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ELEMENTS

OF

GEOLOGY

BY

CHARLES LYELL, F.R.S.

VICE-PRESIDENT OF THE GEOLOGICAL SOCIETY OF LONDON, ETC. ;
AUTHOR OF "PRINCIPLES OF GEOLOGY."

" It is a philosophy which never rests— its law is progress : a point which yesterday was invisible a its goal
today, and will be its starting-post to-morrow." — Edin. Seo.

"Part of the present Treatise was written originally in the form of a supplement to my former work enti-
tled ' Principles of Geology,' and was intended for the use of those students who found certain chapters in
the 'Principles' obscure and difficult, for want of preliminary information. I afterwards considered that it
would not be incompatible with this object to enlarge the ' Elements' into a separate and independent treatise,

to serve as an Introduction to Geology proper The volume now offered to the public, is neither

an epitome of the 'Principles,' nor an abridgement of any part of that work." — Extract from the Author**
Preface.

FIRST AMERIC
FROM THE FIRST LONDON

PHILADELPHIA:

JAMES KAY, JUN. AND BROTHER, 122 CHESTNUT STREET.

PITTSBURGH: C. H. KAY & CO.

1839.

iv PREFACE.

which an attempt is made to point out the bearing on geology
of the modern changes of the earth, and to which is prefixed
a history of the opinions which have been entertained in this
science, from the times of the earliest writers to the present
day.

The volume, therefore, now offered to the public, is neither
an epitome of the Principles, nor an abridgement of any part
of that work. In some places, where I thought it desirable
to incorporate in the Elements certain passages of the former
work, I have not abridged what was previously written, but
have expanded it, giving fuller explanations, and additional
wood-cuts, in the hope of rendering it more intelligible to the
beginner.

> Through the kindness of two of my friends I have been
enabled to refer frequently to two works, not yet before the
public, Mr. Darwin's Journal of Travels in South America,
1832 to 1836, &c., and Mr. Murchison's Silurian System ;
the last of which was presented to me complete, with thei ex-
ception of the maps and plates, and will shortly be published.
Mr. Darwin's Journal was finished, and ready for publica-
tion, some time before the printing of my MS. had begun, but
is still detained, to the great regret of the scientific world,
because it is to form part of a larger work, including an
account of the Surveys of Captains King and FitzRoy, in
South America.

CONTENTS.

PART I.

CHAPTER I.

ON THE FOUR GREAT CLASSES OF ROCKS — THE AQUEOUS, VOLCANIC,
PLUTONIC, AND METAMORPHIC.

Geology defined — Successive formation of the earth's crust — Classification of
rocks according to their origin and age — Aqueous rocks (p. 15.) — Their
stratification and imbedded fossils — Volcanic rocks, with and without cones
and craters (p. 18.) — Plutonic rocks, and their relation to the volcanic —
Metamorphic rocks, and their probable origin (p. 21.) — The term primitive,
why erroneously applied to the crystalline formations (p. 23.) — Division of
the work into two parts ; the first descriptive of rocks without reference to
their age, the second treating of their chronology.

CHAPTER II.

AQUEOUS ROCKS — THEIR COMPOSITION AND FORMS OF STRATIFICATION.

Mineral composition of strata — Arenaceous rocks — Argillaceous — Calcareous
— Gypsum — Forms of stratification (p. 28.) — Original horizontally — thin
ning out — Diagonal arrangement (p. 31.) — Ripple mark.

CHAPTER III.

ARRANGEMENT OF FOSSILS IN STRATA — FRESHWATER AND MARINE.

Successive deposition indicated by fossils — Limestones formed of corals and
shells — Proofs of gradual increase of strata derived from fossils — Serpula
attached to spatangus (p. 37.) — Wood bored by teredina — Tripoli and semi-
opal formed of infusoria — Chalk derived principally from organic bodies
(p. 41.) — Distinction of freshwater from marine formations — Genera of fresh-
water and land shells — Rules for recognizing marine testacea — Gyrogonite
and chara (p. 48.) — Freshwater fishes — Alternation of marine and fresh-
water deposits — Lym-Fiord.

A * (5)

vi CONTENTS.

CHAPTER IV.

CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.

Chemical and mechanical deposits — Cementing together of particles — Hard,
ening by exposure to air (p. 52.) — Concretionary nodules — Consolidating
effects of pressure — Mineralization of organic remains (p. 55.) — Impressions
and casts how formed — Fos^iTwood — Gb'ppert's experiments — Precipitation
of stony matter most rapid where^ putrefaction is going on — Source of lime
in solution (p. 59.) — Silex derived from decomposition of felspar — Proofs of
the lapidification of some fossils soon after burial, of others when much
decayed.

CHAPTER V.

ELEVATION OF STRATA ABOVE THE SEA — HORIZONTAL AND INCLINED
STRATIFICATION.

Why the elevated position of marine strata should be referred to the rising up
of the land, not to the going down of the sea — Upheaval of extensive masses

• of horizontal strata (p. 63.) — Inclined and vertical stratification — Anticlinal
and synclinal lines — Examples of bent strata in east of Scotland (p. 66.) —
Theory of folding by lateral movement — Dip and strike (p. 69.) — Structure
of the Jura — Rocks broken by flexure — Inverted position of disturbed strata
(p. 73.) — Unconformable stratification — Fractures of strata — Polished sur-
faces— Faults — Appearance of repeated alternations produced by them
(p. 77.) — Origin of great faults.

CHAPTER VI.

DENUDATION, AND THE PRODUCTION OF ALLUVIUM.

Denudation defined — Its amount equal to the entire mass of stratified deposits
in the earth's crust — Horizontal sandstone denuded in Ross-shire — Levelled
surface of countries in which great faults occur (p. 81.) — Connexion of
denudation and alluvial formations — Alluvium, how distinguished from rocks
in situ (p. 84.) — Ancient alluviums called diluvium — Origin of these —
Erratic blocks and accompanying gravel (p. 86.) — Theory of their transporta-
tion by ice.

CHAPTER VII.

VOLCANIC ROCKS.

Trap rocks — Name, whence derived — Their igneous origin at first doubted
— Their general appearance and character — Volcanic cones and craters,
how formed (p. 91.) — Mineral composition and texture of volcanic rocks —
Varieties of felspar — Hornblende and augite — Isomorphism (p. 94.) — Rocks,
how to be studied — Basalt, greenstone, trachyte, porphyry, scoria, amygda-
loid (p. 96.) lava, tuff— Alphabetical list, and explanation of names and
synonyms, of volcanic rocks (p. 99.) — Table 'of the analyses of minerals most
abundant in the volcanic and hypogene rocks.

CONTENTS. vii

CHAPTER VIII.

VOLCANIC ROCKS — continued.

Trap dikes — sometimes project — sometimes leave fissures vacant by decom-
position— Branches and veins of trap— Dikes more crystalline in the centre
(p. 105.) — Foreign fragments of rock imbedded — Strata altered at or near
the contact — Obliteration of organic remains — Conversion of chalk into
marble — and of coal into coke (p. 108.) — Inequality in the modifying influ-
ence of dikes — Trap interposed between strata — Columnar and globular
structure (p. 110.) — Relation of trappean rocks to the products of active
volcanos (p. 114.) — Submarine lava and ejected matter corresponds gene-
rally to ancient trap.

CHAPTER IX.

PLUTONIC ROCKS — GRANITE.

General aspect of granite — Decomposing into spherical masses — Rude colum-
nar structure — Analogy and difference of volcanic and plutonic formations

— Minerals in granite, and their arrangement — Graphic and porphyritic
granite (p. 121.) — Occasional minerals — Syenite — Syenitic, talcose, and
schorly granites — Eurite — Passage of granite into trap — Examples near
Christiania and in Aberdeenshire — Analogy in composition of trachyte and
granite — Granite veins in Glen Tilt, Cornwall, the Valorsine, and other
countries (p. 123.) — Different composition of veins from main body of granite

— Metalliferous veins in strata near their junction with granite (p. 128.) —
Apparent isolation of nodules of granite — Quartz veins — Whether plutonic
rocks are ever overlying — Their exposure at the surface due to denudation
(p. 131.)

CHAPTER X.

METAMORPHIC ROCKS.

General character of metamorphic rocks — Gneiss — Hornblende-schist — Mica-
schist — Clay-slate (p. 133.) — Quartzite — Chlorite-schist — Metamorphic
limestone — Alphabetical list and explanation of other rocks of this family

— Origin of the metamorphic strata (p. 135.) — Their stratification is real
and distinct from cleavage — On joints and slaty cleavage (p. 138.) — Sup-
posed causes of these structures — how far connected with crystalline action.

CHAPTER XI.

METAMORPHIC ROCKS — continued.

Strata near some intrusive masses of granite converted into rocks identical
with different members of the metamorphic series — Arguments hence de-
rived as to the nature of plutonic action (p. 146.) — Time may enable this
action to pervade denser masses — From what kinds of sedimentary rock
each variety of the metamorphic class may be derived (p. 150.) — Certain
objections to the metamorphic theory considered.

viii CONTENTS.

PART II.

CHAPTER XII.

ON THE DIFFERENT AGES OF THE FOUR GREAT CLASSES OF ROCKS.

Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically
— Lehman's division into primitive and secondary — Werner's addition of
a transition class — Neptunian theory (p. 153.) — Hutton on igneous origin
of granite — How the name of primary was still retained for granite — The
term " transition," why faulty — The adherence to the old chronological
nomenclature retarded the progress of geology — New hypothesis invented
to reconcile the igneous origin of granite to the notion of its high antiquity
(p. 155.) — Explanation of the chronological nomenclature adopted in this
work, so far as regards primary, secondary, and tertiary periods.

CHAPTER XIII.

ON THE DIFFERENT AGES OF THE AQUEOUS ROCKS.

On the three principal tests of relative age — superposition, mineral character,
and fossils — Change of mineral character and fossils in the same continuous
formation — Proofs that distinct species of animals and plants have lived at
successive periods — Test of age by included fragments (p. 162.) — Frequent
absence of strata of intervening periods — Principal groups of strata in west-
ern Europe — Tertiary strata separable into four groups, the fossil shells of
which approach nearer to those now living in proportion as the formation
is more modern (p. 164.) — Terms Eocene, Miocene, and Pliocene — Identi-
fications of fossil and recent shells by M. Deshayes — Opinions of Dr. Beck
(p. 168.)

CHAPTER XIV.

RECENT AND TERTIARY FORMATIONS.

How to distinguish Recent from Tertiary strata — Recent and Newer Pliocene
strata near Naples — near Stockholm and Christiania — in South America, on
coasts of Chili and Peru — Rocks of Recent period, with human skeleton, in
Guadaloupe (p. 171.) — Shells of living species, with extinct mammalia, in
loess of the Rhine — Recent and newer Pliocene deposits in England —
Older Pliocene strata in England — Crag — Red and Coralline crag — their
fossils in part distinct (p. 175.) — their strata unconformable — belong to the
same period — London clay (p. 178.) — Its shells and fish imply a tropical
climate — Tertiary mammalia — Fossil quadrumana.

CONTENTS. ix

CHAPTER XV.

CRETACEOUS GROUP.

White chalk — Its marine origin shown by fossil shells — Extinct genera of
cephalopoda — Sponges and corals in the chalk — No terrestrial or fluviatile
shells, no land plants — Supposed origin of white chalk from decomposed
corals (p. 186.) — Single pebbles, whence derived — Cretaceous coral-reef in
Denmark (p. 188.) — Maestricht beds and fossils — Origin of flint in chalk —
Wide area covered by chalk (p. 191.) — Green-sand formation and fossils —
Origin of — External configuration of chalk (p. 193.) — Outstanding columns
or needles — Period of emergence from the sea — Difference of the chalk of
the north and south of Europe — Hippurites — Nummulites (p. 198.) —
Altered lithological character of cretaceous formation in Spain and Greece —
Terminology.

CHAPTER XVI.

WEALDEN GROUP.

The Wealden, including the Weald clay, Hastings sand, and Purbeck beds —
Intercalated between two marine formations — Fossil shells freshwater, with
a few marine — Cypris — Fish — Reptiles (p. 202.) — Birds — Plants — Section
showing passage of Wealden beneath chalk — Junction of Wealden and
Oolite — Dirt-bed (p. 205.) — Theory of gradual subsidence — Proofs that the
Wealden strata, notwithstanding their thickness, may have been formed in
shallow water (p. 208.) — Geographical extent of Wealden — Bray near
Beauvais — Relation of the Wealden to the Lower Green-sand and Oolite
(p. 211.)

CHAPTER XVII.

OOLITE AND LIAS.

Subdivisions of the Oolitic group — Fossil shells — Corals in the calcareous

divisions only — Buried forest of Encrinites in Bradford clay (p. 216.)

Changes in organic life during accumulation of Oolites — Characteristic
fossils — Signs of neighbouring land and shoals (p. 221.) — Supposed cetacea
in Oolite •— Oolite of Yorkshire and Scotland (p. 223.)

CHAPTER XVIII.

OOLITE AND LIAS — continued.

Mineral character of Lias — Name of Gryphite limestone — Fossil fish

Ichthyodorulites — Reptiles of the Lias (p. 227.) — Ichthyosaur and Plesiosaur

— Newly-discovered marine Reptile of the Galapagos Islands (p. 229.)

Sudden death and burial of fossil animals in Lias — Origin of the Oolite and
Lias, and of altejnating calcareous and argillaceous formations (p. 232.) —
Physical geography (p. 234.) — Vales of clay — Hills and escarpments of
limestone.

x CONTENTS.

CHAPTER XIX.

NEW RED SANDSTONE GROUP.

Distinction between New and Old Red sandstone — Between Upper and Lower
New Red — Muschelkalk in Germany (p. 237.) — Fossil plants and shells of
New Red Group, entirely different from Lias and Magnesian limestone —
Lower New Red and Magnesian limestone (p. 239.) — Zechstein in Germany
of the same age — General resemblance between the organic remains of the
Magnesian limestone and Carboniferous strata — Origin of red sand-stone and
red marl (p. 242.)

CHAPTER XX.

THE COAL, OR CARBONIFEROUS GROUP.

Carboniferous strata in the south-west of England — Superposition of Coal-
measures to Mountain limestone — Departure from this type in north of
England and Scotland — Freshwater strata (p. 244.) — Intermixture of fresh-
water and marine beds — Sauroidal fish — Fossil plants (p. 247.) — Ferns and
Sigillariae — Lepidodendra — Calamites (p. 250.) — Coniferae — Stigmarise.

CHAPTER XXI.

CARBONIFEROUS GROUP Continued, AND OLD RED SANDSTONE.

Corals and shells of the Mountain limestone — Hot climate of the Carboniferous
period inferred from the marine fossils of the Mountain limestone and the
plants of the Coal (p. 254.) — Origin of the Coal-strata — Contemporaneous
freshwater and marine deposits — Modern analogy of strata now in progress in
and around New Zealand — Vertical and oblique position of fossil trees in the
Coal (p. 257.) — How enveloped — How far they prove a rapid rate of depo-
sition—Old Red sandstone (p. 262.) — its subdivisions — its fossil shells and
fish.

CHAPTER XXII.

PRIMARY FOSSILIFEROUS STRATA.

Primary Fossiliferous or Transition Strata — Term "Grauwacke" — Silurian
Group — Upper Silurian and Fossils (p. 265.) — Lower Silurian and Fossils —
Trilobites (p. 267.) — Graptolites — Orthocerata — Occasional horizontal! ty of
Silurian Strata — Cambrian Group (p. 270.) — Endosiphonite.

CHAPTER XXIII.

ON THE DIFFERENT AGES OF THE^VOLCANIC ROCKS.

Tests of relative age of volcanic rocks — Test by superposition and intrusion —
By alteration of rocks in contact — Test by organic remains (p. 273.) — Test
of age by mineral character — Test by included fragments — Volcanic rocks
of the Recent and Newer Pliocene periods (p. 275.) — Miocene — Eocene —
Cretaceous — Oolitic (p. 278.) — New Red sandstone period — Carboniferous
— Old Red sandstone period — Silurian — Upper and Lower Cambrian peri-
ods (p. 280.) — Relative ages of intrusive traps.

CONTENTS.

CHAPTER XXIV.

ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.

Difficulty in ascertaining the precise age of a plutonic rock — Test of age by
relative position — Test by intrusion and alteration — Test by mineral compo-
sition— Test by included fragments — Recent and Pliocene plutonic rocks,
why invisible (p. 283.) — Tertiary plutonic rocks in the Andes — Granite
altering cretaceous rocks (p. 286.) — Granite altering Lias in the Alps and in
Sky — Granite of Dartmoor altering Carboniferous strata — Granite of the Old
Red sandstone period — Syenite altering Silurian strata in Norway (p. 289.) —
Blending of the same with gneiss — Most ancient plutonic rocks — Granite
protruded in a solid form (p. 291.) — On the probable age of the granite of
Arran in Scotland.

CHAPTER XXV.

ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.

Age of each set of metamorphic strata twofold (p. 294.) — Test of age by fossils
and mineral character not available — Test by superposition ambiguous —
Conversion of dense masses of fossiliferous strata into metamorphic rocks —
Limestone and shale of Carrara — Metamorphic strata of modern periods in
the Alps of Switzerland and Savoy (p. 296.) — Why the visible crystalline
strata are none of them very modern — Order of succession in metamorphic
rocks (p. 299.) — Uniformity of mineral character — Why the metamorphic
strata are less calcareous than the fossiliferous (p. 301.)

Lately Issued,

BY THE SAME PUBLISHERS,

First American, from the Fifth London Edition, Illustrated by 226 Wood Engravings,
and 16 Plates and Maps — 2 vols. royal 8vo,

PRINCIPLES OF GEOLOGY,

BY CHARLES LYELL, F.R.S.,

BEING

AN INQUIRY HOW FAR THE FORMER CHANGES OF THE EARTH'S SUR-
FACE ARE REFERABLE TO CAUSES NOW IN OPERATION, ETC.

The following Notices may serve to show the high character which the above
work enjoys, as the Standard Text Book for advanced Students of Geology, both
in Great Britain and the United States : —

From the London Quarterly Review.— ' We hail with the greatest satisfaction the appear-
ance of Mr. Lyell's work, which henceforward, we can hardly doubt, will mark the begin-
ning of a new era in Geology. The title of the book shows that it is an attempt to place
the study of the science on its true basis— to explain the former changes of the earth's sur-
face by reference to causes now in operation. The mode in which this undertaking has been
executed, is most satisfactory, and confirms the high reputation Mr. Lyell enjoys for zeal and
accuracy in observation, and an intimacy with many of the branches of science and natural
history which bear upon Geology. It exhibits, also, together with much literary research and
elegance of language, a luminous arrangement and powers of analytical reasoning, which
we should be glad to meet with more frequently in the contributions to our scientific know-
ledge. Incorporated with his arguments, and the details extracted from other sources, Mr.
Lyell has, moreover, communicated a great body of original observations of much interest,
collected during the tours he has recently made for scientific purposes on the continent.- We
cannot but express our obligations to him for the great addition he has made to our knowl-
edge of nature, and the beneficial influence it is likely to have in communicating a right
direction and a philosophical spirit of induction to geological inquiry."

