fernando Noronha, in a senii-Huiii MiaM;. 
surface might have caused a wedgi 
cracks at the bottom, and that tin* 
the neighbourhood of the heated 
succeeding yeai*s be driven back in 

Tahiti {Otaheite). — I visited imrr • "'■ ■ j 

western side of this island, and tiiip !■■ " ■" , ,. 

I Ml 

• Spallanzani, Dolomieu and iiBftBP*^**'***- - - 

the Italian volcanic IslandB. iMfkvr^ .i.iii 

Islands is redeposited in the ftrnn ♦^ . .i \\\i 

Ponces). These authors likeviw '-• — - . . .»i i 

it is now experimentally knov^ *** ^ lU. ntu 

ahle to disBolve silica. . i . . i .i i iiva 





or volcanic roc;kt<. Near the coast there are sevenU nuMl 
of basalt, eonie aboiiutiln;^ with large crystals of anghe n 
taraiahed ohvuie, others compact and earthy, — some slight^ 
vesicular, and others occasionBlly amygdaloidal. Tha 
rocks are generally iniicli decomposed, and to my sarpiiseti 
found in several isectiuni^, that it was impossible to distii^;nid 
even approximately, the line of separation between d 
decayed lava and the alternating beds of tuff. Since d 
specimens have become dry, it is rather more easy to distil 
guish the decomposed igneous rocks, from the sedimentai 
tutfs. This gradation in character between rocks havii| 
snch widely different origins, may I think be explained h 
the yielding under pressure of the softened sides of the ves 
cular cavities, whicli in many volcanic rocks occupy a laig 
proportion of their bulk. As the vesicles generally increat 
in size and number in the upper parts of a stream i 
lava, so would the etfectfl of their compression increase ; tt 
yielding, moreover, of each lower vesicle must tend to distni 
all the softened matter above it. Hence we might expect I 
trace a perfect gradation from an unaltered crystalline roe 
to one in which all the particles (although originally formis 
part of the same solid mass) had undei^one mechanical di 
placement ; and such particles could hardly be distinguishc 
from others of similar composition, which had been deposit 
as sediment. As lavas are sometimes laminated in the 
upper parts, even horizontal hnes, appearing like those i 
aqueous deposition, could not in all cases be relied on as a 
criterion of sedimentary origin. From these considerations 
it is not surprising, that formerly many geologists believed in 
real transitions fi-om aqueous deposits, through wacke, into 
igneous traps. 

In the valley of Tia-auru, the commonest rocks are basaltt 
with much olivine, and in some cases almost composed 
large crystals of augite. I picked up some specimens, 
much glassy feldspar, approaching in character to trach" 
There were also many large blocks of vesicular basalt, 
the cavities beautifully lined with chabasie (?), and radiating 



TAHITI. 27 

bundles of mesotype. Some of these specimenB presented a 
cnrious appearance, owing to a number of the vesicles being 
half filled up with a white, soft, earthy mesotypic mineral, 
which intumesced under the blow^pipe in a remarkable man- 
ner. As the upper surfaces in all the half-filled cells arc 
exactly parallel, it is evident, that this substance has sunk to 
the bottom of each cell firom its weight. Sometimes^ however, 
it entirely fills the cells. Other cells are either quite filled, 
or lined, with small crystals, apparently of chabasie ; these 
crystals, also, firequently 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 the earthy mineral to sink from its gravity 
to the bottoms 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 dis- 
turbance ; the strata are separated from each other by thick, 
compact beds of conglomerate, in which the firagments are 
large, some being rounded, but most angular. From the 
character of these beds, from the compact and crystalline 
condition of most of the lavas, and firom 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. Ellis found marine remains 
at a considerable height, which he believes were interstratified 
with volcanic matter, as is likewise described to be the case 
by Messrs. Tyerman and Bennett at Huaheine, an island in 

* MacCulloch, however, has described and given a plate of (Geolog. 
Trans. Ist Series, vol. iv. p. 225) a trap rock, with cavities filled up 
horizontally with quartz and chalcedony. The upper halves of these cavi- 
ties are often filled by layers, which follow each irregularity of the sur- 
face, and by little depending stalactites of the same siliceous substances^ 



S8 



UAUBITItTB. 



[chap. II> 



thia same archipelago. Mr. Stiitchbury also discovered dmt 
the summit of one of the loftiest moantains of Tahiti, at &t 
height of several thousand feet, a stratum of semi-fossil coi^ 
None of these remains liave been spocifically examined. Ob 
the coast, where masses of coral-nx'k would have aSoriei 
the clearest evidence, I looked in vain for any signs of recent 
elevation. For references to the above autliorities, and for 
more detailed reasons for not believing that Tahiti has been 
recently elevated, I must refer to my volume (p. 138) on the 
Structure and Distribution of Coral Reefs. 

Mauritiug. — Approaching this island on the northern of 
north-western side, a curved chain of bold mountains, sur- 
mounted by rugged pinnacles, is seen to rise from a smootb 
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 mountains, and npcm 
examination this view, at least with respect to the inferitv 
parts of the border, is found to be perfectly correct, Serenl 
authors* have described masses of upraised coral-roek roand 
the greater i»art of the circumference of the island. Setweeo 
Tamarin Bay and the Great Black River, I observed, in com- 
pany with Capt. Lloyd, two hillocks of coral-rock, formed in 
their lower part of hard calcareous sandstone, and in theiropper 
of great blocks, slightly aggregated, of Astrsea and M adrep<H«, 
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 
18"; they had a water-worn appearance, and tliey rose 
abruptly from a smooth surface, strewed with rolled debris of 
oi^anic remains, to a height of about twenty feet. The 
Officier du Roi, in his most interesting tour in 1768 romid 
the island, has described masses of upraised coral-rocks, still 

• Captain Carmichael, in Hooker's Bot. Misc. vol, ii, p. 301. Cap- 
tain Lloyd has lately, in the Proceedings of the Geological Solely 
""^ol. iii. p. 317). described carefully some of these masses. In the 
" vsyage a I'lsle de France par im Officier du Roi," many intereetiag 
facts arc given on this subject. Consult also " Voyage aux Quatre Islei 
d'Afrique, par M. Bory St. Vincent." 



MAURITIUS. 

retaining that moat-like structure (p. 54 of iny volume on 
Coral-Reefs) which is characteristic of the living reefs. On 
the coast northward of Port Louis, I found the lava con- 
cealed for a considerable space inland, by a conglomerate of 
corals and shells, like those on the beach, but in parts con- 
solidated by red ferruginous matter. M. Bory St. Vincent has 
described similar calcai'eous beds over nearly the whole of 
the plain of Pamplemousses, Near Port Louis, when turning 
over some lai^e 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 found several shells 
of serpala still adhering to their under sides. 

The jagged mountains near Port Louis rise to a height 
of between 2000 and 3000 feet : they consist of strata of 
basalt, obscurely sepaj-ated from each other by firmly aggre- 
gated beds of fragmentary matter ; and they are intersected 
ty 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, bo tliat they even fuse into a 
pale coloured glass. On the banks of the Great River, a. 
section is exposed nearly 500 feet deep, worn through 
numerous thin sheets of the lava of this series, which are 
separated from each other by beds of scorise. 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 Miheu 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 themorecompact and crystal- 
line basalt, form the main skeleton of the island. M. Bailly* 
• Voyage aux Terrcs Australes, torn. i. p. 64. 



30 

states that they all "se devplopment aut«ur d'elle 
line ceinture d' immenses remparts, loiites atTcctant une pnittl 
plus ou molns inclin4>e vers le rivage de la mer, tandJs u 
coDtraire, que vers le centre de I'ile elles preseotent nnt 
coupe abrupte, et souvent taillee A pic. Toutes ces mont^Dn 
sont fornixes do couches parallvles inclmces du centre it 
rile vers la wier." These statementH have been dUpuled 
though not in detail, by M. Quoy, in the voyage of Freycinet 
As far as my limited lueaiiH of observation went, I faanil 
theoi perfectly correct,* The inouiifaina on tlie N.W. siilcof 
the island, which I examined, namely, La Ponce, PcUf 
Botts, Corps de Garde, Les Mamellea, and ^ijtpareiitly 
another furtlier southward, have precisely the cxtermii shape 
and stratification dcHcrifx'd hy M. Bailly. They form abort 
a quarter of his girdle of ramparts. Although these moan- 
tains now stand quite detached, being separated from oA 
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 heautifid map of 
the Mauritius, published by the Admiralty from a Frcndi 
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 those just described. 
Whether the girdle was ever complete may well be doubted; 
but from M. Bailly's statements, and my own observations, 
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 tliirteeen geographical miles 
in length. M. Bailly boldly supposes that this enormous 

* M' Lesson, in his account of tliis island, in the voyage of the Co- 
quille, Beems to follow M. Baillj's views. 



gulf, wliicli has since been filled up to a great extent by 
streams of modern lava, was formed by tlie 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 geolo- 
gical history. At both islands, mountains of similar external 
form, stratification, and (at least in their upper beds) compo- 
sition, follow in a curved chain the coast-line. These moun- 
tains 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 wide breaches between the moun- 
tains, were in both cases worn by the waves, during their gra- 
dual 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, moreover, recent cones of eruption 
are ecattered around the circumference of the island ; but at 
ndther have eruptions taken place witfiin the period of 
history. As remarked in the last chapter, it is probable that 
tbese ancient basaltic mountains, which resemble (at least in 
many respects) the basal and disturbed remnants of two 
gigantic volcanos, owe their present form, structure, and 
position, to the action of similar causes. 

AY. Paul's Rocks. — 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" 15' west lon- 
gitude. Its highest point is scarcely fifty feet above the level 
of the sea; its outline is irregular, and its entire circumfer- 
ence barely three-quarters of a mile. This httle 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 tliis circmnstaace, which is tb 
most remarkable point in its history (as will hereafter hr 
referred to), properly ought to exclude it from the present 
volume. It is composed of rocks, unlike any ivhich I ban 
met with, and which I cannot characterize hy any name, ud 
must therefore describe. 

The simplest, and one of the most ahuodant kuids, ii 
a very compact, heavy, greenish- black rock, having u 
angular, irregular fracture, with some points jugt hard 
enough to scratch glass, and infusible. This variety p8£»a 
into others of paler green tints, less hard, hut with a men 
crystalline fracture, and translucent on their edges ; and 
these are fusible into a green enamel. Several other vari- 
eties are chiefly characterized, by containing innumerable 
threads of dark-green serpentine, and by having calcareou 
matter in their interstices. These rocks have an obscure, con- 
cretionary ^structure, and are fidl of variously -coloured angular 
pseudo fragments. These angular pseudo fragments consia 
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 vesicukr, 
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 cal- 
careous ones, wliich contain minute fragments of shells, an 
clearly of subsequent origin to the others. 

A glossy incrustation. — Extensive portions of these rocki 
are coated by a layer of a glossy polished substance, with a 
pearly lustre and of a grayish 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 numer- 
ous exceedingly thin layers, their aggregate thickness being 
about the tenth of an inch. It is considerably harder than 
calcareous spar, but can be scratched with a knife ; under 
the blow-pipe it scales off, decrepitates, slightly blackens, 
emits a fetid odour, and becomes strongly alkaline : it does 



h 



8T. Paul's rocks. 33 

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, stalaetitical 
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 enamel of 
teeth, but more translucent, and so hard as just to scratch 
plate-glass. This substance slightly blackens under the 
blow-pipe, 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 forma- 
tion 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 arboresent form, this substance singularly resembles 
some of the branched species of Nullipora. Under the 
blow-pipe, it behaves like the specimens from Ascension ; but 
it is less hard and glossy, and the surface has not the shrunk 
appearance. 

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



D 



34 



[chap. UL 



CHAPTiiR III. 



ASCENSION. 



Basaltic lavas — Numerous craters truncated on the same side — Smffdsr 
structure of volcanic bombs — Aeriform explosions — Ejected gramiiefn^ 
ments — Trachytic rocks — Singular veins — Jasper ^ its manner offvmt 
tion — Concretions in pumiceous tuff-— Caleetreous deposits and frmnklt- 
cent incrustations on the coast — Remarkable laminated beets, altematis§ 
tcith^ and passing into obsidian ^"Origin of obsidian — JLamimaiion tf 
volcanic rocks. 



This island is sitaated in the Atlantic ocean, in lat. 8^8. 
long. 14° W. It has the form of an irregular triangle, (see 
accompanying map,) each side being about six miles ib 
length. Its highest point is 2,870 feet* above the leyel rf 
the sea. The whole is yolcanic, and, from the absence of 
proofs to the contrary, I believe of suba'erial origin. Tbt 
fundamental rock is everywhere of a pale colour, generally 
compact, and of a feldspathic nature. In the S. E. poitioa 
of the island, where the highest land is situated, well charae- 
terized trachyte, and other congenerous rocks of that varyiog 
family, occur. Nearly the entire circumference is covered isf 
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 

* Geographical Journal, vol. v. p. 243. 



BASALTIC ROOKS. 

to have possessed singularly little fluidity ; tlieir side walls 
and lower ends being very steep, and even as much as between 
twenty and thirty feet in height. Their surface is extra- 
ordinarily rugged, and from a short distance appears as if 
studded with small craters. These projections consist of 
broad, irregularly conical, hillocks, traversed by fissm*ea, 

, ^and composed of the same unequally scoriaceous basalt 
I with the surrounding streams, but having an obscure ten- 
■ dency to a columnar structure ; they rise to a height between 
ten and thirty feet above the general surface, and have been 

informed, as I presume, by the heaping up of the viscid lava 

^at points of greater resistance. At the base of several of 
1 these hillocks, and occasionally likewise on more level parts, 

j solid ribs, composed of an gulo- 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 super- 
ficial fragments from these basaltic streams, present singu- 
larly convoluted forms; and some specimens could hardly 
I>e 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 
obhquely, and they all slo])ed towards the 8.E., whence the 
trade- wind blows,* This structure no doubt has been 
caused, by the ejected fragments and ashes beiiig always 
blown, during eruptions, in greater quantity towards one 

• M. Lesson, in the Zoology of the Voyage of the Coquille (p. 490), 
has observed this fact. Mr. Hennah (Geolog. ProceedingB, 1835, p. 189) 
farther remarks, that the most extensive beds uf ashes at AscenBioji 
invariably oecur on the leeward nide of the island. 



36' ASCENSION. [chap. lit. 

side, than towards the other. M, Moreau de Jonn^s has 
made a similar observation with respect to the volcanic ori- 
fices in the West Indian island: 

Volcanic bombs. — These occur in great numhera strewed OH- 
the ground, and some of them lie at cunsideralile 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 tg 
the tail of a comet), irregular, studded with projectii^ 
points, and even concave. Their surfaces are rough and 
fissured with branching cracks ; their internal structure it 
either irregularly seoriaceous and compact, or it presents s 

teymmetrical and very curious appearance. An irregular 
segment of a bomb, of this latter kind, of which I fotmd 
several, is accurately represented in the accompanying 
woodcut. 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 outsidt 
tliey gradually decrease in size. This part is succeeded by a^ 



' » 



VOLCANIC BOMBd. 

well-defined shell of compact lava, havi'-.j t : - 
thickness of about the thinl of an in*-:. . l: 
overlaid by a somewhat thicker cijutIl- .' : 
lava (the cells varj-ing from the fiftieth • • •:.• .. 
an inch in diameter), which forms thf f- w::t. 
line separating the shell of compart Ul 
scoriaceous crust is distinctlv detiiie<i. 'J 
very simply explained, if we suppo-^.' a ::-«: — 
aceous matter, to be projected with a r^- .' 
through the air; for whilst the exTir:.<t. •■- . 
became solidified (in the state we n « - . 
fugal force, by relieving the pre*— iir*- • * . 
the bomb, would allow the heiit»-«] \h-- •- 
cells; but these being driven by th*r -^ .... : 
alreadv-hardened crust, would W:or;j». v.- 
to this part, smaller and smaller or .«-- 
they became packed into a stA'ul. e^f.. » 
know that chips from a grindstorie* '. 
made to revolve with sutficient velv .» 
that the centrifugal force would im.* ,//. . 
structure of a softened bomb, in th* ..., . .,. ^ 
Geologists have remarked, that tii< i-,. ^ , - ,.• 
at once bespeaks the history of i*- <.* -^ ,>«-^ 
see that the internal structure: ';*i/. r^j 
plainness, of its rotatory mov»iiij«rij* 

M. Bory St. Vincentf ha« 'ii:««;iA*^ 
from the Isle of BourlxiO. him*.:. 
structure; his explanation, ii^*«r»*' 
rightly), is very diiFerent fr^jiij Iuai mjgr- 
he supposes that they hav^ ioa^^tk , mt^^ 
sides of the crater. M. B^uu«itr. 
singular little balls of ol^iui^tii, 
inches in diameter, whicij u*: jMitti*? - 
the ground : their foriii au «<««*• - " 

• Nichol's ArcfiiU:«Mtfe •» = , 

t Voyage aux (<uiufe 
t Voyage es ^ i t i K m r.-^ 






38 



[rH*F. 



muoli Bwolk'n in the middle, and ev^ti spindle-shaped: tb 

Llurfnce is regularly marked with concentric ritlges and ft 

trows, all of which on the same ball are at right angles 1 

rbocaxis: their interior is compact and glassy. M.I 

Buppoeee that mafises of lava, when soft, were shot into i 

air, with a rotatory movement round the same axis, and tl 

the form and superficial ridges of the hombs were thiu p 

diiced. Sir Tliumaft Mitchell has given me what at first a 

pears to be the half of a ntitch flattened oval liall of obsidiBa 

it ha§ a singular artificial-like appearance, which is welirt 

presented (of the natural siie)in the accompanying woodcv 




r It was found in its jiresent state, on a great sandy plain be- 
1 tween the rivers Darling and Murray, in Australia, and at 
I the distance of several hundred miles from any known voi- 
I eanic region. It seems to have been embedded in some 
I reddish tufaceous matter; and may have been transported 
I either by the aborigines or by natural means. The extemsl 
I Saucer consists of compact obsidian, of a bottle-green colour, 
and is filled with finely-cellular black lava, much less trans- 
parent and glassy than the obsidian. The external surface 
is marked with four or five, not quite, perfect ridges, which 
are representeil rather too distinctly in the woodcut. Here 
then we have the external structure described liy M, Beu- 



40 



ASCENSION. 



[CHAy. 



N 



siderable dislocations have taken place, and an oval ciTO 
has been formed by subsidence. This sunken space lies 
the north-eastern foot of Green Mountain, and is well r 
Rented in the accompanying map. Its longer axis, t 
is connected witli a N, E. and S. W, line of fissnre, ia ti 
fifths of a nautical mile in length; its sides are ; 
perpendicular, except in one spot, and about 400 fee* 
height ; they consist in the lower part, of a pale basalt n 
feldspar, and in the upper part, of the tufi" and loose ejec 
fragments ; the bottom is smooth and level, and under aim 
any other climate, a deep lake would have been formed hi 
From the thickness of the bed of loose fragments, n 
which the surrounding country is covered, the amotmt 
aeriform matter, necessary for their projection, must hi 
been enormous ; hence we may snppose it probable, that ai 
the explosions, vast subterranean caverns were left, and tj 
the ialling in of the roof of one of these, produced the h 
here described. At the Galapagos Archipelago, pits ol 
similar character, but of a much smaller size, 
occur at the bases of small cones of eruption. 

Ejected granitic fragments. — In the neighbourhood of C 
Mountain, fragments of extraneous rock are not unfreqneR 
found embedded in the midst of masses of scorise. ISt 
Evans, to whose kindness I am indebted for much infbn 
tion, gave m,e several specimens, and I found others myi 
They nearly all have a granitic structure, are brittle, '. 
to the touch, and apparently of altered colours. Fbv^ 
white syenite, streaked and mottled with red ; it consist 
well crystallized feldspar, numerous grains of quartz, 
brilliant, though small, crystals of hornblende. The feldi 
and hornblende in this and the succeeding cases, have l 
determined by the reflecting goniometer, and the quartz 
its action under the blow-pipe. The feldspar in these ejec 
fragments, like the glassy kind in the trachyte, is from 
cleavage a potash-feldspar. Secondly, a brick-red maa 
feldspar, quartz, and small dark patches of a decaj 
mineral ; one minute particle of which, I was able to ascer- J 



tain by its cleavage, to be hornblende. Thirdly, a mass of 
confusedly crystallized white feldspar, with little nests of a, 
dark coloured mineral, often carious, externally rounded, 
having a glossy fracture, but no distinct cleavage : from 
comparison with the second specimen, I have no doubt that 
it b fused hornblende. Fourthly, a rock, which at first 
appears a simple aggregation of distinct and large-sized 
crystals of dusky- 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 rare f to find granitic rocks ejected from vol- 
canos with their minerals uncharixfed, 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 

" ProfeBSOr Miller has been so kind as to examine this mineral. He 
obtained two good cleavages of 86° Sff and 86= Sff. The mean of 
Keveral, which I nia^e, was 86° Sff. Prof. Miller states that these 
crjstals, when reduced to a fine powder, are soluble in hydrochloric 
acid, leaving some undisaolved silex behind ; the addition of oxalate of 
ammonia gives a copious precipitate of lime. He further remarks, that 
according to Voa Kobell, anorthite (a mineral occurring in the ejected 
fragmeniB at Mount Somna) is always white and transparent, so that if 
this be the case, these crystals from Ascension muft be considered as 
Labrador feldspar. Prof. Miller adds, that he has seen an account, in 
" Erdmann's Journal fiir technische 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 mioeralogists, of this mine- 
ral : the author attributed this difference to an error in the analysis of 
Labrador feldspar, which is very old. 

t Daubeny, in his work on Volcanos (p. 386), remarks that this is 
the case; and Humboldt, in his Personal Narrative {vol. i. p. 236), 
says, " In general, the oiasses of known primitive rocks, I mean those 
vhich 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 nepheliae, mica, and 
pyroxene." 



49 



[en A 



layprs of a concretionary iiitchstone pagsing into obt 
TIiPM> layern are parallel, slightly tortuous, aud ahon 
they thin out at their ends, and resemble in form, the layi 
of cjuartz in gneiss. It is probable that these small einbedd 

L fragments were not separately ejected, but were eutangled 

I A fluid volcanic rock, allied to olraidian; and we shall p 
lently see tliat several varieties of this latter series of n 

I iBssume a laminated structure. 

Trachytic teri's of Rocks. — These occupy the more « 

I rated and central, and likewise the south-eaatem parts of i 

■ island. The trachyte is generally of a pale-brown ctAat 
I stained with small darker patclies ; it contains broken N 
I bent crystals of glassy feldspar, grains of specular iron, m 
LUack microscopical points, which latter, from being eaa 
■ftised, and then becoming magnetic, I presume are hot 

■ Mende. The greater number of the hills, however, i 
composed of a quite wliite, friable stone, appearing Q 
a trachytic tuff. Obsidian, hornstone, and several Jcinds 
himinated feldspattiic rocks, are associated vrith the trachyt 
There is no distinct stratification ; nor could I distinguiab 
crateriform structure in any of the hills of this series, 
aiderable dislocations have taken place; and many fissui 
in these rocks are yet left open, or are only partially f 
with loose fragments. Within the space,* maiidy formed 
trachyte, some basaltic streams have burst forth; and i 
far from the summit of Green Mountain, there is one strei 
of quite black, vesicular basalt, containing minute crystals 
glassy feldspar, which have a rounded appearance. 

The soft white stone above-mentioned, is remarkable t 
its singidar resemblance, when viewed in mass, to a i 
mentary tuff: it was long before I could persuade myi 
that such was not its origin ; and other geologists have I 
perplexed by closely similar formations in trachytic regioB 

lis space ib nearly included by a line sweeping round Grei 
IJountain, and joining the hills, called the Weather Port Signal, Holj 
., and that denominated (improperly in a geological sense) *'tl 
!r of an old volcano." , 



In two cases, this wliite earthy stone formed Isolated bills ; 
in a third, it wae associated with columnar and laminated 
trachyte; hut I waa unable to trace an actual junction. It 
contains numerous crystals of glassy feldspar and hlack 
microscopical specks, and is marked with small darker 
patches, exactly as in the surrounding trachyte. Its hasis, 
however, when viewed under the microscope, is generally 
quite earthy ; hut sometimes it exhibits a decidedly crystal- 
line structure. On the hill marked " Crater of an old 
volcano," it passes into a pale greenish-gray variety, differ- 
ing only in its colour, and in not being so earthy; the 
passes was in one case effected insensibly; in another, it 
was formed hy numerous, rounded and angular, masses of the 
greenish variety, being embedded in the white variety ; — in 
this latter case, the appearance was very much like tliat of a 
sedimentary deposit, torn up and abraded, during the depo- 
sition 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 scoriaeeous rocks, the cells of some of which are 
partially filled with the white earthy stone; they likewi.se 
include some huge blocks of a cellular porphyry.* These 
fragments project from the weathered surface, and per- 
fectly resemble fragments embedded in a true sedimentary 
tuff. But as it is known, that extraneous fragments of cellu- 
lar rock are sometimes included in columnar trachyte, in 
phonoIite,+ and in other compact lavas, thia circumstance is 
not any real argument for the sedimentary origin of the white 
earthy stone. J The insensible passage of the greenish 

• The porphyry is dark-coloured ( it contains numerous, often frac- 
tured, crystals of while opaque feldspar, also decnmpoBing crystals of 
oxide of iron ; its vesicles include tnaases of delicate, hair-like, cryslals, 
apparently of anakitne. 

t D'Aiibuisson Traill de Geognosie, torn. ii. p. 54S. 

I Dr. Daiibeiiy (on VoJcanos, p. ISO) seems lo have been led to he- 



44 ABCBNftlON. [chap. I 

variety into the white one, and likewise the more abrn 
passage hj fragincntfl of the former being' embedded j 
the latter, might result from slight ditferences in the comH 
sition of the same mass of molten stone, and from the abn 
action of one such part still fluid, on another part a 
solidified. The curiously fonncd reins have, I behei 
been formed by silicious matter being subsequently segivi] 
gated. 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 niasees of solid trachyte. 
Moreover, as I have remarked, the microscope occasional!; 
reveals a crystalline structure in the apparently earthj 
basia. On the other hand, the partial decomjxisition of suel 
great masses of trachyte, forming whole moantaina, 
undoubtedly a circumstance of not easy explanation. 

Veins in the earthy trachytic masses. — Tliese veins are es" 
traordinarily numerous, intersecting in the most complicatej' 
manner both coloured varieties of the earthy trachyte : (Iwy 
are best seen on the flanks of the " Crater of the old volcano." 
They contain crystals of glassy feldspar, black microscoinail 
Bpecks and little dark stains, precisely as in the surrounding 
rock; but the basis ia very different, being exceeding 
hard, compact, somewhat brittle, and of rather less easy fuubi 
lity. The veins vary much, and suddenly, from the tenth q 
an inch to one inch in thickness ; they oflen thin ont, not 
only on their edges, but in their central parts, thus leaTin^ 
round, irregular apertures; their surfaces are m^;e(L 

lie»e that certain trachytic formations of Ischifl and of the Puy 
Ddme, which closely resemble these of Ascension, were of sedimenti 
origin, chiefly from the frequent presence in them "of scoriform p 
tions, 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 ; be considers the subject 
deserving of further attention. 



SILICEODB &imr£R AND JA5PEB. 

They are inclined at every possible angle with the horizon, 
or are horizontal ; they are generally curvilinear, 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 gronnd, 
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 de- 
scribed, any veins like these ; but in form they resemble the 
ferruginous seams, due to some process of segregation, occur- 
ring 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 abundant source of 
silica, and as these plate-like veins differ from the trachyte, 
only in their greater hardness, brittleness, and less easy fusi- 
bility, it appears probable that their origin is due to the 
segregation or infiltration of siliceous matter, in the same 
manner as happens with the oxides of iron in raany sedi- 
mentary rocks. 

Siliceous sinter and jasper. — The siliceous sinter is either 
quite white, of Ihtle 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 largo 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 
honeycombed, sometimes adheres to the projecting plate- 
like veins, described in the last paragraph. The jasper 



A»CB!t8IOIf. 



f OHAf. ■! 



N 



of an ochre yellow or red colour; it occurs in large 
irregular masses, and sometimes in veins, both in the altered 
trachyte and in ao associated mass of scoriaceous basalt. 
The cells of the scoriaceous basalt arc liued 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 parts, irregular angular patches 
of the red jasper are included, the edges of which tnsensiblj 
blend into the surrounding mass ; other patches occur 
having an intermediate character between perfect jasper 
and the ferruginons, 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 scoriaceom 
basalt are tilled and lined with layers of chalcedony. 
Small fragments of the jasper, examined under the micro- 
scope, 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 facta, — namely, the blending of the jasper into the 
semi-decomposed basalt, — its occurrence in angular patches, 
which clearly do not occupy pre-existing hoUowa in the 
rock, — and its containing little vesicles filled witli chalcedony, 
like those in the scoriaceous lava, — only on the supposi- 
tion that a fluid, probably the same fluid which deposited 
the chalcedony in the air-ceUs, removed in those parU 
where there were no cavities, the ingredients of the basaltic 
rock, and left in their place, silica and iron, and thus pro- 
duced the jasper. In some specimens of sihcified 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 liollows, apparently pro- 
duced by decay, were filled with concentric layers of chal- 
cedony ; in this case, there can be little doubt that the 
same fluid deposited the homogeneous base and the chal- 



SILICEOUS BIKTSR AKD JASPER. 4/ 

cedonic layers. After these considerations, I cannot doubt, but 
that the jasper of Ascension may be viewed as a volcanic rock 
eiUcified, 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 removed and replaced by atoms of sUica, 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 esamtnation of these rocks, and to the con- 
clusion here given, from having heard the Rev. Professor 
Henslow express a similar opinion, regarding the origin 
in trap- rocks of many chalcedonies and agates. Sili- 
ceous deposits seem to be very general, if not of universal 
occurrence, in partially decomposed trachytic tuffs ;-t- and as 
these hills, according to the view above given, consist of 
trachyte softened and altered in situ, the presence of free 
ffllica in this case, may be added as one more instance to the 
list. 

Concretions in pvmiceoui tuff. — 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 ghallow, saucer-like summit, nearly half a mile 
in diameter. This hollow has been nearly filled up with 

• Beudant (Voyage en Hongrie, torn. iii. p. 502, 504) describes kid- 
ney-shaped massEB of jasper- opal, which eilher blend into the surround- 
ing trachytic conglomerate, or are embedded in it like cbalk-flinls ; and 
he compares them with the fragments of opalized wood, which are 
abundant in this same formation. Beudant, however, appears to have 
viewed the process of their formation, lather as one of simple infiltra- 
tion, than of molecular eschange ; but the presence of a concretion, 
wholly different from the surrounding matter, if not formed in a, pre- 
existing hollow, dearly seems to me to require, either a molecular or 
mechanical displacement of the atoms, which occupied the space after- 
wards filled by it. The jasper-opal of Hungary pusses into chalcedony, 
and therefore in this case, as in that of Ascension, jasiier seems to be 
intimately related in origin with chalcedony. 

t Bendanl (Voyage Min. torn, iii, p. 507) enumerates cases in Hun- 
gary, Germany, Central France, Italy, Greece, and Mexico. 



nunjr gucccflsive she^ttt of ashes and scorise, of difiert 
colours, and slightly consolidated . Each successive, 
shaped, layer crops out all round the mar^n, forming 
many rings of various colours, and giving to the hill a 
taatic appearance. The out^r ring is broad, and of a «1 
colour ; hence it resouibles a course round which horses 
been exercised, and has received the name of the 
Riding School, by which it is most generally known. ' 
successive layers of ashes must have fallen over the ■ 
surrounding country, but they have all been blown i 
except in this one hollow, in which probably moisture 
mulated, either during an extraordinary year when rail 
or during the storms often accompanying volcanic erupti 
One of the layers of & pinkish colour, and chiefly di 
from small, decomposet! fragments of pumice, is remarki 
from containing numerous concretions. These are gei 
spherical, from half-an-inch to three inches in diameter 
they are occasionally cylindrical, lite those of iron-pyritea 
the chalk of Europe. They consist of a very tough, 
pact, pale-brown stone, with a smooth and even fracti 
They are divided into concentric layers, by thin white 
titions, resembling the external superficies ; six or eight 
Bueh layers are distinctly defined near the outside ; but those 
toward the inside generally become indistinct, and blend into 
a homogeneous 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 consoli- 
dated ashes. The concretions contain a small proportion 
of carbonate of hme : a fragment placed under the blow- 
pipe decrepitates, then , whitens and fuses into a blebhy 
enamel, but does not become caustic. The surrounding 
ashes do not contain any carbonate of lime ; hence the con- 
cretions have probably been formed, as is so often the case, 



(iby the aggregation of this substance. I have not met with 
' any account of abniJar concretiona ; and considering their 
.great toughness and compactness, their occun-enee in a bed, 

,. which probably has been subjected only to atmospheric 

y 1 moisture, ia remarkable. 

Formation of calcareous rocks on the sea-coast. — On several 
of the sea-beaches, there are immense accumulations of 
small, weU-rounded pai-ticles of shells and corals, of white, 
yellowish, and pink colours, interspersed 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 tliis purpose : and I saw one 
mass, divided into even layers lialf-an-inch in thickness, 
which were so compact, 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 sheila 
are embedded in these agglutinated masses ; and I have 
examined even a large fragment under a microscope, without 
teing able to discover the least vestige of strias 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, w^as entirely dissolved, with 
the exception of some flocculent animal matter ; its specific 
gravity was 2'63, The specific gravity of ordinary lime- 
stone varies from 2'6 to 275 ; pure Carrara marble was 
foand by Sir H, De la Bechef to be 27. It is remarkable 

• The eggs of the turtle being buried by the parent, sometimes be- 
come enclosed in the solid rock. Mr. Lyell has given a figure (Priuci- 
plcB of Geology, book iii. ch. 17.) of some eggs, containing the boneEof 
young turtles, found thus entombed. 

t Researches in Theoretical Geology, p. 12. 



50 A8CBN8ION. [CHAP. HI. 

that these rocks of Ascension, formed close to the sor&ce,' 
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 in- 
formed by Lieut. Evans, is caused by a change in the pre- 
vailing 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 disappears, being either re- 
dissolved, when the water is less charged with lime, or more 
probably is mechanically abraded. Lieut. Evans has ob- 
served these facts, during the six years he has resided at 
Ascension. The incrustation varies in thickness in different 
years: in 1831 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 re- 
moved 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, causes 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 Lieut. 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 incrustation. — In many respects 
this is a singular deposit ; it coats throughout the year the 
tidal volcanic rocks, that project from the beaches com-, 
posed of broken shells. Its general appearance is well ^e- 



CALCAREOUS INCHC8TATION. 



presented in the accompanying woodcut ; but the fronds or 

discs, of wliicli 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 gray or jet black colour ; their fonii is irregular, 
generally circular, and from the tenth of an inch to one inch 
and a-balf 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 
tbey 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 fiised 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 ! 

The substance forming the fronds, has a very compact and 
often almost crystalline fracture ; the edges being translu- 
cent, and hard enough easily to scratch calcareous spar. 
Under the blow-pipe it immediately becomes white, and 
emits a strong animal odour, like that from fresh shells. It 



•V2 AsrsNsioN. [chap. in. 

is chiefly composed of carbonate of lime ; when placed in 
muriatic acid it froths much, leaving a residue of sulphate cf 
lime, and of an oxide of iron, together with a black powder, 
which is not soluble in lieated acids. This latter substance 
seems to be carbonaceous, and is evidently the colonring 
matter. The suli)hate of lime is extraneous, and occurs in 
distinct, excessively minute, lamellar plates, studded on tlie 
surfaces of the fronds, and embedded between the fine layen 
of which they are composed ; when a fragment is heated in 
the blowpipe, these lamellae are immediately rendered visible. 
Tlie 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 deep^ 
corroded, by the dissolving power of the waves, into sharp 
ridges, and then are coated with successive layers of the 
glossy, gray, calcareous incrustation. The inequalities of the 
primary support affect 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 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 <^ 
Ascension ; in this case a basis was not afforded by any pro- 
jecting extraneous particles. Although the incrustation at 
Ascension, is persistent throughout the year ; yet from the 
abraded appearance of some parts, and from the fresh ap- 
pearance 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, to- 
gether with its composition, I think there can be no doubt, 
that its origin is due to the dissolution and subsequent depo* 



CALCAREOUS INCRUBTATIOH. 

sition of the matter, conipoaing the rounded particles of 
shells and corals.* From this source it derives its ammal 
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 gray varnish. Its darkness varies a Uttle, 
but the jet blackness of some of the fronds and of the botry- 
oidal masses, seems due to the translueency of the successive 
gray layers. There is, however, this singular circumstance, 
that when deposited on the under side of ledges of rock or 
in fissures, it appears always to be of a pale, pearly gray 
colour, even when of considerable thickness : hence one is 
led to suppose, that an abnndance of light is necessary to the 
development of the dai'k colour, in the same manner as 
eeeiuB to be the case with the upper and exposed surfaces of the 
shells of living moUusca, which are always dark, compared 
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 
blow-pipe, — in the degree of hardness and translueency of 
the edges, — and in the beautiful polish of the surface,t rival- 

" The selenile, as I have remarked, is extraneous, and must have 
been derived from the sea-wa(cr. It is an interesting circumstance 
thus to find the waves of the ocean, auSicicntly 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, resembling in form those of carbonate of lime, 
are formednear 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 scoria. In 
these latter cases, the salt and gypsum appear to be volcanic products. 

t From the fact described in my Journal of Researches (p. 12), 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 Orinooco and 
Nile), becoming finely polished where the surf acts, I presume that 
the surf in this instance, also, is Ihe polishing agent. 



54 ASCENSION. [chap. IU. 

ling when in a fresh state that of the finest Oliya, there is t 
striking analogy between this inorganic incrustation and the 
shells of living molluscous animals.* This appears to me 
to be an interesting physiological fact.f 

Singular laminated beds aUemating toith and pcLSsing niif 
obsidian. — 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 modem 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 the thickest fonns 
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 vaiieties 
may be noticed, but these insensibly blend into each othff 
by endless gradations. 

First, — A pale gray, irregularly and coarsely laminated^ 

* In the section descriptive of St. Paul's Rocks, I have described a 
glossy, pearly substance, which coats the rocks, and an allied 8tal>fr 
titical incrustation from Ascension, the crust of which resembles flie 
enamel of teeth, but is hard enough to scratch plate glass. Both theie 
substances contain animal matter, and seem to have been derived fffM 
water infiltering through birds' dung. 

t Mr. Homer and Sir David Brewster have described (Philosoplucil 
Transactions, 1836, p. 65) a singular " artificial substance, resembliiig 
shell.'* It is deposited in fine, transparent, highly polished, browB- 
coloured laminse, possessing peculiar optical properties, on the inside d 
a vessel, in which cloth, first prepared with glue and then with lime,* 
made to revolve rapidly in water. It is much softer, more transparo^ 
and contains more animal matter, than the natural incrustation at 
Ascension ; but we here again see, the strong tendency which carbonate 
of lime and animal matter evince to form a solid substance allied to 
shell. 

X This term is open to some misinterpretation, as it may be applied 
both to rocks divided into laminse of exactly the same compositioih 
<ind to layers firmly attached to each other, with no fissile tendency, W 



OBBIDIAN FORMATION. 55 

■feeling rock, resembling clay-slate wliicli has been in 
tntact with a trap-dike, and with a fracture of about the 
same degree of crystalline structure. ITiis rock, as well aa 
the following varieties, easily fuse into a pale glass. The 
greater part is honey-combed with irregular, angular, cavi- 
ties, so that the whole has a carious appearance, and some 
fragments 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 wrap round, one behind the other, the 
elongated carious hollows. 

Secondly, — A bluish gray or pale brown, compact, heavy, 
homogeneous stone, with an angular, uneven, earthy frac- 
ture; viewed, however, under a lens of high power, tlie 
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 tliem : they frequently expand into exceedingly 
thin cavities, which are often only just perceptible with a lens. 
The matter forming the white lines becomes better crys- 
tallized in these cavities, and Prof. Miller waa fortunate 
enough, after several trials, to ascertain that the white 
crystals, which are the largest, were of quartz,* 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 crystallization, and some fine, whit«, granular, crys- 
compoaed of different minerals, or of different shades of colour. The 
term laminated, in this chapter, is applied in these latter senses i where 
a homogeneous rock splits, as in the former sense, in a given direction, 
like clay-slate, I have used the term fissile. 

" Professor Miller informs me that the crj-slals which he measured 
had the faces P, z, m of the figure (147) given hy Haidinger iu 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. 



60 AftCBNsioiv. [chap. m. 

The small nodules also of obsidian are aometinies eztemiDf 
markcMl with ridges and furrows, parallel to the laminitioi 
oF tlic mass, )>ut always less plainly than the sphflerolitei 
Those* olisidian nodules are generally angular, with tbor 
edges blunted ; they are often impressed with the form of 
the adjoining spha*rulites, than which they are always larger; 
the separate nodules seldom appear to have drawn each otber 
out l)y exerting a mutual attractive force. Had I not fonndin 
some cases, a distinct centre of attraction in these nodules of 
obsidian, I should have been led to have considered them is 
residuary matter, left during the formation of the peail- 
stone, in which they are embedded, and of the sphflemlitie 
glol)ules. 

The sphuirulites and the little nodules of obsidian in these 
rocks, so closely resemble in general form and structure, con- 
cretions in sedimentary deposits, that one is at once tempted 
to attribute to them an analogous origin. They resemble ordi- 
nary concretions in the following respects, — in their extenial 
form — in the union of two or three, or of several, into an irre- 
gular mass, or into an even-sided layer, — in the occasional in- 
tersection 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, radi- 
ating 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 mag- 
nesian limestone, described by Professor Sedgwick.* Con- 
cretions 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 cer- 
tain points of attraction. Guided by this fact, I have endea- 
voured to discover whether obsidian and the spheerulites (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 
* Geological Transactions, vol. iii. part i. p. 37. 



OeSfDIAN FOSHATION. 



61 



lalyses, that obsidian contains on an average 76 per cent, of 
lea; from one analysis, that sphaerulit^s contain 79'12; 
two, that niarekanite containB 79'25 ; and from two 
ther analyses, that pearlstone contains 75'62 of silica.* 
ow, the constituent parts of trachyte, aa far as they can be 

stinguished, consist of feldspar, containing 66'21 of silica; 

"^Or of albite containing 69'09 ; of hornblende containing 
*'\65'27,-t- and of oxide of iron : so that the foregoing glaaay 
■'* concretionary substance, all contain a larger proportion of 
silica tlian that occurring in ordinary feldspathic or trachytic 
Tocks. D'Auhuisson,J also, has remarked on the large pro- 
portion of silica compared with alumina, in six analyses of 
obsidian and pearlstone given in Brongniart's Mineralogy. 
Hence I conclude, that the foregoing concretions have been 
fermed 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 
aurrounding mass, and thus producing the different con- 
cretionary varieties. From the well-known effects of rapid 
cooling^ in giving glassiness of texture, it is probably neces- 
sary that the entire mass, in cases like that of Ascension, 
should have cooled at a certain rate; but considering the 
repeated and complicated alternations, of nodules and thin 
layers of a glassy texture with other layers quite stouy or 
le, all witliin the space of a few feet or even inches, 
Iiardly possible that they could have cooled at different 
and thus have acquired their different textures. 
natural sphaerulites in these rocks, || very closely 

• The foregoing analyse 
alogie, torn, ii. p. 113 ; an 
Mineralogy. 

+ These analyseB are taken from Von Kobell's, Grundziige der Miiier- 
alogie, 1838. 

J Tmitfi de Giogn. torn, ii, p. 535. 

S This is seen in the maniifiiclory of common glass, and in Gregory 
Watts'a experiments on molten trap ; also on the natural surfaces of 
lava-streams, and on the Bide-walls of dikes. 

( I do not know whether it is generally known, that bodies having 



e taken from Beu dan t Traits de Miner- 
ne analysis of obsidian, from Phillips's 



62 ASCENSION. [chap. III. 

resemble those produced in glass, when slowly cooled, h 
some fine specimens of partially devitrified glass, in the 
possession of Mr. Stokes, the sphaerulites 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 homo- 
geneous and stony, with only an obscure concretionary 
structure. In the same specimen, also, sphserulites diflfering 
slightly in colour and in structure, occur embedded close 
together. Considering these facts, it is some confirmatioB 
of the view above given of the concretionary origin of the 
obsidian and natural sphserulites, to find that M. Dartigues,* 
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 aggregated. He is led 
to believe that this takes place, from the difficulty in re- 
melting sphaerulitic glass, without the whole be first tho- 
roughly pounded and mixed together ; and likewise from the 
fact, that the change takes place most readily in glass com- 
posed of many ingredients. In confirmation of M. Dartigoes' 
view, I may remark, that M. Fleuriau de Bellevue+ found that 
the sphaerulitic portions of devitrified glass were acted on both 
by nitric acid and under the blow-pipe, in a different manner 
from the compact paste in which they were embedded. 

exactly the same appearance as sphserulites, 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 -like zones of colour in the agate. In the obsidian of 
Ascension, the halves of a sphserulite often lie in different zones of 
colour, but they are not cut off by them, as in the agate. 

* Journal de Physique, tom. 59 (1804), pp. 10, 12. 

t Idem, tom. 60 (1805), p. 418. 



ODSIDIAIf FORMATION. 



63 



iparison of the obsidian Beds and altematinff strata of 
with those of other countriet. — I have been struck 
Haih 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 Mexico and Peru,+ 
and likewii^e the descriptions given by several authors}; of 
the trachytic regions in the Italian islands, agree with my 
ohservationa at Ascension. Many passages might have 
been transferred 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 Uke jasper .§ They all agree in 
the nodular or concretionary character of the obsidian, and 
of the passage of these nodoles into layers. They all refer 
to the repeated alternations, often in undulatory planes, of 
glassy, pearly, stony, and crystalline layers : the crystalline 
layers, however, seem to be much more perfectly developed- 

• Voyage en Hongrie, torn. i. p. 330 ; torn. ii. pp. 221 & 315 ; torn. 
iii. pp. 369,371,377, 381. 

t Esaai G^ognostique, pp. 1 76, 326, 328. 

HP. Serope,in Geological Transactions, vol. ii. (second series) p. 1!)5. 
Molt, also, Dolimieu's Voyage aux isles Lipari, and B'Auliuissun 
P de GfogD. tom. ii. p. 534. 
^n Mr Stokes' fine coUectiiin of obsidians from Mexico, I observe 
t the sphtenilites are generally much larger than those of Ascension; 
they are generally white, opaque, and are united into distinct layers : 
there are many singular varieties, differenl 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 hecome studded with minute white sphterulites, 
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 
Bphffirulites unite and form layers; the white ones always being irre- 
gularly disseminated. Some specimens at the Geological Society, 
said to belong to an obsidian formation from Mexico, have an 
earthy fracture, and are divided in the finest parallel laminie, by specks 
of a black mineral, like the augitic or homblendic specks in the rocka 



64 ASCBN8ION. [chap. IIL 

at Ascension, tlian in the above-named countries. Humboldt 
compares some of the stony beds, when viewed from a dis- 
tance, 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 perfecdy 
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 sphaerulitic 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 hardfy 
doubt that in all these cases, the obsidian and the sphaera- 
lites owe their origin to a concretionary aggre^tion of 
the silica, and of some of the other constituent elements, 
taking place whilst the liquified 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.*}- In 
these cases, the superficial parts are the most 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 pro- 
portion of silica is nearly the same as in the nodular or con- 
cretionary 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. DufrenoyJ that the 
exterior and interior parts of the same stream of lava, some- 
times difier considerably in their composition. Even should 
the whole body of the stream of obsidian turn out to be 

* Beudant*s Voyage, torn. iii. p. 373. 

t For Teneriffe, see Von Buch Descript. des isles Canaries, p. 1 84 and 
190 ; for the Lipari Islands, see Dolimieu's Voyage, p. 34 ; for Iceland, 
see Mackenzie's Travels, p. 369. 

t M^moires pour servir a une descript. G^olog. de la France, torn, 
iv. p. 371. 



UMINATtON OF TBACHTTIC HOCKS. 



65 



siiuikrly composed with nodular obsidian, it would only he 

necessary, in accordance with the foregoing facts, to suppose 

that lava in these instances had been ernpted with its ingre- 

I di ents mixed in the same proportion, as in the concretionary 

bidian. 



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 Mesico in Mr. Stokes' collection, 
with its external surface weathered* into ridges and fiirrows, 
corresponding with the zones of different degrees of glassi- 
ness : Huraboldt,f moreover, found on the Peak of Teneriffe, 
a stream of obsidian divided by very thin, alternating, layers 
of pumiee. 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. ScropeJ has described a remarkable columnar tracliyte 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 
colonrs ; 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 un- 

" MacCulloch states (Classification of Rocka, p. 531), that the exposed 
surfaces of the pitchstone dikes in Arran arc furrowed, " with undu- 
lating linea, resembling certain varieties of marbled paper, and which 
evidentlyreeult from some corresponding differenceof laminar structure." 

t Personal Narrative, vol. i. p. 222. 

t Geological TransactionB, vol. ii. (second scries) p. 195. 



66 LAMINATION OP VOLCANIC ROCKS [cHAP, III. 

aterraptedly for a great length in a vertical direction, and ap- 
parently parallel to the walls of the dike-like mass. Yon 
Bach* has described at Teiiciiffe, a stream of lava contuoing 
innumerable, thin, plate-like crystals of feldspar, which are 
arrang'cd like white threads, one behind the other, and which 
mostly follow the same direction : Doliuiieuf also states, tlml 
the gray 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. Phonolke, 
which I may observe is often, if not always, an injected 
rock, also, often has a fissile structure ; this is general^ 
due to the parallel position of the embedded crystals of feld- 
spar, but sometimes, as at Fernando Noronha, seems to be 
nearly independent of their presence.J From these facts 
I we see, that various rocks of the feldspathic series haie 
lather a laminated or fissile structure, and that it ocean 
ffboth in masses, which have been injected into overlying 
I fttrata, and in others which have flowed as streams of lava. 
The laniins of the beds, alternating with the obsidian at 
J Ascension, dip at a high angle under the mountain, at tbe 
I Ixise of which they are situated ; and they do not appear as 
iff they had been inclined by violence. A high inclination is 
t'Common to these beds in Mexico, Peru, and in some of 
■ Hie Italian Islands ;% on the other hand, in Hungary, the 

* Description des Ilea Canuies, p. 1B4. 

t Voyage ruk Ilea de LipMri, pp. 35 and 85. 

t In this case, and in Ihat of the lisisilo pumice-stone, the structure is 
very different from that in the foregoiog cases, where the lamins con- 
sifit of alternate layers of different composition or texture. In aoaa 
Bedimentary formations, however, which apparently are homogeneoui 
and fissile, as in glassy clay-slate, there is reason to believe, according 
to D'AubiiiBson, that the lominie arc really due to escessirely thia 
alternating, layers of mica. 

S See Phillipe' Mineralogy, for the Italian Islands, p. 136. For, 

Mexico and Pern, see Humboldt's Essai G^ognostique. Mr. Edward*, 

also, describes the high inclinal.ion of the obsidian rocks of the Ceno 

I del Nataja in Mexico, in the Vroe. of the Geolog, Soc, for June, 1838. 



OF THE THACHYTIC SERlEt'. 

layers are horizontal ; the iammee, 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. 1 doubt whether in any of these eases, the 
lamins 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 laminre ; 
and some of the layers, separating the sphrerulitic globules, 
have a scored appearance, as if produced by the grating of 
the globules, I have seen a specimen of zoned obsidian 
from Mesico, in Mr. Stokes' collection, with the surfaces of 
the best-defined layers streaked or furrowed with parallel 
lines ; and these lines or streaks precisely resembled 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 aph^enilites in laminated obsidian rocks from 
Mexico, and Mr. Scrope has 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 
• Geological TranBactions, vol. ii. (Becond seriee) p. 200, &c. Theee 
embedded fragments, in some instances, consist of the iBminated 
trachyte brokea off and " enveloped in those parts, which still re- 
mained liquid." Bcudant, also, frequently tefets, in hia great work on 
Hungary (torn. iii. p. 386), to trachytic tocks, irregularly spotted with 
fragments of the same varieties, which in other parts form the parallel 
ribhons. In these cases, we must suppose, that after part of the molten 
mass had assumed a laminated structure, a fresh irruption of lava broke 
mass, and involved fragments, and that subsequently the wliolc 
lelaminated. 

F 2 



LAMINATION OP VOLCANIC DOCSB (CBAP. tU 

rate air-cell, or each fibre in pnraice-stone,* ehonlii bi 
■D out ill iLc direction of the moving mass ; it is by m 

Mins at first obvious why such air-cells and fibres shonU 
i be arranged by the movement, in the same planes, in h 
absolutely straight and parallel to each other, and ofteiol 
fxtrcme tenuity ; and still less obvious is it, why sach layeM 
should come to be of slightly different composition i 
diiferent teaturea. 

In endeavouring to make out the caose of the lamina^ 
of these igneous feldspatbic rocks, let us return to the fiut 
80 minutely described at Ascension, We there see, 
some of the thinnest layers are chiefly formed by numeros^ 
exceedingly minute, though perfect, crystals of different 
minerala ; that other layers are formed by the union of d" 
ferent kinds of concretionary globules, and that the layen 
thuB formed, often cannot be distinguished from the ordinuj 
feidspathic and pitchstone layers, composing a large portiofl 
of the entire mass. The fibrous radiating structure of thj 
Bphterulites seems, judging from many analogous cases, ti 
connect the concretionary and crystalline forces : the s 
rate crystals, also, of feldspar all lie in the same pandit 
planes-i" These allied forces, therefore, have played t 
portant part in the lamination of the mass, but they cannot In 
considered the primary force ; for the several kinds of no 
dules, both the smallest and largest, are internally zoi 
with excessively fine shades of colour, parallel to the L 
nation of the whole ; and many of them are, also, extemallj 
marked in the same direction with parallel ridges and fiiB 
rows, which have not been produced liy weathering. 

Some of the finest streaks of colour in the stony layer^ 
alternating with the obsidian, can be distinctly seen 

• Dolimieu's Voyage, p. 64, 

t The formation, indeed, of a large crystal of any mineral ii 
of mised composition, implies an aggregation of the requisite i 
allied to concretionary action. The cause of the eryBtala of feldspar i 
these rocks of AscenBioii, being all placed lengthways, is protably t" 
same with that, which elongates and Hattcne all the hro^rn Bphssmlili 
globules (which behave like feldspar under the blow-pipe) in this M 
direction. 