From Silliman's American Journal of Science and Arts.—" The rapidity with which new
editions of this excellent Work have appeared, sufficiently evinces the estimation in which
it is held. We are indebted to the industry, good judgment, and great science of Mr. Lyell,
for a lucid and highly interesting exhibition of facts, and for a logical and candid discussion
of principles. He has done much to recal Geologists from extravagant speculations, and to
allure them back to a course of strict induction ; thus placing Geology side by side with the
other sciences of observation. The publication of Mr. Lyell's Work forms a new era in
Geology ; it must be studied by every person who would be acquainted with the present im-
proved state of the science ; and happily the study will prove no task ; for the lucid and
beautiful style of the author, embellished by occasional classical flowers, gives this Work
almost as peculiar a character, as its novel philosophy."

From the Transactions of the Geological Society of Pennsylvania.— "It can scarcely be
necessary to say any thing in praise of this Work. Its appearance will always form an
epoch in the history of Geology. Up to that time the doctrine which assumed the causes of
changes, whether of a destroying or productive character, actually in progress on the surface
of the globe, to be utterly inadequate to explain, scarcely even to illustrate, the earlier changes
of which that surface exhibits such striking traces, held almost undisputed sway in the geologi-
cal circles. Mr. Lyell. applying himself to the elucidation of the existing causes of change,
and their probable influence on the older geological formations, with an industry and research
which are joined to the happiest powers of description and command of language, has pro-
duced a Work not only of the highest interest to the scientific world, but of the most popular
and fascinating nature to the general reader."

(12)

ELEMENTS OF GEOLOGY.

PART I.

CHAPTER I.

ON THE FOUR GREAT CLASSES OF ROCKS — THE AQUEOUS, VOLCANIC,
PLUTONIC, AND METAMORPHIC.

Geology defined — Successive formation of the earth's crust — Classification
of rocks according to their origin and age — Aqueous rocks — Their stratification
and imbedded fossils — Volcanic rocks, with and without cones and craters —
Plutonic rocks, and their relation to the volcanic — Metamorphic rocks, and
their probable origin — The term primitive, why erroneously applied to the
crystalline formations — Division of the work into two parts ; the first descrip-
tive of rocks without reference to their age, the second treating of their chro-
nology.

OF what materials is the earth composed, and in what man-
ner are these materials arranged 1 These are the inquiries with
which Geology is occupied, a science which derives its name
from the Greek, y»?, ge^ the earth, and toyoj, logos, a discourse.
Such investigations appear, at first sight, to relate exclusively to
the mineral kingdom, and to the various rocks, soils, and metals,
which occur upon the surface of the earth, or at various depths
beneath it. But, in pursuing these researches, we soon find our-
selves led on to consider the successive changes which have
taken place in the former state of the earth's surface and inte-
rior, and the causes which have given rise to these changes; and,
what is still more singular and unexpected, we soon become
engaged in researches into the history of the animate creation,
or of the various tribes of animals and plants which have, at dif-
ferent periods of the past, inhabited the globe.

All are aware that the solid parts of the earth consist of dis-
tinct substances, such as clay, chalk, sand, limestone, coal, slate,
granite, and the like ; but previously to observation it is com-
monly imagined that all these had remained from the first in the
state in which we now see them, — that they were created in their
present form, and in their present position. Geologists have
B (13)

14 LYELL'S ELEMENTS OF GEOLOGY.

Successive Formation of the Earth's Crust.

come to a different conclusion. They have discovered proofs
that the external parts of the earth were not all produced in the
beginning of things, in the state in which we now behold them,
nor in an instant of time. On the contrary, they have acquired
their actual configuration and condition gradually, under a great
variety of circumstances, and at successive periods, during each ,
of which distinct races of living beings have flourished on the
land and in the waters, the remains of these creatures still lying
buried in the crust of the earth.

By the " earth's crust," is meant that small portion of the
exterior of our planet which is accessible to human observation.
It comprises not merely all of which the structure is laid open
in mountain precipices, or in cliffs overhanging the river or the
sea, or whatever the miner may reveal in artificial excavations ;
but the whole of that outer covering of the planet on wliich we
are enabled to reason by observations made at or near the sur-
face. These reasonings may extend to a depth of several miles,
perhaps ten miles ; but even then it may be said, that such a
thickness is no more than ^^th Part °f tne distance from the
surface to the centre. The remark is just ; but although the
dimensions of such a crust are, in truth, insignificant when com-
pared to the entire globe, yet they are vast and of magnificent
extent in relation to man, and to the organic beings which peo-
ple our globe. Referring to this standard of magnitude, the
geologist may admire the ample limits of his domain, and admit,
at the same time, that not only the exterior of the planet, but the
entire earth, is but an atom in the midst of the countless worlds
surveyed by the astronomer.

Now the materials of this crust are not thrown together con-
fusedly, but distinct mineral masses, called rocks, are found to
occupy definite spaces, and to exhibit a certain order of arrange-
ment. The term rock is applied indifferently by geologists to
all these substances, whether they be soft or stony, for clay and
sand are included in the term, and some have even brought peat
under this denomination. Our older writers endeavoured to
avoid offering such violence to our language, by speaking of the
component materials of the earth as consisting of rocks and soils.
But there is often so insensible a passage from a soft and inco-
herent state to that of stone, that geologists of all countries have
found it indispensable to have one technical term to include both,
and in this sense we find roche applied in French, rocca in Ita-
lian, and felsart in German.

The beginner, however, must constantly bear in mind, that the
term rock by no means implies that a mineral mass is in an
indurated or stony condition.

PART I. CHAPTER I. 15

Classification of Rocks Aqueous Rocks.

In order to classify the various rocks which compose the
earth's crust, it is found most convenient to refer, in the first
place, to their origin, and in the second to their age. I shall
therefore begin by endeavouring briefly to explain to the student
how all rocks may be divided into four great classes by refer-
ence to their different origin, or, in other words, by reference to
the different circumstances and causes by which they have been
produced.

The first two divisions, which will at once be understood as
natural, are the aqueous and volcanic, or the products of watery
and those of igneous action.

Aqueous rocks. — The aqueous rocks, sometimes called the
sedimentary, or fossiliferous, cover a larger part of the earth's
surface than any others. These rocks are stratified, or divided
into distinct layers, or strata. The term stratum means simply
a bed, or any thing spread out or strewed over a given surface ;
and we infer that these strata have been generally spread out by
the action of water, from what we daily see taking place near
the mouths of rivers, or on the land during temporary inunda-
tions. For, whenever a running stream, charged with mud or
sand, has its velocity checked, as when it enters a lake or sea,
or overflows a plain, the sediment, previously held in suspension
by the motion of the water, sinks, by its own gravity, to the
bottom. In this manner layers of mud and sand are thrown
down one upon another.

If we drain a lake which has been fed by a small stream, we
frequently find at the bottom a series of deposits, disposed with
considerable regularity, one above the other; the uppermost,
perhaps, may be a stratum of peat, next below a more dense and
solid variety of the same material ; still lower a bed of laminated
shell-marl, alternating with peat or sand, and. then other beds of
marl, divided by layers of clay. Now if a second pit be sunk
through the same continuous lacustrine formation, at some dis-
tance from the first, we commonly meet with nearly th^. same
series of beds, yet with slight variations ; some, for example, of
the layers of sand, clay, or marl, may be wanting, one or more
of them having thinned out and given place to others, or some-
times one of the masses first examined is observed to increase in
thickness to the exclusion of other beds.

The term "formation" which I have used in the above
explanation, expresses in geology any assemblage of rocks which
have some character in common, whether of origin, age, or
/ composition. Thus we speak of stratified and unstratified, fresh-
water and marine, aqueous and volcanic, ancient and modern,
metalliferous and non-metalliferous formations.

16 LYELL'S ELEMENTS OF GEOLOGY.

Aqueous Origin of certain Rocks.

In the estuaries of large rivers, such as the Ganges and the
Mississippi, we may observe, at low water, phenomena analogous
to those of the drained lakes above mentioned, but on a grander
scale, and extending over areas several hundred miles in length
and breadth. When the periodical inundations subside, the
river hollows out a channel to the depth of many yards through
horizontal beds of clay and sand, the ends of which are seen
exposed in perpendicular cliffs. These beds vary in colour, and
are occasionally characterized by containing drift-wood or shells.
The shells may belong to species peculiar to the river, but are
sometimes those of marine testacea, washed into the mouth of
the estuary during storms.

The annual floods of the Nile in Egypt are well known, and
the fertile deposit of mud which they leave on the plains. This
mud is stratified, the thin layer thrown down in one season
differing slightly in colour from that of a previous year, and
being separable from it, as has been observed in excavations at
Cairo, and other places.*

When beds of sand, clay, and marl, containing shells and
vegetable matter, are found arranged in the same manner in the
interior of the earth, we ascribe to them a similar origin ; and
the more we examine their characters in minute detail, the more
exact do we find the resemblance. Thus, for example, at vari-
ous heights and depths in the earth, and often far from seas,
lakes, and rivers, we meet with layers of rounded pebbles com-
posed of different rocks mingled together. They are like the
pebbles formed in the beds of torrents and rivers, which are car-
ried down into the sea wherever these descend from high grounds
bordering a coast. There the gravel is spread out by the waves
and currents of the ocean over a considerable space ; but during
seasons of drought the torrents and rivers are nearly dry, and
have only power to convey fine sand or mud into the sea. Hence,
alternate layers of gravel and fine sediment accumulate under
water, and such alternations are found by geologists in the inte-
rior of every continent.f

If a stratified arrangement, and the rounded forms of pebbles,
are alone sufficient to lead us to the conclusion that certain rocks
originated under water, this opinion is farther confirmed by the
distinct and independent evidence of ^fossils, so abundantly
included in the earth's crust. By a fossil is meant any body, or
the traces of the existence of any body, whether animal or vege-

* See Silliman's Amer. Journ. of Sci. vol. xxviii. 1835 ; also Principles of
Geology, Index, " Nile, " Rivers," &c.

t See Principles of Geology, by the author ; refer to « Magnan,' and ' Conglo-
merates, in the Index.

PART I. CHAPTER I. 17

Aqueous Origin of certain Rocks.

table, which has been buried in the earth by natural causes.
Now the remains of animals, especially of aquatic species, are
found almost everywhere imbedded in stratified rocks. Shells
and corals are the most frequent, and with them are often asso-
ciated the bones and teeth of fish, fragments of wood, impres-
sions of leaves, and other organic substances. Fossil shells of
forms such as now abound in the sea, are met with far inland,
both near the surface and at all depths below it, as far as the
miner can penetrate. They occur at all heights above the level
of the ocean, having been observed at an elevation of from 8000
to 9000 feet in the Alps and Pyrenees, more than 13,000 feet
high in«the Andes, and above 15,000 feet in the Himalayas.

These shells belong mostly to marine testacea, but in some
places exclusively to forms characteristic of lakes and rivers.
Hence, we conclude that some ancient strata were deposited at
the bottom of the sea, while others were formed in lakes and
estuaries.

When geology was first cultivated, it was a general belief that
these marine shells and other fossils were the effects and proofs of
the general deluge. But all who have carefully investigated the
phenomena have long rejected this doctrine. A transient flood
might be supposed to leave behind it, here and there upon the
surface, scattered heaps of mud, sand, and shingle, with shells
confusedly intermixed ; but the strata containing fossils are not
superficial deposits, and do not cover the earth, but constitute the
entire mass of mountains. It has been also the favourite notion
of some modern writers, who are aware that fossil bodies cannot
all be referred to the deluge, that they, and the strata in which
they are entombed, may have been deposited in the bed of the
ocean during a period of several thousand years which inter-
vened between the creation of man and the deluge. They ima-
gine that the antediluvian bed of the ocean, after having been
the receptacle of many stratified deposits, became converted, at
the time of the flood, into the lands which we inhabit, and that
the ancient continents were at the same time submerged, and be-
came the bed of the present sea. This hypothesis, however
preferable to the diluvial theory, as admitting that all fossiliferous
strata were slowly and successively thrown down from water, is
yet wholly inadequate to explain the repeated revolutions which
the earth has undergone, and the signs which the existing con-
tinents exhibit, in most regions, of having emerged from the
ocean at an era far more remote than four thousand years from
the present time. It will also be seen in the sequel, that many
distinct sets of sedimentary strata, each several hundreds or
thousands of feet thick, are piled one upon the other in the earth's

B *

18 LYELL'S ELEMENTS OF GEOLOGY.

Volcanic Rocks Volcanic Origin of certain Rocks.

crust, each containing their peculiar fossil animals and plants,
which are distinguishable, with few exceptions, from species now
living. The mass of some of these strata consists almost entirely
of corals, others are made up of shells, others of plants turned
into coal, while some are without fossils. In one set of strata
the species of fossils are marine ; in another, placed immediately
above or below, they as clearly prove that the deposit was form-
ed in an estuary or lake. When the student has more fully ex-
amined into these appearances, he will become convinced that
the time required for the origin of the actual continents must have
been far greater than that which is conceded by the theory above
alluded to, and that no one universal and sudden conversion of
sea into land will account for geological appearances.

We have now pointed out one great class of rocks, which,
however they may vary in mineral composition, colour, grain,
or other characters, external and internal, may nevertheless be
grouped together as having a common origin. They have all
been formed under water, in the same manner as sand, mud,
shingle, banks of shells, coral, and the like, and are characterized
by stratification or fossils, or by both.

Volcanic rocks. — The division of rocks which we may next
consider, are the volcanic, or those which have been produced,
whether in ancient or modern times, not by water, but by the
action of fire, or subterranean heat. These rocks are for the
most part unstratified, and are devoid of fossils. They are more
partially distributed than aqueous formations, at least in respect
to horizontal extension. ; Among those parts of Europe where
they exhibit characters not to be mistaken, I may mention not
only Sicily and the country round Naples, but Auvergne, Velay,
and Vivarais, now the departments of Puy de Dome, Haute
Loire, and Ardeche, towards the centre and south of France, in
which we find several hundred conical hills, having the forms of
modern volcanos, with craters more or less perfect on many of
their summits. These cones are composed, moreover, of lava,
sand, and ashes, similar to those of active volcanos. Streams of
lava may sometimes be traced proceeding from the cones into
the adjoining valleys, where they choke up the ancient channels
of rivers with solid rock, in the same manner as some modern
flows of lava in Iceland have been known to do, the rivers either
flowing beneath, or cutting out a narrow passage on one side of
the lava. Although none of these French volcanos have been
in activity within the period of history or tradition, their forms
are often very perfect. Some, however, have been compared to
the mere skeletons of volcanos, the rains and torrents having
washed their sides, and removed all the loose sand and scoriae,

PART I. CHAPTER I. 10

Volcanic Origin of certain Rocks.

leaving only the harder and more solid materials. By this ero-
" sion, and by earthquakes, their internal structure has occasion-
ally been laid open to view, in fissures and ravines ,• and we then
behold not only many successive beds and masses of porous lava,
sand, and scoriae, but also perpendicular walls, or dikes, as they
are called, of volcanic rock, cutting through the other materials.
Such dikes are also observed in the structure of Vesuvius, Etna,
and other active volcanos. They have been formed by the
pouring of melted matter, whether from above or below, into
open fissures, and they commonly traverse deposits of volcanic
tuff.) a substance produced by the showering down from the air,
or incumbent waters, of sand and cinders, first shot up from the
interior of the earth by explosions of volcanic gases.

Besides the parts of France above alluded to, there are other
countries, as the north of Spain, the south of Sicily, the Tuscan
territory of Italy, the lower Rhenish provinces, and Hungary,
where spent volcanos may be seen with cones, craters, and often
accompanying lava-streams.

There are also other rocks in England, Scotland, Ireland, and
almost every country in Europe, which we infer to be of igneous
origin, although they do not form hills with cones and craters.
Thus, for example, we feel assured" that the rock of Staffa, and
that of the Giant's Causeway, called basalt, is volcanic, because
it agrees in its columnar structure and mineral composition with
streams of lava which we know to have flowed from the craters
of volcanos. We find also similar basaltic rocks associated with
beds of tuff in various parts of the British Isles, and forming dikes,
such as have been spoken of; and some of the strata through
which these dikes cut are occasionally altered at the point of
contact, as if they had been exposed to the intense heat of melted
matter.

The absence of cones and craters, and long narrow streams
of lava, in England and elsewhere, is principally attributed by
geologists to the eruptions having been formerly submarine, just
as a considerable proportion of volcanos in our own times burst
out beneath the sea. But this question must be enlarged upon
more fully in the chapters on Igneous Rocks, in which it will
also be shown, that as different sedimentary formations, contain-
ing each their characteristic fossils, have been deposited at suc-
cessive periods, so also volcanic sand and scoriae have been
thrown out, and lavas have flowed over the land or bed of the
sea, at many different epochs, or have been injected into fissures ;
so that the igneous as well as the aqueous rocks may be classed
as a chronological series of monuments, throwing light on a
succession of events in the history of the earth.

20 LYELL'S ELEMENTS OF GEOLOGY.

Plutonic Rocks and their Origin.

Plutonic rocks. — We have now therefore pointed out the ex-
istence of two distinct orders of mineral masses, the aqueous and
the volcanic : but if we examine a large portion of a continent,
especially if it contain within it a lofty mountain range, we rarely
fail to discover two other classes of rocks, very distinct from
either of those above alluded to, and which we can neither assimi-
late to deposits sucli as are now accumulated in lakes or seas,
nor to those generated by ordinary volcanic action. •- The mem-
bers of both these divisions of rocks agree in being highly crys-
talline and destitute of organic remains/; The rocks of one divi-
sion have been called plutonic, comprehending alt the granites
and certain porphyries, which are nearly allied in some of their
characters to volcanic formations. < The members of the other
class are stratified and often slaty, and have been called by some
the crystalline-schists. In these are included gneiss, micaceous-
schist (or mica-slate,) hornblende-schist, statuary marble, the
finer kinds of roofing slate, and other rocks afterwards to be
described.