OF THE THACHYTIC BEHIES. 69 

incipient crystallization of the constituent minerala. 
extent to which the minerals have crystallized, can, 
, he distinctly seen to he connected with the greater or 
size, and with the numher, of the minute, flattened, cre- 
nulated 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 crystallization is much 
iavoured hy space. Hence, I conclude, that, if in a mass of 
cooling volcanic rock, any cause produced in parallel planes 
a numher of minute fissures or zones of less tension, (which 
from the pent-up vapours would often be expanded into 
crenulated air-cavitiea,) the crystallization of the constituent 
parts, and probably the formation of concretions, would be 
fiaperinduced 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 tliin alternate layers of 
obsidian and pumice described hy Humboldt, and of the 
small, flattened, crenulated air-cells in the laniinated 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' col- 
lection, I have seen a Vieautiful 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 de- 
pending on the number of microscopically 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 
difierent tension, in masses semi-liquefied by heat. In a 



fragment of devitrified glass, I have obaeired layers of 
splisralttca which appeared, from the manner in which thej 
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 pro- 
duced the fissures, continued whilst the injected rock 
remained fluid. Guided, however, by Professor Forbes't 
clear description of the zoned structure of glacier-iee, fiu" 
the most probable explanation of the laminated structure rf 
these teldspathic rocks appears to be, that they have been 
stretched whilst slowly flowing onwards in a pasty conditionj; 
in precisely the same manner as Professor Forbes believes, 
that the ice of moving glaciers is stretched and fissured, In 
both cases, 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 laminse are ren- 
dered distinct by the subsequent congelation of infiltrated 
water, in the stony feldspathic lavas, by subsequent crystal- 
line and concretionary action. The fragment of glas^ 
obsidian in Mr. Stokes' collection, which is zoned widi 
minute air-cells, must strikingly resemble, judging from 
Professor Forbes' descriptions, a fragment of the zoned 
ice ; and if the rate of cooling and nature of the mass hsd 
been favourable to its crystallization or to concretionary 
action, we should here have had the finest parallel zones i 
diSerent composition and texture. In glaciers, the lines 

" Mem. pour aervir, &c,, tom, iv. p. 131. 

t Edinburgh New Phil. Journal, 1842, p. 350. 

i I presume that this ia nearly the same esplanation which L 
Bcrope had in his mind, when he speaks (Geolog, Transact. toI. ii 
second series, p. 228) of the ribboned structure of his trachytic rocb, I 
having arisen, from " a linear extension of the mass, while in a state of I 
imperfect liquidity, coupled with a concretionary process." 



OP THS TBACHYTIC HBRIEB. 



71 



poroQS ice and of minute crevices seem to be due to an 
incipient stretching, caused by the central parts of the frozen 
stream moving faster thaji the sides and bottom, which are 
retarded by fiiction : 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 feld- 
spathic lavas with horizontal laminfe, we see an analogous 
case ? All geologists, who have examined trachytie regions, 
bave 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 characterized, 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 pos- 
sessed a high degree of fluidity, are not,* bke the feldspatbic 
lavas, divided into laminte of different composition and texture. 
Moreover, in the augitic series, there never appears to he 
any tendency to concretionary action, which we have seen 
plays an important part in the lamination of rocks of the 
trachytie series, or at least in rendering that structtire 
apparent. 

"Whatever may be thought, of the explanation here 
advanced of the laminated structure of the rocks of 
the trachytie 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 oside of iron, — and 

Basaltic lavas, and many other rocks, are not unfrequently divided 
I thick lamios or jilatcs, of the same composition, which are either 
curved ; these heing crossed by vertical lines of fiKSure, 
acs become united iato columns. This structure seems related, 
jorigin, to that by which many rocks, both igneous and Bedimeatary, 
1>econie traversed by parallel systems of fissures. 



r 



72 LAMINATION OF TRACHYTIC ROCKS. [CHAP. III. 

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 certaiiily 
known, that the laminse have originally been formed with 
their present high inclination. Facts of this nature are mani- 
festly of importance, with relation to the structural 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 rV. 



ST. HELENA. 

Lavas of the /eldspaihte, basallic, and submarine series — Section of Flag' 
alaff Hill andofthe Bam — Dikes — Turk's Cap and ProsperoKS Bays — 
Basaltic ring — Central crateriform ridge, teilh an rntemal ledge and a 
parapet — Canes of phonolite — Superficial bedx of calcareous sandstone — 
Extinct lattd-shtils — Beds of detritus — Elevation of the land — Denuda- 
tion — Craters of elevation. 

The whole island is of volcanic origin ; its circumference, 
according to Beatson,* is about twenty-eight milea. The cen- 
tral 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 esisted as streams of 
lava, but the greater number were probably ejected in the 
form of scorisB and ashes : other beds of a purple tint, por- 
phyritic with crystal-shaped patches of a white, soft sub- 
stance, which are now unctuous, and yield, like wax, a 
pohshed streak to the nail, seem once to have existed as solid 
claystone-porphyryes : the red argillaceous beds generally 
have a brecciated structure, and no doubt have been formed 
by the decomposition of scorite. Several extensive streams, 
however, belonging to this series, retain their stony character- 
• Governor Beatson's Account of St. Helena. 



»T. HKLK5A. 



tlieiie are either of a blackish-green colour, vitL i 
acicular crystala of feldsjiar, or of a very pale tint, I 
almost contpoMd of niiaiite, often scaly, cr^'stals of feld 
abounding witli microscopical black specks; they are gi 
rally compact and laminated; others, however, of i 
composition, are cellular and somewhat decomftosed. 
of these rocks contain large crystals of feldspar, or havefl 
harsh fracture peculiar to trachyte. These feldspathic bi 
and tuffs, are the uppermost or those last erupted; 
merahle dikes, however, and great masses of molten n 
have suhsecjuently been injected into them. ThevconTOJ 
as they rise, towards the central curved ridge, of which ■ 
point attains the elevation of 2700 feet. Tliis ridge ii 
liighest land m the island ; and it once fonited the no 
rim of a great crater, whence the lavas of this series fl 
from its ruined condition, from the southern half han 
been removed, and from the violent dislocation which ] 
whole island has undergone, its structure is rendered * 
obscure. 

Basaltic serieg. — ^The margin of the island is formed b 
rude circle of great, black, stratified, ramjiarts of b 
dipping seaward, and worn into cliffs, which are often n 
perpendicular, and vary in height from a few hundred feet ill 1 
two thousand. This circle, or rather horse-shoe sliaped ring; I 
is open to the south, and is breached by several other h 
spaces. Its rim or summit generally projects little i 
the level of the adjoining inland country ; and the more n 
feldspatiiic lavas, sloping down from the central heig 
generally abut against and overlap its inner margin ; 
north-western side of the island, however, they apM 
(judging from a distance) to have flowed over and e 
cealed portions of it. In some parts, where the baf 
ring has been breached, and the black ramparts { 
detached, the feldspathic lavas have passed between t 
and now overhang the sea-coast in lofty cliffs. The basall| 
rocks are of a black colour and thinly stratified ; they i 
generally highly vesicular, but occasionally compact ; s 



EAmuato hATAs. 76 

of them contain Dumeroas ciystab of glsasy feldspar and 
octahedrons of titaniferous iron ; others aboandwith crystals 
of sngite and grains of olivine. The resides are frequently 
lined with niinatc crystals (of chabasie?) and even become 
unygdaloidal vith Iheni. The stnntus are separated from 
each other by cindeiy matter, or by a bright red, friable, 
nliferous tuff, which is marked by successive lines like 
those of aqueous deposition ; and sometimes it has an 
obst^ure, concretionary structure. The rocks of this basaltic 
series occur nowhere except near the coast. In most volcanic 
tlislricts the tracbytic lavas are of anterior origin to the 
hasallic ; but here we see, that a great pile of rock, closely 
fslated in composition to the trachytic family, has been 
Wipted subsequently to the basaltic strata : the nuiubcr, 
however, of dikes, abounding with large crystals of augitc, 
with which the feldspatbic lavas have been injected, shows 
perhaps, some tendency to a return to the more usual order 
of superposition. 

Basal tubmarine lavas. — ^The lavas of this basal series He 
immediately beneath both the basaltic and feldspathic rocks. 
According to Mr, Scale,* they may be seen at intervals on 
the Bca-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 ehccts are either closely united together, or 
are separated from each other by beds of scoriaceoiis rock 
and of laminated tuff, frequently containing well rounded 
\ fragments. The interstices of these beds are filled with 
I gypsum and salt ; the gypsum also, sometimes occurring in 
j thin layers. From the large quantity of these two sub- 
stances, from the presence of rounded pebbles in the tuffs, 
and from the abundant amygdaloids, I cannot doubt that 

• "Geognosy of the Island of St. Helena."' Mr. Scale liaa con- 
I Btructed a gigantic model of St. Helena, well worth vUiting, which is 
now deposited at Addiscombe College, in Surrey. 



m 



HT. BBL£NA 



[chap, n 



th(.-BG bagal volcanic strata flowed beneath the sea. Tl 
remark ought perhaps to be extended to a part of the snpi 
iuuunihent basaltic rocks ; hut on this point, I was not at 
to obtain clear evidence. The strata of the basal 
wherever I exaniiiied them, were intersected by an e: 
ordinary number of dikes. 

Flagstaff Hill and the Bam. — I will now describe soi 
the more remarkable sections, and will comjnence with 
two hills, which form the principal external feature on t 
north-eastern side of tlie island. The square, angular « 
line, and black colour of tlie Bam, at once show that 
belongs to the basaltic series ; whilst the smootli, coni 
figure, and the varied bright tints of Flagstaff Hill, rend 
it equally clear, that it is composed of the softened, fd 
spathic rucks. These two lofty hills are connected (as 
shown in the accompanying wood-cut) by a sharp rit^ 
No. 8. 



-ru^l^ 




The double Hum npi 



which is composed of the ruhbly lavas of the basal sej 
The strata of this ridge dip westward, the inclination 
coming less and less towards the Flagstaff; and the n] 
feldspathic strata of this hill can be seen, though with son 
difficulty, to dip conformably to the W.8.W. Close to ti 
Bam, the strata of the ridge are nearly vertical, but s 
much obscured by innumerable dikes ; under thb hHl, thi 
probably change from being vertical, into being inclined in 
an opposite direction ; for the upper or basaltic strata, whit 
are about 800 or 1000 feet in tluckness, are inclined norti 
eastward, at an angle between thirty and forty degrees. 
This ridge, and likewise the Barn and Flagstaff Hills, ai 



FIAGSTAPF HILL AND THE 1 



^terlaced 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 large 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, unhke true pitchstone, friscs 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 they seem to preserve a nearly uniform thickness rf- 
Mr. Scale states, that one near the Barn, in a height of 
1260 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 instances 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 & degree I never saw equalled any where 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, inter- 
laced by other dikes. 

Prom the complexity produced by the dikes, from the 

■ This circumstance has been observed (Lyell, Principles of Geology, 
vol. iv. chap. x. p. 9) in the dikes of the Atrio del Cavallo, hut appar- 
ently it ia not of very common occurrence. Sir G. Mackenzie, however, 
Btates (p. 372, Travels in Iceland) that all the veins in Iceland have a 
" black vitreous coating on their sides." Capt. Carmichnel, speaking 
of the dikes in Tristan D'Acunha, a volcanic island in the southern 
Atlantic, says (Linnsan Transactions, vol. sii. p. 485) that their sides, 
" where they come in contact with the rocks, are invariably in a semi- 
vitrified state." 

t Geognosy of the Island of St. Helena, plate 5. 



78 BT. HELENA. [CHAP. IV, 

Mgh 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 beaa I 
the case, that the summit of Flagstaff Hill, once formed thtfl 
lower extremity of a sheet of lava and ashes, which wert"" 
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 Flag- 
staff Hill, must have been upturned at least twelve hnndred^ 
feet, and probably much more, for the great truncated dikolfl 
on its summit show that it has been largely denuded. T^H 
summit of this hill now nearly equals in height the crate^H 
form ridge; and before having been denuded, it was probaH^^ 
higher than this ridge, from which it is separated by ^M 
broad and much lower tract of country : we here, thereforen 
see that the lower extremity of a set of lava-streams have beaKj 
tilted up to as great a height as, or perhaps greater height th^^f 
the crater, down the flanks of which they originally flowc^H 
I believe that dislocations on so grand a scale are extrem^^J 
rare* in volcanic districts. The formation of such nnmbeflH 
of dikes in this part of the island, shows that the sur&e^l 
must here have been stretched to a quite extraordinaMw 
degree : this stretching, on the ridge between Flagstaff aaaj 
Bam Hills, probably took place subsequently (thoo^H 
perhaps immediately so) to the strata being tilted ; for hadH 
the strata at that time extended horizontally, they would iiu 
all probability have been fissured and injected transTersely:^] 
instead of in the planes of their stratification, Althoo^kl 
the space between the Barn and Flagstaff Hill, presents tm 
distinct anticlinal line extending north and south, and thou^^l 
most of the dikes range with much regularity in tbe saiiufl 

' M, Constant Prevoet (Mem. de la Soc. G^olog. torn, ii.) obaerrcffl 
lliat " tes produjts volcaniques n'ant que localcment et rarement mGaM 
iitangi le sol, d travels lequel ils Be aont fait jour." ^fl 



TUBKS' CAP AND PHOBPEBOrS BAYB. 79 

nevertheleBs, at only a mile due south of the ridge, the 
Btrata lie undisturbed. Hence the disturbing force seems to 
have acted under a point, rather than along a line. The 
manner in which it has acted, is probably explained by the 
structure of Little Stony-top, a mountain 2000 feet high, 
situated a few miles southward of the Barn; we there see, 
even from a distance, a dark- coloured, sharp, wedge of com- 
pact columnar rock, with the bright-coloured feldspathic 
Btrata, 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 
tkat a similar body of rock Ues injected, beneath the ridge 
connecting tlie Barn and Flagstaff, the structure there ex- 
hibited would be explained, 

Turks' Cap and Prospei-ous 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, ou the brown-coloured, por- 
phyritic beds of the basal series, and in another part, on a 
fissured mass of highly scoriaeeous and amygdaloidal rock, 
which seems to have formed a small point of eruption be- 
neath the sea, 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 Prosperous 
Hill and the Bam, as represented in this wood-cut, consists 

No. 9. 

Flegitaff HLU. 



of lofty cliffs, composed of the lavas of the upper or feld- 
spathic series, which rest, though unconfomiably, on the 



(itMl ■iiliiiiitriric Htmtn, ax we have seen that Aef it 
FlH((Ntnfl' Kill. DiH'iTf^iitly, however, from in that UI,A| 
iiligMT Mtrntrt nrc iiciirly liorizontal^ gently rising Bbwh^I 
Intrlor iif llir inliiiid ; ami ihcy sre compoeed of gnod 
hWk, nr iiKtrc ooiiimuiil}', pule-brown, i iim|ii In 
inntJ'od (>r Nuni'iird ami Iiighly coloured maCOK Tli 
liriiiTii-nildiiruiI, c'oin{>act lavoH, consist almost tate i j ^ 
Htimll [{'■■■■■'■''''''■K tcaIcK, or of minute acicolar 
of relilN|mr, pliircil l'Ihiw tiy the side of each other, 
•lH)iiii(liiig willi niiiiiito black opecks, appareutlj of ha 
lilnndo. Till' Imwiltic Mrnta of Prosperous Hill project o 
R IIiiIk hImivii lliit level of the gently-eloping, fehUpadk 
HtrnHiii*, whii'li wind round iind abut against their aptu 
iidK""> 'Ibc ■iii'lirintion of tbc basaltic strata seems to be |i 
(frcfir, til have Iicmii ciiiiiti'd by ibeir having flowed down 
»lo|ii<, and lliey iiiiiitt have bctMi tilted into their present po^ 
ti"n, biifoNi tlin (irupdon of the feldspathic streams. 

ilnmUir rintj. — Proeeeding round the island, the larasl 
tlirt iippiir wriitN, Houtliwurd of Prosperous Hill, ovei 
thn ana In lolty prtHUpicuv. Further on, the headlanil, calM 
(tritat Mt»iiy>liip, in coiupoaed, as I believe, of basalt; 
I^trijt HftitKo Point, on tbc itdand side of which, the coIouni< 
IkkIh abut. On the Huiithoru side of the island, we seeUi^ 
baNullic Ntrala of tbu Suntb Ilarn, dipping obliquely seawaid 
lit a eontildiTublu angle; thiu headland, aleo, stands a 1 
ftbuve tlie b-vel of the more modern, feldspathic lavas, Fttfs 
ther on, a large t>|iuec of coast, on each side of Sandy B^y^ 
IiaM been much denuded, and iherc seems to be left only tl 
basal wreck of the groat, central crater. The basaltic strata 
rcapi>ear, with their seaward dip, at the foot of the hill calledi 
Man -and- Horse ; and thence they are continued along thft 
whole north-western coast to Sugar- Loaf Hill, situated nesK 
to the Flagstaff; and they everywhere liave the same 6 
ward inclination, and rest, in some parts at least, on tl# 
lavas of the basal series. We thus see tliat the circumfis^ 
ence of the island, is formed by a much-broken ring, tiC 
■""♦'■"r a horse-shoe of basalt, open to the south, and i ' 



RIWO OF BASALTIC CLIPFS. 81 

rupted on the eastern side by many wide breacliea. The 
breadth of this marginal fringe on the north-western side, 
Trhere 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 lee- 
wai'd. 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 np 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. 
line, coincident with the present longest axis of the island. 

The central curved ridge. — This ridge consists, as before 
remarked, of gray feldspathic lavas, and of red, brecciated, 
ai^iliaceous tuffs, like the beds of the upper coloured series. 
The gray 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 over- 
hang the coast at Prosperous Bay. Considerable intervals of 
time appear to have elapsed, judging from the marks of de- 
nudation, 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 
tuff, covered by gray, decomposed, feldspathic lavas. 



62 



ST. KBLBMA. 



[OHAP. IT. 



with only n thin earthy layer interposed between them, 
un Btljoining jiart, I noticud a trap-dike, four feet wide, 
off and covered uj* hy the fuUlspathic lava, as is r» 
in the wood-cut. The ridge ends on the eastern nde 

No. 10. 



t—Onj fl-lilifUMc Un. 

a—i lartq-, one inch In UilcliDMi, ot t rti'UiU earthy muirr 

B— BrecclUed, red. Brglltuwaui liilT. 

a hook, which is not represented clearly enough in any B 
which I have seeu; towards the western end, it gradni 
elopes down and divides into several suliordinate ridj 
Tile best defined portion between Diana's Peak : 
Nest Lodge, which supports the highest pinnacles in 
island varying from 2000 to 2700 feet, is rather lees t' 
three miles long in a straight hue. Throughout this q 
the ridge has a uniform appearance and structure ; its i 
Tature resembles that of the coast-line of a great bay, l 
made up of many smaller ciir^'es, all open to the s( 
The northern and outer side is supported by narrow ri^ 
or buttresses, which slope down to the adjoining com" 
The inside is much steeper, and is almost precipitous ; 
formed of the basset edges of the strata, whicli gently • 
cline outwards. Along some parts of the inner sitle, a 1 
way beneath the summit, a flat ledge extends, which u 
in outline the smaller curvatures of the crest. Ledgej 
this kind occur not unfrequently within volcanic craters, i 
their formarion seems to be due to the sinking down ( 
level sheet of hardened lava, the edges of which rem 
(like the ice round a pool, from which the water has b 
drained) adhering to the sides.* 

• A most remarkable instance of this alructnre is described in El 
Polyneaian Researches (second edit), where an admirable drawing 



CBNTRAL CURVED RIDOE. H^S 

In some parts, the ridge is surmounted ))y a wall or para- 
pet, perpendicular on both sides. Near ])iana*s Pt^ak tliirt 
wall is extremely narrow. At the Cialapa^i^os Ar(*lii|»('la};o 
I observed parapets, havinio^ a (piite similar structiiro and 
appearance, surmounting several of the craters ; oiu\ wliicli 
I more jMirticuhirly examined, was composed of >i;lossy, hmI 
scoriae firmly cemented together ; being externally pcrp^'iuli- 
cnkr, and extending round nearly the whole circiinifcTciire 
of the crater, it rendered it almost iuacoessible. The Peak 
of Teueriffe and Cutojmxi, accor<lin«i^ to Ifiimhohlt, are 
sinularly constructed; he states* that '*at their summits a 
circQiar wall surrounds the crater, which wall, at a distan<*o, 
luis the ap{>earance of a small cylinder placed on a tnineated 
cone. On Cotojiaxif this peculiar structure is visible to the 
naked eye at more than 2000 toises' distance; an<l no ])erson 
luM ever reached its crater. On the Peak of Tenerilie, tho 
parapet is so high, that it would be impossible to reaeh the 
calderoy if on the eastern side there did not (;xist a breiieh.*' 
The origin of these circular parapets, is probalily due to the 
heat or vapours from the crater, ])enctrating and hardening 
the sides to a nearly equal depth, and aflerwards to the 
mountain being slowly acted on ])y the w(*atlu*r, which would 
leave the hardened part, projecting in the form of a cylinder 
or circular parapet. 

From the points of structure in the; central ridge, now 
enumerated, — ^namely, from the convergc^nce towards it of 
the beds of the upper series, — from the lavas ihora bec<nn- 
ing highly cellular, — from the flat ledg(% (.'xt<*nding along itM 
inner and precipitous side, like that witlihi some still active 
craters, — from the parapet-like wall on its summit, -and 
lastly, from its peculiar curvature, unlike that of any com- 
mon line of elevation, I cannot donlit that this curved ridg(« 
forms the last remnant of a great crater. In endeavouring, 

given of the successive ledges or terraces, on the borders of tho Itnuirnup 
crater at Hawaii, in the Sandwich iHlandn. 

* Personal Narrative, vol. i. p. 171 • 

t Humboldt's Pietoresque Atlas, folio, i)l. 10. 

u 2 



•4 ??. ggLf^ j.. [chap. IV. 

ho-r^T:r. :: -n*-? .'* '■ni*^^ ri-ifzf?- -^ne i* soon baffled; its 
▼-r*r<rn -rT'^::i:_'7 x^iiiio—j *c ^t?* 'iowix. and branching 
■"•.: rirr? r.«:^T^. t\"-:l'> : "-r "S«fa-o.xijit : tbe eastern end 
L-? ii:7»r rirrr-L "■•:: i: > i-j i -"rrLe cercer defined. Some 
a: :»7:in-«'e? >!■: zi-r •; *::-. Ti>^, tha: the soatbem wall of 
:ir :n:rr ■ ine: -be rresr!!': Sir? n^ear Ne^ Lodcre: in this 
rsL^ "sZk iTi'.rT misT biT-f rtftr?! TitrorlT three miles lono^. and 
*••:-: a =ilc- An*! i-ialr in Vrvar-Tli. Had the denudation of 
ti-r rj:^-^. a::i-i Tr.^ i-e^v.riir^siTi'rn •>? hs eonstitnent rocks, 
Tr:-^tei-^i a f-ew ^'r^?? rirrii^r. an-i had this ridge, like 
•rrT-rT^I .:i-rr znn^ .z Th«r >Li:>i. l^tn bn>ken ap by grest 
c:k*> ai.'i ::iis?-erj ■:' ir.'-i^r.<A niict^r, we ^hoold in vainhaTe 
er.:-=A-. .-ir^i :.:• .Ii-h.i ttt :t* :ne namre. Even now we 
hiTr: irr-7.. rLa: a: Fli^^'-i- HlL the lower extremitv and 
IT- -'•r ':_•:&::: p->r:::'n *. f . "r sheet of the empted matter, hts 
Ij'f^T. npLeiTel. TO a? iT^it a hei^rhr a< the crater down 
w:,;cb it n>-arei, and pr.-':si* ly even to a srreater height. It 
Vi ir.t'r.'^rin^ the* to trace the steps, by which the stractaie 
of a T'/icanic district be\N"»mes obscured, and finally ohK- 
XhTsnTfA : v.* near to tliis bst sta^e is St. Helena, that I 
b*rl:eve ao one has hitherto sus|vcte«l. that the central ri<^ 
or ax-T of tbe Island, is the last wreck of the crater, whence 
t:;'; Tft^/rt modem volcanic streams were poured forth. 

Tilt: 'ip'^^.x hollow space or valley southward of the central 

f:'}n':f\ ridjre, across which the half of the crater must once 

hixvt: (:xU:Tiflf:f], Is formed of bare, water-worn hillocks and 

t'A'j:/^, of red, yellow, and bro\*-n rocks, mingled together in 

'rlja//*-likc' confosion, interlaced by dikes, and without any 

r«?jfijjar stratification. Tbe chief part consists of red deeom- 

ff^/iiistif scoriae, associatefl with various kinds of tuff and 

y^rllow ari^ilaceous beds, full of broken crystals, those of 

ao^fit/j }}i6u'^ particularly large. Here and there masses of 

*»!l(lily r^;Ilij]ar and amygdaloidal lavas protrude. From 

''«'r of tbe* ridges, in the midst of the valley, a conical preci- 

'•f bill, ^sailed Lot, boldly stands up, and forms a most 

r and conspicuous object. It is composed of phono- 

ifJcfl in one part into great curved laminae, in another, 



' PHONOLITE. 



85 



into angular concretionary balls, and in a third part, into 
outwardly radiating columna. At its base the strata of lava, 
tuff', and scorise, dip away on all sides:* the uncovered 
portion is 197t feet in height, and its horizontal section 
gives an oval figure. The phonolite is of a greenish-gray 
colour, and is full of minute acicular crystals of feldspar ; in 
most parts it has a conclioidal fracture, and is sonorous, yet 
it is erennlated with minute air-cavities. In a S.W. direc- 
tion 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 hne of 
fissure. It is, moreover, remarkable, 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 
iar as is apparent, with any hne 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. Beside those just mentioned, and the hypo- 
thetical one beneath Flagstaft' Hill, there is Little Stony-top 

• Abich, in hisViewa of VeauviuB (plate vi.), lias shown the manner 
in which beds, under nearly similar circumBtances, are tilted up. The 
upper beds are more turned up than the lower ; and he accounts for 
this, hy showing' that the lava insinuates itsi^lf horizontally between 
the lower beds. 

fThis 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 he 1444 




1^^^ era]il 
^^H qnali 



ST. nstem. [chap. tf. 

Pand others, as I hare rea.son to believe, at the Man-Bud* 

Horse, and at High Hill. Most uf these masses, if not aS 

of them, have been injected, subsequently to tlie last Tole&nie 

erD]itions from the central crater. ITie formation of conical 

jeses of rock on lines of fissure, the walls of which are iB 

noBt cases parallel, may probably be attributed to ine- 

r qualities in the tension, causing small transverse fissures; ami 

B [JointB of intersection, the edges of the strata would 

naturally yield, and }>e easily turned npwanls. Finally, 

I may remark, that hills of pbouolite everywhere are apt* 

to assume singular and even grotesque shapes, like that uf 

Lot: the peak at Fernando Noronha offers an instance; 

at St. Jago, however, tlie cones of jiLonolite, though tapffl 

ing, liave a regular form. Supposing, as seems jjrohabl 

that all such hillocks or oliehsks have originally 

injected, whilst liquefied, into a mould formed }>y yieldni| 

strata, as certainly has been the case with Lot, how are WS 

to account for the frequent abmptness ami singularity o( 

their outlines, compared with similarly injected masses i 

, greenstone and basalt? Can it be due to a less perfeC 

_ degree of fluidity, which is generally supposed to be chants 

^^H^ teristic of tlie allied, trachytic lavas ? 

^^^B Superfieial deposits. — Soft calcareous sandstone occurs i 
^^Hiextensive, though thin, superficial beds, both on the nort 
^^^Hand southern shores of the island. It eonsista of i 
^^^K-juinute, equal-sized, rounded particles of shells, and t 
^^^P organic bodies, which partially retain their yellow, brown, i 
^^^ pink coloiu^, and occasionally, though very rarely, prea 
r an obscure trace of their original external forms. I in v 

endeavoured to find a single unrolled fragment of a ahd 
The colour of the particles, is the most obvious charactfi 
by which their origin can be recognized, the tints being a 
I fected (and an odour produced) by a moderate heat, in 

same manner as in fresh sheila. The particles are cementi 
together, and are mingled with some earthy matter: 

• D'Aubuisson, in his Traits de G^ognosie (torn. ii. p. 540), 
lulj remarkB that tbia is the case. 