As it is admitted that nothing strictly analogous to these crys-
talline productions can now be seen in the progress of formation
on the earth's surface, it will naturally be asked, on what data
we can find a place for them in a system of classification founded
on the origin of rocks. First then, in regard to the plutonic
class, a passage has been traced from various kinds of granite
into different varieties of rocks decidedly volcanic; so that if the
latter are of igneous origin, it is scarcely possible to refuse to
admit that the granites are so likewise. Secondly, large masses
of granite are found to send forth dikes and veins into the con-
tiguous strata, very much in the same way as lava and volcanic
matter penetrate aqueous deposits, both the massive granite and
the veins causing changes analogous to those which lava and
volcanic gases are known to produce. But the plutonic rocks
differ from the volcanic, not only by their more crystalline tex-
ture, but also by the absence of tuffs and breccias, which are the
products of eruptions at the earth's surface/1 They differ also by
the absence of pores or cellular cavities, which the entangled gases
give rise to in ordinary lava. From these and other peculiari-
ties it has been inferred, that the granites have been formed at
great depths in the earth, and have cooled and crystallized slowly
under enormous pressure where the contained gases could not
expand. The volcanic rocks, on the contrary, although they
also have risen up from below, have cooled from a melted state
more rapidly upon or near the surface. From this hypothesis
of the great depth at which the granites originated, has been de-
rived the name of " Plutonic rocks," which they have received

PART I. CHAPTER I. 21

Metamorphic Rocks

to distinguish them from the volcanic. The beginner will easily
conceive that the influence of subterranean heat may extend
downwards from the crater of every active volcano to a great
depth below, perhaps several miles or leagues (see Frontispiece,)
and the effects which are produced deep in the bowels of the
earth may, or rather must be distinct ; so that volcanic and plu-
tonic rocks, each different in texture, and sometimes even in
composition, may originate simultaneously, the one at the sur-
face, the other far beneath it.

Although granite has often pierced through other strata, it
has rarely, if ever, been observed, to rest upon them as if it had
overflowed. But as this is continually the case with the volca-
nic rocks, they have been styled from this peculiarity, " overly-
ing" by Dr. MacCulloch ; and Mr. Necker has proposed the
term " underlying " for the granites, to designate the opposite
mode in which they almost invariably present themselves.

Metamorphic rocks. — The fourth and last great division of
rocks are the crystalline strata or schists, called gneiss, mica-
schist, clay-slate, chlorite-schist, marble, and the like, the origin
of which is more doubtful than that of the other three classes.
They contain no pebbles or sand or scoriae, or angular pieces of
imbedded stone, and no traces of organic bodies, and they are
often as crystalline as granite, yet are divided into beds, corre-
sponding in form to those of sedimentary formations, and are
therefore said to be stratified. The beds sometimes consist of an
alternation of substances varying in colour, composition, and
thickness, precisely as we see in stratified fossil iferous deposits.
According to the theory which I adopt as most probable, and
which will be afterwards more fully explained, the materials of
these strata were originally deposited from water in the usual
form of sediment, but they were subsequently altered by subter-
ranean heat, so as* to assume a new texture. It is demonstrable,
in some cases at least, that such a complete conversion has
actually taken place. I have already remarked that alterations,
such as might be produced by intense heat, are observed in strata
near their contact with veins and dikes of volcanic rocks.
These, however, are on a small scale ; but a similar influence
has been exerted much more powerfully in the neighbourhood
of plutonic rocks under different circumstances, and perhaps in
combination with other causes. The effects thereby superin-
duced on fossiliferous strata have sometimes extended to a dis-
tance of a quarter of a mile from the point of contact. Through-
out the greater part of this space the fossiliferous beds have
exchanged an earthy for a highly crystalline texture, and have
lost all traces of organic remains. Thus, for example, dark

22 lAELL'S ELEMENTS OF GEOLOGY.

Metamorphic Rocks and their Origin.

limestones, replete with shells and corals, are turned into white
statuary marble, and hard clays into slates called mica-schist and
hornblende-schist, all signs of organic bodies having been oblite-
rated.

Although we are in a great degree ignorant of the precise
nature of the influence here exerted, yet it evidently bears some
analogy to that which volcanic heat and gases are capable of
producing ; and the action may be conveniently called plutonic,
because it appears to have been developed in those regions where
plutonic rocks are generated, and under similar circumstances
of pressure and depth in the earth. Whether electricity or any
other causes have co-operated with heat to produce this influ-
ence, may be matter of speculation, but the plutonic influence
has sometimes pervaded entire mountain masses of strata. The
phenomena, therefore, being sometimes on so grand a scale, we
must not consider that the strata have always assumed their
crystalline or altered texture in consequence of the proximity of
granite, but rather that granite itself, as well as the altered strata,
have derived their crystalline texture from plutonic agency.

In accordance with this hypothesis I have proposed (see
Principles of Geology,) the term " Metamorphic" for the altered
strata, a term derived from j«fa, meta, trans, and ftop^, morphe,
forma.

Hence there are four great classes of rocks considered in
reference to their origin, — the aqueous, volcanic, plutonic, and
metamorphic, all of which may be conceived to have been
formed contemporaneously at every geological period, and to be
now in the progress of formation. By referring to the Frontis-
piece, the reader will perceive what relative positions the mem-
bers of these four great classes A, B, C, D, may occupy in the
earth's crust, while in the course of simultaneous production.
Thus, while the aqueous deposits A, which are expressed by the
yellow colour, have been accumulating in successive strata at the
bottom of the sea, the volcanic cone B, has been piled up during
a long series of eruptions, and the other igneous rocks coloured
purple have also ascended from below in a fluid state. Some of
these last have been poured forth into the sea, and there mingled
with aqueous sediment. On pursuing downwards either the
small dikes or large masses of volcanic rock, we find them pass
gradually into plutonic formations, D, which are coloured red,
and which underlie all the rest. These last again are seen to
be in contact with a zone of contemporaneous metamorphic
strata, C, coloured blue, which they penetrate in numerous veins.

In that part of the section which is uncoloured, a more ancient
series of mineral masses are seen, belonging also to the four

PART I. CHAPTER I. 23

Contemporaneous origin of Four Classes of Rocks.

great divisions of rocks. The strata from a to i represent as
many distinct aqueous formations, which have originated at dif-
ferent periods, and are each distinguished by their peculiar fos-
sils. The mass v v is of volcanic origin, and was formed at one
of those periods, namely, when the strata g were deposited. The
strata m m are ancient metamorphic formations, and the rocks
1, 2, are plutonic, also ancient, but of different dates.

Now it will be shown in the course of this volume, that por-
tions of each of these four distinct classes of rocks have origin-
ated at many successive periods. It is not true, as was formerly
supposed, that all granite, together with the crystalline or meta-
morphic strata, were first formed, and therefore entitled to be
called " primitive," and that the aqueous and volcanic rocks were
afterwards superimposed, and should, therefore, rank as second-
ary in the order of time. This idea was adopted in the infancy
of the science, when all formations, whether stratified or unstra-
tified, earthy or crystalline, with or without fossils, were alike
regarded as of aqueous origin. At that period it was naturally
argued, that the foundation must be older than the superstructure.
Granite, as being the lowest rock, must have been first " precipi-
tated from the waters of the primeval ocean which originally
invested the globe," then the crystalline, and finally the fossilife-
rous strata, together with other associated rocks, were deposited.
But when the doctrine of the igneous origin of granite was
generally adopted, the terms primitive and primary, as embra-
cing the plutonic and metamorphic rocks, should at once have
been banished from the nomenclature of geology. For after it
had been first proved that granite had originated at many differ-
ent epochs, some antecedent, others subsequent to the origin of
many fossiliferous strata, it was also demonstrated that strata
which had once contained fossils, had become metamorphic at
different periods ; in other words, some of the rocks termed pri-
mary were newer than others which were called secondary. A
question, therefore, has arisen, whether the lower crystalline por-
tions of the earth's crust, partially modified as they have been,
and renewed from time to time, are newer or older, regarded as
a whole, than the sedimentary and volcanic formations. Have
the operations of decay and repair been most active above or
below 1 The same question might be asked1 with respect to the
relative antiquity of the foundations and the buildings in certain
ancient cities, such as Venice or Amsterdam, which are supported
on wooden piles — whether in the course of ages have the wooden
props, or the buildings of brick, stone, and marble which they
support, proved the most durable ? Which have been renewed
most frequently ? for the piles, when rotten, can be removed one

24 LYELL'S ELEMENTS OF GEOLOGY.

Contemporaneous origin of Four Classes of Rocks.

after the other without injury to the buildings above. In like
manner the materials of the lower part of the earth's crust may
pass from a solid to a fluid state, and may then again become
consolidated ; or sedimentary strata may assume a new and meta-
morphic texture, while the strata above continue unchanged, or
retain characters by which their claim to high antiquity may be
recognized. During such subterranean mutations, the earth-
quake may shatter and dislocate the incumbent crust, or the
ground may rise or sink slowly and insensibly throughout wide
areas ;* or there may be volcanic eruptions here and there ; but
the great mass may not undergo such an alteration as to be re-
generated and composed of new rocks.

As all the crystalline rocks may, in some respects, be viewed
as belonging to one great family, whether they be stratified or
un stratified, it will often be convenient to speak of them by one
common name. But the use of the term primary would imply
a manifest contradiction, for reasons which the student will now
comprehend. Ic is indispensable, therefore, to find a new name,
one which must not be of chronological import, and must express,
on the one hand, some peculiarity equally attributable to granite
and gneiss (to the plutonic as well as the altered rocks,) and,
on the other, must have reference to characters in which those
rocks differ, both from the volcanic and from the unaltered sedi-
mentary strata. I have proposed in the Principles of Geology
the term " hypogene" for this purpose, derived from vrto, under,
and ywoptu,, to be born ; a word implying the theory that granite,
gneiss, and the other crystalline formations are alike nether-
formed rocks, or rocks which have not assumed their present
form and structure at the surface. It is true that all metamorphic
strata must have been deposited originally at the surface, or on
that part of the exterior of the globe which is covered by water ;
but, according to the views above set forth, they could never have
acquired their crystalline texture, unless they had been modified
by plutonic agency under pressure in the depths of the earth.

From what has now been said, the reader will understand that
the four great classes of rocks may each be studied under two
distinct points of view : first, they may be studied simply as
mineral masses deriving their origin from particular causes, and
having a certain composition, form, and position in the earth's
crust, or other characters both positive and negative, such as the
presence or absence of organic remains. In the second place,
the rocks of each class may be viewed as a grand chronological
series of monuments, attesting a succession of events in the for-
mer history of the globe and its living inhabitants.

* See chap. 5.

PART I. CHAPTER II. 25

Division of the Work Mineral Composition of Stratified Rocks.

I shall accordingly divide this work into two parts, in refer-
ence to these two modes of considering each family of rocks. In
the first part, the characters of the aqueous, volcanic, plutonic,
and metamorphic rocks will be described, without reference to
their ages, or the periods when they were formed. In the second,
their different ages will be considered, and I shall endeavour to
explain the rules according to which the chronology of rocks in
each of the four classes may be determined.

CHAPTER II.

AQUEOUS HOCKS — THEIR COMPOSITION AND FOEMS OP STRATIFICATION.

Mineral composition of strata — Arenaceous rocks — Argillaceous — Calcareous —
Gypsum — Forms of stratification — Original horizontality — thinning out — Diagonal
arrangement — Ripple mark.

FIRST, then, in pursuance of the arrangement explained in
the last chapter, we have to examine the aqueous or sedimentary
rocks, which are, for the most part, distinctly stratified, and con-
tain fossils. We are to consider them with reference to their
mineral composition, external appearance, position, mode of ori-
gin, and other characters which belong to them as aqueous
formations, without reference to their age, or the various geolo-
gical periods when they may have originated.

I have already given an outline of the data which lead to the
belief that the stratified and fossiliferous rocks were originally
deposited under water ; but, before entering into a more detailed
investigation, it will be desirable to say something of the ordinary
materials of which such strata are composed. These may be
said to belong principally to three divisions, the arenaceous, the
argillaceous, and the calcareous, which are formed respectively
of sand, clay, and carbonate of lime. Of these, the arenaceous,
or sandy masses, are chiefly made up of siliceous or flinty grains;
the argillaceous, or clayey, of a mixture of siliceous matter, with
a certain proportion, about a fourth in weight, of aluminous
earth;5 and, lastly, the calcareous rocks or limestones consist of
carbonic acid and lime.

Arenaceous or siliceous rocks. — To speak first of the sandy
division : beds of loose sand are frequently met with, of which
the grains consist entirely of silex, which term comprehends all
purely siliceous minerals, as quartz and common flint. Quartz
is silex in its purest form ; flint usually contains some admixture
of alumine and oxide of iron. The siliceous grains in sand and
c

26 LYELL'S ELEMENTS OF GEOLOGY.

Mineral Composition of stratified Kocks.

sandstone are usually rounded, as if by the action of running
water ; but they sometimes, though .more rarely, consist of small
crystals, as if they had been chemically precipitated from a fluid
containing silex in solution.

Sandstone is an aggregate of such grains, which often cohere
together without, any visible cement, but more commonly are
bound together by a slight quantity of siliceous or calcareous
matter, or by iron or clay. In nature there is every intermediate
gradation, from perfectly loose sand, to the hardest sandstone.
In micaceous sandstones mica is abundant ; and the thin silvery
plates into which that mineral divides, are arranged into layers
parallel to the planes of stratification, giving a slaty or laminated
texture to the rock.

When sandstone is coarse-grained, it is usually called grit.
If the grains are rounded, and large enough to be called pebbles,
it becomes a conglomerate, or pudding-stone, which may consist
of pieces of one or of many different kinds of rock. A conglom-
erate, therefore, is simply gravel bound together by a cement.

Argillaceous rocfcs.-r— Clay, strictly speaking, is a mixture of
silex or flint, with a large proportion, usually about one-fourth,
of the substance called alumine, or argil ; but, in common lan-
guage, any earth which possesses sufficient ductility, when
kneaded up with water, to be fashioned like paste by the hand,
or by the potter's lathe, is called a clay ; and such clays vary
greatly in their composition, and are, in general, nothing more
than mud derived from the decomposition or wearing down of
various rocks. The purest clay found in nature is porcelain
clay, or kaolin, which results from the decomposition of a rock
composed of felspar and quartz, and is almost always mixed
with quartz.* Shale has also the property, like clay, of be-
coming plastic in water : it is a more solid form of clay, having
been probably condensed by pressure. It usually divides into
thin laminse.

One general character of all argillaceous rocks is to give out
a peculiar odour when breathed upon, which is a test of the pre-
sence of alumine, although it does not belong to pure alumine,
but, apparently, to the combination of that substance with oxide
of iron.f

Calcareous rocks. — This division comprehends those rocks
which, like chalk, are composed of lime and carbonic acid.

* The kaolin of China consists of 71.15 parts of silex, 15.86 of alumine, 1.92 of
lime, and 6.73 of water, (W. Phillips, Mineralogy, p. 33.) ; but other porcelain
clays differ materially, that of Cornwall being composed of CO parts of alumine
and 40 of silex.

t See W. Phillips's Mineralogy, " Alumine."

Shells and corals are also formed of the sanW5ii^eiits1.with ifie
addition of animal matter. To obtain pure lime, it is necessary
to calcine these calcareous substances, that is to say, to expose
them to heat of sufficient intensity to drive off the carbonic acid,
and other volatile matter, without vitrifying or melting the lime
itself. White chalk is often pure carbonate of lime ; and this
rock, although usually in a soft and earthy state, is sometimes
sufficiently solid to be used for building, and even passes into a
compact stone, or a stone of which the separate parts are so
minute as not to be distinguishable from each other by the naked
eye.

Many limestones are made up entirely of minute fragments of
shells and coral, or of calcareous sand cemented together. These
last might be called " calcareous sandstones ;" but that term is
more properly applied to a rock in which the grains are partly
calcareous and partly siliceous, or to quartzose sandstones, having
a cement of carbonate of lime.

The variety of limestone called " oolite" is composed of numer-
ous small egg-like grains, resembling the roe of a fish, each of
which has usually a sma*ll fragment of sand as nucleus, around
which concentric layers of calcareous matter have accumulated.

Any limestone which is sufficiently hard to take a fine polish
is called marble. Many of these are fossiliferous ; but statuary
marble, which is also called saccharine limestone, as having a
texture resembling that of loaf-sugar, is devoid of fossils, and a
member of the metamorphic series.

Siliceous limestone is an intimate mixture of carbonate of
lime and flint, and is harder in proportion as the flinty matter
predominates.

The presence of carbonate of lime in a rock may be ascer-
tained by applying to the surface a small drop of diluted sul-
phuric, nitric, or muriatic acids ; for the lime, having a stronger
chemical affinity for any one of these acids than for the carbonic,
unites itself immediately with them to form new .compounds,
thereby becoming a sulphate, nitrate, or muriate of lime. The
carbonic acid, when thus liberated from its union with the lime,
escapes in a gaseous form, and froths up or effervesces as it
makes its way in small bubbles through the drop of liquid. This
effervescence is brisk or feeble in proportion as the limestone is
pure or impure, or, in other words, according to the quantity of
foreign matter mixed with the carbonate of lime. Without the
aid of this test, the most experienced eye cannot always detect
the presence of lime in rocks.

The above-mentioned three classes of rocks, the arenaceous,
argillaceous, and calcareous, pass continually into each other,

28 LYELL'S ELEMENTS OF GEOLOGY.

- Forms of Stratification.

and rarely occur in a perfectly separate and pure form. Thus
it is an exception to the general rule to meet with a limestone as
pure as ordinary white chalk, or with clay as aluminous as that
used in Cornwall for porcelain, or with sand so entirely com-
posed of siliceous grains as the white sand of Alum Bay in the
Isle of Wight, or sandstone so pure as the grit of Fontainebleau,
used for pavement in France. More commonly \ve find sand
and clay, or clay and marl, intermixed in the same mass. When
the sand and clay are each in considerable quantity, the mixture
is called loam. If there is much calcareous matter in clay, it is
called marl; but this term has unfortunately been used so
vaguely, as often to be very ambiguous. It has been applied to
substances in which there is no lime ; as, to that red loarmisually
called red marl in certain parts of England. Agriculturists were
in the habit of calling any soil a marl, which, like true marl,
fell to pieces readily on exposure to the air. Hence arose the
confusion of using this name for soils which, consisting of loam,
were easily worked by the plough, though devoid of lime.

Marl slate bears the same relation to marl which shale bears
to clay, being a calcareous shale. It is very abundant in some
countries, as in the Swiss Alps. Argillaceous or marly lime-
stone is also of common occurrence.

There are few other kinds of rock which enter so largely into
the composition of sedimentary strata as to make it necessary to
dwell here on their characters. I may, however, mention two
others, — magnesian limestone or dolomite, and gypsum. Mag-
nesian limestone is composed of carbonate of lime and carbo-
nate of magnesia : the proportion of the latter amounting in
some cases to nearly one half. It effervesces much more slowly
and feebly with acids than common limestone. In England this
rock is generally of a yellowish colour ; but it varies greatly in
mineralogical character, passing from an earthy state to a white
compact stone of great hardness. Dolomite, so common in many
parts of Germany and France, is also a variety of magnesian
limestone, usually of a granular texture.