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 oc- 
cupy; and no doubt they thus originated, which explains 
the equal size and minuteuess 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 dnst could he 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 tliat of Ascension, was favourable to the accumu- 
lation of shelly detritus. Some of the beils of this limestone 
are between 600 and 700 feet above tlie sea ; but part of this 
height may possibly he due to an elevation of the land, sub- 
sequent 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, stalagmitie limestone. At the Sugar- Loaf quarry, 
fragments of rock on the ailjoining slopes,* have been 
thickly coated by successive fine layers of calcareous matter. 
It is singular, that many of these pebbles have their entire 
surfaces coated, without any point of contact having been 
left uncovered ; hence, tliese pebbles must have been bfted 
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 

• In the earthy detritus on several parts of this hill, irregular masses 
of very impure, crystallized sulphate of lime occur, is 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 600 and TOO feet. 



[chap. IV. 

pebbles. Von Buch has described a eoinpact limestone at 
Lanzarote, which seems perfectly to resemble the stalagmittc 
deposition just mentioned : it coats pebbles, and in parts b 
finely oolitic : it forms a far-exteuded layer, fi-om one inch to 
two or three feet in thickness, 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,* that it is not found in hollows, but only on the 
nnhroken and inclined surfaces of the mountain. He be- 
lieves, 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 t 
through finely comminuted shells : for when sand is bloK 
on a much exposed coast, it always tends to accumulate o 
broad, even surfaces, whicli offer a uniform resistance to A 
winds. At the neighbouring island, moreover, of Fenrt 
v'entura,'t- there is an earthy limestone, which, according t 
Vou Buch, is quite similar to specimens which he has B 
from St. Helena, and which he believes to have been forin^ 
by the drifting of shelly detritus. 

The upper beds of the limestone, at the above-mentione 
quarry on the Sugar-Loaf Hill, are softer, finer-grained an 
less pure, than the lower beds. They abound with fra^ 
ments of land-shells, and with some perfect ones ; they c<M 
tain, also, the bones of birds, and the large egg8,J apparent) 
of water-fowl. It is probable that these upper beds r 
mained long in an unconsolidated form, during which tim 
these terrestrial productions were embedded. Mr. G, 1 
Sowerby has kindly examined three species of land-shelll 
which I procured from this bed, and his descriptions ar 

* Description des Isica Canaries, p. 293. 

t Idem, pp. 314 and 374. 

t Colonel Wilkes, in a catalogue presented with some specimens I 
the Geological Society, states that as many as ten eggs were found b 
one person. Dr. Buckland has remarked (Reolog. Trans, vol. v. p. 4Ji 
on these eggs, j. 



EXTINCT LAND-8HELLB. 89 

giyen in the Appendix. One of them is a Succinea, identical 
with a species, now living abundantly on the island : the two 
others, namely, Cochlogena fossilisy and Helix biplicatoy 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-shelh. — Land-shells, all of which ap- 
pear to be species now extinct, occur embedded in earth, in 
several parts of the island. The greater num])er have been 
found at a considerable height on Flagstaff Hill. On the 
N.W. side of this hill, a rain-channel exposes 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 l(»ss ]>lnck 
earth, abounding with young and old shells, and witli their 
fragments: part of this earth is slightly consolidated by 
calcareous matter, apparently due to the partial (le(*oin))o- 
sitioQ of some of the shells. Mr. Scale, an intelligcMit resi- 
dent, who first called attention to these shells, gav(* me ii 
large collection from another locality, where tlui Hlu»llrt 
appear to have been embedded in very black earth. Mr. (1. 
R. Sowerby has examined these shells, and has <leH(*ril)ed 
4em in the Appendix. There are seven sjMJcit's, namely, 
^ne Cochlogena, two species of the genus Co('lili<M)pa, and 
ft>ur of Helix : none of these are known in a n^'ent state, or 
tave been found in any other country. The* smalh^r sjiecieH 
"^ere picked out of the inside of the large shells of the 
Cochlogena auris-vulpina. This la8t-mentione<l speci<»s is in 
^^aany respects a very singular one; it was classc>d, ev(?n by 
-^LAmarck, in a marine genus, and having thus bec^n mistaken 
*V)r a sea-shell, and the smaller accompanying species having 
^een overlooked, the exact localities where it was fontid, 
l^ave been measured, and the elevation of this island tlnm 
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 procMircnl from ano- 
t:Tier locality by Mr. Scale. As this Cochlogcma is a large 



ST. HELENA. 



[ClIAP. 



Pud conspicuous shell, I particularly enquired troro sever 
intelligent counlryinen nhetlier they had ever seeo it alire 
they all assurcil me that they had not, and they would a 
even believe tliat it was a land animal : Mr. Scale, moreove 

^who was a collector of sheik all his life at St. Helena, neni 
net with it alive. Possibly some of the smaller epecies mi 

I torn out to be yet living kinds ; but, on the Other hand, i 
two laud-shells which are now living on the island i 
numbers, do not occur embedded, as far as is yet knowi 

. with the extinct species. I have sliown iu my Journal,* thi 
; extinction of these land-shells possibly may not be i 
incicnt event j as a great change took place in the state t 

■tile island about 120 years ago, when the old trees died, an 

Iwere not rejilaced by young ones, these being destroyt 
' the goats and hogs, wliich had run wild iii numbc 

' from the year 1602. Mr. Seale states, that on. Flagsti 
Hill, where we have seen that the embedded laud-shells a 
especially numerous, traces are everywhere discoveraU 
which plainly indicatfi that it was onee thickly duthed n 
; at present not even a bush grows there. The tiiifl 

Kbed of black vegetable mould wliich covers the shell-bed, a 

Ktiie flanks of tliia hill, was probably washed down I 

Upper part, as soon as the trees perished, and the a 

Biftfibrded by them was lost. 

violation of the la/td. — Seeing that the lavas of the hai 
leries, which are of submarine origin, are raised above t 

Mevel of the sea, and at some places to the height of mail 
hundred feet, I looked out for sujierficial signs of tt 
elevation of the land. The bottoms of some of the gorge 
which descend to the coast, are filled up to the depth of aboi 
a hundred feet, by rudely divi'led layers of sand, muddy cla 
and fragmentary masses ; in these lieils, Mr. Seale has fi 
the bones of the tro|iic-bird and of the albatross ; the f 
now rarely, and the latter never visiting the island. Fro 
the difference l)etween these layers, and the sloping piles i 
detritus which rest on them, I suspect that they wei 
* Journnl of Keseorches, p. 582, 



ELETATION AND DENUDATION, 

deposited, wlien the gorges stood beneath the sea, Mr. Seale, 
moreover, has shown tliat some of the fissure-like gorges,* 
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 : never- 
theless, in a bay-like depression on the table-laud behind 
Prosperous Bay, at the height of about 1000 feet, there are 
flat-topped masses of rock, which it is scarcely conceivable, 
could have been insiilatetl from tlie 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 
2000 feet high, presents, according to Mr. Seale, a perfect 
net-work of truncated dikes ; on liills like the Flagstaff, 
formed of sofl rock, we might suppose that the dikes had 
been worn down and cut off by meteoric agency, tut we can 
hardly suppose this possible with the hard, basaltic strata of 
the Barn. 

Coast denudation. — The enormous cliffs, in many parts 
between 1000 and 2000 feet in height, with which this prison- 
like island is surroimded, 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, Seale, can be traced to 
the shore, and are found to be the continuations of certtun 
well-known great dikes. The swell of the Atlantic ocean 
has obviously been the active power in forming these cliffs ; 

• A fiesure-like gorge, near Stony-top, is said by Mr, Seale to be 840 
feet deep, and ouly 115 feet in width. 



and it is interesting to olwerve tliat the lesser, thongb stOl 
great, heiglit of tlie cliffx on the leeward and partially pro- 
tected side of the island, (extending from the Sugar-Loaf 
Hill to South West Point,) corresponds with the lessor 
degree of exposure. ^Vhen reflecting on the comparativdy 
low coasts uf Riuuj volcanic islands, which also stand exposed 
in the open ocean, and are apparently of considerable 
antiquity, t)ie mind recoils from an attempt to grasp the 
Dumber of centuries of exposure, necessary to have ground 
into mud and to have disjiersed, the enormous cubic mass of 
hard rock, which has been pared otf the circumference ot 
this island. The contrast in the superficial state of St. 
Helena, compared with the nearest island, namely. Ascen- 
sion, is very striking. At Ascension, the surface of the 
lava-streams are glossy, as if just poural forth, their bonn- 
daries are well defined, and they can often be traced to 
perfect craters, whence they were erupted ; ui the course of 
many long walks, I did not observe a single dike ; and the 
coast round nearly tlie entire circumference, is low, and has 
been eaten back (thougli 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 tliirty 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 tlie valleys only, but the surface of some of the 

• In the Nautical Magazine for 1835, p. 642, and for 1838, p. 361, 
ajid in ihe Comptca Hendus, April, 1838, accounts are given of a series 
of volcanic phenomena — earthquakes — troubled water — floating scome 
and columnE of Bmoke — which have been observed at intervala since 
the middle of the last century, in a space of open sea between longi- 
tudes 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 proceis 
of formation in the middle of the Atlantic: a line joining St, Heku 
and Ascension, prolonged, iutersccta this slowly nascent focus ^b|dy 



highest hills, are interlaced by worn-down dikes, and, in 
many places, the denuded summils 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, 8t, Jago, and Manrilius. 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 escarpements steep 
towards the interior of the island, and their strata dip out- 
wards. I was able to ascertaiu, only in a few cases, the 
inclination of the beds ; nor was this easy, for the stratifi- 
cation was generally obscure, except when viewed &om a 
distance. I feel, however, little doubt that according to the 
researches of M. Elie de Beaumont, their average inclination 
18 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. AH 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 mountains, 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,— tbe 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 



rRATBRS OP BLBVATIOH, 



[chap. IV 



Vupted ; but one eeea only a vast boUow platform Btretched 
l)ent>ath, or piles of matter of more recent origin. 

Tiieee basaltic mountains come, I presume. Into tliee 
of Craters of elevation : it is immaterial whether the r 
L were ever completely formed, for the portions which i 
I exist, have so uniform a structure, that, if they do not f(nl 
ifragmentB of tnie craters, they cannot be classed with onl 
nary lines of elevation. With respect to their ori^n, a 
having read the works of Mr. Lyell,* and of MM. 
Prevost and Virlet, I cannot believe, that the gjeat centi 
hollows have been formed by a simple dome-8haped elevatiQ 
and the consequent arcliin^r of the strata. On the other h 
I have very great difficulty in admitting, that these basalt 
mountains are merely the basal fra^^enta of great voica 
of which the summits have either been blown off, ormorepf 
bably swallowed up by subsidence. These rings are in so 
instances so immense, as at St. Jago and at Mauritius, 8 
their occurrence is so frequent, that I can hanlly persui 
myself to adopt this explanation. Moreover, I suspect tl 
the following circumstances, from their frequent concu 
rence, are someway connected together, — a connection i 
implied in either of tlie above views ; namely, first, the broki 
state of the ring, showing that the now detached portioi 
have been exposed to great denudation, and in some c 
perhaps, rendering it probable that the ring never ' 
entire ; secondly, the gi-eat amount of matter erupted ( 
the central area, after or during, the formation of the i 
and thirdly, the elevation of the district in mass. As &r I 
relates to the inclination of the strata being greater t 
that, which the basal fragments of ordinary volcani 
naturally possess, I can reaflily believe that this incUnatij 
might have been slowly acquired by that amount of elevade 
of which, accorduig to M. Elie de Broaumont, the numei 
upfilled fissures or dikes are the evidence and the c 
a view equally novel and important, which we owe to t 
researches of that geologist on Mount Etna. 



' Principles of Geology (fifth edit.), vol. ii. p. 1?!. 



CRATERS OF ELEVATION. 

A conjecture, including the above circumstances, occurred 
to me, when, — with my iiuud fully convinced from the plieno- 
mena of 1 835 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, 
wliere the horizontally upraised, calcareous stratum dips into 
Hie sea, directly beneath a cone of subsequently erupted 
laya, The conjecture is, that during the slow elevation of a 
Tolcanic district or island, in the centre of which one or 
more orifices continue open, and thus relieve the subter- 
ranean forces, the borders are elevated more than the 
central area ; and that the portions thus upraised, do not 
B^pe gently into the central, less elevated area, as does the 
calcareous stratum under the cone at St, Jago, and as does a 
lai^e part of the circumference of Iceland, + but that they 

• 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 dis- 
tricts immediately Burro iinding several of the great vents in the Cordillera 
remained quiescent ; the suhtcrranean forces being apparently relieved 
by the eniptions, which then recommenced with great violence. An 
event of somewhat the same kind, but on an infinitely smaller scale, 
appears to have taken place, according to Abieh (Views of Vesuvius, 
plates i. and is.), within the great crater of Vesuvius, where a platform 
on one aide of a fissure waa raised in mass twenty feet, whilst on the 
other Bide, a train of small volcanoa burst forth in eruption. 

t 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 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 12°. The 



96 CRATERS OP ELEVATION. [CHAP. IV. 

are soparated from it by curved faults. We might expect 
from wliat 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 ex- 
]>osed 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 connected 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. 

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 suspect- 
ing 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," like 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 volcanos, 
and the frequent association of volcanic and fresh-water strata, seem to 
indicate that the areas around volcanos 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. 



CHAPTER V. 



/gland— Crnters composed of a peculiar kind of luff'—Sirtatt 
bagaUie cralers, with holloas at tlieir bases — Albemarle Idatid, Jlvid 
lavas, their eompasition — Craters of luff, inclination of their exterior 
diverging strata, and structure of their interior converging strata — 
James Island, segment of a small basaltic crater; fluiditif and eompo- 
aition of its lava streams, and of its ejected fragments — Concluding Te~ 
marh on the craters of tuff, and en the breached condition of their 
aouthem sides — Mineralogieal composition of the rocks of the archipe- 
lago — Elevation of the land — Direction of the fissures of eruption. 

This archipelago is situated under th« Equator, at a distance 
of between five and six hundred miles from the weet coast 
^^B|£oatli America. It consists of five principal islands, and 
^^^nereral small ones, which together are equal in area,* hut 
WBt ia extent of land, to Sicily conjointly with the Ionian 
islaiidB, They are all volcanic: on two, craters have been 
Been 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 height 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 spira- 
cles to huge caldrons, several miles in circumference; they 
are extraordinarily numerous, so that I should think, if 
enumerated, they would be found to exceed two thousand ; 

* I exclude from this measuTement, the small volcanic islands of 
Culpepper and Wenman, lying seventy miles northward of the group. 
Craters were visiUe on all the islands of the group, except on Towers 
Island, which is one of ihe lowest ; this island is, however formed of 
volcanic rocks. 



98 GALAPAGOS ABCHTPSLAeo. [CHAP. '^. 

tliey are formed either of scoriro and lava, or of a bro i^ l 
coloured tuff; and these latter craters are in several respefst^t 
romorkable. The whole group was surveyed by the nfflp p— -r 
of the Beagle. I visited myself four of the principal islancr3: 
and received specimens from all the others. Under the he^^^a 
of the different islands, I will describe only that wh^Hi< 
appears to me deserving of attention. 
No. 11. 






nartennwA J- C^ O 




ARCHIPELAGO. 



Chatham Island. Craters composed of a sinffular JbW ^ 
tuff. — ^Towards the eastern end of this island, there occur ti 
craters, composed of two kinds of tuff; one kind being fnab 
like slightly consolidated ashes ; and the other compact, i 
of a different nature from any thing, which I have met v 
described. This latter substance, where it is best characterize 
is of a yeUowish -brown colour, translucent, and with a lut 
somewhat resembling resin ; it is brittle, with au anj 
rough, and very irregular fracture, sometimes, however, I 
slightly granular, and even obscurely crystalline: 



ALTERED TUFF. 



9y 



'eadilj be scratched with a knife, yet some points are hard 
ugh just to mark common glass; it fiisea with ease into a 
tlackish-green glasB. The mass contains numerous broken 
srystals of olivine and augite, and small particles of black 
md brown scoriie : it is often trarersed by thin eeama of 
Icareous matter. It generally affects a nodular or con- 
sretionary structure. In a hand specimen, this substance 
ronld certainly be mistaken for a pale and peculiar variety 
if pitchstone ; but when seen in mass, its stratification, and 
he numerous layers of fragments of basalt, both angular 
Lnd rounded, at once render its subaqueous origin evident. 
ji examination of a series of specimens, shows that this 
«in-like substance, results from a chemical change on small 
particles of pale and dark -coloured, scoriaeeous rocks ; and 
ius change could be distinctly traced in different stages, 
round the edges of even the same particle. The position 
oear the coast, of all the craters composed of this kind of tuff 
peperino, and their breached condition, renders it pro- 
bable 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 tliink it highly probable, that much the 
greater part of the tuff has originated, from the trituration of 
fragments of the gray, 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 ? I 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 cal- 
careous bottom, into the sea, the outermost film, which in 
other parts resembles pitchstoue, is changed, apparently by 
its contact with the carbonate of lime, into a resin-like sub- 
Btance, precisely like the best characterized specimens of 
the tuff from this archipelago.* 

' The concretions containing lime, which I have described at 
ii2 



OAI^PAGOa ARCHIPELAGO. [CHAP. 

To Tetnm to the two craters : one of them standa at I 
distance of a league from tlie coast, the intervening tri 
consisting of a calcareous tuff, apparently of submaii 
origin. This crater consists of a circle of liills, some ■ 
which stand quite detached, but all have a very regular, qn 
qu^ versal dip, at an inclination of between thirty and for 
degrees. The lower beds, to the thickness of several hundi 
feet, consist of the resin-hke stone, with embedded fragmei 
of lava. The upper beds, which are between thirty bj 
forty feet in thickness, are composed of a thinly stratifiet 
fine-grained, harsh, friable, brown-coloured tuff, or pepe* 
rino.* A central mass without any stratification, whic 
must formerly have occupied the hollow of the crater, but I 
now attached only to a few of the circumferential 1 ~ 
consists of a tuff, intermediate in character between t 
with a resin-like, and that with an earthy fracture, 
mass contains white calcareous matter in small patchi 
The second crater (520 feet in height) must have t 
until the eruption of a recent, great stream of lava, as 
separate islet ; a fine section, worn by the sea, shows a grai 
iunnel-shaped mass of basalt, surrounded by steep, slopin 
flanks of tuff, having in parts an earthy, and in otht 
eemi-resinous fracture. The tuff is traversed by aev^ 
broad, vertical dikes, with smooth and parallel sides, 
I did not doubt were formed of basalt, until I actually bro 
off fragments. These dikes, however, consist of tuff U 
that of the surrounding strata, but more compact, and w 
a smoother fracture ; hence we must conclude, that fissul 
were formed and filled up with the finer mud or tuff f. 

AficeusioD, as formed in a. bed of ashes, present some degree of reGeni 
blance to this suhstance, hut they have not a resinous fracture. At St 
Helena, also, 1 found veins of a somewhat similar, compact, bat nOA- 
resinoua substance, occurring in a hed of pumiceous ashes, apparently 
free from calcareous matter ; in neither of these cases could heat have 

• Those geologists wlio restrict the tenn of tuff, to ashes of a white 
colour, resulting from the attrition of feldspathic lava.s, would call these 
brown-coloured strata "peperino." 



! crater, before its interior was occupied, as it now ia, by a 
*lidified pool of basalt. Other fissures have been subse- 
quently formed, parallel to these singular dikes, and are 
firely filled with loose rubbish. The change from ordinary 
*oriaceou9 particles to the substance with a semi-reBinous 
*acture, could be clearly followed in portions of the compact 
p^lff of these dikes. 

At the distance of a few miles from these two craters, 

lada the Kicker rock, or islet, remarkable from its singular 

It is unstratified, and is composed of compact tuff, 

V parts having the resin-like fracture. It is probable that 

No. 12. 



SMALL BASALTIC CHATEHS. 




THE KICXEB BOCK,— 400 feet bigh. 

amorphous mass, like that similar mass in the case first 
iescribed, once filled up the central hollow of a crater, and 
that its flanks, or sloping walls, have since been worn quite 
away by the sea, in which it stands exposed. 

Small hasultic craters, — A bare, undulating tract, at the 
eastern end of Chatham Island, is remarkable from the num- 
ber, proximity, and form of the small basaltic craters with 
which it is studded. They consist, either 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 30 to 150 yards, and rise from about 60 to 100 feet 
above the level of the surrounding plain. From one small 
lence, I counted sixty of these craters, all of which were 
within a third of a mile from each other, and many were 
much closer. 1 measured the distance between two very small 



craters, and found thgt it was only thirty yards firom lii 
suininit-rim of cue, lo die riiu of tlie other. Small streana 
of black, basaltic lava, contaiDing; olivine and moch glaaef 
feldspar, have flowed from Diany, but not from all of these 
craters. The surfaces of the more recent streams were as- 
ceediugly rugged, and were crossed by great fissnrea; the 
older streams were only a little less rugged ; an»i they n 
all blended and mingled together in complete confiision. ' 
different growth, however, of the trees on the streams, ( 
plainly marked their different ages. Had it not been i 
this latter character, the streams could in few cases ban 
been distinguished ; and consequently, this vride nndulatoii 
tract might Lave, (as probably many tracts have,) been e 
neously considered as formed by one great deluge of lavij' 
instead of by a nmltitude of small streams, erupted from 
many small orifices. 

In several parts of this tract, and especially at the base fli 
the small craters, there are circular pits, with perpendicula 
sides, from twenty to forty feet deep. At the foot of o 
small crater, there were three of these pits. They have 
probably been formed, by the falling in of the roofs of BnuB 
In other parts, there are mammiforui hillock^ 
which resemble great bubbles of lava, with their summitt 
fissured by irregular cracks, which appeared, upon entering 
them, to be very deep; lava has not flowed from thes* 
hillocks. There are, also, other very regular, mammifor 
hillocks, composed of stratified lava, and surmounted ] 
circular, steep-sided hollows, which, I suppose have beeq 
formed by a body of gas, first, arching the strata into one Ol 
the bubble-like hillocks, and then, blowing off its t 
These several kinds of hillocks and pits, as well as 
numerous, small, scoriaeeous craters, all show that this ti 
has been penetrated, almost like a sieve, by the ] 



' M. Elie de Beaumont has degeribed (Mem. pour aervir, &c 
»|v. p. 113) many "petits cirques d'iboulement " on Etna, of m 
Cwhich the origiti is hiatorically known. 



probabl 

^^^» which 1 
^^^B fissured 
^^" them, 1 



PLDIDITY OP DIFFEHENT LAVAS. 103 

Tapours. The more regular hillocks could only liave 
heaved up, whilst the lava was in a softeued state.* 

Albemarle Ibiand. — ^This island consists of five, great, 
flat- topped craters, which, together with the one on the adjoin- 
ing island of Narborough, singularly reaemble each other, in 
form and height. The southern one is 4700 feet liigh, two 
others are 3720 feet, a third only 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 e:Kamined one of these great streams of lava, which is 
remarkable from the evidence of its former high degree of 
fluidity, especially whea its composition is considered. Near 
the sea-coast tiiis 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-ineh, in 

• Sir G. Mackenzie (Travels in Iceland, p. 389 to 392) haa 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 com- 
poses the uppermost stratum; and the same fact is afiirtQed by Von. 
Bach (DeBcript. des Isles Canaries, p. 159), with respect to the basaltic 
stream near Rialcjo, in Teneriffe, It appears singular that it should 
be the upper streams that are chiefly cavernous, for one sees no reason 
wliy 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 Ihem, of 
bodies of gas P 

t In the Cordillera of Chile, I have seen lava very closely resembling 
this variety at the Galapagos Archipelago. It coutained, however, be- 
Hides the albile, well-formed crystals of augite, and the base (perhaps in 
consequence of the aggregation of the augitic particles) was a shade 



P166 



OAUPAOOB ABOBIPBLAOO. 



[chap. 



I of tuff rise. The lower beda are formed of compact tuff, a 
I petLTiDg like a subaqueous deposit; whilst the tipper bed^ 
I louQil the entire circumference, consist of a harsh, &iaUft 
tuff, of little specific gravity, bat often containing fragments 
of rock in layers. Tliis upper tuff contains 
pisolitic balls, about the size of small bullets, which difibS 
from the surrounding matter, only in being slightly hardeti 
and finer grained. The beds dip away very regularly on sT 
ndes, at angles varying, as I found by measurement, from 2 
I to 30 degrees. The external surface of the crater slopes al 
I ft nearly similar inclination ; and is formed by slightly conre 
ribs, like those on the shell of a pecten or scallop, which becoai 
broader as they extend from the mouth of the crater to il 
, base. These ribs are generally from eight to twenty feet i 
breadth, but sometimes they are as much as forty feet bro&i 
and they resemble old, plastered, much flattened vaults, n 
the plaster scaling off in plates : they are separated from 
each other by gullies, deepened by alluvial action. At tb 
upper and narrow ends, near the mouth of the crater, thi 
ribs often consist of real hollow passages, like, but rati 
smaller than, those often formed by the cooling of the cr 
of a lava-stream, whilst the inner parts have flowed i 
ward ; — of which structure I saw many examples at Chatbari 
Island. There can be no doubt, but that these hollow ribs Oi 
vaults have been formed in a similar manner, namely, by ti 
setting or hardening of a superficial crust on streams i 
mud, which have flowed down from the upper part of ti 
crater. In another part of this same crater, I saw op({ 
concave gutters, between one and two feet ivide, which &] 
peared to have been formed by the hardening of the lowi 
surface of a mud-stream, instead of, as in the former cu 
of the upper surface. From these facts, I tliink, it is certain 
that the tuff must have flowed as mud.* This mad i 

* This conclusion is of Bomc interest, because M. Dufr^noy (Mu 
I |iour scrvir, torn, iv, p. 274) has argued from strata of tuiF, appi 
of similar composition -with that here described, being inclined | 
Bnglea between 18° aud 20^, tliat Monte Nuevo and some other c 



CSATEBB OP TTPP. 107 

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 com- 
posed of fragments of lava, abut, like a consolidated talus, 
against the inside walls. They rise to a height of between 
100 and 150 feet, above the surface of the internal brine- 
lake ; they dip inwards, and are inclined at an angle vary- 
ing from 30 to 36 degrees. They appear to have been 
formed beneath water, probably at a period when the sea 
occnpied the hollow of the crater, I was surprised to ob- 
serve, that beds having this great inclination, did not, as far 
as they could be followed, thicken towards their lower 
extremities. 

Banks' Cove. — This harbour occupies part of the interior 
No. 13. 




of Southern Italy, have been formed by upbeaval. From the Sasta 
given above, of the vaulted character of the separate rills, and from the 
tuff not extending in horizontal sheets round these craleriform hills, no 
one will suppose that the strata have here been produced by elevation j 
and yet we see that their incliiAlion 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 aboFe 30°. 



Pl08 



OALwiPAOOS ARCHIPELAGO. 