Gypsum. — Gypsum is a rock composed of sulphuric acid,
lime, and water. It is usually a soft whitish-yellow rock, with
a texture resembling that of loaf-sugar, but sometimes it is
entirely composed of lenticular crystals. It is insoluble in acids,
and does not effervesce like chalk and dolomite, the lime being
already combined with sulphuric acid, for which it has a stronger
affinity than for any other. Anhydrous gypsum is a rare variety,
into which water does not enter as a component part. Gypseous
marl is a mixture of gypsum and marl.

Forms of stratification. — A series of strata sometimes con-

PART I. CHAPTER II. 29

Alternations Sorting Power of Water.

sists of one of the above rocks, sometimes of two or more in
alternating beds. Thus, in the coal districts of England, for
example, we often pass through several beds of sandstone, some
of finer, others of coarser grain, some white, others of a dark
colour, and below these, layers of shale and sandstone or beds
of shale, divisible into leaf-like laminse, and containing beautiful
impressions of plants. Then again we meet with beds of pure
and impure coal, alternating with shales, and underneath the
whole, perhaps, are calcareous strata, or beds of limestone,
filled with corals and marine shells, each bed distinguishable
from another by certain fossils, or by the abundance of particu-
lar species of shells or zoophytes.

This alternation of different kinds of rock produces the most
distinct stratification ; and we often find beds of limestone and
marl, conglomerate and sandstone, sand and clay, recurring
again and again, in nearly regular order, throughout a series of
many hundred strata. The causes which may produce these
phenomena are various, and have been fully discussed in my
treatise on the modern changes of the earth's surface.* It is
there seen that rivers flowing into lakes and seas are charged
with sediment, varying in quantity, composition, colour, and
grain, according to the seasons ; the waters are sometimes flooded
and rapid, at other periods low and feeble ; different tributaries,
also, draining peculiar countries and soils, and therefore charged
with peculiar sediment, are swollen at distinct periods. It was
also shown that the waves of the sea and currents undermine the
cliffs during wintry storms, and sweep away the materials into
the deep, after which a season of tranquillity succeeds, when
nothing but the finest mud is spread by the movements of the
ocean over the same submarine area.

It is not the object of the present work to give a description of
these operations, repeated as they are, year after year, and cen-
tury after century; but I may suggest an explanation of the
manner in which some micaceous sandstones have originated,
those in which we see innumerable thin layers of mica dividing
layers of fine quartzose sand. I observed the same arrangement
of materials in recent mud deposited in the estuary of La Roche
St. Bernard in Brittany, at the mouth of the Loire. The sur-
rounding rocks are of gneiss, which, by its waste, supplies the
mud : when this dries at low water, it is found to consist of brown
laminated clay, divided by thin seams of mica. The separation
of the mica, in this case, or in that of micaceous sandstones, may

* Consult Index to Principles of Geology. " Stratification," " Currents,"
« Deltas," « Water," &c.

30 LYELL'S ELEMENTS OF GEOLOGY.

Horizontality of Strata.

be thus understood. If we take a handful of quartzose sand,
mixed with mica, and throw it into a clear running stream, we
see the materials immediately sorted by the water, the grains of
quartz falling almost directly to the bottom, while the plates of
mica take a much longer time to reach the bottom, and are car-
ried farther down the stream. At the first instant the water is
turbid, but immediately after the flat surfaces of the plates of
mica are seen alone reflecting a silvery light, and they descend
slowly, to form a distinct micaceous lamina. The mica is the
heavier mineral of the two ; but it remains longer suspended,
owing to its great extent of surface. It is easy, therefore, to
conceive how the intermittent action of waves, currents, and
tides, may sort the sediment brought down from the waste of a
granitic country, and throw down the mica, layer after layer,
separately from the mud or sand.

Original hcrizontality. — It has generally been said that the
upper and under surfaces of strata, or the planes of stratification,
as they are termed, are parallel. Although this is not strictly
true, they make an approach to parallelism, for the same reason
that sediment is usually deposited at first in nearly horizontal
layers. The reason of this arrangement can by no means be
attributed to an original evenness or horizontally in the bed of
the sea ; for it is ascertained that in those places where no matter
has been recently deposited, the bottom of the ocean is often as
uneven as that of the dry land, having, in like manner, its hills,
valleys, and ravines. Yet if the sea should sink, or the water
be removed near the mouth of a large river where a delta has
been forming, we should see extensive plains of mud and sand
laid dry, which, to the eye, would appear perfectly level, al-
though, in reality, they would slope gently from the land towards
the sea. This tendency in newly-formed strata to assume a
horizontal position, arises principally from the motion of the
water, which forces along particles of sand or mud at the bottom,
and causes them to settle in hollows or depressions, where they
are less exposed to the force of a current than when they are
resting on elevated points. The velocity of the current and the
motion of the superficial waves diminish from the surface down-
wards, and is least in those depressions where the water is deep-
est. A good illustration of the principle here alluded to, may be
sometimes seen in the neighbourhood of a volcano, when a sec-
tion, whether natural or artificial, has laid open to view a suc-
cession of various-coloured layers of sand and ashes, which have
fallen in showers upon uneven ground. Thus, let A, B (Fig. 1.)
be two ridges, with an intervening valley. These original in-
equalities of the surface have been gradually effaced by beds of

PART I. CHAPTER II.

81

Original Horizon tali ty Thinning out.

sand and ashes c d e, the surface at e being quite level. It will
be seen, that although the materials of the first layers have ac-
commodated themselves, in a great degree, to the shape of the
ground A B, yet each bed is thickest at the bottom. At first, a
j?jp. j great many particles would be car-

ried by their own gravity down the
steep sides of A and B, and others
would afterwards be blown by the
wind as they fell off the ridges, and

would settle in the hollow, which would thus become more and
more effaced as the strata accumulated from c to e. This level-
ling operation may, perhaps, be rendered more clear to the stu-
dent, by supposing a number of parallel trenches to be dug in a
plain of moving sand, like the African desert, in which case the
wind would soon cause all the signs of these trenches to disap-

pea

and the surface would be as uniform as before. Now, water

in motion can exert this levelling power on similar materials
more easily than air, for almost all stones lose in water more
than a third of the weight which they have in air, the specific
gravity of rocks being in general as 2^ when compared to that
of water, which is estimated at 1. But the buoyancy of sand
or mud would be still greater in the sea, as the density of salt
water exceeds that of fresh.

Yet, however uniform and horizontal may be the surface of
new deposits in general, there are still many disturbing causes,
such as eddies in the water, and currents moving first in one
and then in another direction, which frequently cause irregulari-
ties. We may sometimes follow a bed of limestone, shale, or
sandstone, for a distance of many hundred yards continuously ;
but we generally find at length that each individual stratum thins
Fig. 2.

Section of strata of sandstone, grit, and conglomerate.

out, and allows the beds which were previously above and below
it to meet. If the materials are C^MMC, as in grits and con-
glomerates, the same beds can rarely be traced many yards
without varying in size, and often coming to an end abruptly.
(See Fig. 2.)

There is also another phenomenon of frequent occurrence.
We find a series of larger strata, each of which is composed of
a number of minor layers placed obliquely to the general planes
of stratification. To this diagonal arrangement the name of

32

LYELL'S ELEMENTS OF GEOLOGY.

Diagonal Stratification, and its Causes.

" false stratification" has been given. Thus in the annexed sec-
tion (Fig. 3.) we see seven or eight large beds of loose sand,

Fig. 3.

Section of sand at Sandy Hall, near JBiggleswade, Bedfordshire,
Height twenty feet. (Green-sand formation.)

yellow and brown, and the lines a, &, c, mark some of the prin-
cipal planes of stratification, which are nearly horizontal. But
the greater part of the subordinate laminae do not conform to
these planes, but have often a steep slope, the inclination being
sometimes towards opposite points of the compass. When the
sand is loose and incoherent, as in the case here represented, the
devia^n from parallelism of the slanting lamina? cannot possi-
bly be accounted for by any rearrangement of the particles
acquired during the consolidation of the rock. In what manner
then can such irregularities be due to original deposition ? We
must suppose that at the bottom of the sea, as well as in the beds
of rivers, the motions of waves, currents, and eddies often cause
mud, sand, and gravel to be thrown down in heaps on particular
spots, instead of being spread out uniformly over a wide area.
Sometimes, when banks are thus formed, currents may cut pas-
sages through them, just as a river forms its bed. Suppose the
bank A (Fig. 4.) to be thus formed with a steep sloping side, and
Fig. 4. B

mmm

C D

the water being in a tranquil state, the layer of sediment No. 1-

PART I. CHAPTER II.

33

Ripple Mark, how formed.

is thrown down upon it, conforming nearly to its surface. After-
wards the other layers, 2, 3, 4, may be deposited in succession,
so that the bank B C D is formed. If the current-then increases
in velocity, it may cut away the upper portion of this mass down
to the dotted line e (Fig. 4.), and deposit the materials thus
removed farther on, so as to form the layers 5, 6, 7, 8. We have
now the bank B C D E (Fig. 5.,) of which the surface is almost

Fig. 5.

level, and on which the nearly horizontal layers 9, 10, 11, may
then accumulate. The opposite slope of the diagonal layers of
successive strata, in the section Fig. 3., may be accounted for by
changes in the direction of the tides and currents in the same
place.

Fig. 6.

Slab of ripple-marked (new red} sandstone from Cheshire.

The ripple mark, so common on the surface of sandstones of
all ages (see Fig. 6.) and which is so often seen on the sea-shore

34 LYELL'S ELEMENTS OF GEOLOGY.

Ripple Mark, how formed.

at low tide, seems to originate in the drifting of materials along
the bottom of the water, in a manner very similar to that which
may explain the inclined layers above described. This ripple is
not entirely confined to the beach between high and low water
mark, but is also produced on sands which are constantly covered
by water. Similar undulating ridges and furrows may also be
sometimes seen on the surface of drift snow and blown sand.
The following is the manner in which I once observed the motion
of the air to produce this effect on a large extent of level beach,
exposed at low tide near Calais. Clouds of fine white sand
were blown from the neighbouring dunes, so as to cover the
shore, and whiten a dark level surface of sandy mud, and -this
fresh covering of sand was beautifully rippled. On levelling all
the small ridges and furrows of this ripple over an area several
yards square, I saw them perfectly restored in about ten minutes,
the general direction of the ridges being always at right angles
to that of the wind. The restoration began by the appearance
here and there of small detached heaps of sand, which soon
lengthened and joined together, so as to form long sinuous ridges
with intervening furrows. Each ridge had one side slightly
Fig. 7.

inclined, and the other steep ; the lee side being always steep,
as b, c, — d, e; the windward side a gentle slope, as a, &, — c, dt
Fig. 7. When a gust of wind blew with sufficient force to drive
along a cloud of sand, all the ridges were seen to be in motion
at once, each encroaching on the furrow before it, and, in the
course of a few minutes, filling the place which the furrows had
occupied. The mode of advance was by the continual drifting
of grains of sand up the slopes a b and c d, many of which
grains, when they arrived at b and d, fell over the scarps b c and
d e, and were under shelter from the wind ; so that they remained
stationary, resting, according to their shape and momentum, on
different parts of the descent, and a few only rolling to the bot-
tom. In this manner each ridge was distinctly seen to move
slowly on as often as the force of the wind augmented. Occa-
sionally part of a ridge, advancing more rapidly than the rest,
overtook the ridge immediately before it, and became confounded
with it, thus causing those bifurcations and branches which are
so common, and two of which are seen in the slab Fig. 6. We
may observe this configuration in sandstones of all ages, and in
them also, as now on the sea-coast, we may often detect two sys-

PART I. CHAPTER III. 35

Arrangement of Fossils in Strata.

terns of ripples interfering with each other ; one more ancient
and half effaced, and a newer one, in which the grooves and
ridges are more distinct, and in a different direction. This cross-
ing of two sets of ripples arises from a change of wind, and the
new direction in which the waves are thrown on the shore.

CHAPTER III.

ARRANGEMENT OF FOSSILS IN STRATA — FRESHWATER AND MARINE.

Successive deposition indicated by fossils — Limestones formed of corals and
shells — Proofs of gradual increase of strata derived from fossils — Serpula
attached tospatangus — Wood bored byteredina — Tripoli and semi-opal formed
of infusoria — Chalk derived principally from organic bodies — Distinction of
freshwater from marine formations — Genera of freshwater and land shells —
Rules for recognizing marine testacea — Gyrogonite and chara — Freshwater
fishes — Alternation of marine and freshwater deposits — Lym-Fiord.

HAVING in the last chapter considered the forms of stratifica-
tion so far as they are determined by the arrangement of inor-
ganic matter, we may now turn our attention to the manner in
which organic remains are distributed through stratified deposits.
We should often be unable to detect any signs of stratification or
of successive deposition, if particular kinds of fossils did not
occur here and there at certain depths in the mass. At one level,
for example, bivalve shells of some one or more species pre-
dominate ; at another, some univalve shell, and at a third, corals ;
while in some formations we find layers of vegetable matter
separating strata.

It may appear inconceivable to a beginner how mountains,
several thousand feet thick, can have become filled with fossils
from top to bottom ; but the difficulty is removed when he reflects
on the origin of stratification, as explained in the last chapter,
and allows sufficient time for the accumulation of sediment. He
must never lose sight of the fact that, during the process of depo-
sition, each separate layer was once the uppermost, and covered
immediately by the water in which aquatic animals lived. Each
stratum, in fact, however far it may now lie beneath the surface,
was once in the state of loose sand or soft mud at the bottom of
the sea, in which shells and other bodies easily became enveloped.

By attending to the nature of these remains, we are often
enabled to determine whether the deposition was slow or rapid,
whether it took place in a deep or shallow sea, near the shore or
far from land, and whether the water was salt, brackish, or fresh.

36

LYELL'S ELEMENTS OF GEOLOGY.

Gradual Deposition indicated by Fossils.

Some limestones consist almost exclusively of corals, and their
position has evidently been determined by the manner in which
the zoophytes grew ; for if the stratum be horizontal, the round
spherical head of certain species is uppermost, and the point of
attachment directed downwards. This arrangement is sometimes
repeated throughout a great succession of strata. From what
we know of the growth of similar zoophytes in modern reefs, we
infer that the rate of increase was extremely slow, and some of
the fossils must have flourished for ages like forest trees, before
they attained so large a size. During these ages, the water re-
mained clear and transparent, for such zoophytes cannot live in
turbid water.

In like manner, when we see thousands of full-grown shells
dispersed every where throughout a long series of strata, we can-
not doubt that time was required for the multiplication of suc-
cessive generations ; and the evidence of slow accumulation is
rendered more striking from the proofs, so often discovered, of
fossil bodies having lain for a time on the floor of the ocean after
death, before they were imbedded in sediment. Nothing, for
example, is more common than to see fossil oysters in clay, with
serpulae, acorn-shells, corals, and other creatures, attached to the
inside of the valves, so that the mollusk was certainly not buried
Fig. 8. in argillaceous mud the

moment it died. There
must have been an interval
during which it was still
surrounded with clear wa-
ter, when the testacea, now
adhering to it, grew from
an embryo state to full ma-
turity. Attached shells,
which are merely external,
like some of the serpulse
in the annexed figure, (Fig.
8.) may often have grown
upon an oyster, or other
shell, while the animal with-
in was still living ; but if
they are found on the in-
side, it could only happen
after the death of the in-
habitant of the shell which
affords the support. Thus,
in Fig. 8., it will be seen

Fossil Gryjifuza, covered both on the outside and
inside with fossil serpula;.

PART I. CHAPTER III.

37

Gradual Deposition indicated by Fossils.

Fig. 9.

. 10.

that two serpulse have grown on the interior, one of them ex-
actly on the place where the adductor muscle of the Gryphtra
(a kind of oyster) was fixed.

Some fossil shells, even if simply attached to the outside of
others, bear full testimony to the conclusion above alluded to,
namely, that an interval elapsed between the death of the crea-
ture to whose shell they adhere, and the burial of the same in
mud or sand. The sea-urchins, or Echini, so abundant in white
chalk, afford a good illustration. It is well known that these
animals, when living, are invariably covered with numerous
spines, which serve as organs of motion, and are supported by
rows of tubercles, which last are only seen after the death of the
sea-urchin, when the spines have dropped off. In Fig. 10. a

living species of Spa-
tangus, common on
our coast, is repre-
sented with one-half
of its shell stripped
of the spines. In Fig.
9, a fossil of the same
genus from the white

Recent Spatangus7witk the chalk of England
spines removed from one shows the naked Slir-

&.tp!ne and tubercles, f*<* which the > indi-
natural size. Vlduals of this family,

a. The same magnified, exhibit when denuded
of their bristles. The full-grown Serpula, therefore, which now
adheres externally, could not have begun to grow till the Spa-
tangus had died, and the spines were detached.

Now, the series of events here attested by a single fossil may
be carried a step farther. Thus, for example, we often meet
with a sea-urchin in the chalk, (see Fig. 11.), which has fixed to
it the lower valve of a crania, an extinct genus
of bivalve mollusca. The upper valve (6 Fig.
11.) is almost invariably wanting, though occa-
sionally found in a perfect state of preservation
in white chalk at some distance. In this case,
we see clearly that the sea-urchin first lived
from youth to age, then died and lost its spines
which were carried away. Then the young
a. Echinus from the Crania adhered to the bared shell, and perish-

va?ie' onSJ ^ranTa ed in its turn ? after which the UPPer Valve Was

attached. separated from the lower before the Echinus

' cSia detached. the became enveloped in chalky mud.
It may be well to mention one more illustration of the manner
D

Serpula attached to
fossil Spalangus
from the chalk.

38

LYELL'S ELEMENTS OF GEOLOGY.

Gradual Deposition indicated by Fossils.

in which single fossils may sometimes throw light on a former
state of things, both in the bed of the ocean and on some ad-
joining land. We meet with many fragments of wood bored by
ship-worms ac various depths in the clay on which London is
built. Entire branches and stems of trees, several feet in length,
are sometimes dug out, drilled all over by the holes of these
borers, the tubes and shells of the mollusk still remaining in the
cylindrical hollows. In Fig. 13., e, a representation is given of
a piece of recent wood pierced by the Teredo navalis, or com-
mon ship-worm, which destroys wooden piles and ships. When

Fig. 12.

Fig. 13.

Fossil and recent wood drilled by perforating mollusca.

Fig. 12. a. Fossil wood from London clay, bored by Teredina.

b. Shell and tube of Teredina personata, the right hand figure the ven-

tral, the left the dorsal view.
Fig. 13. e. Recent wood bored by Teredo.

d. Shell and tube of Teredo navolis, from the same. •

c. Anterior and posterior view of the valves of the same detached from

the tube.

the cylindrical tube d has been extracted from the wood, a shell
is seen at the larger extremity, composed of two pieces, as shown
at c. In like manner, a piece of fossil wood (a, Fig. 12.) has
been perforated by an animal of a kindred but extinct genus
called Teredina by Lamarck. The calcareous tube of this mol-
lusk was united, and, as it were, soldered on to the valves of the
shell, (b) which therefore cannot be detached from the tube, like
the valves of the recent Teredo. The wood in this fossil sped-

PART I. CHAPTER III.

Infusoria in Tripoli.

men is now converted into a stony mass, a mixture of clay
and lime ; but it must once have been buoyant and floating in
the sea, when the TeredinaB lived upon it, perforating it in all
directions. Again, before the infant colony settled upon the drift
wood, the branch of a tree must have been floated down to the
sea by a river, uprooted, perhaps, by a flood, or torn offatid cast
into the waves by wind ; and thus our thoughts are carried back
to a prior period, when the tree grew for years on dry land,
enjoying a fit soil and climate.

It has been already remarked that there are rocks in the inte-
rior of continents, at various depths in the earth, and at great
heights above the sea, almost entirely made up of the remains
of zoophytes and testacea. Such masses may be compared to
modern oyster-beds and coral reefs ; and, like them, the rate of
increase must have been extremely gradual. But there are a
variety of stony deposits in the earth's crust now proved to have
been derived from plants and animals of which the organic ori-
gin was not suspected until of late years, even by naturalists.
Great surprise was therefore created by the recent discovery of
Professor Ehrenberg of Berlin, that a certain kind of siliceous
stone, called tripoli, was entirely composed of millions of the
skeletons or cases of microscopic animalcules. The substance
alluded to has long been well known in the arts, being used in
the form of powdef for polishing stones and metals. It has been
procured, among other places, from Bilin, in Bohemia, where a
single stratum, extending over a wide area, is no less than 14
feet thick. This stone, when examined with a powerful micro-
scope, is found to consist of the siliceous cases of infusoria, united
together without any visible cement. It is difficult to convey an

Fig. 14. Fig. 15. Fig. 16.

These figures are magnified nearly 300 limes, except the lower figure of G. ferru-
ginea (Fig. 16, a,) which^is magnified 2000 times.

idea of their extreme minuteness ; but Ehrenberg estimates that
in the Bilin tripoli there are 41,000 millions of individuals of the
Gaillonella distans (see Fig. 15.) in every cubic inch, which
weighs about 220 grains, or about 187 millions in a single grain.
At every stroke, therefore, that we make with this polishing
powder, several millions, perhaps tens of millions, of perfect fos-
pils are crushed to atoms.

40

LYELL'S ELEMENTS OF GEOLOGY.

Infusoria in Tripoli Fossil Infusoria.

The shells or shields of these infusoria are of pure silex, and
their forms are various, but very marked and constant in partic-
ular genera and species. Thus, in the family Bacillaria, (see
Fig. 14.,) the fossil species preserved in tripoli are seen to exhi-
bit the same divisions and transverse lines which characterize the
living shells of kindred form. With these, also, the siliceous
spiculse or internal supports of the freshwater sponge, or Span-
gilla of Lamarck, are sometimes intermingled (see the needle-
shaped bodies in Fig. 18.) These flinty cases and spiculae,
although hard, are very fragile, breaking like glass, and are
therefore admirably adapted, when rubbed, for wearing down
into a fine powder fit for polishing the surface of metals.

Fig. 18.

Fig. 17.

Besides the tripoli,
which is formed ex-
clusively of infusoria,
there occurs in the
upper part of the
great stratum at Bi-
lin another heavier
and more compact
stone, a kind of se-
mi-opal, in which in-
numerable parts of
infusoria and spicu-
lse of the Spongilla
are filled with, and
cemented together by
siliceous matter. It
is supposed that the
shells of the more
delicate animalcules
have been dissolved
by water, and have
thus given rise to this

Fragment of semi-opal from the great bed of tripoli, Bilin. Opal, in which the

Fig. 17. Natural size. more durable fossils

Fig. 18. The same magnified, showing circular artic-are preserved like in-

ulations of a species of Gaillonella, and spi-sects in amber. This

cu^ of Spongilla. opinion is confirmed

by the fact that the small shells decrease in number and shafp-
ness of outline in proportion as the opaline cement increases in
quantity.

In the Bohemian tripoli above described, as in that of Planitz

PART I. CHAPTER III. 41

Microscopic Fossils Infusoria in Flints.

in Saxony, the species of infusoria are freshwater ; but in other
countries, as in the tripoli of the Isle of France, they are of ma-
rine species, and they all belong to formations of the tertiary
period, which will be spoken of hereafter. (See Part II.)

A well-known substance, called bog-iron ore, often met with
in peat mosses, has also been shown by Ehrenberg to consist of
innumerable articulated threads, of a yellow ochre colour, com-
posed partly of flint and partly of oxide of iron. These threads
are the cases of a minute animalcule, called Gaillonella fcrru-
ginea (Fig. 16.)

It is clear that much time must have been required for the
accumulation of strata to which countless generations of infuso-
ria have contributed their shells ; and these discoveries lead us
naturally to suspect that other deposits, of which the materials
have usually been supposed to be inorganic, may in reality have
been derived from microscopic organic bodies. That this is the
case with the white chalk, has often been imagined, this rock
having been observed to abound in a variety of marine fossils,
such as shells, echini, corals, sponges, Crustacea, and fishes.
Mr. Lonsdale, on examining lately, in the museum of the Geo-
logical Society of London, portions of while chalk from different
parts of England, found, on carefully pulverizing them in water,
that what appear to the eye simply as white grains were, in fact,
well-preserved fossils. He obtained about a thousand of these
from each pound weight of chalk, some being fragments of mi-
nute corallines, others entire Foraminifera and Cytherinse. The
annexed drawings will give an idea of the beautiful forms of

CylherincB and Foraminifera from the chalk.
Fig. 19. Fig. 20. Fig. 21. Fig-. 22.

Cytherina. Portion of Lenticulina, Lam. Discorbis.

jg§ Nodosaria. (Operculina, D'Orb.)

many of these bodies. The figures a a represent their natural
size, but, minute as they seem, the smallest of them, such as a,
Fig. 22., are gigantic in comparison with the cases of infusoria
before mentioned. There is, moreover, good reason to believe
that the chambers into which these Foraminifera are divided are
actually often filled with hundreds of infusoria ; for many of the
minute grains which they contain, and which compose the envel-
oping chalk, have been observed, under a powerful microscope,
to consist of circular discs, like the articulations of Gaillonella,
before represented in Fig, 18. The bodies alluded to were cal-

D *

42 LYELL'S ELEMENTS OF GEOLOGY.

Slow Deposition of Strata.

careous ; but Ehrenberg has discovered others in the flints of the
chalk, which, like the infusoria in tripoli, are siliceous. These
forms are especially apparent in the white coating of flints, often
accompanied by innumerable needle-shaped spicula? of sponges :
and the same are occasionally visible in the central parts of
chalk flints where they are of a lighter colour. After reflecting
on these discoveries, we are naturally led on to conjecture that,
as the formless cement in the semi-opal of Bilin has been de-
rived from the decomposition of animal remains, so also even
those parts of chalk flints in which no organic structure can be
recognized may nevertheless have constituted a part of micro-
scopic animalcules.

" The dust we tread upon was once alive !" — Byron.

How faint an idea does this exclamation of the poet convey of
the real wonders of nature ! for here we discover proofs that the
calcareous and siliceous dust of which hills are composed has
not only been once alive, but almost every particle, albeit invisi-
ble to the naked eye, still retains the organic structure which, at
periods of time incalculably remote, was impressed upon it by
the powers of life.

As 1 have dwelt upon the proofs of the slowness with which
fossiliferous strata in general have been produced, I may remark
that some writers have argued, from the appearances of certain
deposits containing coal, that sedimentary rocks of great thick-
ness have been accumulated with rapidity. This conclusion has
been drawn chiefly from a remarkable phenomenon, — the posi-
tion of the trunks of fossil trees intersecting obliquely, and often
at right angles, the planes of many strata. For a full examina-
tion of this question, the reader is referred to the chapter on the
carboniferous formations, in the sequel ; and I shall merely say
here, that, although partial deposits have been thrown down in
the spots where these fossil trees occur in a comparatively short
lapse of time, yet we can by no means infer that a similar rate
of increase of carboniferous rocks prevailed simultaneously over
a wide area. On the other hand, the vegetable origin of coal is
now universally admitted by geologists ; and, when we discuss
the probable manner in which the terrestrial plants from which
it was derived were imbedded in marine shale and sandstone, we
shall find it necessary to suppose a long succession of operations.

Freshwater and marine fossils. — Strata, whether deposited
in salt or fresh ivater, have the same forms ; but the fossils are
very different in the two cases, for the same reason that aquatic
animals which frequent lakes and rivers are distinct from those
inhabiting the sea. As an example of English strata character-
ized by freshwater fossils, I may point out a formation which

PART I. CHAPTER III. 43

Distinction of Freshwater from Marine Formations.

extends over the northern part of the Isle of Wight, composed
of marl and limestone more than fifty feet thick. The shells are
principally, if not all, of extinct species ; but they are of the
same genera as those now abounding in ponds and lakes, either
in our own country or warmer latitudes.

In many parts of France, as in Auvergne, for example, strata
of limestone, marl, and sandstone occur, hundreds of feet thick,
which contain exclusively freshwater and land shells, together
with the remains of terrestrial quadrupeds. The number of land
shells scattered through some of these freshwater deposits is
exceedingly great ; and there are even districts where the rocks
scarcely contain any other fossils except snail-shells (helices ;)
as, for instance, the limestone on the left bank of the Rhine,
between Mayence and Worms, at Oppenheim, Findheim, Buden-
heim, and other places. In order to account for this phenome-
non, the geologist has only to examine the small deltas of tor-
rents which enter the Swiss lakes when the waters are low,, such
as the newly-formed plain where the Kander enters the Lake of
Thun. He there sees sand and mud strewed over with innume-
rable dead land shells, which have been brought down from val-
leys in the Alps in the preceding spring, during the melting of
the snows. Again, if we search the sands on the borders of the
Rhine, in the lower part of its course, we find countless land
shells mixed with others of species belonging to lakes, stagnant
pools, and marshes. These individuals have been washed away
from the alluvial plains of the great river and its tributaries,
some from mountainous regions, others from the low country.

Although freshwater formations are often of great thickness,
yet they are usually very limited in area when compared to
marine deposits, just as lakes and estuaries are of small dimen-
sions in comparision with seas.

We may distinguish a freshwater formation, first, by the
absence of many fossils almost invariably met with in marine
strata. For example, there are no corals, no sea-urchins, and
scarcely any other zoophytes ; no chambered shells, such as the
nautilus, nor microscropic Foraminifera. But it is chiefly by
attending to the forms of the mollusca that we are guided in
determining the point in question. In a freshwater deposit, the
number of individual shells is often as great, if not greater, than
in a marine stratum ; but there are fewer species and genera.
This might be anticipated from the fact that the genera and spe-
cies of recent freshwater and land shells are few when contrasted
with the marine. Thus, the genera of true mollusca according
to Blainville's system, excluding those of extinct species and
those without shells, amount to about 200 in number, of which

44

LYELL'S ELEMENTS OF GEOLOGY.

Distinction of Freshwater from Marine Formations.

the terrestrial and freshwater genera scarcely form more than a
sixth.

Almost all bivalve shells, or those of acephalous mollusca, are
marine, about ten only out of ninety genera being freshwater.
Among these last, the four most common forms, both recent and
fossil, are Cyclas, Cyrena, Unio, and Anodonta (see figures ;)
the two first and two last of which are so nearly allied as to pass
into each other.

Fig. 23.

Fig. 24.

Cyclas obanoata ; fossil. Hants.

Fig. 25.

Cyrena trigonula ; fossil. Grays, Essex.

Fig. 26. Fig. 27.

Anodonta Cordierii;
fossil. Paris.

dnodonta latimargi-
natus; recent. Bahia.

Unio littoralis ;
recent. Auvergne.

Lamarck divided the bivalve mollusca into the Dimyary, or
those having two large muscular impressions in each valve, as
ab in the Cyclas, Fig. 23., and the Monomyary, such as the oys-

Fig. 28.

Ostrea vesicularis (Orypht
globosa, Sow.) ; chalk.

nia, and Neritina.

ter and scallop, in which there is only one
of these impressions, as is seen in Fig. 28.
Now, as none of these last, or the ummus-
cular bivalves, are freshwater, we may at
once presume a deposit in which we find
any of them to be marine.

The univalve shells most characteristic
of freshwater deposits are, Planorbis, Lim-
nea, and Paludina. (See figures.) But to

« these are occasionally added Physa, Succi-
nea, Ancylus, Valvata, Melanopsis, Mela-

(See figures.)

PART I. CHAPTER III.

45

Distinction of Freshwater from Marine Formations.

Fig. 29.

Fig. 30.

Fig. 31.

Planorbis euomphalus ;
fossil. Isle of Wight.

Fig. 32.

Succinea elovgnta ;
fossil. Loess, Rhine.

Fig. 36.

Limnea longiscata ;
fossil. Hants.

Paludina lento, ;
fossil. Hants

Fig. 33.

Fig. 34. Fig. a5.

Ancylus elegans ;
fossil. Hants.

Valvata ; Physa

fossil. hypnorum,

Grays, Essex. recent.

Fig. 37.

Fig. 38.

Fig. 39.

Auricula ;
recent. Ava.

Melanopsis buc-

cinoidea; recent.

Asia.

In regard to one of these, the Ancylus (Fig. 33.,) Mr. Gray
observes that it sometimes differs in no respect from the marine
Siphonaria, except in the animal. The shell, however, of the
Ancylus is usually thinner.*

Some 'naturalists include Neritina (Fig. 40.) and the marine
Nerita (Fig. 41.) in the same genus, it being scarcely possible
Fig. 40. Fig. 41.

Neritina globulus. Paris basin. Jftrita granulosa. Paris basin.

* Gray, Phil. Trans. 1835, p. 302.

46

LYELL'S ELEMENTS OF GEOLOGY.

Distinction of Freshwater from Marine Formations.

Fi 42 to distinguish tne two °y S°°d generic characters.
But, as a general rule, the fluviatile species are small-
er, smoother, and more globular than the marine;
and they have never, like the Neritse, the inner mar-

fin of the outer lip toothed or crenulated. (See
ig. 41.)

A few genera, among which Cerithium (Fig. 42.)
is the most abundant, are common both to rivers and
the sea, having species peculiar to each. Other
genera, like Auricula (Fig. 36.), are amphibious, liv-
ing both in freshwater and on land.

The terrestrial shells are all univalves. The most
abundant genera among these, both in a recent and
fossil state, are Helix (Fig. 45.), Cyclostoma, Pupa,
(Fig. 44.), Clausilia, Bulimias (Fig. 43.), and Acha-
^na » which two last are nearly allied and pass into
each other. The same may be said with almost
equal truth of Pupa and Clausilia.
Fig. 43. Fig. 44. Fig. 45.

Paris basin.

Pupa muscorum.

Helix plebcium.

Fig. 46.

Bulimus
lubricus.

Att recent ; and also fossil from Loess of Rhine.

The Ampullaria (Fig. 46.) is another genus of shells, inhabit-
ing rivers and ponds in hot countries. Many
fossil species have been referred to this genus,
but they have been found chiefly in marine for-
mations, and are suspected by some concho-
logists to belong to Natica and other marine
genera.

All univalve shells of land and freshwater
species have entire mouths ; .and this circum-
stance may often serve as a convenient rule for
distinguishing freshwater from marine strata;
since, if any univalves occur of which the mouths are not entire,
we may conclude that the formation is marine. The aperture
is said to be entire in such shells as the Ampullaria and the land
shells figured in this page, when its outline is not interrupted by
an indentation or notch'such as that in Ancillaria (Fig. 48.) ; or
is not prolonged into a canal, as that seen at a in Pleuromota
(Fig. 47.).

The mouths of a large proportion of the marine univalves

from the Jumna,

PART I. CHAPTER III.

47

Distinction of Freshwater from Marine Formations.

Fig. 47.

Fig. 48.

Pleurotomata

rotata.

Subap. hills,

Italy.

Jlncillaria sululata. London clay.

have either these notches or canals, and all these species are,
without exception, carnivorous ; whereas nearly all testacea hav-
ing entire mouths, are plant-eaters, whether the species be
marine, freshwater, or terrestrial.

There is, however, one genus which affords an occasional
exception to one of the above rules. The Cerithium (Fig. 42.,)
although provided with a short canal, comprises some species
which inhabit salt, others brackish, and others fresh water.

Among the fossils very common in freshwater deposits, are
the shells of Cypris, a minute crustaceous animal, having a shell
much resembling that of the bivalve mollusca.* Many minute
living species of this genus swarm in lakes and stagnant pools in
Great Britain ; but their shells are not, if considered separately,
conclusive as to the freshwater origin of a deposit, because
another kindred genus of the same order, the Cytherina of
Fig. 49. Fig. 50.

Chara medicaginula ; Chara elastica ; recent. Italy.

fossil. Isle of Wight, a, Sessile seed-vessel between the division of the

a, Seed-vesels, leaves of the female plant.

magnified 20 b, Transverse section of a branch, with five seed-
diameters, vessels magnified, seen from below upwards.

6, Stem magnified.

* See figures in chap, on Wealden, Port II.

48 LYELL'S ELEMENTS OF GEOLOGY.

Distinction of Freshwater from Marine Formations.

Lamarck (see Fig. 19. p. 41.,) inhabits salt water; and, although
the animal differs slightly, the shell is undistinguishable from that
of the Cypris.

The seed-vessels of Chara, a genus of aquatic plants, are
very frequent in freshwater strata. These seed-vessels were
called, before their true nature was known, gyrogonites, and
were supposed to be shells. (See Fig. 49. a.)

The Charse inhabit the bottom of lakes and ponds, and
flourish mostly where the water is charged with carbonate of
lime. Their seed-vessels are covered with a very tough integu-
ment, capable of resisting decomposition; to which circumstance
we may attribute their abundance in a fossil state. The annexed
figure (Fig. 50.) represents a branch of one of many new spe-
cies found by Professor Amici in the lakes of northern Italy.
The seed-vessel in this plant is more globular than in the British
Charse, and therefore more nearly resembles in form the extinct
fossil species found in England, France, and other countries.
The stems, as well as the seed-vessels, of these plants are found
both in modern shell marl and in ancient freshwater formations.
They are generally composed of a large tube surrounded by
smaller tubes ; the whole stem being divided at certain intervals
by transverse partitions or joints. (See 6, Fig. 49.)