[chap. 1 



[ of a shattered crater of tuff, larger tlian that last described, 

I All the tuff is compact, and includes numerous fi-agments of 

I lava ; it appears like a suhaqueoua deposit. The most re- 

j markable feature in this crater, is the great derelopment of 

I strata, convei^ing inwards, as in the last case, at a considoq 

J ahle inclination, and often deposited in irregular, i 

layers. These interior, converging beds, as well \ 

proper, diverging, crateriform strata, are represented in 1 

foregoing rude, sectional sketch of the headlands, fom 

ing this Cove. The internal and external strata differ litd 

in composition, and the former have evidently resulted f 

the wear and tear, and redeposition, of the matter fom 

the external, crateriform strata. From the great develop 

I ment of these inner beds, a person walking round the rim of 

' this crater, might fancy himself on a circular, anti-clind 

, fidge, 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 

e fiiture age, be left standing alone, — a case which 

' might at first perplex a geologist.* 

JamesIsland.— Two craters of tuff on this island, are the 
only remaining ones which require any notice. One of them 
lies a mile and a-half inland from Puerto Grande ; it is cir- 
cular, about the third of a mile in diameter, and 400 feet in 
depth. It differs fi-om all the other tuff-craters which I 
examined, in having the lower part of its cavity, to t 
height of between 100 and 150 feet, formed by a precipito 
wall of basalt, giring to the crater the appearance of havinj 
I burst through a sohd sheet of rock. The upper part of d 
[ crater consists of strata of the altered tuff, with a e 

' 1 believe that this case actually occurs in the Azores, where I 
j ■Webeter(DeBcription, p. 185) has described a baBin-formed, little is 
I composed of strata of tuff, dipping inwards and bounded estemally ij 
steep sea-worn cliffs. Dr, Daiibeny supposes (on Volcanos, p. ; 
that this cavity must have been formed by a circular subsidence. 
It appears to me far more probal4e, that we here have strata, -which 
were originally deposited within the hollow of a crater, of which the 
1 walls have sioce been removed by the sea. 



' A SMALL CHATEH. 



109 



resinous fracture. Its bottom is 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 1200 feet above the level of the sea, and the 
interior hollow is 600 feet deep. Its external, sloping sur- 
fece presented a curious appearance, from the smoothness of 
the wide layers of tutf, 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 con- 
dition, consisting of little more than half a circle, open to 
the south ; its great size is probably due, in part, to internal 
d^jTadation, 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 tbe 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; 
and of an irregular pinnacle, about fifteen feet high, of red- 
dish-brown, vesicular basalt, abounding with large crystals of 
glassy albite, and with fused augite. This pinnacle, and 
some adjoining paps of rock on tbe beach, represent the 

No. 14. 




110 



OACAPAOOS ARCHIPBLAOO. 



[CBW. *. 



axis of the crater. The alreams of lava can be followed i^ 
a little ravine, at right angles to the coast, for between Ua 
and fifteen yards, where they are hidden by detritus : along 
the beach they are visible for nearly eiglity yards, and 1 it 
not believe that they extend much further. The three k>v< 
streams are unif«d to the pinnacle; and at the point I 
junction, (as is shown in the accompauying rude sketch, n 
on the spot), they are slightly arched, as if in the act i 
flowing over the lip of the crater. The six upper str 
no doubt, were originally united to this same coluuui, ! 
it was worn down by the sea. The lava of these streams; 
of similar composition with that of the pinnacle, excepti^ 
that the crystals of albite appear to be more comminnte 
and the grains of fused augite are absent. Each stream^ 
separated from tlie one above it, by a few inches, or at i 
by one or two feet in thickness, of loose, fragmentary sec 
apparently derived from the abrasion of the streams, in p 
ing over each other. All these streams are very remarkabk 
from their thinness. I carefully measured several of thei^ 
one was eight inches thick, but was firmly coated with t 
inches above, and three inches below, of red scoriaceons T 
(which is the case with aU the streams), making altogethef 
thickness of fourteen inches : this tliickneas was preservei 
quite uniformly, along the entire length of the section, 
second stream was only eight inches thick, including b< 
the upper and lower scoriaceons surfaces. Until examini 
this section, I bad not thought it possible, that lava coi^ 
have flowed in such uniformly thin sheets, over a surface i 
from smooth. These little streams closely resemble in cofl 
position, that great deluge of lava at Albemarle Island, whk 
likewise must have possessed a high degree of fluidity. 

Pseudo-extraneous, ejected fragments. — In the lava and ii 
the scorifB of this little crater, I found several fn^ment 
which, from their angular form, their granular structui 
their freedom from air-cells, their brittle and burnt conditi 
closely resembled those fragments of primary rocks, whi 
are occasionally ejected, as at Ascension, from volct 



BJBCTED FRAGMENTS. 



These fragments consist of glassy albite, mucli macklerl, 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 pinna- 
cle, 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 crystallized. 
Although there is so wide a difference in appearance, be- 
tween 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 
fiimilar composition with the fragments. Besides the speci- 
men above alluded to, in which we see a fragment becoming 
sligbtly 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 are 
throughout, in an intermediate condition. The paste seems 
to consist of the augite more perfectly fused, or, more pro- 
bably, merely disturbed in its softened state by the move- 
ment of the mass, and mingled with the oxide of iron and 
■with finely comminuted, glassy albite. Hence probably it is, 
that the fused augite, which is abundant in the pinnacle, dis- 
appears 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 in the frag- 




tUPAOOS ARCHIPBLAOO. 



[CEAP. 



I mcnts, they appear to be leas abundant : this, howeve 
I would naturally happen, from the intumescence of t 
I augittc base, and its consequent, apparent increase in bu! 
It is interesting thus to trace the steps, by which a compn 
granular rock becomes converted into a vesicular, pseudo-pa 
phyritic lava, and finally into red scoriae. The stmcti 
and composition of the embedded fragments, show that tl 
are parts, either of a mass of primary rock, which has and 
gone considerable change from volcanic action, or m 
probably of the crust of a body of cooled and cryst 
lava, which has afterwards been broken up and re-Uquefii 
the crust being less acted on by the renewed heat i 
movement. 

CoHcluding remarks on the tuff-craters. — These craters, ft 
[ the peculiarity of the reein-like substance which enl 
largely uito their composition, from theii' structure, tl 
size and number, present the most striking feature in 1 
geology of this Arcliipelago. The majority of them, £ 
either separate islets, or promontories attached to the k] 
islands ; and those which now stand at some little d 
from the coast, are worn and breached, as if by the i 
of the sea. From tliis general circumstance of their positi 
and from the small quantity of ejected ashes in any | 
of the Archipelago, I am led to conclude, that the tnff 1 
been chiefly produced, by the grinding together of fragnu 
of lava within active craters, eommumcating with tie t 
In the origin and composition of the tuff, and in the freaa 
' presence of a central lake of brine and of layers of salt, I 

craters resemble, though on a gigantic scale, the ' 
I or hillocks of mud, which are common in some parts 
Italy and in other countries." Their closer conneeti 
however, in this Archipelago, with oi-dinary volcanic act: 

" D'AiibuJBaon's Trailfi de G^ognosie, torn. i. p. 189, I may fm 
that I saw at Terceira, in the Aaores, a crater of tuff or peperino ■ 
similsir to these of the Galapagoa Archipelago. From the descrii 
given in Freycinet's Voyage, similar ones occur at the Sandi 
lElaods ; and probably they are present in many other places. 



BREACHED CRATERS. 



K. 

^^^■pwu by the pools of solidified baEalt, -with whicti they 

^^^^■Kmietimes filled up. 

^1^^^ at first appears very singular, that all the craters 
fiirmed 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,* and now exist aa mere 
crescents quite open to the south, with occasionally a few 
points of rock marking their former circumference ; of the 
remaiuing sixteen, some form promontories, and others 
stand at a little distance inland from the shore ; but all, 
have their southern sides either the lowest, or quite broken 
down. Two, however, of the sixteen, had their northern 
Bides 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 
Bcorise. The explanation is simple i at this Archipelago, 
the waves from the trade-wind, and the swell propagated 
firom the distant parts of the open ocean, coincide in direc- 
tion, (which is not the ease in many parts of the Pacific,) 
and with their united forces attack the southern aides of all 
the islands; and consequently the southern elope, even when 
entirely formed of hard basaltic rock, ia 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 fi-om the 
■worn condition of many of these craters, it is probable, that 
* These consiBt of the three Croeeman Islets, the largeEt of which is 
600 feet in height ; Enchanted Island ; Gardner Island (7(iOfeet high) ; 
Champion Island (331 feel high) ; Enderbj Island; Dmltle Island i 
two islets near Indefatigable Island ; and one near James Island. 
A Becond crater near James Island (with a salt-lake in its centre) has 
il8 southern side only ahout twenty feet high, whilst the other parts of 
the circurafercaee arc about 300 feet in height. 



^ 



QALAPAGOS ARCHIPELAGO. [CQAP. T^ 

sfime have been entirely washed away. As there i 
ri?ason to suppose, that the craters formed of scoriffi a 
lava were erupted whilst staTidiug in the sea, we can » 
why the rale does not apply to them. At Ascenuon, I 
was shown, that the mouths of the craters, which are t 
all of terrestrial origin, have been affected by the t 
wind ; and this same power might here, also, aid in i 
the windward and exposed sides of some of the craters, or^- ^ 
nally the lowest. 

Mineralogical composition of tlie rockt, — In the northern 
islands, tlie basaltic lavas seem generally to contain more albite 
than they do in the southern half of the Archipelago ; hut al- 
most all the streams contain some. The albite is not uu&e- 
quently associated with olivine. I did not observe in any speci- 
men distinguishable crystals of hornblende or angite ; I except 
the fused grains in theejected 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 ; bnt in 
every case the basis fuses into a black enamel. Beds of 
ashes and far-ejected scoriee, as previously stated, are almOBl 
absent ; nor did I see a fragment of obsidian or of pumice. 
Von Buch* believes that the absence of pumice on Moimt 
Etna, is consequent on the feldspar being of the Liabrador 
variety ; if the presence of pnmiee depends on the constitu- 
tion of the feldspar, it is remarkable, that it should be 
absent in tliis archipelago, and abundant in the Cordillera of 
South America, in both of which regions, the feldspar is of 
the albitic variety. Owing to the absence of ashes, and tlie 
general indecomposable character of the lava in this Archi- 
pelago, the islands are slowly clothed with a poor vegetation, 
and the scenery has a desolate and frightful aspect, 

Eleoation 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, cou- 
* Description des Isles Canaries, p. 328, 



FISSURES OP EHUPTION. 



115 



^^^Hbg recent shells; but they occurred at the height of 
^H^^'a few feet above high-water mark. One of the officers 
gave me some fragments of sheUs, which he found embedded 
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 h«ght, in an 
eruption of mud; but as in one instance, they were asso- 
ciated 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 
recognized 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 acta during 
the heaviest gales. This appeared, at first, good evidence in 
favour of the elevation of the land ; but it was quite decep- 
tive, 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 Tolling up 
rounded blocks to a great height, above the line of their ordi- 
nary action. As the little chtf 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 probable, that the rounded boulders, now lying 
on its summit, are merely the remnant of those, which had 
been rolled up during storms, to their present height. 

ZHrection of tltejissures of eruption. — The volcanic orifices 
in this group, cannot be considered as indiscriminately scat- 
tered. Three great craters on Albemarle Island form a 
well marked line, extending N.W. by N. and S.E. by 8. 
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 tl»e 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 
I 3 



116 0ALAPA008 ARCHIPELAGO. [CHAP. V. 

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 lines 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 archipelagos 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 archipelagos : the 
highest, is the great mound on the south-western extremity 
of Albemarle Island, which exceeds by barely a thousand 
feet, several other neighbouring craters. 




CHAPTER VI. 

TRACHYTE AND BASAI.T. — DISTRIBUTION OF VOLCANIC ISLBa. 

The saiking of cryslaU in Jluid lata — Specific gravity of the constituent 
parts of trachyte and of basalt, and their consequent separaliim — Obsi- 
dian — Apparent non-ieparalion of the elements of plulonic roeks — 
Origin of trap-dikes in the piutonie teries — Diatribulion of volcanic 
islands ; their prevalence in the great oceans — They are generally 
arranged in lineS'^The central volcanas of Fan Buch doubtful — Fol- 
eanie islands bordering continents — Antiquity of volcanic islands, and 
their elevation in mass — 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, ia formed by the wreck of a large crater, 
mentioned in the last chapter, of which the interior Las been 
filled up by a pool of basah-, about 200 feet in thickness. 
This basalt is of a gray 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, Vou Buch* has described a stream of 
obsidian on the peak of TeneriS'e, 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 
* Deaeription dea IbUb Canaries, pp, 190 and 191. 



118 TRACHTTB AND BA8ALT. [CHAP. VI. 

precipitate themselves to the bottom of the crucible. In 
these cases, I presume there can be no doubt,* that the 
crystals sink from their weight. The specific gravity 
of feldspar variesf from 2*4 to 2*68, whilst obsidian seems 
commonly to be from 2'3 to 2*4 ; and in a fluidified state, its 
specific gravity would probably be less, which would fiwriK- 
tatc the sinking of the crystals of feldspar. At James 
Island, the crystals of albite, although no doubt of less 
weight than the gray 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. Dr6e, is worthy of further 
consideration, as throvnng 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 ^ven; 
and he has overlooked one very 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 

* In a mass of molten iron, it is foand (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 sur- 
face. 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 Dufr^noy (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. 

t I have taken the specific gravities of the simple minerals from Yon 
Eobell, 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 2*42. 



F 



ND BASALT. 



119 



atoms, whetlier 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 granulea, it is 
easy to perceive that from the lessened friction, their sinking 
or floating power would be greatly increased. Oil 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 separation, has lately 
been made ; when lead containing a small proportion of 
eilver, is constantly stirred whilst cooling, it becomes granu- 
lated, and the grains or 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 undis- 
turbed, although kept fluid for a length of time, the two 
metals show no signs of 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 11'35: the 
granulated lead, which sinks, is never absolutely pure, and 
the residual fluid metal contains, when richest, only -j-f^ part 
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 difference in gravity between the lead, when granular 
thpngh still hot, and when fluid. 

In a body of liquefied volcanic rock, left for some time 
without any violent disturbance, we might expect, in accord- 

• A full and interesting account of this discovery, by Mr. Pattinson, 
was read before the British Association in September, 183S. In some 
alloys, according to Turner (Chemistry, p. 210), the heaviest metal 
sinks, and it Hppears 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 snr- 
priscd at the atoms separating, without either substance being granu- 
lated. 



120 TRACHYTB AND BASALT. [CHAP. VI. 

ance with the above facts, that if one of the constituent 
minerals became aggregated into crystals or granules, or had 
been enveloi)ed in this state from some previously existing 
mass, such crystals or granules virould rise or sink, according 
to their specific gravity. Novr 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 trachytic 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 2*74 ; 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 gravi- 
ties from 4*8 to 5*2. Hence crystals of feldspar, enveloped 
in a mass of liquefied, but not highly vesicular lava, vvould 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 feld- 
spar, 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 vol- 
canic 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 erup- 

• 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. Jameson 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. 



tioas, however, from moat 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 in- 
tervals, from its summit, and basaltic streams from its base : 
this seems to have taken place at TenerifFe,* I need only 
further remark, that from violent disturbances the separa- 
tion of the two series, even under otlierwise favourable con- 
ditions, would naturally often be prevented, and likewise 
their nsual 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. 

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 Te- 
nerifle ; at this latter place, obsidian has never flowed from 
a leas 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 fre- 
quent presence, and the abundance of this mineral, in the 
lavas of the trachytic series, as observed in previous parts of 
this volume. 

•ConsaltVon Buch's well-known nnd admirable De*cri))(i'o» Physique 
of this island, which might serve as a model of descrii)tive geology. 




TBACHYTB AND DAHAtT. 



[chap. 1 



I 



An objection to the foregoing theory, will, perlinpB, \ 
(Iratm, from the plutonic rocks not being separated into t 
evidently distinct series, of different specific gravities; 
though, like the volcanic, they have been liquefied, 
answer, it may first be remarked, that we have rto evidei 
of the atoms of any one of the constitoent minerals in ^ 
plutonic series, having been aggregated, whilst the ( 
remained fluid, which we have endeavoured to i 
an almost necessary condition of thi-ir separation; on I 
contrary, the crystals have generally impressed i 
with their forms.* 

]n the second place, the perfect tranquillity, under n 
it is probable that the plutonic masses, buried at pro 
depths, have cooled, would, most likely, be highly un&val 
able to the separation of their constituent minerals ; 
the attractive force, which during the progressive c 
draws together the molecules of the different minerals, has ' 
power suilicient 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 disturb- 
ance, which would probably occur in most volcanic foci, and 
which we have seen does not prevent the separation of gra- 

" The crystalline paste of phonolite, is frequently penetrated Ly long 
needles of hornblende ; from which it appean, that the hornblende, 
though the more fusible mineral, has crystallized before, or at the 
■ame time with, a more refiractory substance. Phonolite, as far 34 laj 
obserrations 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 without repeated and violent disturbances. 
Those geologists who have doubted whetlier granite could have been 
formed by igneoua liquefaction, because minerals of different degrees of 
fusibility impress each other with their forms, could not have been 
aware of the fact of crystallized hombleoUe penetrating phonoUte,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. Gcolog. Transact- 
p.439; and L'lnstitut, with respect to quartz, 1839. p. 161. 



TRACHYTE AND BABALT. 123 

nulea of lead from a mixture of molten lead and silver, or 
erystals of feldspar from streama of lava, by breaking and 
dissolving the less perfectly formed globules, would permit 
ithe more perfect and therefore unbroken crystals, to sink or 
tise, according to their specific gravity. 

Although in plutonic rocks two distinct apeciee, cor- 
iresponding to the trachytic and basaltic series, do not exist, 
mnch suspect, that a certain amount of separation of their 
eonstituent 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 
ft district in an extensive granitic region, without discovering 
dikes ; I may instance the numerous trap-dikeB, in several 
districts of Brazil, ChUe, and Australia, and at the Cape of 
Good Hope : many dikes likewise occur in the great gra- 
nitic tracts of India, in the nortli 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 gra- 
nitic and metamorphic series, and by their more fluid parts, 
consisting chiefly of hornblende, oozing out, and being sucked 
into such fissures ? At Bahia, in Branil, in a district com- 
posed of gneiss and primitive greenstone, I saw many dikes, 
of a dark augitic (for one crystal certainly was of this mi- 
neral) or hornblendic rock, which, as several appearances 
clearly proved, either had been formed before tlie surround- 
ing mass had become solid, or liad' together with it been 
afterwards thoroughly softened.* On both sides of one of 

• Portions of theae dikes have been broken off, and are now sur- 
rounded by the primary rocka, with their laminre conformably winding 
ronnd them. Dr. Hubba,Td, also, (Silliman's Jouma!, Tol. sxxiv. p, 1 19), 
has described an interlacement of trap-veins in the granite of (he 



124 THE DISTRIBUTION OP [CHAP. 

these dikes, the gneiss was penetrated to the distance 
several yards, by numerous, curvilinear threads or streakf 
dark matter, which resembled in form clouds of the cI 
called cirrhi-comse ; some few of these threads could 
traced to their junction with the dike. When examin 
them, I doubted whether such hair-like and curvilinear vc 
could have been injected, and I now suspect, that insteac 
having been injected from the dike, they were its feeders, 
tlie foregoing view of the origin of trap-dikes in vndely 
tended granitic regions, far from rocks of any other i 
mation, be admitted as probable, we may further admit 
the case of a great body of plutonic rock, being impelled 
repeated movements into the axis of a mountain-chain, t 
its more liquid constituent parts might drain into deep \ 
unseen abysses; afterwards, perhaps, to be brought to 
surface under the form, either of injected masses of grc 
stone and augitic porphyry,* or of basaltic eruptions. Mi 
of the difficulty which geologists have experienced, w] 
they have compared the composition of volcanic with j 
tonic formations, will, I think, be removed, if we may belii 
that most plutonic masses have been, to a certain ext 
drained of those comparatively weighty and easily liquet 
elements, which compose the trappean and basaltic serie 
rocks. 

On the distribution of volcanic islands. — During my inve 

White Mountains, which he thinks most have been formed when 1 
rocks were soft. 

• Mr. Phillips (Lardner's Encyclop. vol. ii. p. 115) quotes 
Buch's statement, that augitic porphyry ranges parallel to, ani 
found constantly at the base of, great chains of mountains. Humb 
also, has remarked the frequent occurrence of trap-rock, in a sin 
position ; of which fact I have observed many examples at the footoi 
Chilian Cordillera. The existence of granite in the axes of ^ 
mountain chains is always probable, and I am tempted to supj 
that the laterally injected masses of augitic porphyry and of trap, 
nearly the same relation to the granitic axes, which basaltic lavas 
to the central trachytic masses, round the flanks of which they 1 
so frequently been erupted. 



VOLCANIC ISI^NDS. 



125 



coral-reefs, I had oceaaion to consult the works 
taany voyagers, and I was mvariahly struck with the fact, 
Hiat with rare exceptions, the iimunierable islands scattered 
throughout the Pacific, Indian, and Atlantic Oceans, were 
-composed either of voleanic, or of modern coral-rocks. It 
Would be tedious to give a long catalogue of all the volcanic 
ialanda ; but the exceptions which I have found, are easily 
enumerated : in the Atlantic, we have St. Paul's Rock, de- 
scribed in this volume, and the Falkland Islands, composed 
of quartz and clayslate ; hut these latter islands are of con- 
fflderahle 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 aa is known) to the primary class ; 
New Zealand, which contains much volcanic rock and some 
active voleanos, from its size cannot be classed with the 
Bmall islands, which we are now considering. The presence 
of a small quantity of non-volcanic rock, as of clay-slate on 
three of the Azore8,+ 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 archi- 
pelagos, 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 extension of that law, and the effect 

• 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 Eodriguez, in the Indian Ocean, is 
composed. 

t This is stated on the authority of Count "V. de Bedeniar, with 
respect to Flores and Graciosa (Charlsworth Magazine of Nat. Hisl. 
vol. i. p. 557). St. Maria has no volcanic rock, according to Captain 
Boyd (Von Buch's Descript. p. 365), Chatham Island has been de- 
B«riLed 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. 




THH DISTRIBCTIOM OP [cHAP. ' 

of tho«e same oaosea, whtHhf r chemical or merbanie&l, fin 
which It rMiilM, thai a vsAt niajonty of the volcanos noff 
action, siHnd eitlier as islands in the eea., or near its si 
Tliix fact of tlie oceaii- islands being so generally ToIcaniCi 
also, interesting, in relation to the nature of the 
chune on our continents, which are comparatirely sell 
volcanic ; and yet we are led to suppose, that wheie 
continents now stand, an ocean once extended. Do vole 
eruptions, we may ask, reach the surface more rea 
through fissures, formed during the first stages of the i 
version of the bed of the ocean into a tract of land t 
Looking at the charts of the nnmerons volcanic 
lagos, we see that the islands are generally arranged, ei( 
in single, double, or treble rows, in lines which are 
qnently curved in a slight degree.* Each separate 
either rounded, or more generally elongated in the n 
direction with the group in which it stands, but sometii 
transversely to it. Some of the groups which are not mi 
elongated, present little symmetry in their forms; M. ViH 
states that this is the ease with the Grecian Archipelago; 
such groups I suspect, (for I am aware how easy it is 
deceive oneself on these points), that the vents are genei 
arranged on one line, or on a set of short parallel li 
intersec6ng at nearly right angles another line, or set 
lines. The Galapagos Archipelago offers an example of' 
structure, for most of the islands and the chief orifices 
the largest island, are so grouped as to fall on a set of 
ranging about N.W. by N,, and on another set 
about W.S.W.: in the Canary Arcliipelago, we have a 
structure of the same kind : in the Cape de Verde 
which appears to be the least symmetrical of any c 
volcanic archipelago, a N.W, and S.E. line formed by 



' Profeasorg WUliam and Henry Darwin Rogers have lately ini 
mnch, in a tnemoif read before the American Association, on the ; 
larly curved lines of elevation in parts of the Appalachian range. 

+ Bulletin de la Soc. Gcolog. torn. iii. p. 110, 



W VOLCANIC ISLANDS. 127 

Balands, if prolonged, would intersect at right angles a 
fcurved line, on whicli the remaining islands are placed. 
r Von Buck* has classed all volcauos under two heads. 
Homely, central volcanos, round which numerous eruptions 
pave taken place on all sides, in a manner almost regular, 
liiid volcanic chains. In the examples given of the first 
nlase, as far as position is concerned, I can see no grounds 
Bbr their being called ' central' ; and the evidence of any 
nifference in mineralogical nature, between central volcanos 
hnd volcanic chains, appears slight. No doubt some one 
Island in most small volcanic archipelagos, is apt to be 
Considerably higher than the others ; in a similar manner, 
■whatever the cause may be, that on the same island, one 
Kent is generally higher than all the others. Von Buch 
Boes not include in his class of volcanic chains, small archi- 
belagos, in which the islands are admitted by him., as at the 
l&.zores, to be arranged in lines ; but when viewing on a 
■nap of the world, tow perfect a series exists, from a few 
polcanic islands placed in a row, to a train of linear archi- 
pelagos following each other in a straight line, and so on to 
B great wall like the Cordillera of America, it is difficult to 
n>elieve, that there exists any essential difference between 
short and long volcanic chains. Von Buch stateaf that hia 
xrolcanic chains surmount, or are closely connected with, 
inountain* ranges of primary formation : but if trains of 
unear archipelagos are in the course of time, by the long 
feontinued action of the elevatory and volcanic forces, con- 
certed into mountain-ranges, it would naturally result, that 
fhe inferior primary rocks would often be uplifted and 
jbrought into view, 

ji 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 con- 
liected with them. In the case of Juan Fernandez, situated 
130 miles from the coast of Chile, there was undoubtedly a 

• Description dcs Isles Canaries, p. 324. 
t Idem, p. 393. 



128 THB DISTRIBUTION OF [CHAP. TI. 

connexion between the volcanic forces acting under this 
island, and under the continent, as was shown during the 
earth([iiake 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 Archipelagos, and 
the best marked line of the Canary Islands. If these fects 
be not merely accidental, we see that many scattered vol- 
canic islands and small groups are related, not only by 
l)roximity, 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 archipelagos, the orifices are seldom in acti- 
vity 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 comparatively 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 sufiered, 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 connexion, 
moreover, between volcanic eruptions and contemporaneous 
elevations in mass* has, I think, been shown to exist, in my 
work on Coral Reefs, both from the frequent presence of 

* A similar conclusion is forced on us, by the phenomena, which 
accompanied the earthquake of 1835, at Conception, and which are 
detailed in my paper (vol. v. p. 601) in the Geological Transactions. 



VOLCANIC I3LANDS. 129 

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 Albemarle Island, which vents do not fall on the same 
line ; at the Canary Islands, eruptions have taken place in 
Teneriffe and Lanzarote ; and at the Azores, on the three 
parallel lines of Pico, St. Jorge and Terceira. Believing 
that a mountain-axis diflfers essentially from a volcano, only 
in plutonic rocks having been injected, instead of volcanic 
matter having been ejected, this appears to me an interest- 
ing 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. 



Neit South Walen — Sandstone formation — Embedded pseudo-Jr 
aha le — Siralificalion — Cu rrenl-ctea voge — Great valleys.— 
Land — Pala-oS'iic formation — Netcer /ormation tcith volcanic r 
Trnrertin with leaves of exlinet plants — Elevation of tka land.— 
Zealand — King George's SouTtd— Superficial ferrvgtJtous beds — Sit- 
perfieial calcareous deposits, with easts of branchet — Tlieir origin frm 
drifted particles of shells and corals — Their extent. — Caps of G ^ 
Hope — Junction of the granite and clay-sttUe — Sandstone formatim. 1 

The Beagle, in her homeward voyage, touched at Tff 
Zealand, Australia, Van Dieiiien's Land, and the Cape 
Good Hope. In order to confine the third Part of 
Geological Observations to South America, I will hi 
briefly describe all that I observed at these places, worthy j 
the attention of geologists. 