It is not uncommon to meet with layers of vegetable matter,
impressions of leaves, and branches of trees, in strata containing
freshwater shells ; and we also find occasionally the teeth and
bones of land quadrupeds, of species now unknown. The man-
ner by which such remains are occasionally carried by rivers
into lakes, especially during floods, has been fully treated of in
the " Principles of Geology."*

The remains of the fish are occasionally useful in determining
the freshwater origin of strata. Certain genera, such as carp,
perch, pike, and loach, (Cyprinus, Perm, Esox, and Cobitis,)
as also Lebias, being peculiar to freshwater. Other genera con-
tain some freshwater and some marine species, as Coitus, Mugil,
and Anguilla, or eel. The rest are either common to rivers
and the sea, as the salmon ; or are exclusively characteristic of
salt water. The above observations respecting fossil fishes are
applicable only to the more modern or tertiary deposits ; for in
the more, ancient rocks the forms depart so widely from those of
existing fishes, that it is very difficult, at least in the present state
of science, to derive any information from icthyolites, respecting
the element in which strata were deposited.

j£ (

*See Index, " Fossil ization."

PART I. CHAPTER I

Alternations of Marine and Freshwater

The alternation of marine and freshwater
a small and large scale, are facts well ascertained
When it occurs on a small scale, it may have arisen from the
alternate occupation of certain spaces by river-water and the
sea ; for in the flood season the river forces back the ocean and
freshens it over a large area, depositing at the same time its sedi-
ment ; after which the salt water again returns, and, on resum-
ing its former place, brings with it sand, mud, and marine shells.

There are also lagoons at the mouths of many rivers, as the
Nile and Mississippi, which are divided off by bars of sand from
the sea, and which are filled with salt and fresh water by turns.
They often communicate exclusively with the river for months,
years, or even centuries ; and then a breach being made in the
bar of sand, they are for long periods filled with salt water.

The Lym-Fiord in Jutland offers an excellent illustration of
analogous changes ; for, in the course of the last thousand
years, the western extremity of this long frith, which is 120
miles in length, including its windings, has been four times fresh
and four times salt, a bar of sand between it and the ocean hav-
ing been as often formed and removed. The last irruption of
salt water happened in 1824, when the North Sea entered, kill-
ing all the freshwater shells, fish, and plants ; and from that
time to the present, the sea-weed Fucus vesiculosus, together
with oysters and other marine mollusca, have succeeded the
Cyclas, Limnea, Paludina, and Charre.*

But changes like these in the Lym-Fiord, and those before-
mentioned as occurring at the mouths of great rivers, will only
account for some cases of marine deposits resting on freshwater
strata. When we find, as in the south-east of England, a great
series of freshwater beds, resting upon one marine formation of
great thickness, and again covered by another more than 1000
feet thick, we shall find it necessary to seek for a different expla-
nation of the phenomena.f

* See Principles of Geology, Index, " Lym-Fiord."
t See account of Wealden, Part II.

50 LYELL'S ELEMENTS OF GEOLOGY.

Consolidation of Strata.

CHAPTER IV.

CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.

Chemical and mechanical deposits — Cementing together of particles — Hard-
ening by exposure to air — Concretionary nodules — Consolidating effects of pres-
sure—Mineralisation of organic remains — Impressions and casts how formed —
Fossil wood — Goppert's experiments — Precipitation of stony matter most rapid
where putrefaction is going on — Source of lime in solution — Silex derived from
decomposition of felspar — Proofs of the lapidification of some fossils soon after
burial, of others when much decayed.

HAVING spoken in the preceding chapters of the forms of
stratification, both as dependent on the deposition of inorganic
matter and the distribution of fossils, I may next treat of the
consolidation of stratified rocks, and the petrifaction of imbedded
organic remains.

Chemical and mechanical deposits. — A distinction has been
made by geologists between deposits of a chemical, and those
of a mechanical, origin. By the latter name are designated beds
of mud, sand, or pebbles produced by the action of running
water, also accumulations of stones and scoriae thrown out by a
'volcano, which have fallen into their present place by the force
of gravitation. But the matter which forms a chemical deposit
has not been mechanically suspended in water, but in a state of
solution until separated by chemical action. ; In this manner car-
bonate of lime is often thrown to the bottom of lakes and seas
in a solid form, as may be well seen in many parts of Italy,
where mineral springs abound, and where the calcareous stone,
called travertin, is deposited. In these springs the lime is usually
held in solution by an excess of carbonic acid, or by heat if it
be a hot spring, until the water, on issuing from the earth, cools
or loses part of its acid. The calcareous matter then falls down
in a solid state, encrusting shells, fragments of wood and leaves,
and binding them together.*

In coral reefs, large masses of limestone are formed by the
stony skeletons of zoophytes ; and these; together with shells,
become cemented together by carbonate of lime, part of which
is probably furnished to the sea-water by the decomposition of
dead corals. Even shells of which the animals are still living,

* See Principles of Geology, Index, " Calcareous Springs," &c

PART I. CHAPTER IV. 51

Consolidation of Strata.

on these reefs, are very commonly found to be encrusted over
with a hard coating of limestone.*

If sand and pebbles are carried by a river into the sea, and
these are bound together immediately by carbonate of lime, the
deposit may be described as of a mixed origin, partly chemical,
and partly mechanical.

Now, the remarks already made in Chapter II. on the original
horizontally of stratd are strictly applicable to mechanical depo-
sits, and only partially to those of a mixed nature. Such as are
purely chemical may be formed on a very steep slope, or may
even encrust the vertical walls of a fissure, and be of equal
thickness throughout. ; but such deposits are of small extent, and
for the most part confined to vein-stones.

Cementing of particles. — It is chiefly in the case of calca-
reous rocks that solidification takes place at the time of deposi-
tion. But there are many deposits in which a cementing process
comes into operation long subsequently. We may sometimes
observe, where the water of ferruginous or calcareous springs
has flowed through a bed of sand or gravel, that iron or carbo-
nate of lime has been deposited in the interstices between the
grains or pebbles, so that in certain places the whole has been
bound together into a stone, the same set of strata remaining in
other parts loose and incoherent.

Proofs of a similar cementing action are seen in a rock at
Kelloway in Wiltshire. A peculiar band of sandy strata, belong-
ing to the group called Oolite by geologists, may be traced
through several counties, the sand being for the most part loose
and unconsolidated, but becoming stony near Kelloway. In this
district there are numerous fossil shells which have decomposed,
having for the most part left only their casts. The calcareous
matter hence derived has evidently served, at some former period,
as a cement to the siliceous grains of sand, and thus a solid
sandstone has been produced. If we take fragments of many
other argillaceous grits, retaining the casts of shells, and plunge
them into dilute muriatic or other acid, we see them immediately
changed into common sand and mud; the cement of lime,
derived from the shells, having been dissolved by the acid.

Traces of impressions and casts are often extremely faint. In
some loose sands of recent date we meet with shells in so
advanced a stage of decomposition as to crumble into powder
when touched. It is clear that water percolating such strata may
soon remove the calcareous matter of the shell ; and, unless
circumstances cause the carbonate of lime to be again deposited,

* See Principles of Geology, Index, "Travertin," "Coral reefs," &c.

52 LYELL'S ELEMENTS OF GEOLOGY.

Consolidation of Strata.

the grains of sand will not be cemented together ; in which case
no memorial of the fossil will remain. The absence of organic
remains from many aqueous rocks may be thus explained.

In some conglomerates, like the puddingstone of Hertfordshire,
flinty pebbles and sand are united by a siliceous cement so firmly,
that if a block be fractured the rent passes as readily through
the pebbles as through the cement.

It is probable that many strata became solid at the time when
they emerged from the waters in which they were deposited, and
when they first formed a part of the dry land. A well-known
fact seems to confirm this idea ; by far the greater number of the
stones used for building and road-making are much softer when
first taken from the quarry than after they have been long
exposed to the air. Hence it is found desirable to shape the
stones which are to be used in architecture while they are yet
soft and wet, and whil§ they contain their " quarry-water," as it
is called ; also to break up stone intended for roads when soft,
and then leave it to dry in the air for months that it may harden.
Such induration may perhaps be accounted for by supposing the
water, which penetrates the minutest pores of rocks, to deposit
on evaporation carbonate of lime, iron, silex, and other minerals
previously held in solution. These particles, on crystallizing,
would not only be deprived themselves of freedom of motion,
but would also bind together other portions of the rock which
before were loosely aggregated. On the same principle wet sand
and mud become as hard as stone when frozen ; because one
ingredient of the mass, namely, the water, has crystallized, so as
to hold firmly together all the separate particles of which the
loose mud and sand were composed.

Dr. MacCulloch mentions a sandstone in Sky, which may be
moulded like dough when first found ; and another from China,
which is compressible by the hand when immersed in water. But
it is not merely these compounds which readily admit water to
penetrate into them ; some simple minerals, says the same wri-
ter, which are rigid and as hard as glass in our cabinets, are
often flexible and soft in their native beds ; this is the case with
asbestos, sahlite, tremolite, and calcedony, and it is reported also
to happen in the case of the beryl.*

The marl recently deposited at the bottom of Lake Superior,
in North America, is soft, and often filled with fresh- water shells ;
but if a piece be taken up and dried, it becomes so hard that it
can only be broken by a smart blow of the hammer. If the lake
therefore was drained, such a deposit would be found to consist

* Dr. MacCulloch, Syst. of Geol. vol. i. p. 123.

PART 1. CHAPTER IV. 53

Concretionary Structure in Stratified Rocks.

of strata of marlstone, like that observed in many ancient Euro-
pean formations, and like them containing freshwater shells.*

It is probable that some of the heterogeneous materials which
rivers transport to the sea may at once set under water, like the
artificial mixture called pozzolana, which consists of fine volca-
nic sand charged with about 20 per cent, of iron, and the addi-
tion of a small quantity of lime. This substance hardens and
becomes a solid stone in water, and was used by the Romans in
constructing the foundations of buildings in the sea.

Consolidation in these cases is brought about by the action of
chemical affinity on finely comminuted matter previously sus-
pended in water. After deposition similar particles seem to exert
a mutual attraction on each other, and congregate together in
particular spots, forming lumps, nodules, and concretions. Thus
in many argillaceous deposits there are calcareous balls, or sphe-
rical concretions, ranged in layers parajlel to the general strati-
fication ; an arrangement which took place after the shale or
marl had been thrown down in successive laminse ; for these
p. -, laminse are often traced in the

°' __^ concretions, remaining parallel to

those of the surrounding uncon-
solidated rock. (See Fig. 51.)
Such nodules of limestone have
often a shell or other foreign
Calcareous nodules in Lias. body in the centre.f

Among the most remarkable examples of concretionary
structure are those described by Professor Sedgwick as abound-
ing in the magnesian limestone of the north of England. The
spherical balls are of various sizes, from that of a pea to a
diameter of several feet, and they have both a concentric and
radiated structure, while at the same time the laminse of original
deposition pass uninterruptedly through them. In some cliffs
this limestone resembles a great irregular pile of cannon-balls.
Some of the globular masses have their centre in one stratum,
while a portion of their exterior passes through to the stratum
above or below. Thus the larger spheroid in the annexed sec-
Fig. 52. tion (Fig. 52.) passes from the stra-
tum b upwards into a. In this
instance we must suppose the depo-
sition of a series of minor layers,
first forming the stratum &, and
afterwards the incumbent stratum a;

Spheroidal concKtwns^inmagncsian ^^ & movement of the particles

* Principles of Geology, Index, " Superior, Lake."
t See De la Beche's Geological Researches, p. 95.

54 LYELL'S ELEMENTS OF GEOLOGY.

Consolidation of Strata by Pressure and Heat.

took place, and the carbonates of lime and magnesia separated
from the more impure and mixed matter forming the still unconso-
lidated parts of the stratum. Crystallization, beginning at the
centre, must have gone on forming concentric coats around the
original nucleus, without interfering with the laminated structure
of The rock. As to the radiations from a centre, it is a phenome-
non which, however singular, is common in spherical concretions
of various mineral ingredients.

When the particles of rocks have been thus re-arranged by
chemical forces, it is sometimes difficult or impossible to ascer-
tain whether certain lines of division are due to original deposi-
tion or to the subsequent aggregation of similar particles.

p. _„ Thus suppose three strata of grit,

A, B, C, are charged unequally with
f calcareous matter, and that B is the

III

Bi-

most calcareous. If consolidation
takes place in B, the concretionary
action may spread upwards into a
part of A, where the carbonate of lime is more abundant than in
the rest ; so that a mass, d, e, jf, forming a portion of the supe-
rior stratum, becomes united with B into one solid mass of stone.
The original line of division df, e, being thus effaced, the line,
d,f, would generally be considered as the surface of the bed B,
though not strictly a true plane of stratification.

Pressure and heat. — When sand and mud sink to the bottom
of a deep sea, the particles are not pressed down by the enormous
weight of the incumbent ocean ; for the water, which becomes
mingled with the sand and mud, resists pressure with a force
equal to that of. the column of fluid above. The same happens
in regard to organic remains which are filled with water under
great pressure as they sink, otherwise they would be immediately
crushed to pieces and flattened. Nevertheless, if the materials
of a stratum remain in a yielding state, and do not set or solidify,
they will be gradually squeezed down by the weight of other
materials successively heaped upon them, just as soft clay or
loose sand on which a house is built may give way. By such
downward pressure particles of clay, sand, and marl may become
packed into a smaller space, and be made to cohere together
permanently.

Analogous effects of condensation may arise when the solid
parts of the earth's crust are forced in various directions by those
mechanical movements afterwards to be described, by which
strata have been bent, broken, and raised above the level of the
sea. Rocks of more yielding , materials must often have been

PART I. CHAPTER IV.

55

Mineralization of Organic Remains.

forced against others previously consolidated, and, thus com-
pressed, may have acquired a new structure.

But the action of heat at various depths in the earth is proba-
bly the most powerful of all causes in hardening sedimentary
strata. To this subject I shall refer again when treating of the
metamorphic rocks, and of the slaty and jointed structure.

Mineralization of organic remains. — The changes which
fossil organic bodies have undergone since they were first imbed-
ded in rocks, throw much light on the consolidation of strata.
Fossil shells in some modern deposits have been scarcely altered
in the course of centuries, having simply lost a part of their
animal matter. But in other cases the shell has disappeared, and
left an impression only of its^ exterior, or a cast of its interior
form, or, thirdly, a cast of the shell itself, the original matter of
which has been removed. These different forms of fossilization
may easily be understood if we examine the mud recently thrown
out from a pond or canal in which there are shells. If the mud
be argillaceous, it acquires consistency on drying, and on break-
ing open a portion of it we find that each shell has left impres-
sions of its external form. If we then remove the shell itself,
we find within a solid nucleus of clay, having the form of the
interior of the shell. This form is often very different from that
of the outer shell. Thus a cast such as a, Fig. 54., commonly
called a fossil screw, would never be suspected by an inexperi-
Fig. 54. Fig. 55.

Phasianella Heddingtonensis,
and cast of the same. Coral Rag.

Trochus Anglicus, and
cast. Lias.

enced conchologist to be the internal shape of the fossil univalve,
b, Fig. 54. Nor should we have imagined at first sight that the
shell a and the cast &, Fig. 55, were different parts of the same
fossil. The reader will observe, in the last-mentioned figure
(&, Fig. 55.), that an empty space shaded dark, which the shell
itself once occupied, now intervenes between the enveloping
stone and the cast of the smooth interior of the whorls. In such

56 LYELL'S ELEMENTS OF GEOLOGY.

Mineralization of Organic Remains.

cases the shell has been dissolved and the component particles
removed by water percolating the rock. If the nucleus were
taken out, a hollow mould would remain, on which the external
form of the shell with its tubercles and striae, as seen in a, Fig.
55., would be seen embossed. Now if the space alluded to
between the nucleus and the impression, instead of being left
empty, has been filled up with calcareous spar, pyrites, or other
mineral, we then obtain from the mould an exact cast both of the
external and internal form of the original shell. In this manner
silicified casts of shells have been formed ; and if the mud or
sand of the nucleus happen to be incoherent, or soluble in acid,
we can then procure in flint an empty shell which is the exact
counterpart of the original. This cast may be compared to a
bronze statue, representing merely the superficial form, and not
the internal organization ; but there is another description of petri-
faction by no means uncommon, and of a much more wonder-
ful kind, which may be compared to certain anatomical models
in wax, where not only the outward forms and features, but the
nerves, blood-vessels, and other internal organs are also shown.
Thus we find corals, originally calcareous, in which not only the
general shape, but also the minute and complicated internal
organization are retained in flint.

Such a process of petrifaction is still more remarkably exhi-
bited in fossil wood, in which we often perceive not only the
rings of annual growth, but all the minute vessels and medullary
rays. Many of the minute pores and fibres of plants, and even
those spiral vessels which in the living vegetable can only be
discovered by the microscope, are preserved. Among many
instances I may mention a fossil tree, seventy -two feet in length,
found at Gosforth near Newcastle, in sandstone strata associated
with coal. But cutting a transverse slice so thin as to transmit
light, and magnifying it about fifty-five times, the texture seen
in Fig. 56. is exhibited. A texture equally minute and compli-
cated has been observed in the wood of large trunk* of fossil
•p- gg trees found in the Craigleith quarry near

Edinburgh, where the stone was not in
the slightest degree siliceous, but consisted
chiefly of carbonate of lime, with oxide
of iron, alumina, and carbon. In some
examples the woody fibre is partially pre-
served, but it has entirely vanished from
others.

Texture of a tree from the ^ attemPtmg tO exP^in the prOCCSS of

coatetrata^magnifcd^^vi- petrifaction in such cases, we may first
tham-) assume that strata are very generally per-

PART I. CHAPTER IV. 57

Mineralization of Organic Remains.

mealed by water charged with minute portions of calcareous,
siliceous, and other earths in solution. In what manner they
become so impregnated will, be afterwards considered. If an
organic substance is exposed in the open air to the action of the
sun and rain, it will in time putrefy, or be dissolved into its com-
ponent elements, which consist chiefly of oxygen, hydrogen, and
carbon. These will readily be absorbed by the atmosphere or be
washed away by rain, so that all vestiges of the dead animal or
plant disappear. But if the same substances be submerged in
water, they decompose more gradually ; and if buried in earth,
still more slowly, as in the familiar example of wooden piles or
other buried timber. Now, if as fast as each particle is set free
by putrefaction in a fluid or gaseous state, a particle equally
minute of carbonate of lime, flint, or other mineral, is precipi-
tated, we may imagine this inorganic matter to take the place
just before left unoccupied by the organic molecule. In this
manner a cast of the interior of certain vessels may first be
taken, and afterwards the walls of the same may decay and
suffer a like transmutation. Yet when the whole is lapidified, it
may not form one homogeneous mass of stone or metal. Some
of the original ligneous, osseous, or other organic elements may
remain mingled in certain parts, or the lapidifying mineral itself
may be so crystallized in different parts as to reflect light differ-
ently, and thus the texture of the original body may be faithfully
exhibited.