New South Wales. — My opportunities of observation 
sisted of a ride of ninety geographical miles to Bathurat, 
a W.N.W, direction from Sydney. The first thirty 
from the coast passes over a sandstone country, broken 
in many places by trap-rocks, and separated by a 
esearpement overhanging tlie river Nepean, from the j 
sandstone platform of the Blue Mountains. This upper plat- 
form is 1000 feet high at the edge of tlie esearpement, and 
rises in a distance of 25 mdes to between 3000 and 4O00 
feet above the level of the sea. At this distance, the road 
descends to a country rather leas 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 ijuartz, and intersected in some jjlaees by trap- 
dikes. Near tlie Downs of Bathnrst, I passed over much 
pale-brown, glossy clay-slate, witli the sliatteied lamintB 
running nortli and south : I mention iViis fact, because 
Captain King informs nie, that in the country a hundred 
mLJea southward, near Lake George, the mica-slate ranges 
so invariably north and south, that the inliabitants 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 thick- 
ness; it consists of 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 
GloisopteriB Brownii, a fern which so frequently accompanies 
the coal of Austraha. 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 frei^uently 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 metamorphic 
action, I was surprised at observing, that in some specimens 
nearly all the grains of quartz were so perfectly crystallized 
with brilliant facets, that they evidently had not in their 
present form been aggregated in any previously existing 
K 2 



¥ 



[chap, ■ 

rock.* It is difficult to imairine liow these crystals coul 
have been formed ; one can hardly believe that they irei 
sejiaratfly precipitated in tiieir present crystalUzed Bta 
Is it possible, tliat rounded grains of quartz may have b 
acted on by a fluid corroding their surfaces, and depo^tii 
on tliem freeb silica 1 I may remark, that in the s 
formation of the Cape of Good Hope, it is evident, 
silica lias been profusely deposited from aqueous solution. 
In several parts of the sandstone, I noticed patches i 
Bfaale, which might at the first glance have been mista 
for extraneous fragments ; their horizontal laminae, hoi 
ever, being parallel with those of the sandstone, showed tl 
they were the remnants of thin, continuous beds. One snC 
fragment (probably the section of a long narrow strip) a 
in the face of a cliff, was of greater vertical thickness t 
breadth, which proves that this bed of shale must hai 
been in some slight degree consolidated, after having I 
deposited, and before being worn away by the curreuli 
IHach patch of the shale shows, also, how slowly many i 
the successive layers of sandstone were deposited. Thea 
pseudo-fragments of shale will perhaps explai 
cases, the origin of apparently extraneous fragments j 
crystalline metaraorphic rocks, I mention this, becaa 
I found near Rio dc Janeiro a well-defined angular fragmei 
seven yards long by two yards in breadth, of gneiss conta 
ing garnets and mica in layers, enclosed in the ordinal 
stratified, porphyritic gneiss of the country. The laminffi 
the fragment and of the surrounding matrix, ran in exact 
the same direction, but they dipped at different angles, 
do not wish to afHrm that this singular fragment (a solita; 

• I have lately seen, in a paper by Smith (the father of Engli 
geologises), in the Magazine of Natural History, that the grains 
quartz in the mill-Blone grit of England are often crystalliaed. ! 
David BrettBter, in a paper read before the British Aseociatioii, Ift 
States, that in old decomposed glass, (he sileK and metals separate il 
concentric rings, and that the silex regains its crystalline Btcucture^l 
IE shown by its action on lighL 



SANDSTONE PLA.TFORMS. 133 

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 suffiL'ient analogy between the two cases to 
render such an explanation possible. 

Stratification of the escarpement. — The strata of the Blue 
MonntainB appear to the eye horizontal ; but they probably 
have a similar inclination with the surface of the platfonn, 
which slopes from the west towards the escarpement over 
the Nepean, at an angle of one degree, or of one hundred 
feet in a mile.* The strata of the escarpement dip almost 
conformably with its steeply incUned face, and with so much 
regularity, that they appear as if thrown into their present 
position ; but on a more earefiil examination, they are seen 
to thicken and to thin out, and in the upper part to be suc- 
ceeded and almost capped by horizontal beds. These 
appearances render it probable, that we here see an original 
escarpement, 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 examin- 
ing accurate charts of sea-coasts, where sediment is accumu- 
lating, will be aware, that the surfaces of the banks thus 
formed, generally slope from the coast very gently towards 
a certain line iu the offing, beyond which the depth in most 
cases suddenly becomes great. I may instance the great 
banks of sediment within the West Indian Arcbipelago,+ 
which terminate in submarine slopes, inclined at angles of 
between 30 and 40 degrees, and sometimes even at more than 

• Thia is slated on the authority of Sir T. MitchelJ, in his Travels, 

Toi. ii. p. aw. 

■}■ I have described these very curious baniia in the Appendix (p, 196) 
to my volume on the structure of Coral Reefs. I liave 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 hanks in all parts of the 
West Indies, have the same general form of surface. 



134 KSW 90UTD WALES. [CBAP. 1 

40 degrees : every ouc knows how Bteeii such a elope wodd 
appear on the laud. Banks of tliia nature, if uplifted, 
woulil pruliably have nearly the same external form as the 
platform of tlic Blue Mountains, where it abruptly terminaUd 
over the Nepean, 

Current clrurage. — The strata of sandstone in the low 
coast country, and likewise on the Blue Mountains, are 
often divided by cross or current lamiuse, which dip In dif- 
ferent directions, and frequently at an angle of forty-fiye 
dt^ees. Most authors have attributed these cross Jajers 
to successive umall accnmulations on an inclined snr&ce; 
hut from a careful exauunation in some parts of the New Bed 
sandstone of England, I believe that such layers generallj 
form parts of a series of curves, Uke gigantic lidal-ripples, 
the tops of which have since been cut olf, either by near^ 
horizontal layers, or by another set of great ripples, tbe 
folds of which do not exactly coincide with those belov 
them. It is well known to surveyors that mud and sand 
are disturbed during etonns at considerable depths, at least 
from 300 to 450 feet,* so that the nature of the bottom even 
becomes temporarily chaDge<l ; the bottom, also, at a deptli 
between 60 and 70 feet, has been obseryedf to be broadly 
rippled. One may, therefore, be allowed to suspect, fhnn 
the appearances just mentioned in the New Red sandstone, 
that at greater depths, the bed of tlie ocean is heaped np 
during gales into great ripple-like furrows and depressioiu. 
which are afterwards cut off by tlie currents during more 
tranquil weather, and again furrowed during gales. 

Valleys in the sandstone phtforma. — The grand valleys, lij 
which the Blue Mountains and the other sandstone pl*t- 
forms of this part of Australia are penetrated, and whidi 
long offered an insuperable obstacle to the attempts of the 
most enttrprising colonist to reach the interior country, 
form the most striking feature in the geology of New Soatb 
' See Martin White, on Soundings id the British Ch&nne], j^. 4 



I 




QBEAT VAUiEYB. 



"Wales. They are of grand dimensions, and are bordered 

by continuous lines of lofty cliffs. It ia 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 per- 
pendicular, that he can strike with a atone (as I have tried) 
the trees growing, at the depth of between 1000 and 1500 
feet below bim ; 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 line rising up to the same height with that 
on which he stands, and formed of the same horizontal 
strata of pale sandstone. The bottom of these valleys are 
moderately level, and the fell of the rivers flowing in them, 
according to Sir T, Mitchell, is gentle. The main valleys 
often send into the platform great bay-like 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 then- upper parts, 
they generally contract towards their mouths to such a 
degree, as to become impassable. The Surveyor- General, 
Sir T. Mitchell,* in vain endeavoured, first on foot and then 
by crawhng between the great fallen 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 beUeved to be nowhere less than 3000 feet above 
the level of the sea. When cattle are driven into the valley 
• Travels in Australia, vol. i, p. 154.—I must express my obligation 
to Sir T, Mitchell, for several interesting personal communications on 
ihe subject of theee great Tslleye of New South Wales. 




NBW aotnii wAUts. [cbu>. va 

t the Wolgan by a path (which I descended) partly cnt b 
the colonists, they caniiot escape; for ihis valley is in eveiy 
other part surrounded by perpendicular cliSs, and «^ 
miles lower down, it contracts, from an average width of h 
a mile, to a mere chasm impassable to man or bea^t. Sit- 
T, Mitchell* states, that the great valley of the Cox i 
with all its brunches, contracts, where it unites with tfatf 
Nepean, into a gorge 2200 yards wide, and about 1000 U 
in depth. Other similar cases might have been added. 

The first impression, from seeing the correspondence 
the horizontal strata, on each side of these valleys and gretf 
amphi theatrical depressions, is that they have been in chief 
part hollowed out, like other valleys, by aqueous erosion; 
but when one reflects on the enormous amount of stonc^ 
which on this view must have been removed, in most of th 
above cases through mere gorges or chasms, one is led t 
ask whether these spaces may not have subsided. But Cor 
sidering the form of the irregularly branching valleys, anj 
of the narrow promontories, projecting into them from t 
platforms, we are compelled to abandon this notion. Tt 
attribute these hollows to alluvial action, would be pr«i 
posterous; nor does the drainage from the summit-levd 
always fall, as I remarked near the Weatherboard, into tl 
head of these valleys, but into one side of their bay-Ill 
recesses. Some of the inhabitants remarked to me, thai 
they never viewed one of these bay-like recesses, with 1 
headlands receding on both hands, without being strud 
with their resemblance to a bold sea-coast. This is certainl 
the case ; moreover, the numerous fine harbours, with thai 
widely branching arms, on the present coast of New Souti 
Wales, which are generally connected with the sea by i 
narrow mouth, from one mile to a quarter of a mile i 
width, passing through the sandstone coast-cliffs, present I 
likeness, though on a miniature scale, to the great valleys o 
the interior. But then immediately occurs the startling 
diificulty, why has the sea worn out these great, thou^ 
■ Idem, vol. ii. ii. 358. 



QttBAT VALLEYS. 

elKumscribed, 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 
fonns, and that the sides of such banks are so steep (as 
before stated) that a comparatively small amount of subse- 
quent erosion would form them into chffs : that the waves 
have power to form high and precipitous cliffs, even in land- 
locked harbours, I have observed in many parts of South 
America. In the Red Sea, banks with an extremely 
irregular outhne 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, 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 ia hardly possible to douttj 
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 miglit 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. 

• See the Appendix (pp. 192 and 196) to the Part on Coral Reefs. 
The feet of the sea heaping up mud round a suhmarine 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 cliffs,would naturally bethought to have been once connected 



va;< dibhss'b land. [chap. vn. 



Van Itiemen't Land. 



Tlie southern ]>art of this island is mainly formed of 
mountains of greenstone, which oBen iteaunies a eyenitic 
character, and contains much hyiierstheue. These moun- 
tains, in their lower half, are generally encased by EtnU 
containing numerous small corals and some shells. These 
shells have been examined by Mr. G, B. Sowerby, and are 
described in the Aj>|>endix ; they consist of two speciee of 
Prodiieta, and of six of Hpirifera; two of these, iiamelj, 
P. rugata and S. rohindata, resemble, as far aa their imperfect 
condition allows of comparison, British mountain -limestone 
ehells. Mr. Lonsdale has had the kindness to examine the 
corals ; they consist of six uudescribed species, belonging to 
three genera. S|iecies of these genera occur in the Silurian, 
Devonian, and Carboniferous strata of Kurope. Ur, 
Lonsdale remarks, that all these fossils have undoubted!; i 
Palffiozoic character, and that probably they correspond in 
age to a division of the system, above the Silurian fi)^ 
mationa. 

The strata containing these remains are singular from the 
extreme variability of their miner alogical composition. 
' Every intermediate form is present, between Hiuty-slate, 
clay-slate passing into gray-wacke, pure limestone, sand- 
stone, and porcellanic rock ; and some of the beds can onlj 
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 thickness : the upper few hundred feet usually consist 
of a siliceous sandstone, containing pebbles and no organic 
remains ; the inferior strata, of which a pale flinty slate ifi 
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 whiW 
soft calcareous matter is quarried, and is used for white- 
washing houses. From infoi-mation given to me by Mr. 
Frankland, the surveyor-general, it appears that this Palteo- 
zoic formation is found in different parts of the whole if-lancl ; 




VOLCANIC ROCK8. 139 

the eame autliority, I may add that on the north-eastern 
feoest, and in Basa' Straits primary rocks extensively occur. 
The shores of Storm Bay are skirted, to the height 
of a few hundred feet, by strata of sandstone, contain- 
ing pebbles of the formation just described, with its 
characteristic fossils, and therefore belonging to a sub- 
sequent 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 Ho- 
tart Town, I ohser\-ed one dike, nearly a hundred yards 
in width, ou one side of which the strata were tilted at an 
angle of 60°, and on the other they were in some parts 
vertical, and bad been altered by the eifecta 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 
was a mass of breceiated 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 argiUaceoua 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 I 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. 

• Chlorophffiite, descrilied by Dr. MacCulloch (Western Islands, 
vol. i. p. 504) as occurring in a basaltic amygdedoid, differs from this 
substance, in Temaining unchanged before the blow-pipe, and In black- 
ening from exposnre to the air. May we suppose that oliviae, in nndcr- 
going the remarkable change described at St, Jago, paeses through 
several states P 



140 



TAM DIEMEKS l^Sb. 



[chap. ■ 



Traverlia with extinct plantt. — BeMnd Hobart Town 
is a small quarry of a liard travertin, the lower strata 
which abound with distinct impressions of leaves. 5 
Robert Brown has had the kindness to look at my spe( 
mens ; he informs me that there are four or five kinds, 
of which he recognizes as belonging to existing speci 
The most remarkable leaf is palmate, like that of a fan- 
and no plant having leaves of this structure has hithi 
been dwcovered in Van Diemen's Land. The other 
do not resemble the most usual form of the Euealyptna, 
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 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 occasional layers of chalcedonic 
nodules, like those of cliert in our Greensand. From the 
pureness of this calcareous rock, it has been searched for in 
other places, but has never been found. From this 
stance, 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 subsequently 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 wacke and 
a white, earthy, alumino-calcareous substance. Hence 
would appear, as if a volcanic eruption had taken place 
the borders of the pool, in which the calcareous matter 
depositing, and had broken it up and drained it. 

Elevation of the land. — Both the eastern and western shoi 
of the Bay, in the neighbourhood of Hobart Town, are 
most parts covered, to the height of tliirty feet above 
level of high-water mark, with broken shells, mingled i 
pebbles. The colonists attribute these shells to the at 
gines having carried them up for food: undoubtedly, tl 






141 



^^^^feftnj large mounds, as was pointed out to me by Mr. 

^^^Htkland, whicli have been thus formed ; but I think from 
tfie numbers of the shells, from their frequent small size, 
from the manner in whieh they are thinly scattered, and 
&om 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 serpulse 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 be- 
tween the land and water, has been detected on almost all 
the land on this side of the globe. Capt. 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. Clarket finds proofs of the elevation of the 
land, to the amount of 400 feet, at the Cape of Good Hope. 
In the neighbourhood of the Bay of Islands in New Zea- 

• It would appear that some changes ate now in progress in Ralph 
Bay, foe I was assured liy an intelligent farmer, that oysters were 
formerly abundant in it, but that about the year 1834 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 Ihe 
southern United States. At Chiloe, in South America, I heard of a 
Bimilar loss, sustained by the inhabitants, in the disappearance from one 
part of the coast of an edible species of Ascidia. 

+ Proceedings of the Geological Society, vol. iii. p. 420, 



ri43 



KIN<1 CEOROBE BOUND. 



[CHlP, 1 



I 



land,* I oWrveJ that the chores were Bcattered to 
boight, as at Van Diemen's Land, with sea-shells, whicli the 
Ctilouists attribute to the natives. Whatever may have been 
the origin of these shells, I cannot doubt, after having seen a 
lection of the valley of the Tliames River (37° S,), drawn by 
the Rev. W.Williams, that the land haa been there elevated: 
on the opposite sides of this great valley, three "step-like ter. 
races, composed of an enormous accumulation of rouuded 
pebbles, exactly correspond with each other: the escarp*- 
ment of each terrace is about fifty feet in height. No one 
aftfir having examined the terraces in the valleyei on the 
western shores of South America, which are strewed will 
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 manifesl 
" that the sea has left many places bare, which were once 
covered by its waters." 

King Gforffe's Sound. 

This settlement is situated at the south-western angle of tin 

Australian coutinent : the whole country is granitic, with the 

• I will here give a catalogue of the rocks which I met with near Ihe 
Bay of Islands, in New Zealand ; — Ist, Much basaltic lava, and scori- 
form rocks, forming diEtincI; craters ; — 2nd, A castelluted hiil of hori- 
zontal strata of flesli-coloured limestone, showing when fractured dis- 
tinct crystalline faccls : the rain has acted on tliis rock in a remarkalile 
manner, corroding ila surface into a miniature model of an Alpine 
country : I ohserved here layers of chert and clay iroR-stone [ and in 
the bed of a stream, pebbles of clay-slate i — 3rd, The shores of ihe Bay 
of Islands are formed of a feldspathicrock, of a blueish-gray colour, often 
much decomposed, with an angular fracture, and crossed by numerous 
ferruginous seams, but without any distinct stratification or cleaTsga 
Some varieties are 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. x\. pp. 202, 205. 




aUPERFICIAL PEHKnOmorS BEDri. 

eonstituent minerals sometimeB 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 liills, appearing to be com- 
posed of great folding layers, strikingly resembles, on a small 
scale, those corapoaed of gneise-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 pri- 
mary districts, the occurrence of systems of dikes parallel 
and close to each other. 

Superficial ferruginous beds. — The lower parts of the 
country are everywhere covered by a bed, following the ine- 
qualities of the surface, of a honeycombed sandstone, 
abounding with oxides of iron. Seds of nearly similar 
composition are common, I believe, 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 iine-grained, 
mbbly, 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, whicii 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 honey-combed. 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 
tends to become aggregated, either in the form of a 
shell, or of a network. The origin of these superficial beds, 
,^ough sufficiently obscure, seems to be due to alluvial 
1 detritus abounding with iron. 




KtKu uedrob's aoCNu. [cBAF. vn. 

Superficial calcareous depotit. — A calcarcoas depoat on the 
snmuiit of Bald Head, contaiaiDg branched bodies, any- 
\wseil by some- aatliors to have been corals, has been cele- 
brated by the de»:ri])tioiis of many distin^uisheil voyagers,* 
It foldH round and conceals irregular hummocks of granite, 
at the height of 600 feet above the level of the sea. It 
Taries much in thickness ; where stratiiied, the beds are 
often inclined at high angles, even aa much as at 30 degrees, 
and they dip in all directions. These beds are somei 
crossed by oblique and even-sided lamime. The depoat 
consists either of a fine, white, calcareous powder, in whicli 
not a trace of structure can be discovered, or of exceedingly 
minute, rounded grains, of brown, yellowish, and pnrpM 
colours ; both varieties being generally, but not alwa)'*, 
mixed with small particles of quartz, and being cemented 
into a more or less perfect stone. The rounded calcarcooa 
grains, when heated in a slight degree, instantly lose thor 
colours ; in this and in every other respect, closely resem- 
bling those minute, equal-sized particles of shells and corals, 
which at St. Helena have been drifted up the sides of 
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 Ucrt/e unbroken 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 sub- 
stalagmitict rock, which, even when the stone on each side 
" I visited this hill, in company with Captain FitzRoy, and we came 
to a similar conclusion regarding these branching bodies. 

t 1 adopt this term from Lieut. Nelson's excellent paper on the 
Bermuda Islands (Geolog. Trans, vol. v, p. 106), for the hard, compael. 
cream or brown-coloured stone, without any crystalline Btruoture, 
wliich BO often nccompaniea superficial calcareous accuroulations. 1 
have observed such superficial beds, coated with Bub-slalogmilic rock, 
at the Cape of Good Hope, in several parts of Chile, and over wide 



lUPERPlCIAL CALCAHEOne DEPOSIT. I4fi 

iffitains particlea of quartz, is entirely free from them : 
nence we must suppose that these layers, as well as certain 
veia-hke masses, have heen formed by rain dissolving the 
calcareous matter and re-precipitating it, as has happened at 
St. Helena. Each layer probably marks a fresh surface, 
-wheii the, DOW firmly cemented, particles existed as loose 
Band. 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 

i-shell ; but bleached shells of the Helix melo, an existing 
land species, abound in all the strata ; and I likewise found 
aaother Hehx, and the case of an Oniscus, 

The branches are absolutely un distinguishable 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 fimier than the 
surrounding matter, with the central parts filled, either with 
friable calcareous matter, or with a sub-stalagmitic variety ; 
this central part is also frequently penetrated by hnear 
crevices, sometimes, though rarely, containing a trace of 

spaces ia La Plata and Patagonia. Soma of tLese beds have been 
formed from decayed shells, Lut the origin of the greater number is 
sufficiently obscure. The causes which determine water to diasolve 
lime, and then soon to redeposit it, are not, I think, known. The sur- 
face of the sub-Btalagmitic layers appears always to be corroded by the 
■water. As all the above-mentioned countries have a long dry 
an, compared with the rainy one, I should have thought that the 
presence of the sub-stalagmite was connected with the climate, had not 
Lieut. Nelson found this substance forming under sea-water. Disinte- 
grated 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 ou 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. 



KINO OKOBOBB BOUND. 



[chap, T 



woody matter. These calcareous, branching bodies, ) 
to have l>een formed, by fine calcareous matter being waslu 
into the casts or cavities, left by the decay of branches t 
roots of thickets, buried under drifted saud. The whole 8 
face of the hill is now undergoing disintegration, and 1 
the casts, which are compact and hard, are left projecti] 
In calcareous sand at the Cape of Good Hope, I found t 
casts, described by Abel, quite similar to these at Bald 
Head ; but their centres are ofl«n filled with black car- 
bonaceous matter, not yet remoyed. 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, look back to a period when the land stood lowert I 
of which the French naturahsts* found evidence in upraised I 
shells of recent species, for the drifting on Bald Head of thttM 
calcareous and quartzose sand, and the consequent embt 
ment of the vegetable remains. There was only one ap^fl 
pearance which at first made me doubt concerning i~ 
origin of the cast, — namely, that the finer roots from ( 
ferent stems sometimes became united together into nprigld 
plates or veins ; but when the manner is borne in mind, ^ 
which fine roots often fill up cracks in hard earth, and thi 
these roots would decay and leave hollows, as well as t 
stems, there is no real difficulty in this case. Besides ( 
calcareous branches from the Cape of Good Hope, I hafS 
seen casts, of exactly the same forms, from Madeira^ i 



• See M. Peroa'a Voyage, torn. i. p. 'iOi 

t Dr. J. Macaulay has fully described (Eding, New Phil. , 
vol. Kxix. p. 350) the casts from Madeira. He considers {differenth; 
from Mr. Smith of Jordan Hill) these bodies to be corals, and the cal- 
careous deposit to be of subaqueous origin. His arguments chiel^y res' 
(foe hie remarks on their structure are vague) on the great quantity o/ 
the colcareoua matter, and on the casts containing animal matter, i " 



er, aim 



r.jbi^ aJCBi^v^ jsp^iff 



fiom Bermaim; m iS^ jaeaes- 7101%. '^ 4nir y tnu tdiug c&J- 
eueoos rockfw jm i ging frtm. iiit int»aiiHSftf fM«lH«rve£ tn- 
Lieat. NekoB, are Skrnw: fimiLiL*. nf » xLtiir f^nlastraJ 
fcMrmatxm. BAciaug {il -aie 4Crvizii«3miiL .if ^ 6e7k:»^ cti: 
Bald Head , o e tiie irriiETiJtrix nr^tnanjiir kj*?^ cif frcft*- 
stalagmitie rock, — <ai ia«er uudunLJT «:s«;i. iluc rcauidtid 
pardcleSy mpparoiiT ^/fcft-fttittlk luc tr^njt. — fjL ^dte aAnciH 
dance of laad ihcflr tkrvKiivin titf: 2ust4«E. — aai^ frTaJV. o^ 
the mbfiohle naem*JbaMat <i vl^ «L>j«re:>ii§ «K^f. 1^:* \h^ 
stnmpa, roots, aad l«aaiebe» of lida kind ^rf* tc^vuqkqi. vlodk 
woiiU growon flaad4u2MJL^ I ULiiik TLer% <^ 
doubt, notwithrtandtig liie ci&nst c^tcidcw of €«c«ie antlKv^. 
that a tme fiev of tbdr ongps htaf beies Lie7>e gireii. 

Calcareooft deporsh^. Eke thesf^t <A Kiitg G^orpe'f Sound, 
are of rasl extent on tiie Anstralian «liore$w Y^r. Fltton 
remarks, that " reeent ealeamcmi brema (hj which term all 
these depools are indndedj va£ fouid daring Bandin's 
Toyage, OTer a space c^ no less than 25 degrees of latitude 
and an equal extent c^ longitnde, on the soathem, western, 
and nordi-westem coasts.'** It appears also firom M. 

shown by their erolriiig ammonia. Had Dr. Macaulaj seen the enor- 
mons masBCS of rolled paiticles dL sheUs and corals on the* beach of 
Ascension, and especiall j od eoral-reefii ; and had he reflected on the 
cflfects of kmg-eontinned, gentle winds, in drifting up the finer particles^ 
he woold hardlj hare adraneed the argument of quantity, which is 
leldom tmstworthy 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 analyzed for Dr. 
Uacanlay part of a cast, and he found it composed of — 

Carbonate of lime 73*15 

SiUca 11-90 

Phosphate of lime 8 81 

Animal matter 4*25 

Sulphate of Hme a trace 

9811 

♦ For ample details on this formation^ consult Dr. Fitton's Appendix 
^o Capt. King's Voyage. Dr. Fitton is inclined to attribute n con- 
^etionary origin to the branching bodies : I may remark, that 1 havo 

L 2 



148 CAPE OT GOOD HOPS, [cHAP. VH 

Peron, with whose observations and opimons on the ori^ 
of the calcareous matter and branching casts, mine entirely 
accord, that the deposit is generally much more continuons, 
than near King George's Sound, At Swan River, Arch- 
deacon Scott* 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 
crow-bars being let down, to consist of white calcareous 
matter. Hence it seems that along this coast, as at Ber- 
muda and at Keeling Atoll, submarine and subaerial depo- 
sits 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-reeta of the Indian and Pacific Oceans. In other parts 
of the world, for instance in South America, there are super- 
Jicial calcareous deposits of great extent, in which not a trace 
of organic structure is discoverable; these observations ■ 
would lead to the enquiry, whether such deposits may notj^H 
also, have been formed from disintegrated shells and conUsf.fl 

Cape of Good Hope. 
After the accounts given by Barrow, Carmichael, Basil 
Hall, and W. B. Clarke of the geology of tliis district, I 
shall confine myself to a few observations on the junction of 
the three principal formations. The fundamental rock is 
granite,t overlaid by clay -slate : the latter is generally hard, j 

Been in beds of sand in La Plata, cylindrical Btems, which no doubt tbal 
originated [ but they differed much in appearance from these at I 
Head, and the otlier places above Bpecified. 

• Proceedings of the Geolog. Soc. vol. i. p. 320. 

"t" In Beveral places I observed in the granite, small dark-coloutrf^ 
balls, composed of minute scales of black mica in a tough bas 
another place, 1 found crystals of black Bcliorl radiating from a common 
centre. Dr. Andrew Smith found iu the interior parts of the country, 
some beautiful specimens of granite, with silvery mica radiating or 
rather braDching, like moss, from central points. At the Geologi ' 



^ii*t=ii J» IB ta^Kns ^?a rii^.v- l^-jtx -4* 



jvnn. — ■■■gM"ii»y umimf ^au^ rr 'nur% r sihf?^ 
pa- mil. i#?-i«* tr" -iUrnL*' rr^-^-^aJinif. i;*n- 
fVus. ^:ir^^4iHZf «- •••'nmi'itiiiifti Trim. KruiK 

to Ae 4e^o. -rf .'•Hair" if**:, nni i lau*— riuinr-f»c suniho/ntf^ 
Eke nek. 'nnsa, la^ ]*s?s im??arf.-*n, l i^tr^t^ i^ smii; 
iir" 1. 4iRj«cirr* iiriiuniiii. I ▼ti*' £Tiiti*»£ y^ 1^ 

m 

tke giaMJae - aatc diirr-ciaa^ - 'ijh iirrit? l: tin ihscsoir^ .r ji 
qwter rf m mi* iraiL ^di* *?:«ir_ ▼itfr* -rif; rrkimi *,Tc*«tr!s 
OB tke kadcL lainor^ iiriiWiiJT. ria rraim i> XLorl T»^«:r<fr 

fine. Ai m lc» £*?a-u»- f*:aii* :c -di* ••**af :c Kii^-jiiij^ irv ^c a 

anes of c«4k«t. widkc '.cij«r* c^ :«^i?in:TY\T <c»."^:Taf\*> \V::i:» 
a hmidred Tmrd* rf Tit £ra to- i^ mxrre. :ikr ci*x-siji:v* 
consists of spTeial Trnzirat* : «c€Zke oc«Bpftci wirii jk i:r:^ of 
purple, odier^ g5*giw.^g wrii utizmeTVMZf m:::nTe !«^^i's x>f 
miea and imperfecdr rry^ialEiad teUspstr ; ^nHiie i^^j^nirvlx 
granahr, odMT§ porj^hrTidc with smalL eJon^rftUxl >{k>I:!^ \\f a 
soft white nuiMfraL which heins easUr cv>rroded, sives lo ihi* 
TarietT m Teacolar appearance. Cla^e to ihe srniiuu\ ihe 
clay-slate is changed into a dark-colour^. laniiiijiu\) nvk^ 
hairing a granular firactnre, which is doe to im|)t^ri^vt orvsiids 
of feldspar, coated br minote, brilliant scales of mica. 

The actual junction between the granitic and cl«y-sl«to 
districts, extends over a width of about 200 yanls, luu) ihxu- 
sists of irregular masses and of numert>us dikes of gmiu!t\ 
entangled and surrounded by the day-slate : nuv^l of fho 
dikes range in a N.W. and S.E. line, parallel to tho oU^ivii^* 
of the slate. As we leave the junction, thin IhmIs, mu\ h\H\\\\ 
mere films of the altered clay-slate are stn^n, quito iHoluttHl, 
as if floating, in the eoarsely-erystallizeil granito; but altliou^li 
completely detached, they all retain traces of tlit* nniltu^ni 

Society, there are specimens of granite with crystnUiicd fcldii^mr 
branching and radiating in like manner. 



CAPE OF COOD HOPE. 



[c, 



N.W. and S.E. cleav^e. This fact has been observed t 
other similar cases, and has been advanced by some eminegj 
geologists,* as a great difficulty on the ordinary theory, \ 
granite having been injected whilst liquefied ; but if ^ 
reHect on the probable state of the lower surface of a Itasf 
nated mass, like clay-elate, after liaving been violently arch4 
by a body of molten granite, we may conclude that it wool 
be ftdl of fissures parallel to the planes of cleavage ; 
that these would bo filled with granite, so that wherever t] 
fissures were close to each other, mere parting layers ( 
wedges of the slate would depend into the granite. Shonl 
therefore, the whole body of rock afterwards become v 
down and denuded, the lower ends of these depende| 
masses or wedges of slate, would be left quite isolated in tJ 
granite ; yet, they would retain their proper lines uf cleavagj 
from having been united, whilst the granite was fluid, ^ 
continuous covering of clay-slate. 

Following, in company with Dr. A. Smith, the line J 
junction l.ictween the granite and the slate, as it stretcha 
inland, in a S.E. direction, we came to a place, where the 
slate was converted into a fine-grained, perfectly character- 
ized gneiss, composed of yellowish-brown granular feldspar, 
of abmidant black brilliant mica, and of few and thin 
lamince of quartz. Prom the abimdaaice of the mica in this 
gneiss, compared with the small quantity and excessively 
minute scales, in which it exists hi the glossy clay-slate, we 
must conclude, that it has been here formed by the meta- 
morphic action, — a circumstance doubted, under nearly 
similar circumstances, by some authors. The laminaa of the 
clay-slate are straight ; and it was interesting to observe, that 
as they assumed the character of gneiss, they became unda- 1 
latory with some of the smaller flexures angular, like thft] 
laminee of many true metamorphic schists. 

Sandstone formation. — ^This formation makes the 

• See M. Keilhau's Theory on Granite, translated in the Sdiabc 
New Philosophical Journal, vol. xxiv. p. 402. 



SANDSTONE FORMATION. 151 

ig feature in the geology of Southern Africa. The 
1 many parts horizontal, and attain a thickness 
of about 2000 feet. The sandstone varies in character ; it 
contains little earthy matter, but is often stained with iron ; 
some of the beds are very flne-grained and quite white ; 
others are as compact and homogeneous as quartz rock. In 
some places I observed a breccia of quartz, with the frag- 
ments 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 thickness of the reeemented crystals of the 
granite, on which the great pile of sandstone immediately 
rested, 

Mr. Schomburgk has describedt a great sandstone forma- 
tion 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 containing 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 

* The Rev. W. B. Clarke, however, slates, to my surprise, (Geolog. 
Proceedings, vol. ill. p- 422,) that the sandstone in some parts ia pene- 
trated by granitic dikes : such dikes must belong to an epoch altogeiher 
subsequent to that, when ihc molten granite acted on the clay-slate. 

+ Geogrjphkal Journal, vol, x, p. 246. 



132 CAPE OF GOOD HOPE. [CHAP. VII. 

the 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. 



APPENDIX. 



DESCRIPTION OF FOSSIL SHELLS. 

Br G. B. SOWERBY, Esq., F.L.S, 



Shells from a tertiary deposit, beneath a great basaltic 
stream, at 9t. Jago in the Cape de Verde Archipelago, 
referred to at p. 4 of this volume. 



1. LiTTORENA Planaxis. G. SoTcerby. 

Tesld iuhovald, eragtd, heeigatd, anfractihuM qttatuor, tpiraliiir 

tlriatU J aperlurd guboeald ; lahio eolumellariinfimdque parte 

anfraetui ultimi planatii : long. 0-G, lat. Q'iS, poll. 

In stature and nearly in form this resembles a Email periwinkle ; 

it differs, however, very materially in having the lower part of the 

la^ volution, and the columellar tip as it vrere cut ofT and flattened, 

as in the Purpune, Among the receat sheila from the same locality, 

is one which greatly resembles this, and vthich may be identical, but 

which ia a very young aliell, and cannot therefore be strictly com- 

2. Cebithidm shvlvm. G. Sowerby. 

Tettd ohlongo-turritd, luhventricosd, apice rululato, an/raettbut 

decern lemter tpiraliter ttriatU, primi» terie utticd tubereulorum 

inttructit, intermediit irr^ularilir obtoleti lubereali/erit, 

ultimo longi majori absque tuberculit, tulcis duobux/eri batali- 

bus imtructo : labii exterai margins interao intm crenulalo : 

long, I'S, lat. Q-7,poll. 

This species resembles bo nearly one of the shells brought to- 

getlicT by Lamarck, under the name of Ccrithium Vertagua, ttiat at 



154 



APPENDIX. 



first sight I thought it might be identical with it ; it may be es 
distinguished, liowever, by its being destitute of the fold in 
centio of the columella so conspicuous in those shells. Thero 1 
only one specimen, which has unfortunately lost tiie lower part o 
the outer lip, so that it is impossible to describe the form of t 
aperture. 



3. Venus 



G. Soicurbi/, 



Tetld rotundatd, vmtrieoad, IcBviuteuld, eraatd ; eoetis ohhait, 
latiuiculU, concentrieU, antich potticegue tuherculatim Kilulit ; 
ared cardinali poitied alterai valvm latiuieuM; impreiaiont I 
ndiumbonali poHtcd drculari ; long. 1-8, alt. 1-8, Uu. 1"5,'J 
poll, 

A shell which is intermediate in its characters, taking its place ] 
between the Vemts ner-raeosa of the British Channel and the ] 
V, rotalina of Bang of the western coast of Africa, but sufficiently | 
distinguished from botli by its broad, obtuse, concentric ribs, wliioli ] 
are divided into tubercles both before and behind. It is also of a ] 
more circular form than either of those species. 

The following Shells, from the same bed, as far as they caal 
be distinguished, are known to be recent species. 

4. Pgbpdba Fuccs. 

5. AmFHISESUA AnSTBAL 

e. CoNira vENULATos. Lam. 

7. FiaSPRELLA COARCTATA, L 

8. Pbrna, — two odd valves, but in such condition that it cannot 
be identified. 

9. OsTBEA cORNUcoPi.TS. Lam. 

10. Akca ovata. Lam. 

11. Patella NiGRiT A. Badgin. 

12. TOREITELLA BIClHQnLATA ? Lai 

13. Strombtjs, — too mnch worn and mutilated to be identified. 

14. HippOHYx RADiATA. Gray. 

15. Natica hbbb. Vfdendeanei, 

16. Peoten, which in form resembles operculars, but which icl 
distipgnishable by several characters. There is only a single valveJ 
wherefore I cannot consider myself warranted to describe it. 

17. PtiPA STJBDiAPnANA. King. 

18. Tmocbus — indeterminable. 



APPENDIX. 



EXTINCT LAND-SHELLS FROM ST. HELENA. 



The following six species were found associated together, at 
the bottom of a thick bed of mould ; the two last species, 
namely, the Cochlogena fossilis and Helix biplicata were 
found, together with a species of Succinea now living on 
the Island, in a very modern calcareous sandstone. These 
Shells are referred to at p. 89 of this volume. 



1. CocHLooESA AuRis-TCLPiNA. De Fer, 
Tliis species is well described and figured in Martini and Chem- 
tat^a eleventh volume, Chemnitz expresses doubts as to what 
genua it might properly be referred, and also a strong opinion un- 
favourable to the conclusion tliat it should be regarded aa a land- 
shell. His specimens were bought at a public auction in Hamburg, 
having been sent there by the late G. Humphrey, who appears to 
have been very well acquainted witlt their real locahty, and who 
sold them for land-shells. Chemnitz, however, mentions one speci- 
men in Spengler's collection, in a fresher condition than his own, and 
which was said to be from China. The representation wliich he 
baa given is taken from this individual, and appears to me to have 
been only a cleaned specimen of the St. Helena shell. It is easy to 
suppose that a sbell from St. Helena might have been either acci- 
dentally or interestedly, after passing through two or three hands, 
sold as a Chinese shell. I think it is not possible tjiat a shell of 
this species could have been really found in Cliina ; and among the 
immense quantities of shells that come to tliis country from the 
Celestial Empire, I have never seen one. Chemnitz could not bring 
himself to establish a new genus for tbe reception of this remarkable 
shell, though he evidently could not collate it with any of the then 
known genera, and though he did not think it a land-sbell, he has 
called it Atiris^vulptna. Lamarck has placed it as the second 
species of his genus Strulhiolaria, under tbe name of crenulata. 
To this genus it does not however bear any affinity ; and there can 
bo no doubt about the correctness of De Ferussac's views, who 
places it in the fourth division of his sub-genus C'oe/ilti^eiia : and 
Lamarck would have been correct according to his own principles, 



if bo hod i>kced U with his Auricula, A variet}r of this specia 
occun, which may b« characterized u fulluws: — 

CocuLOGENA Auiii»-viiLPii(A, var. 
T«*tA tuhpyramidtili, apertard l/reviori, lahto tenuMri: Itmg, 

1 '68, apertarm 076, lat. 087, poll. 
Obs. — The ptoportiuns of this, difior from those of tbo niul 
variety, which are as follows:— Length 1'65, of the aperture I-, 
width 0-9(1 inches. It 1* worthy of observation, tliat all the sheila 
of thia variety came from a different part of the island, from tlie 
fotrgoing specimens. 

2, CocHLuoEKA FoastLiB. G. Sowerby. 
Te*tS Muagd, cratnHMuld, tpirtS mbacumitiatd, obtusd, atijrac- 
lilitu tent; tubventrieotit, leviter itriatU, tuturd pro/uruli 
impreud ; apcrturd tubosata ; perilremate confinuo, iidiin- 
crtu$alo; uml/Uieo paroo : lottff. 0-8, lat, 0-37, poll. 
This species is of the stature of C. Guadaloapentu, bat may 
easily be distinguished by the form of Ihe volutions and the deeply- 
marked suture. The specimens vary a little in tlieir proportions. 
Tliia Bpeeiea was not obtained by Mr. Darwin, but is from tlie 
oollection of the Geological Society. 

]. COCULICDPA BDBPLICATA, G. Sowtrb^. 
Tetid olhngd, tulaeuminato-pyramiiMi, apice obluao, anfrae- 
tibiu «oo«m Imeihut, poitice tubplicatis, auturd erenulatS; 
aperlurd otald, pottici aeuld, labio txterno tmmij columdli 
olttolete tubtruncatd: umbilico tnitiimo: long. 0*93, lat. 02S, 
poll. 
This and the following are placed with De FeruasBc's sub-genua 
Cochlicopa, because thoy are most nearly related to his CocAlicopa 
follieuluf. As species they are, however, both perfectly distinct, 
being much larger, and not shining and smooth like C. IPollieuliu^ 
which is found in tlio South of Europe and at Ikladeira. Some 
very young sheila and an egg were found, which I conjecture to 
belong to this species. 

2. Cochlicopa terebellum. G. Sweerby. 
Tettd oblongd, cj/lindraceo-pj/ramidali, apiee obcuiitueulo, aTi/rae- 
tibut septenii, UevHu* ; nilurd postici erenulald ; aperturd 
osali, poitkc aculd, labio externa tenui, anlicl decliri ; colu- 



APPENDIX. 167 

melld obsolele truneatd, umhiUco minimo; long. 0'77, liU. 

0-25, poll. 
This species differs from the last in being more cylindrical, and in 
being neatly free, when full grown, from the obtuse folds of the pos- 
terior volutioui, as well as in the form of the aperture. The young 
abells of this species are longitudinally striated, and they have some 
Tery obsolete longitudinal folds. 

I. Helix bilamellata. G. Soieerbjf. 

Tettd orhieulato-depressd, aptrd pland, anfractibu$ tenU, ultimo 

tubtiit ventrieoto, supeme angulari; wmhilico parvo ; aperturd 

*emilunari, gupemi extiit angulatd, labia ea:terno tenui ; inCemo 

pUeU duabui tpiralibua, poglicd mc^ori : long, 0' 1 5, lat. 0'33, 

poll. 

The young shells of this species have very different proportions 

from those marked above, their aiig being nearly as great as their 

width. The largest specimen is white, with irregular ferruginous 

rays. This is very different from any known recent species, although 

there are several to which it appears to have some analogy, such as 

Helix epigtglmm, or Guokiana, &dA. H. gularii: in both of these, 

however, the internal spiral plaits are placed within the outer wall 

of the shell, and not upon tlie inner lamina, as iu Helix bilamellata. 

There is another recent species, which is somewhat analogous to 

this ; it is as yet undescribed, and differs from this and from 

Cookiana, in tlio circumstance of its possessing four internal spiral 

plaits, two of wbicli are placed within the outer, and two upon the 

inner wall of the shell; it was brought from Tahiti, in the Beagle. 

2. Helix polyodon. O. Somerbg. 
Te»td orbiculatosubdepreetd, an/raelibue sex, rotundatU, Hriatit ; 
aperturd semilunari, labia interna plieis tribus tpiralibut, 
poiUcis gradalim majorihug, easterno inliit dentibu* guinqite 
imtructo; umhilieo mediocri s fcw^, 0'07, lat. 0'15,poll. 
This is somewhat related to Helix contoria, of De Feruasoc, Moll, 
terr. et fluv. Tab. 51 A, f 2 j but differs from it in several par- 
ticulars. 

3. Helix sfurca. G. Soroerby. 
Tettd fuborbiculari, spird mbeonoided, oltusd ; anfractibus gua- 
iMoi" tumidii, ntbsiriatis ; aperturd magnd, peritremate tenui; 
timhilieo parvo, pro/undo ; lon<f. O'l, lat. Q-13, poll. 



Eamly disUiigiiisKod from Helix polgodon, by its widt^ tooililefl 
^rtnre. 

4. Helix biplicata. G. SoKerbi/. 

Tntd orhieulatO'deprestd, anfraetibut guinque roiundatu, rtrwtiij 

aptrturd temiltinari, labio inUnto plieu daubut tpirali 

poilicd majori ; uinlilieo moffno; lonif. O'Oi, lat, O-l. poU. 

Tub must be regarded us perfectly distinct from Helix bilat 

on account of its form ; its umbilicua is mucli larger, its spire is i 

flat, nor is the posterior edge of each volution angular. There i 

specimens, whieh must be referred to tiiia species, found with t 

foregoing apceiea, and with the Cofflo</ena fotnlit, whicU bitter 

ucoctated with a living Succinca, in the raodera calcareous aui 




G. Sotcerbff. 

Producta, teatd guhtrapezi/ormi, eomprettd, parte antied latioH^ 
bilohd, poiticd augustiori, lined cardinali hrevi. 

The most remarkable characters of this epecies, are the Bhotttuai 
of the hinge-line, and the comparative width of the anterior parti 
its outside is ornamented with small, blimt tubercles, irr^ula^ 
placed : it is in limestone, of the ordinary grey colour of moimtai 
limestone. Another specimen, which I suppose to be an impress 
of the inside of the flat valve, is in atone, of a light, nisty-brow 
colour. There is a third specimen, which I believe to be the ii 
pression of the inside of the deeper valve, in a nearly similar e 
accompanied by other shells. 



APPENDIX. 



1. Spimpeha SDBRADIATA. G. Sowerbt/. 
Spirifera, testd ImvUtimd, parte mediand laid, radiU lateralihus 

utriusque lateria paucie, incompicuh. 
The breadth of this ehell is ratber ^eater than its length. The 
rays of the lateral surfaces ore very few and indistinct, and the 
medial labe ia uncommonly large and wide. 

2. Spuufeka botukdata ? PAillipii Geology of Yorkshire, 
pi. is. f. 17. 

Although thia shell is not exactly like the figure aboTO referred to, 
it wonld perhaps be impossible to find any good distinguish ing 
character. Our specimen is much distorted; it is, moreover, an 
example of that sort of accidental variation that shows how little 
dependence ought, in some instances, to be placed upon particular 
characters ; fur the radiating riba of one side of one valve, are much 
more numerous and closer than those on the other side of the same 

3. Spirifera trafbzoidalis. G. Soicerlp. 
Spiri/era, tmtd eultetragond, mediand parte profundd, radiit 
nonnullU, tubineompieuiii ; radiig lateralibug utriusque laterit 
geptem ad octo dUtinelit: long. I '5, lai, 2', poll. 
There are two specimens of thia, in a dark, rusty, gray limestone, 
probably bituminous, 

Spibifeba TBAPEZ0IDAL18, var. ? G. Soieerby. 
Spiri/era, testd radiis lateralihus triparlifiin, divi^ig, lineig inere- 
menti antiquatis, coiteroquin omnind ad Spiri/eram trapezoi- 
dalem simillimd. 
At first I hesitated to unite this to Spiri/era trapezoidalis, but 
observing that at the commencement the radiating ribs were simple, 
and knowing that these are subject to variations, I have thought it 
best merely to distinguish thia specimen as a variety. 



There are several other, probably distinct, species of Spiriforte, 
mt as these are only casts, it is obviously imiiossible to give the 



160 APPENDIX. 

external characters of the species. Since, however, they are very 
remarkable, I have thought it advisable to give a name, together 
with a short description of each. 

4. Spirifera paucicostata. G. Sotcerhy. 

Length equal to about two-thirds of its breadth ; ribs few and 
variable. 

5. Spirifera Vespbrtilio. G. Sowerhy. 

Breadth more than double its length, radiating ribs rather large, 
distinct, and not numerous ; posterior inner surface covered with dis- 
tinct punctulations in both valves. 

6. Spirifera avicula. G. Sowerhy. 

The proportions of this species are very remarkable, inasmuch as 
it appears to have been nearly three times as wide as it is long ; the 
radiating ribs are not very numerous, and the internal posterior sur- 
face of one valve alone (the large valve) has been punctulated. In 
its proportions, it resembles Phillips's Spirifera convoluta* but as 
our Sp» avicula is only a cast of the inside, its proportions are not so 
abnormal as those of Sp, convoluta. 

A specimen which is very much pressed out of its natural shape, 
but which still appears to differ somewhat in its proportions, shows 
not only the cast of the inside, but also the impression of the out- 
side; its radiating ribs are very irregular, and numerous, but it 
must be regarded as doubtful whether some of them be not principal 
and others only interstitial : their irregularity renders it impossible to 
decide. 

* Geology of Yorkshire, Part 2. Plate IX. f. 7. 



APPENDIX. 



BCRIPTION OF SIX SPECIES OF CORALS, 
FROM THE PALEOZOIC FORMATION OF 
VAN DIEMEN'S LAND. 

Bz W. LONBDALE, Ebq., F.G.B. 

]. Stknopoba Tasmaniensm, ap. n.* 
Branched, hrancfieg cylindrical, variously inclined or contorted ; 
tahe* more or leM divergent; mouth oeal, divisional ridges 
ftronfflif luherculated ; indication of succetsive narrotcing in 
each tube, 1 — 2. 
This coral, in ita general mode of growth, resembles Calamopora 
(Stenopora ?) tutnida, (Mr. PliilJips, Geol. Toiksliire, Part II. 
VI. 1, fig. 62), but in the form of the mouth and other atrnctural 
details, the differences are very great. Stenopora Tasmanienti* 
attains considorable dimensions, one specimen being 4j inches in 
length, and half an inch in diameter. 

The branchea have individually great uniformity in their circiim- 
feieiice, but they differ with respect to each other in the same spe- 
nd tliere is no definite method of aubdivision or direction 
of growth. The extremities are occasionally hollow ; and ono 
Bpecimen, about 1 J inches in length, and half an inch in breadth, is 
crushed completely flat. The tubes, in the beat exposed cases, hafe 
considerable length, apringing almost solely from the axis of the 
branch, and diverging very gently till they nearly reach the circum- 
ference, where they bend outwards. In the body of the branch, 

Thoi^b the chnracterB of this genag are unpublished, it bos bepn thought 
advisable pot to give Ihem fully in this notice, a very fow species only having- 
beeo examined. The coral is essentiall; composed of simple tubes, varlooaly 
aggregated and Tadiattug outwards. The mouth is round or obloug, and but- 
Touoded by projecting walls, having aloug the creat a row of tubercles. The 
month or^nally oval la gradually narrowed {nr[v6g) by a band projecting from 
the lanar wall of the tube, and it finally closed. 



162 APPENDIX. 

tho tnWs arc angnlar from lateral interferonce ; but, on approaching 
till} outer eurfacp, tlicjr become otiI in consequence of tbe inter- 
Bpacea produced by the greater divergence. Their diameter is veij 
uniform throughout, with the exception of the narrowing near the 
terminations of the full-grown tubes. Tlie walls in tlie interior of 
tlie branches were apparently very tliin, but there is a relatdrely 
considerable thickness of matter at the circumference. No tracei 
of transverse diaphragms have been noUced within tbc tubes. 

Cases illustrative of the clianges to maturity and final ubliteration 
in tlie oval termination of the tubes are rare, but the following have 
been observed. Where the mouth becomes free and oval, the walk 
are thin and sharp, and perpendicular within the tube. In some 
cases they are in contact ; but, in others, they are separated by 
grooves of variable dimensions, in whicli very minute foramina or 
pores niay be detected. As the mouth approaches towards matu- 
rity, the grooves are more or less filled up, and the walls thicken, t, 
row of very minute tubercles being discoverable along the crtst. At 
this stage, the inner side of the tube ceases to be vertical, being 
lined by a very nanow inclined band. The mature months an 
separated by a bold ridge, generally simple, but not nnfre^nently 
divided by a groove ; the double as well as the single ridge being 
surmounted by a row of prominent tubercles almoat in contact 
with eacli other. Only one example of the lilliog up of the nontba 
has been observed, but it affords satisfactory evidence of a gradnal 
expansion of the inner band, before alluded to, and a final meeting 
in the centre. In this cxtremo state, there is a general blending of 
details, but the tubercles are for the most part distinct. 

In this species, proofs of a narrowing of the mouth previously to 
the formation of the perfect tube, and tbe final contraction, are not 
very prominently exhibited in the long cylindrical straight branchn; 
but near the point where the tubes bent outwards, there is an annu- 
lar indentation, which may be traced successively from cast to cast 
in a lineal direction, parallel to the surface ; and between the pio- 
miaent narrowing and the perfect surface, the walls of the tuba 
were slightly rugose. In another short branch, believed to belong 
to this species, but in which the tubes diverged outwards very 
rapidly, the narrowing is strongly marked, but not to an equal ei- 
tent throughout the spocimon. 

The matrix, in which tiio fossil is imbedded, is a coarse caloa- 
l^us shale, or a gray limestone ; and in which occur also Perwttelh 



APPENDIX. 



2. Stenopoha ovata, ap, n. 

Branched, braTiehes oeal ; tubei relativeli/ tkort, Heergence great ; 
mouths round; eontraotiom or irregularities of grotcth nu- 



The characters of this epeciea have been very imperfectly ascei- 
tained, Tlio branches are not uniformly oval, even in apparently 
the same fragment. The tubes diverged rapidly along the line of 
the major axis, and had but a very limited vertical growth. Their 
casta exhibit a rapid succession of irregularities of dovelopment. 
The mouths, as far as they can be determined, were round or 
alightly oval, and the dividing, tuberculated ridges sharp ; but in 
conaeijuence of the outer surface not being exposed, their perfect 
characters, and tho changes incidental upon growth, could not bo 
ascertained. 

The coral is Imbedded in a dark gray limestone. 

]. Fenestella ampla, sp. n. 

Cup-»haped ; eelluli/erous turface internal; branches dicholi- 

tnoiM, broad, fiat, thin ; meshei oeal ; rotes of cells numerous, 

rarely limited to Ifno, alteittate ; transverse connecting processes 

sometimes cellular ; inner layer of non-eellular surface very 

Jlhrous ; external layer very granular, non-Jihrous ; gemmit- 

liferous vesicle ? small. 

Some of the casta of this coral have a general rescmhlance to 

Fenestella polyporata, aa refreseated in Captain Portlock's Report 

on the Geology of Londonderry, PI. XXII. A. fig. 1 a. Id; biit 

there is no agreement between the Van Diemen's Land fossil, and 

the structure of that species as given ui PI. XXII. figure 3. of 

the same work, or in Mr. PhiUips' original figures, Geology of 

Yorkshire, Part 2, PI. I. figures 19, 20. A general resemblance 

also exists between Fenestella ampla and a coral obtained by Mr. 

Murcliison from the carboniferous limestone of Kossatchi Datclii, on 

the eastern flank of the Ural mountains, but there is again a marked 

difference in structural details. 

Fenestella ampla attained considerable dimensions, fragiuenta ap- 
parently of one specimen covering an area of 4) inches by 3 inches ; 
and it displays considerable massiveness of outline, the branches at 
the points where they dlchotimosa often exceeding the tenth of au 
inch in breadth. 