But the student will ask whether, on chemical principles, we
have reason to expect that mineral matter will be thrown down
precisely in those spots where organic decomposition is in pro-
gress ? The following curious experiments may serve to illus-
trate this point. Professor Goppert of Breslau attempted recently
to imitate the natural process of petrifaction. For this purpose
he steeped a variety of animal and vegetable substances in waters,
some holding siliceous, others calcareous, others metallic matter
in solution. He found that in the period of a few weeks, or even
days, the organic bodies thus immersed were mineralized to a
certain extent. Thus, for example, thin vertical slices of deal,
taken from the Scotch fir (Pinus Sylvestris), were immersed in
a moderately strong solution of sulphate of iron. When they
had been thoroughly soaked in the liquid for several days, they
were dried and exposed to a red-heat until the vegetable matter
was burnt up and nothing remained but an oxide of iron, which
was found to have taken the form of the deal so exactly that
even the dotted vessels peculiar to this family of plants, and
resembling those in Fig. 56., were distinctly visible under the
microscope.

58 LYELL'S ELEMENTS OF GEOLOGY.

Mineralization of Organic Remains.

Another accidental experiment has been recorded by Mr. Pepys
in the Geological Transactions.* An earthen pitcher containing
several quarts of sulphate of iron had remained undisturbed and
unnoticed for about a twelvemonth in the laboratory. At the end
of this time when the liquor was examined an oily appearance
was observed on the surface, and a yellowish powder, which
proved to be sulphur, together with a quantity of small hairs.
At the bottom were discovered the bones of several mice in a
sediment consisting of small grains of pyrites, others of sulphur,
others of crystallized green sulphate of iron, and a black muddy
oxide of iron. It was evident that some mice had accidentally
been drowned in the fluid, and by the mutual action of the animal
matter and the sulphate of iron on each other, the metallic sul-
phate had been deprived of its oxygen ; hence the pyrites and the
other compounds were thrown down. Although the mice were
not fossilized, or turned into pyrites, the phenomenon shows how
mineral waters, charged with sulphate of iron, may be deoxy-
dated on coming in contact with animal matter undergoing putre-
faction, so that atom after atom of pyrites may be precipitated,
and ready, under favourable circumstances, to replace the oxy-
gen, hydrogen, and carbon into which the original body would
be resolved.

The late Dr. Turner observes, that when mineral matter is in
a " nascent state," that is to say, just liberated from a previous
state of chemical combination, it is most ready to unite with
other matter, and form a new chemical compound. Probably
the particles or atoms just set free are of extreme minuteness,
and therefore move more freely, and are more ready to obey any
impulse of chemical affinity. Whatever be the cause, it clearly
follows, as before stated, that where organic matter newly imbed-
ded in sediment is decomposing, there will chemical changes
take place most actively.

An analysis was lately made of the water which was flowing
off from the rich mud deposited by the Hooghly river in the
Delta of the Ganges after the annual inundation. This water
was found to be highly charged with carbonic acid gas holding
lime in solution.f Now if newly deposited mud is thus proved
to be permeated by mineral matter in a state of solution, it is not
difficult to perceive that decomposing organic bodies, naturally
imbedded in sediment, may as readily become petrified as the
substances artificially immersed by Professor Goppert in various
fluid mixtures.

* Vol. i. p. 399, first series.

t Piddinglon, Asiat. Researches, vol. xviii. p. 226.

PART I. CHAPTER IV. 59

Flint of Silicifiecl Fossils, whence derived.

It is well known that the water of springs, or that which is
continually percolating the earth's crust, is rarely free from a
slight admixture either of iron, carbonate of lime, sulphur, flint,
potash, or some other earthy, alkaline, or metallic ingredient.
Hot springs in particular are copiously charged with one or more
of these elements ; and it is only in their waters that silex is
found in abundance. In certain cases, therefore, especially in
volcanic regions, we may imagine the flint of silicified wood and
corals to have been supplied by the waters of thermal springs.
In other instances, as in tripoli and chalk-flint, it may have been
derived in great part, if not wholly, from the decomposition of
infusoria, sponges, and other bodies. But even if this be granted,
we have still to inquire whence a lake or the ocean can be con-
stantly replenished with the siliceous matter so abundantly with-
drawn from it by the secretions of these zoophytes.

In regard to carbonate of lime there is no difficulty, because
not only are calcareous springs very numerous, but even rain-
water has the power of dissolving a minute portion of the calca-
reous rocks over which it flows. Hence marine corals and mbl-
lusca may be provided by rivers with the materials of their shells
and solid supports. But pure silex, even when reduced to the
finest powder and boiled, is insoluble in water. Nevertheless
Dr. Turner has well explained, in an essay on the chemistry of
geology,* how the decomposition of felspar may be a source of
silex in solution, as widely spread as are the felspathic rocks
which form so large a proportion of the volcanic, plutonic, and
metamorphic rocks, and are therefore universal, occurring some-
where in the course of every large river.''

The siliceous earth, which constitutes more than half the bulk
of felspar, is intimately combined with alumine, potash, and
some other elements. The alkaline matter of the felspar has a
chemical affinity for water, as also for the carbonic acid which
is more or less contained in the waters of most springs. The
water therefore carries away alkaline matter, and leaves behind
a clay consisting of alumine and flint. But this residue of the
decomposed mineral, which in its purest state is called porcelain-
clay, is found to contain only a small portion of the silica
which existed in the original felspar. The other part therefore
must have been dissolved and removed; and this can be
accounted for in two ways, first, because silex when combined
with an alkali is soluble in water ; secondly, because silex in
what is technically called its nascent state is also soluble in
water. Hence an endless supply of silica is afforded to the
waters of the sea.

* Jam. Ed. New Phil. Journ. No. 30. p. 246.

60 LYELL'S ELEMENTS OF GEOLOGY.

Process of Petrifaction.

The disintegration of mica also, another mineral which enters
largely into the composition of granite and various sandstones,
may yield silex which may be dissolved in water, for nearly
half of this mineral consists of silica, combined with alumine,
potash, and about a tenth part of iron. The oxidation of this
iron in the air is the principal cause of the waste of mica.

We have still, however, much to learn before the conversion
of fossil bodies into stone is fully understood. Some phenomena
seem to imply that the mineralization must proceed with con-
siderable rapidity, for stems of a soft and succulent character,
and of a most perishable nature, are preserved in flint ; and
there are instances of the complete silicification of the young
leaves of a palm-tree when just about to shoot forth, and in that
state which in the West Indies is called the cabbage of the
palm.* It may, however, be questioned whether in such cases
there may not have been some anti-septic quality in the water
which retarded putrefaction, so that the soft parts of the fturied
substance may have remained for a long time without disintegra-
tion, like the flesh of bodies imbedded in peat.

Mr. Stokes has pointed out examples of petrifactions in which
the more perishable, and others where the more durable portions
of wood are preserved. These variations, he suggests, must
doubtless depend on the time when the lapidifying mineral was
introduced. Thus in certain silicified stems of palm-trees the
cellular tissue, that most destructible part, is in good condition,
all signs of the hard woody fibre having disappeared, and the
spaces once occupied by it being hollow or filled with agate.
Here petrifaction mustfnave commenced soon after the wood was
exposed to the action of moisture, and the supply of mineral
matter must have failed, or the water have become too much
diluted before the woody fibre decayed. But when the latter is
alone discoverable, we must then suppose that an interval of
time elapsed before the commencement of lapidification, during
which the cellular tissue was obliterated. When both structures,
namely the cellular and the woody fibre, are preserved, the pro-
cess must have commenced at an early period, and continued
without interruption till it was completed throughout.!

* Stokes, Geol. Trans, vol. v. p. 212. second series. t Ibid.

PART I. CHAPTER V. 01

Land has been raised, not the sea lowered.

CHAPTER V.

ELEVATION OF STRATA ABOVE THE SEA — HORIZONTAL AND INCLINED
STRATIFICATION.

Why the elevated position of marine strata should be referred to the rising up
of the land, not to the going down of the sea — Upheaval of extensive masses of
horizontal strata — Inclined and vertical stratification — Anticlinal and synclinal
lines — Examples of bent strata in east of Scotland — Theory of folding by lateral
movement — Dip and strike — Structure of the Jura — Rocks broken by flexure —
Inverted position of disturbed strata — Unconfbrmable stratification — Fractures
of strata — Polished surfaces — Faults — Appearance of repeated alternations pro-
duced by them — Origin of great faults.

LAND has been raised, not the sea lowered. — It has been
already stated that the aqueous rocks contaiaing marine fossils
extend over wide continental tracts, and are seen in mountain
chains, rising to great heights above the level of the sea. Hence
it follows that what is now dry land was once under water. But
if we admit this conclusion, we must imagine either that there has
been a general lowering of the waters of the ocean, or that the
solid rocks, once covered by water, have been raised up bodily
out of the sea, and have thus become dry land. The earlier
geologists, finding themselves reduced to this alternative, em-
braced the former opinion, assuming that the ocean was origin-
ally universal, and had gradually sunk down to its actual level,
so that the present islands and continents were left dry. It
seemed to them far easier to conceive that the water had gone
down, than that solid land had risen upwards into its present
position. It was, however, impossible to invent any satisfactory
hypothesis to explain the disappearance of so enormous a body
of water throughout the globe, it being necessary to infer that
the ocean had once stood at whatever height marine shells were
detected. It moreover appeared clear, as the science of Geology
advanced, that certain spaces on the globe had been alternately
sea, then land, then estuary, then sea again, and, lastly, once
more habitable land, having remained in each of these states for
considerable periods. In order to account for such phenomena,
without admitting any movement of the land itself, we are
required to imagine several retreats and returns of the ocean ;
and even then our theory applies merely to cases where the
marine strata composing the dry land are horizontal, leaving

02 LYELL'S ELEMENTS OF GEOLOGY.

Land has been raised, not the Sea lowered.

unexplained those more common instances where strata are
inclined, curved, or placed on their edges, and evidently not in
the position in which they were first deposited.

Geologists, therefore, were at last compelled to have recourse
to the other alternative, namely, the doctrine that the solid land
has been repeatedly moved upwards or downwards, so as per-
manently to change its position relatively to the sea. There are
several distinct grounds for preferring this conclusion. First, it
will account equally for the position of those elevated masses of
marine origin in which the stratification remains horizontal, and
for those in which the strata are disturbed, broken, inclined, or
vertical. Secondly, it is consistent with human experience that
land should rise gradually in some places and be depressed in
others. Such changes have actually occurred in our own days,
and are now in progress, being accompanied in some cases by
violent convulsions, while in others they proceed insensibly, or
are only ascertainable by the most careful scientific observations.

On the other hand, there is no evidence from human experi-
ence of a lowering of the sea's level in any region, and the wa-
ters of the ocean cannot sink in one place without their_ level
being depressed every where throughout the globe.

These preliminary remarks will prepare the reader to under-
stand the great theoretical interest attached to all facts connected
with the position of strata, whether horizontal or inclined, curved
or vertical.

Now the first and most simple appearance is where strata of
marine origin occur above the level of the sea in horizontal posi-
tion. Such are the strata which we meet with in the south of Sicily,
filled with shells of the same species as now live in the Mediter-
ranean. Some of these rocks rise to the height of 2000 feet and
more above the sea. Other mountain masses might be men-
tioned with horizontal strata of high antiquity which contain fos-
sils wholly dissimilar in form to any now known to exist, as in
the south of Sweden, near Lake Wener, where the beds of a
deposit, called Transition or Silurian by geologists, occur in as
level a position as if they had recently formed part of the delta
of a great river, and been left dry on the retiring of the annual
floods. Aqueous rocks of about the same age extend over the
lake-district of North America, exhibiting in like manner a strati-
fication nearly undisturbed. The Table Mountain at the Cape
of Good Hope is another example of highly elevated and per-
fectly horizontal strata, no less than 3500 feet in thickness, and
consisting of sandstone of very ancient data.

Instead of imagining that such fossiliferous rocks were always
at their present level, and that the sea was once high enough to

PART I. CHAPTER V.

Elevation of Land.

cover them, we suppose them to have constituted the ancient bed
of the ocean, and that they were gradually uplifted to their pre-
sent height. This idea, however startling it may at first appear,
is quite in accordance, as above stated, with the analogy of
changes now going on in certain regions of the globe. Thus in
parts of Sweden, for example, and the shores and islands of the
Gulf of Bothnia, proofs have been obtained that the land is ex-
periencing, and has experienced for centuries, a slow upheaving
movement. Play fair argued in favour of this opinion in 1802,
and in 1807 Von Buch, after his travels in Scandinavia, an-
nounced his conviction that a rising of the land was in progress.
Celsius and other Swedish writers had, a century before, declared
their belief that a gradual change had for ages been taking place
in the relative level of land and sea. They attributed the change
to a fall of the waters both of the ocean and the Baltic ; but this
theory has now been refuted by abundant evidence ; for the alter-
ation of relative level has neither been universal nor every where
uniform in quantity, but has amounted in some regions to seve-
ral feet in a century, in others to a few inches, while in the
southernmost part of Sweden, or the province of Scania, there
has been actually a loss instead of a gain of land, buildings hav-
ing gradually sunk below the level of the sea.*

It appears from the observations of Mr. Darwin and others that
very extensive regions of the continent of South America have
been undergoing slow and gradual upheaval, by which the level
plains of Patagonia, covered with recent marine shells, and the
Pampas of Buenos Ayres have been formed. f On the other
hand the gradual sinking of part of the west coast of Greenland,
for the space of more than 600 miles from north to south, during
the last four centuries, has been established by the observations
of a Danish naturalist, Dr. Pingel. And while these proofs of
continental elevation and subsidence by slow and insensible move-
ments have been recently brought to light, the evidence is daily
strengthened of continued changes of level effected by violent
convulsions in countries where earthquakes are frequent. Here
the rocks are rent from time to time, and heaved up or thrown
down several feet at once, and disturbed in such a manner, that
the original position of strata may, in the course of centuries, be
modified to any amount.

* In the first three editions of my Principles of Geology, I expressed many
doubts as to the validity of the alleged proofs of a gradual rise of land in Sweden ;
but after visiting that country in 1834 1 retracted these objections, and published
a detailed statement of the observations which led me to alter my opinion in the
Phil. Trans. 1835, Part I. See also Principles, 5th edition, [1st. Am. Edit.] book
ii. chap. 17.

t See his Journal in Voyage of the Beagle.

04

LYELL'S ELEMENTS OF GEOLOGY.

Rising and Sinking of Land Vertical, Inclined, and Curved Stratification.

It has also been shown by Mr. Darwin, that, in those seas
where circular coral islands abound, there is a slow and con-
tinued sinking of the submarine mountains on which these masses
of coral are based ; while in other areas of the South Sea, where
coral is found above the sea level, arid in inland situations, and
where there are no circular or barrier reefs, the land is on the
rise.*

It would require a volume to explain to the reader the various
facts and phenomena which confirm the reality of these move-
ments of land, whether of elevation or depression, whether ac-
companied by earthquakes or accomplished slowly and without
local disturbance. Having treated fully of these subjects, in the
Principles of Geology, I must assume, in the present work, that
such changes are part of the actual course of nature ; and when
admitted, they will be found to afford a key to the interpretation
of a variety of geological appearances, such as the elevation of
horizontal or disturbed marine strata, the superposition of fresh-
water to marine deposits, and many other phenomena, afterwards
to be described. It will also appear, in the second part of this
volume, how much light the doctrine of a continued subsidence
of land may throw on the manner in which a series of strata
formed in shallow water may have accumulated to a great thick-
ness. The excavation of valleys also, and other effects of denu-
dation, of which I shall presently treat, can alone be understood
when we duly appreciate the proofs now on record of the pro-
longed rising and sinking of land throughout wide areas.

Inclined stratification. — The most unequivocal evidence of a
change in the original position of strata is afforded by their stand-
ing up perpendicularly on their edges, which is by no means a
rare phenomenon, especially in mountainous countries. Thus
we find in Scotland, on the southern skirts of the Grampians,
beds of puddingstone alternating with thin layers of fine sand,

all placed vertically to the horizon.
When Saussure first observed cer-
tain conglomerates in a similar po-
sition in the Swiss Alps, he re-
marked that the pebbles, being for
the most part of an oval shape, had
their longer axes parallel to the
planes of stratification, (See fig.
57.) From this he inferred that
such. strata must, at first, have been

Fig. 57.

Vertical conglomerate and sandstone.

* Proceedings of Geol. Soc. No. 51. p. 552., and his Journal in Voyage of the
Beagle, vol. iii. p. 557.

PART I. CHAPTER V.

65

Curved Strata.

horizontal, each oval pebble having originally settled at the bot-
tom of the water, with its longer side parallel to the horizon, for
the same reason that an egg will not stand on either end if
unsupported. Some few, indeed, of the rounded stones in a con-
glomerate may afford exceptions
to the above rule, for the same
reason that we see on a shingle
beach an occasional oval or flat-
sided pebble resting on its end
or edge. For some pebbles hav-
ing been forced along the bottom
and against each other, may
have settled in this position.

Vertical strata, when they can
be traced continuously upwards
or downwards for some depth,
are almost invariably seen to be
parts of great curves, which may
have a diameter of a few yards,
or of several miles. I shall first
describe two curves of consider-
1 1> able regularity, which occur in
.2 1 Forfarshire, extending over a
- country twenty miles in breadth,
from the foot of the Grampians
to the sea near Arbroath.

The mass of strata here shown
may be nearly 2000 feet in
thickness, consisting of red and
white sandstone, and various co-
loured shales, the beds being dis-
tinguishable into four principal
groups, namely, No. 1. red marl
or shale ; No. 2. red sandstone,
used for building; No. 3. con-
glomerate; and No. 4., grey
paving-stone, and tile-stone, with
green and reddish shale, con-
taining peculiar organic re-
mains. A glance at the section
will show that each of the for-
mations 2, 3, 4, are repeated
thrice at the surface, twice with
a southerly and once with a
northerly inclination or dip, and the beds in No. 1., which arc

LYELL'S ELEMENTS OF GEOLOGY.