164 



APPENDIX. 



Iq iho goncral aspect of the coral a considerable uniformity pie- 
vails, but the branches vary in breadth, swelling out greatly near 
the bifurcations; nevertheless, there is no marked difference of 
character bi'twecn the base and the upper part of the cup, even in 
the number of the rows of cells. 

In the I>e8t state of the cellular surface, which has been noticed, 
the nioutlis of tlie cells are relatively large, round or oval, and are 
defined by a slightly raised margin ; and an undulating, thread-like 
ridge winds between them, dividing the interspaces into lozenge- 
shaped areas. The row^s of cells, immediately preceding the bifurcation, 
sometimes amount to ten, and after the separation generally exceed 
two. The mouths of the lateral rows project into the meshes ; and 
the transverse connecting processes are sometimes cellular. The in- 
terspaces between tlie mouths, as well as the undulating ridges, are 
granular, or very minutely tuberculated. Internally, the cells ex- 
hibit tlie usual oblique arrangement, overlying each other, and ter- 
minating abruptly against the dorsal part of the branch. The per- 
fect casts of the cellular surface give the reverse of the characters 
just noticed, but more generally the impressions dis])lay scarcely a 
trace of any other structure than longitudinal rows of circular 
mouths. 

On the inner layer of the non-cellular surface, twenty well- 
marked parallel fibres, with intermediate narrow grooves or corre- 
sponding casts, may sometimes be detected, and the number is 
alw^ays considerable. The mode of preservation did not permit the 
true nature of the fibres to bo discovered, but in consequence of 
what has been noticed in other species, it is inferred that they are 
tubular. Their range is considerable, but in the specimen, which 
exhibits their structure most fully, they are frequently cut off by 
circular foramina. Tlieir perfect surface is minutely granular. 
The outer layer, or back of the branches, is composed of an uniform 
crust witliout any indications of fibres, but covered with numerous 
microscopic papillee, and corresponding pores penetrating the sub- 
stance of the layer. 

Tlic only indications of gemmuliferous vesicles, are small circular 
pits occasionally situated over the mouth, and agreeing in position 
with the vesicles, which in other cellular genera, have been con- 
sidered as gemmuliferous. In the Russian specimen before alluded 
to, casts of siiDilar pits are very uniformly distributed between the 
cai^ts of the mouths. 

The youngest state of the coral has not been noticed, nor have 



APPEMDIX. 16fi 

'inarkcd changes iticidunt upon age, except tbe grailuni tliitlfen- 
_ of tlie nnn-cellular surface, fay tlie coating OTer of tlie fibrous 

kyer. 

The matris of the spscinicns is a dark grey splintery or an eartliy 

limestone. 

2. Fenestella internata, sp, n. 
C'upsfiaped; eelluH/eroTts surface internal ; braneAes, dicAotimoiu, 
compressed, breadth variable ; meshes oblong, narrow ; rotes of 
celU 2 — 5, divided hy limgitud'vnal ridget: iransmrse connect- 
ing proeeaies short ttilhoitl cells; non-eellular rar/ace, inner 
lager, skarplg Jibroits, outer layer, minutely granular. 
By the delicacy of its striicturej this speciea is easily distinguish- 
able from Fin. ampla; nnd in the rows of cells varying from two 
to five, as well as in their moJe of development, there are further 
well-marked differences. It appears to have attained considerable 
diraonsions, fragments baving been noticed an inch and a-hnlf in 
length and an inch in breadth. 

The brancbea vary in width, swelling out gradually towards the 
bifurcations, but without Any alteration in the form or size of tbe 
meshes; and as far as tbe stnte of the specimens will permit an 
opinion to bo formed, no marked changes occurred during the deve- 
lopment of the cup, except one about to be noticed. On tiie cellu- 
liferons surface of the brandies, considerable, but uniform, altera- 
tions take place between the BucceasiTe bifurcations. For a short 
distance above the point of separation, tiie branch is narrow and 
angnlar, and traversed along the centre by a ridge, and there la only 
one row of cellular mouths on each side. As the branch grew, the 
ridge widened, and ultimately became cellulifoions, a row of mouths 
springing from its place {intemata). The three ranges of cellular 
openings are, in this state of the branch, separated by two ridges, 
and these, as the development advanced, again widened and became 
cellular, tbe five rows being divided by four ridges. This appears 
to be the extreme stage of growth, another bifurcation taking place 
immediately after. In the earliest formed part of the cup only two 
or three rows of mouths prevail ; and where the number is greater, 
a certain amount of irregularity in the linear arrangement is porcep* 
tible, resulting from tbe lateral expansion of the branch. 

In the best preserved specimens, the mouths are relatively large, 
round or oval, and tbe margin is slightly raised. In the middle 
rows they are parallel or near'y parallel, and in the direction of the 



axis of the brancL ; but in the side raws they are often obhqiiely 
placed, inclining towaida the meshea. In these nearly perfect speci- 
mena the dividing ridges are thread-like and slightly waved, but 
there ia no trace of the lozenge-shaped compartmeuta so distinctly 
exhibited in Piiiettalla ampla. The interspaces between the mouths 
are flat or slightly convex. In specimens less finely preserved, or 
deprived of the original surface, the mouths are not uniform in out- 
line, and have no projecting margin. The dividing ridges are also 
relatively broader j and the whole surface, including the transverse 
connecting processes, is granular or minutely tubereulated. 

The inner layer of the non-cellular surface ia sharply fibrous, and 
the same structure may be more or less clearly detected in the 
transverse, connecting processes. The number of fibres on the 
branches do not apparently exceed twelve, and they are in general 
leas numerous. Their range is considerable, additional ones being 
interpolated as tlie branch widens ; and their surface is minutely 
tubereulated. No seporate, circular foramina were noticed. The 
outer layer is uniformly granular, where completed, but every inter- 
mediate state from the sharply fibrous may be traced on the same 
specimen. 

No distinct proofs of gemmuliferous vesiclea have been observed, 
but in a specimen, which is believed to exhibit impreasions of this 
epecies, there are occasionally to be detected, near the mouths, 
hemispherical casts, perfectly rounded on the surface, and evidently 
unoonnected immediately witli the interior of the cells, aud which it 
is presumed may represent those vesicles. Fenetlella intemata' 
appears to be an abundant fossil, one alab nearly eight indiea longi^ 
and sis wide, being covered on both aides with fragments of it, and 
numerous smaller apecimens occur in the collection. The matrix 
is chiefly a coarse gray calcareous shale, but it is sometimes a splin- 
tery limestone, or a bard ferruginoua or light- colon red clay-stone. 

3. Fenestella fossula, sp. n. 

Oup-tAaped, celluUferouii surface inm-nal ; hranckm dicAotimout'. 

ilender; metket oval j roiEi of etlh, tieo ; transverte proeesitt- 

non- cellular ; inner la^/er of non-celluli/erous mi/aee minulely'. 

Jihroui ; external la^er smooth OT granular. 

In general aspect and stnictnral details, this species bears a great 

reaemblauco to Fenestetla Jlustracea of the magnesian limestone of. 

England, {Betepora Jlustracea, Geol. Trans., 2nd Series, Vol. III. 



APPSHDIX. 167 

PL XII. t 8), Imi H difim from it in the peculiar character 
exhibited in the cast of the oellaliferoas soifMse, the nature of whidi 
will be given in noticii^ that sorCaoe. 

The principal ^)ectmen is a nearly perfect cup 1 1 inches in height, 
and about two inches across the widest, compressed part. There 
are no marked rariations of character, but occasionally, irregu- 
larities of growth, due, apparently, to accidents during progressive 
development. 

The following details have been obtained from casts, no perfect 
8ur&ce having been noticed. — The branches had great uniformity of 
dimensions, swelling but very slightly at the distant points of bifur- 
cation, and their thickness was apparently nearly equal to their 
breadth. The cast of the cellular surface is traversed along the 
centre by a sharp narrow trench (/osmla)^ with nearly vertical 
sides, the distinguishing character between this species and Fen, 
flustraeea. The cylindrical casts of the mouths, or the interior of the 
cells, are arranged in a single row on each side of the trench, and 
no increase of number is clearly perceptible at the bifurcations. 
Along the centre of the trench is a row of indentations or minute 
conical pits, a character noticeable in other species, particularly in 
Fen, Jltutracea. They are plainly not casts of cellular openings, 
but of relatively large papillee. Traces of such projections have 
aso been noticed in several other instances. 

The mouths of the cells, in the minute fragment which has been 
obtained exhibiting them, are large, round, slightly projecting, and 
not very distant, and in the same atom is an imperfect keel. The 
remains of the non- cellular surface exhibit no characters requiring 
notice, but indications of a striated and smooth layer have been 
observed. 

The two specimens which afforded these structural details have a 
matrix of dark-coloured, hard limestone. 

Hemitrypa sexanqula, sp. n. 
• Net-work fine^ hexagonal ; me9he9 round in double rows. 

The coral to which the above inefficient characters are applied, is 
imbedded in the shaly surface of a dark, hard limestone. It is 
about an inch in breadth and half an inch in height, and consists of 
two layers of net-work, — one presenting quadrangular meshes, and 
the other hexagonal, with a round, inner area ; and over a consider-' 
able part of the specimen, the quadrangular network has been 



lemOTod, whereby the connexion of the two structures is perfectly 
exposed. 

This fossil is believcHl to agree completely in its essential generio 
characters witli those of Hemitrypa (Pal. Fosa. Cornwall, p. 27), 
but its Htatc of preservation, anil some facilities afforded by it for 
determining structural details, have led to an inference respecting 
its nature somewliat different from that given in the work just 
quoted. 

The inner surface of ffemilft/ptt oculala (Zye. ciV.) is described as 
" tnorted witli radiating lid^rea," haviug intervening " oval depres- 
aitina, which penetrate only half tlirough the substance of the coral, 
and no where reach the outer surface." The equivalent portion of 
the Van Dieinen's Land specimen agrees perfectly with tliis state- 
ment, except in the form of the meshes or depressions; it is, how- 
ever, not iQurely ''like some FenestoUee," but it possesses all Uie 
essential characters of that genus, and is believed to be a fragment 
of Fen. fossula. This inference is drawn from a minute portion 
mechanically detached, and which exhibited a row of large, round, 
projecting, cellular moutlis. The external surface of Hem. oeulata 
is described as " wholly covered with numerous round pores ax 
cells" — "associated in double rows," and the corresponding portion 
oi Hem. K&eatiffala has been ascertained to consist also of a similar 
surface of double rows of round racslics or " pores," but n'ith hexa- 
gonal boundaries ; and tboy are shown, as exhibited by the speuimea 
in its embedded state, to penetrate to tlie surface of the Fonestella 
or quadrangular net-work. 

These details are conceived to he sufficient to establish a generic 
agreement between the Van Diemen's Land coral and Hemitrypa 
oeulata ; and an examination of an Irish specimen of that genus 
has fully confirmed the structural details exhibited in the " inner 
surface" of the specimen to which, provisionally, the name of ffemi- 
trypa texangula is applied. 

Of the true nature of the "external" net-work no opinion is 
rentured, It is formed almost entirely of dark gray, calcareon> 
matter, fiUing apparently an originally cellular structure; but tbeie 
are also a few small patches of the outer covering, consisting of on 
opaque white crust on the surface, which was originally in contact 
■with the external net-work. That it was a parasite little doubt is 
entertained, and the interesting agreement between the space occupied 
by the double row of meshes, and that of the parallel brandies of 
the Fcnestella arises iipparently from the latter having afforded 



APPENDIX. 169 

suitable base lines for attachment. In the Van Diemen's Land 
specimen, the agreement is marked by an increased breadth in the 
net-work, and by a row of projecting points. There is also a re- 
markable agreement between the arrangement of the mouths of the 
Fenestella and the meshes of the " inner" net- work. Similar con- 
forniities are admirably shown in Mr. Phillips's excellent figures 
(Pal. Foss. PI. XIII. f. 38). 

The solid portions of the structure being exceedingly fine, resem- 
bling the thread of the most delicate lace, attempts to discover satis- 
factorily internal characters proved unsuccessful, except in one place^ 
where a true cellular arrangement was believed to be visible.* 
Of the nature of the investing crust, nothing abo has been 
determined. 

Though the name Hemitrypa may be objectionable, as applied to 
the corals under consideration, it has been thought right to retain 
the word, until the full characters of the genus shall have been 
ascertained. 

Falmouth, January^ 1844. 

* A Codrington lens, half-an-inch in diameter, was invariably used in ex- 
amining the corals described in this notice. 



^^^^^P^^IN D E X, ^^^^^1 


Abbl, M., oh CBlcareous caata at the 


Beudant, M., on Ijomto, 37, 3D 


Cape of Good Hope, 146 


on jasper, 47 


Abingdon lalam!, IW 




AliTolbos Islands, incrustation oil, 33 


on obsiiliBQ of Huiisrary, 63 


Aerifonn exploaiona at Asccneion, 39 


on silex in traobyte, 14, 46 


Albatross, driven from St. Hei™a, 00 


Bermuda, calcareous rooks of, 144, 147 


AliJemarle Islaud, 103 


Boie, 139 


Albitc, at the Galapagos Archipelago, 


Bomiis, volcanic, 36 


104 


Borz, St. Vincent, on bombs, 37 


Amygdaloidal ceils, half Blied, 27 


Boulders, alisenee in Australia and tiie 


AjnygdftloidB, caicareouB origin of, 14 


Cape ofGood Hope, 152 


ABeension, 34 


of greenstoue in New Zealaud, 1G3 




Brattle Island, 109 


of, 83 




aiiflence of dikes, freedom from vol- 


substance, 54 


canic action, and state of iavi- 


on decomposed gloss, 132 


Btreame, 92 


Blown, Hr. R., on extinct plants from 


Atddifl, ^tinction of, 141 


Van Diemen's Land, 140 


Atlantic ocean, new volcanic fouus 


ou spiiKrulitlc indies in silicified 


in, 92 


wood, 62 


Ai^ite fused. Ill 


Bncli, Von, on caTemous lava, 103 


Australia, 130 


on central volcanos, 127 


Azores, 24, 125 


on crystals sinking in obsidian, 117 




on laminated lava, 60 


Bahia in Bradl, dikes at, 123 


on obsidian streams, 64 


Bailiy, M., on the mountains of Manri- 


on olivine in basalt, 104 


"^'tius.ao 


on BuperBeial coicareona beds Id 


Bald Head, 144 


tlie Canary Islands, BS 


BttukB' Coife, lOa, 107 




Bam, Tile, St. Helena, 76 




Bualt, flpeciflc gravity of, 130 


Cfllcareons deposit at 8t, Jago, aSected 


iSmltic coast~moDntains, at Mauri- 


by iieat, 3—7 


tius, 29 


fibrous matter, eotanj-ied in streaks 


mt St. Helena, 80 




at Bl. jBgo, 17 


freestone at Ascension, 49 


Beaumont, M. Elie de, on circular .ub- 


incrustations at Ascension, 50 


sideaces in lava, 102 


superficial beds at King George's 




Sound, 144 


on inclination of iftva-strcama, 03 






Cape ofGood Hope, 148 



172 



INDEX. 



Carbonic acid^ expulsion of by heat, 

6,14 
CarmichacI, Capt., on glassy coatings to 

dikes, 77 
Casts, calcareous, of branches, 145 
Cerithiurn, (fossil,) 153 
Chalcedonic nodules, 140 
Chalcedony in basalt and in silicified 

wood, 47 
Chatham Island, 08, 105, 114, 125, 142 
Chlorophaeite, 130 
Clarke, Rev. W., on the Cape of Good 

Hope, 141,148 
Clay-slate, its decomposition and junc- 
tion with granite, at the Cape of 

Good Hope, 140 
Cleavage of clay-slate in Australia, 131 
Cleavage-cross, in sandstone, 134 
Coast-denudation, at St. Helena, 01 
Cochlicopa, (fossil,) 156 
CochlogenOf auriS'Vtdpina, 00, 155 
Columnar basalt, 10 
Comptes Rendus, account of volcanic 

phenomena in the Atlantic, 02 
Conception, earthquake of, 05, 127 
Concretions in aqueous and igneous 

rocks compared, 60 
in tuff, 47 
of obsidian, 60, 64 
Conglomerate, recent, at St. Jago, 21 
Coquimbo, curious rock of, 145 
Corals, fossil, from Van Diemen's Land, 

138, 161 
Crater, segment of, at the Galapagos, 100 
great central one at St. Helena, 81 
internal ledges round, and parapet 



on, 82 



Craters, basaltic, at Ascension, 35 

form of affected by the trade- 
wind, 35 
of elevation, 03 
of tuff at Terceira, 24 
of tuff at the Galapagos Archipe- 
lago, 98, 100, 105, 108 
their breached state, 112 
small basaltic, at St. Jago, 16 
small basaltic, at the Galapagos 
Archipelago, 101 
Crystallization favoured by space, 69 

Dartigues, M. on sphserulites, 62 
Daubeny, Dr., on a basin-formed is- 
land, 108 

on fragments in trachyte, 43 
D'Aubuisson on hills of phonolite, 86 

on the composition of obsidian, 61 

on the lamination of clay-slate, 66 
De la Beche, Sir H., on magnesia in 
erupted lime, 11 

on specific gravity of limestones, 49 



Denudation of coast at St. Helena, 9 
Diana's Peak, St. Helena, 82 
Dieffenbach, Dr., on tlie Chatha 

Islands, 142 
Dikes, truncated on central craterifo: 
ridge of St. Helena, 82 
at St. Helena ; number of; coa 
by a glossy layer ; unifo 
thickness of, 77 
great parallel ones at St. Helena^ 85 
not observed at Ascension, 92 
of tuff, 100 

of trap, in the plutonic series, 12d 
remnants of, extending far into 
sea round St. Helena, 01 
Dislocations at Ascension, 40 

at St. Helena, 78, 84 
Distribution of volcanic islands, 124 
Dollmieu on decomposed trachyte, 25 
on laminated lava, 66, 68 
on obsidian, 63, 64 
Dr^e, M., on crystals sinking in lava, 

117 
Dufr6noy, M.,on the composition of the 
surface of certain lava-streams, 
64,118 
on the inclination of tuff-strata, 106 

Eggs of birds embedded at St. He- 
lena, 88 
of turtle at Ascension, 49 
Ejected fragments at Ascension, 40 

at the Galapagos Archipelago, 110 
Elevation of St. Helena, 90 

the Galapagos Archipelago, 115 
Van Diemen's Land, Cape of Good 
Hope, New Zealand, Australia, 
and Chatham Island, 140 
of volcanic islands, 128 
Ellis, Rev. W., on ledges within the 
great crater at Hawaii, 83 
on marine remains at Otaheite, 27 
Eruption, fissures of, 116, 126, 128 
Extinction of land-shells at St. Helena, 
00 

Faraday, Mr., on the expulsion of car- 
bonic acid gas, 6 
Feldspar, fusibility of, 122 

in radiating crystals, 148 

Labrador, ejected, 41 
Feldspathic lavas, 10 

at St. Helena, 80 

rocks, alternating with obsidian, 55 

lamination, and origin of, 65 
Fenestellttf (fossil,) 163 
Fernando Noronha, 23, 66 
Ferruginous superficial beds, 143 
Fibrous calcareous matter at St. Jago, 
11 



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HtwKiijyptt 'imoL , I'fs 

HcBBAh.. Xr . oa uhc» ax Asccas&Tii. 93 

HoffDOdUL otL 4«i]aip>Q««fi tradiTer. ^ 

Homer. Mr., «n a galcareo iiniwl scb> 
stance, 64 

on faabditj of feldqwr. I^ 
Hubboidy Dr., on dike*, 133 
Humboldt on ejected fngmentsSy 41 

on obsidian fonnationSy 63, 65 

on parapets loond enters, d3 

on spberolites, 67 
Hatton on aznvgdaloids, 14 
Hyalite in decomposed tracbvte, 25 

Iceland, stratification of the cireumfe- 

rential bills, d5 
Islands, yolcanic, distribution of, 134 

their elevation, 128 
Incrustation, on St. Paul's Rocks, 3d 



rrjuiutrz* ^ 



\L-!ihau. H.. m ^moiti*. •Jl 

Zjuik.-*- IX niLin ft 'oiciuitAv M 
IjuniziaftUcn n' 'iicruui: >ivs>« Kt 

lji"aiK "Wsnlh: rn^'tr- n\ ' ' *, Vi* 

^jmnuMtiiiu if "uHkx n\ J>k 
4urnic«»K Ll'f 

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LM»£ik« Xr.w vwn lK^tit»U cvintJ^ if^vJbiv^ 

Maeaulay. Dr«» ^^ ^nUv^ivimu ivmI^ «I 

Maildm^ 141) 
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tm ehKxr^)^M»)t^ \9^ 
on laiulttal^l |Uu^U*h^«H ^^ 
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on stmUfloAticm )n l\>«^)Hud) ik^ 
M«deim> e«l<Mirt>MU« omU Ht| UH 



174 



INDBX. 



Magaiine, Nantieal, — a«c<mnt of toI- 
canic phenomena in the Atlantic, 
92 
Marekanite, 60 
Mauritius, 28 

crater of elevation of, 93 
Mica» in rounded nodules, 2 

origin in metamorphic slate, 150 
radiating form of, 148 
Miller, Prof., on ejected Labrador feld- 
spar, 41 
on quartz crystals in obsidian beds, 
55 
Mitchell, Sir T., on bombs, 38 

on the Australian valleys, 135 
Mud streams, at the Galapagos Archi- 
pelago, 106 

Narborough Island, 103 

Nelson, Lieut., on the Bermuda Islands, 
144, 147 

New Caledonia, 125 

New Bed sandstone, cross-cleavage of, 
134 

New South Wales, 130 

New Zealand, 142 

Nulliporse (fossil), resembling concre- 
tions, 4 

Obsidian, absent at the Galapagos Archi- 
])elago, 114 
bombs of, 37, 38 

composition and origin of, 61, 64 
crystals of feldspar sink in, 117 
its eruption from lofty craters, 121 
passage of beds into, 54, 57 
specific gravity of, 118, 121 
streams of, 64 

Olivine decomposed at St. Jago, 18 
at Van Diemen's Land, 139 
in the lavas at the Galapagos Archi- 
pelago, 104 

Oolitic structure of recent calcareous 
beds at St. Helena, 87 

Otaheite, 25 

Oysters, extinction of, 141 

Panza Island, laminated trachyte of, 65 
Pattinson, Mr., on the separation of lead 

and silver, 119 
Paul's, St., rocks of, 31 
Pearlstone, 60 
Peperino, 100 
P6ron, M., on calcareous rocks of Aus- 

traUa, 146, 148 
Phonolite, hills of, 20, 23, 84 

laminated, 66 

with more fusible hornblende, 122 
Pitchstoue, 58 

dikes of, 65 



Plants extinct, 140 

Plutonic rocks, separation of constitu^^* nt 

parts of by gjravity, 122 
Porto Praya, 1 
Prevost, M. C., on rarity of great dis.ll o- 

cations in volcanic islands, 78 
Producta, 158 

Prosperous Hill, St. Helena, 79 
Pumice, absent at the Galapagos ArcTi/- 

pelago, 114 
laminated, 65, 66,68 
Puy de Ddme, trachyte of, 44 

Quail Island, St. Jago, 3, 8, 10 
Quartz, crystals of, in beds alternating 
with obsidian, 55 
crystallized in sandstone, 132 
fusibiUty of, 122 

rock, mottled from metamorphic 
action with earthy matter, 5 

Red Hill, 9, 10 
Resin-like altered scoriae, 7 
Rio de Janeiro, gneiss of, 132 
Robert, M., on strata of Iceland, 95 
Rogers, Professors, on curved lines of 
elevation, 126 

Salses, compared with tuff craters, 112 
Salt deposited by the sea, 53 

in volcanic strata, 53, 75 

lakes of, in craters, 112 
Sandstone of Brazil, 151 

of the Cape of Good Hope, 151 

platforms of, in New South Wales, 
131, 151 
Schorl, ranting, 148 
Scrope, Mr. P., on laminated trachyte, 
65, 67, 70 

on obsidian, 63 

on separation of trachyte and ba- 
salt, 118 

on silcx in trachyte, 14 

on sphserulites, 67 
Scale, Mr. Geognosy of St. Helena, 75 

on dikes, 91, 92 

on embedded birds* bones, 90 

on extinct shells of St. Helena, 89 
Sedgwick, Professor, on concretions, 60 
Septaria, in concretions in tuff, 48 
Serpulse on upraised rocks, 29 
Seychelles, 125 
Shells, colour of affected by light, 53 

fossil, from St. Helena, 155 

from St. Jago, 153 

from Van Diemen's Land, 138, 158 

land, extinct at St. Helena, 89 

particles of, drifted by the wind, at 
St. Helena, 87 



INDEX. 



175 



Shelly matter deposited by the waves, 

52 
Sian, M., on ripples, 134 
Signal-Post Hill, 8, 13, 15 
Silica, deposited by steam, 25 

large proportion of in obsidian, 61, 

64 
specific gravity of, 121 
Siliceous sinter, 45 
Smith, Dr. A., on junction of granite 

and clay-slate, 149 
Spallanzani on decomposed trachyte, 25 
Specific gravity of recent calcareous 
rocks and of limestone, 49 
of lavas, 120 
Sphserulites in glass and in silicified 
wood, 62 
in obsidian, 58, 67 
Spirifera, 159 

Sowerby's, Mr. G. B., descriptions of 
fossil shells (in Appendix), 153 
on fossil shells from Van Diemen's 

Land, 138 
from St. Jago, 4 
land-shells, from St. Helena, 89 
St. Helena, 73 

crater of elevation of, 93 
St. Jago, crater of elevation of, 93 

effects of calcareous matter on lava, 
99 
St. Paul's Rocks, 31, 125 
Stenopora (fossil), 161 
Stokes', Mr., collection of sphserulites 

and of obsidians, 62, ^ 
Stony-top, Little, 79, 85 

Great, 80 
Stratification of sandstone in New South 

Wales, 133, 137 
Streams of obsidian, 64 
Stutchbury, Mr., on marine remains at 

Otaheite, 28 
Subsided space at Ascension, 40 

Tahiti, 25 

Talus, stratified, withm tuff craters, 107 



Terceira, 24 

Tertiary deposit of St. Jago, 4 
Terchyte, absent at the Galapagos 
Archipelago, 114 

at Ascension, 42 

at Terceira, 24 

decomposition of by steam, 24 

its lamination, 65, 66 

its separation from basalt, 118 

softened at Ascension, 42 

specific gravity of, 120 

with singular veins, 44 
Trap-dikes, in the plu tonic series, 123 

at King George's Sound, 143 
Travertin at Van Diemen's Land, 140 
Tropic -bird, now rare, at St. Helena, 90 
Tuff, craters of, 98, 100, 105, 108 

their breached state, 112 

peculiar kind of, 98, 100 
Turner, Mr., on the separation of molten 

metals, 119 
Tyerman and Bennett on marine re- 
mains at Huaheine, 27 

Valleys, gorge-like, at St. Helena, 91 

in New South Wales, 134 

in St. Jago, 21 
Van Diemen's Land, 138 
Veins in trachyte, 44 

of jasper, 45 
Venus (fossil), 154 
Vincent, Borz St., on bombs, 37 
Volcanic bombs, 36 

island in process of formation in 
the Atlantic, 92 

islands, their distribution, 124 

Wacke, its passage into lava, 26, 139 
Wackes, argillaceous, 3, 18 
Webster, Dr., on a basin-formed island, 
.108 
on gypsum at Ascension, 53 
White, Martin, on soundings, 134 
Wind, effects of on the form of craters, 
35 



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