Curved Strata.

nearly horizontal, are still brought up twice by a slight curva-
ture to the surface, once on each side of A. Beginning at the
north-west extremity, the tile-stones and conglomerates No. 4.
and No. 3. are vertical, and they generally form a ridge paral-
lel to the southern skirts of the Grampians. The superior strata
Nos. 2. and 1. become less and less inclined on descending to
the valley of Strathmore, where the strata, having a concave
bend, are said by geologists to lie in a "trough" or "basin."
Through the centre of this valley runs an imaginary line A,
called technically a " synclinal line," where the beds which are
tilted in opposite directions, may be supposed to meet. It is most
important for the observer to mark such lines, for he will per-
ceive by the diagram, that in travelling from the north to the
centre of the basin, he is always passing from older to newer
beds ; whereas after crossing the line A, and pursuing his course
in the same southerly direction, he is continually leaving the
newer, and advancing upon older strata. All the deposits which
he had before examined begin then to recur in reversed order,
until he arrives at the central axis of the Sidlaw hills, where the
strata are seen to form an arch or saddle, having an anticlinal
line B, in the centre. On passing this line, and continuing to-
wards the S. E., the formations 4, 3, and 2, are again repeated,
in the same relative order of superposition, but with a northerly
dip. At Whiteness (see diagram) it will be seen that the inclined
strata are covered by a newer deposit, a, in horizontal beds.
These are composed of red conglomerate and sand, and are
newer than any of the groups, 1, 2, 3, 4, before described, and
rest unconformably upon strata of the sand-stone group, No. 2.

Fig. 59.

rved strata of slate near St, AWs Head, Berwickshire.

An example of curved strata, in which the bends or convolu-
tions of the rock are sharper and far more numerous within an

PART I. CHAPTER V.

67

Experiments to illustrate curved Strata.

equal space, has been well described by Sir James Hall.* It
occurs near St. Abb's Head, on the east coast of Scotland, where
the rocks consist principally of a bluish slate, having frequently
a ripple-marked surface. The undulations of the beds reach
from the top to the bottom of the cliffs from 200 to 300 feet in
height, and there are sixteen distinct bendings in the course of
about six miles, the curvatures being alternately concave and
convex upwards.

An experiment was made by Sir James Hall, with a view of
illustrating the manner in which such strata, assuming them to
have been originally horizontal, may have been forced into their
present position. A set of layers of clay were placed under a
weight, and their opposite ends pressed towards each other with
such force as to cause them to approach more nearly together.
On the removal of the weight, the layers of clay were found to
be curved and folded, so as to bear a miniature resemblance to
the strata in the cliffs. We must, however, bear in mind, that
in the natural section or sea-cliff we only see the foldings imper-
fectly, one part being invisible beneath the sea, and the other, or
upper portion, being supposed to have been carried away by de-
nudation, or that action of water which will be explained in the
next chapter. The dark lines in the accompanying plan (Fig.

)f

60.), represent what is actually seen of the strata in part of
Fig. 60.

the

line of cliff alluded to ; the fainter lines, that portion which is
concealed beneath the sea level, as also that which is supposed
to have once existed above the present surface.

We may still more easily illustrate the effects which a lateral
thrust might produce on flexible strata, by placing several pieces
of differently coloured cloths upon a table, and when they are
spread out horizontally, cover them with a book. Then apply

* Edin. Trans, vol. vii. pi. 3.

68

LYELL'S ELEMENTS OF GEOLOGY.

Origin of Curved Strata Zigzag Flexures.

other books to each end, and force them towards each other.

The folding of the cloths will exactly imitate those of the bent

strata. (See Fig. 61.)

r gj To inquire whe-

ther the analogous
flexures in strata
have really been
due to similar side-

way movements, or
other exertions of
force, would lead
me farther into the
regions of specula-
tion and conjecture
than might be consistent with the scope of this elementary
work. When the volcanic and granitic rocks are described,
it will be seen that some of them have, when melted, been in-
jected forcibly into fissures, while others, already in a solid
state, have been protruded upwards through the incumbent
crust of the earth, by which a great displacement of flexible
strata must have been caused. It also appears that cavities are
sometimes formed in the interior of the earth, whether by the
removal of matter by volcanic action, or by the contraction of
argillaceous rocks, or other causes. In this manner pliable beds
sinking down, from failure of support, into chasms of less hori-
zontal extent, may have become folded and compressed laterally.
Such subsidences have been witnessed on a small scale in the
undermined ground immediately over coal-pits, from which large
quantities of coal and stone had been extracted.

Fig. 62.

Zigzag flexures of coal near Mons.

Between the layers of shale, accompanying coal, we some-

PART I. CHAPTER V.

Dip and Strike.

times see the leaves of fossil ferns spread out as regularly as
dried plants between sheets of paper in the herbarium of a bota-
nist. These fern leaves, or fronds, must have rested horizontally
on soft mud, when first deposited. If, therefore, tljey and the
layers of shale are now inclined, or standing on end, it is obvi-
ously the effect of subsequent derangement. The proof becomes,
if possible, still more striking when these strata, including vege-
table remains, are curved again and again, and even folded into
the form of the letter Z, so that the same continuous layer of coal
is cut through several times in the same perpendicular shaft.
Thus, in the coal-field near Mons, in Belgium, these zigzag bend-
ings are repeated four or five times, in the manner represented in
Fig 62., the black lines representing seams of coal.*

Dip and Strike. — In the above remarks several technical
terms have been used, such as dip, the unconformable position
of strata, and the anticlinal and synclinal lines, which, as well
as the strike of the beds, I shall now explain. If a stratum or
bed of rock, instead of being quite level, be inclined to one side,
it is said to dip; the point of the compass to which it is inclined
is called the point of dip, and the degree of deviation from a
Y'lff gg level or horizontal line is

fl °' w called the amount of dip, or

the angle of dip. Thus, in
the annexed diagram (Fig.
63.), a series of strata are
inclined, and they dip to the
north at an angle of forty-
five degrees. The strike, or line of bearing, is the prolongation
or extension of the strata in a direction at right angles to the
dip; and hence it is sometimes called the direction of the strata.
Thus, in the above instance of strata dipping to the north, their
strike must necessarily be east and west. We have borrowed
the word from the German geologists, streichen signifying to
extend, to have a certain direction. Dip and strike may be aptly
illustrated by a row of houses running east and west, the long
ridge of the roof representing the strike of the stratum of slates,
which dip on one side to the north, and on the other to the south.
A stratum which is horizontal, or quite level in all directions,
has neither dip nor strike.

It is always important for the geologist, who is endeavouring
to comprehend the structure of a country, to learn how the beds
dip in every part of the district ; but it requires some practice to

Sea Level

* See plan by M. Chevalier, Burat's D'Aubuisson, torn. ii. p. 331.

70 LYELL'S ELEMENTS OF GEOLOGY.

Dip and Strike.

avoid being occasionally deceived, both as to the point of dip and
the amount of it.

If the upper surface of a hard stony stratum be uncovered,
whether artificially in a quarry, or by the waves at the foot of a
cliff, it is easy to determine towards what point of the compass
the slope is steepest, or in what direction water would flow, if
poured upon it. This is the true dip. Perfectly horizontal lines
in the face of a vertical cliff may be the edges of highly inclined
strata, if the observer see them in the line of their strike, their
dip being inwards from the face of the cliff. If, however, we
come to a break in the cliff, which exhibits a section exactly at
right angles to the line of the strike, we are then able to ascertain
the true dip. In the annexed drawing (Fig. 64.), we may sup-
Fig. 64.

Apparent horizontality of inclined strata.

pose a headland, one side of which faces to the north, where the
beds would appear perfectly horizontal, to a person in the boat ;
while in the other side facing the west, the true dip would be seen
by the person on shore to be at an angle of 40°. If, therefore,
our observations are confined to a vertical precipice facing in one
direction, we must endeavour to find a ledge or portion of the
plane of one of the beds projecting beyond the others, in order
to ascertain the true dip.

It is rarely important to determine the angle of inclination
with such minuteness as to require the aid of the instrument
called a clinometer. We may measure the angle within a few
degrees by standing exactly opposite to a cliff where the true dip
is exhibited, holding the hands immediately before the eyes, and
placing the fingers of one in a perpendicular, and of the other in
a horizontal position, as in Fig. 65. It is thus easy to discover
whether the lines of the inclined beds bisect the angle of 90°,

PART I. CHAPTER V.

71

Dip and Strike.

Fig. 65. formed by the meeting of the hands,

so as to give an angle of 45°, or
whether it would divide the space
into two equal or unequal portions.
The upper dotted line may express
a stratum dipping to the north ;
but should the beds dip precisely to
the opposite point of the compass
as in the lower dotted line, it will
be seen that the amount of incli-
nation may still be measured by
the hands with equal facility.

It has been already seen, in de-
scribing the curved strata on the
east coast of Scotland, in Forfarshire and Berwickshire, that a
series of concave and convex bendings are occasionally repeated
several times. These usually form part of a series of parallel
waves of strata, which are prolonged in the same direction
throughout a considerable extent of country. Thus, for exam-
ple, in the Swiss Jura, that lofty chain of mountains has been
proved to consist of many parallel ridges, with intervening lon-
gitudinal valleys, as in Fig. 66., the ridges being formed by

Fig. 66.

Section illustrating the structure of the Swiss Jura.

curved fossiliferous strata, of which the nature and dip are occa-
sionally displayed in deep transverse gorges, called " cluses,"
caused by fractures at right angles to the direction of the chain.*
Now let us suppose these ridges and parallel valleys to run north
and south, we should then say that the strike of the beds is

* See M. Thurmann's work, " Essai sur les Soulevemens Jurassiques du Por-
rentruy, Paris, 1832," with whom I examined part of these mountains in 1835.

72

LYELL'S ELEMENTS OF GEOLOGY.

Anticlinal and Synclinal Lines.

Fig. 67.

Fig. 68.

north and south, and the
dip east and west. A line
| drawn along the summit of
| the ridges A, B would be
| an anticlinal line, and one
g following the bottom of the
I adjoining valleys a syn-
• clinal line. It will be ob-

Oroundplan of the denuded ridge C,fg. 66. Served that SOme of these

ridges, A, B, are unbroken on the summit, whereas one of them,
C, has been fractured along the line of strike, and a portion of
it carried away by denudation, so that the edges of the beds in
the formations a, &, c come out to the day, or, as the miners say,
crop out, on the sides of a valley. The ground plan of such a
denuded ridge as C may be expressed by the diagram Fig. 67.,
and the cross section of the same by Fig. 68. The line D E,
Fig. 67., is the anticlinal line, on each side of which the dip is
in opposite directions, as expressed by the arrows. The emer-
gence of strata at the surface is called by miners their outcrop
or basset.

If, instead of being folded into parallel ridges, the beds form a
boss or dome-shaped protuberance, and if we suppose the sum-
mit of the dome carried off, the ground plan would exhibit the
edges of the strata forming a succession of circles, or ellipses,
round a common centre. These circles are the lines of strike,
and the dip being always at right angles is inclined in the course
of the circuit to every point of the compass, constituting what is
termed a qua-qua-versal dip — that is, turning each way.

In the majority of cases, an anticlinal axis forms a ridge, and
a synclinal axis a valley, as in A, B, Fig. 58. p. 65. ; but there
are exceptions to this rule, the beds
sometimes sloping inwards from either
side of a mountain, as in Fig. 69.

On following the anticlinal line of
the ridges of the Jura, before men-
tioned, A, B, C, Fig. 66., we often
discover longitudinal fissures along the
line where the flexure was greatest.
At the eastern extremity of the Pyrenees a curious illustration
of an analogous phenomenon may be seen on a small scale (Fig.
70.) The strata there laid open, in the sea-cliffs, consist of
marl, grit, and chert, belonging to a formation of the age of the
green-sand of English geologists. Some of the bendings are so
sharp, that fragments of the slaty chert — a hard flinty rock —
taken from the points where they form an angle at a, might be

PART I. CHAPTER V.

73

Bent Strata in Pyrenees Reversed dip of Strata.

Fig,

Fig. 71.

Strata of chert, grit, and marl, near St. Jean de Luz.

used for ridge-tiles on the roof of a house. Although this chert
is now brittle, we must necessarily suppose that it was flexible
when folded into this shape ; nevertheless it must have had some
solidity, for precisely at the angle of flexure there are numerous
cracks filled with calcedony. There are also some veins of
quartz, &, Fig. 70., traversing the same formation which have
filled irregular fissures, probably enfiltered at the same time as
the calcedony above mentioned.

Between San Caterina and Castrogiovanni, in' Sicily, bent and
undulating gypseous marls occur, with here and there thin beds
of solid gypsum interstratified. Sometimes these solid layers
have been broken into detached fragments, still preserving their
sharp edges, (g g, Fig. 71.), while
the continuity of the more pliable and
ductile marls, m m, has not been inter-
rupted.

I shall conclude my remarks on bent
strata by stating, that, in mountainous
regions like the Alps, it is often difficult
for an experienced geologist to deter-
mine correctly the relative age of

, j , J . . n D,

beds by superposition, so often have
the strata been folded back upon themselves, the upper parts of
the curve having been removed by denudation. Thus, if we met
with the strata seen in the section Fig. 72., we should naturally
suppose that there were twelve distinct beds, or sets of beds, No.

1. being the youngest, and No. 12.
the oldest of the series. But this
section may, perhaps, exhibit mere-
ly six beds, which have been fold-
ed in the manner seen in Fig. 73.,
so that each of them are twice repeated, the position of one half
being reversed, and part of No. 1., originally the uppermost,
having now become the lowest of the series. These phenomena
are often observable on a magnificent scale in certain regions in
Switzerland, where there are precipices from 2000 to 3000 feet
in perpendicular height. In the Iselten Alp, in the valley of the

g. gypsum, m. marl.

p.

74

LYELL'S ELEMENTS OF GEOLOGY.

Curved Strata in the Alps Unconformable Stratification

Fig. 73. Lutschine, between Un-

terseen and Grindelwald,
curves of calcareous
shale are seen from 1000
to 1500 feet in height,
in which the beds some-
times plunge down ver-
tically for a depth of
1000 feet and more, be-
fore they bend round
again. There are many

flexures not inferior in dimensions in the Pyrenees, as those near

Gavarnie, at the base of Mont Perdu.

Fig. 74.

Curved strata of the Iselten Alp.

Unconformable stratification. — Strata are said to be uncon-
formable, when one series is so placed over another, that the
planes of the superior repose on the edges of the inferior. In
this case it is evident that a period had elapsed between the pro-
duction of the two sets of strata, and that, during this interval,
the inferior series had been tilted and disturbed. Afterwards the
upper series was thrown down in horizontal strata upon it. If
these superior beds are also inclined, it is plain that the lower
strata have been twice displaced ; first, when they were them-
selves brought into an inclined position, and a second time when
the superior beds were thrown out of the horizontal line.

It often happens that in the interval between the deposition of
two sets of unconforrnable strata, the inferior rock has been de-
nuded, and sometimes drilled by perforating shells. Thus, for

PART I. CHAPTER V.

75

Fissures in Strata.

example, at Autreppe and Gusigny, near Mons, beds of ancient
stone commonly called transition limestone, highly inclined, and
often bent, are covered with horizontal strata of greenish and
whitish marls of the cretaceous formation, which will be men-
tioned in a future chapter. The lowest and therefore the oldest
bed of the horizontal series is usually the sand and conglome-
rate, a, in which are rounded fragments of stone, from an inch

Fig. 75.

Junction of unconformable strata near Mons, in Belgium.

to two feet in diameter. These fragments have often adhering
shells attached to them, and have been bored by perforating mol-
lusca. The solid surface of the inferior limestone has also been
bored, so as to exhibit cylindrical and pear-shaped cavities, as at
£, the work of saxicavous mollusca ; and many rents, as at &,
which descend several feet or yards into the limestone, have been
filled with sand and shells, similar to those in the stratum a.

Fractures of the strata. — Numerous rents may often be seen
in rocks which appear to have been simply broken, the separated
parts remaining in the same places ; but we often find a fissure,
several inches or yards wide, intervening between the disunited
portions. These fissures are usually filled with fine earth and
sand, or with angular fragments of stone, evidently derived from
the fracture of the contiguous rocks.

The face of each wall of the fissure is often beautifully pol-
ished, as if glazed, striated, or scored with parallel furrows and
ridges, such as would be produced by the continued rubbing to-
gether of surfaces of unequal hardness. Those polished surfaces
are called by miners " slicken-sides." It is supposed that the
lines of the striee indicate the direction in which the rocks were
moved. During one of the late minor earthquakes in Chili, the
brick walls of a building were rent vertically in several places, and
made to vibrate for several minutes during each shock, after
which they remained uninjured, and without any opening, al-
though the line of each crack was still visible. When all move-
ment had ceased, there were seen on the floor of the house, at
the bottom of each rent, small heaps of fine brickdust, evidently
produced by trituration.

Faults. — It is not uncommon to find the mass of rock, on one

76

LYELL'S ELEMENTS OF GEOLOGY.

Faults.

side of a fissure, thrown up above or down below the mass with
which it was once in contact on the other side. This mode of
displacement is called a shift, slip, or fault. " The miner," says
Play fair, describing a fault, " is often perplexed, in his subterra-
neous journey, by a derangement in the strata, which changes
at once all those lines and bearings which had hitherto directed
his course. When his mine reaches a certain plane, which is
sometimes perpendicular, as in A B, Fig. 76., sometimes oblique

Fig. 76.

B D

Faults. A B perpendicular, C D oblique to the horizon.

to the horizon (as in C D, ibid.), he finds the beds of rock broken
asunder, those on the one side of the plane having chaftged their
place, by sliding in a particular direction along the face of the
others. In this motion they have sometimes preserved their pa-
rallelism, as in Fig. 76., so that the strata on each side of the
faults A B, C D, continue parallel to one another ; in other cases,
the strata on each side are inclined, as in a, &, c, d, (Fig. 77.),

Fig. 77.

E F, fault or fissure filled with rubbish, on each side of which the shifted
strata are not parallel.

though their identity is still to be recognized by their possessing
the same thickness, and the same internal characters."*

We sometimes see exact counterparts of these slips, on a small
scale, in pits of fine loose sand and gravel, many of whrch have
doubtless been caused by the drying and shrinking of argillace-
ous and other beds, slight subsidences having taken place from

* Playfair, Illust. of Hutt. Theory, $ 42.

PART I. CHAPTER V.

77

Faults.

failure of support. Sometimes, however, even these small slips
may have been produced during earthquakes ; for land has been
moved, and its level, relatively to the sea, considerably altered,
within the period when much of the alluvial sand and gravel
now covering the surface of continents was deposited.

I have already stated that a geologist must be on his guard, in
a region of disturbed strata, against inferring repeated alterna-
tions of rocks, when in fact, the same strata, once continuous,
have been bent round so as to recur in the same section, and
with the same dip. A similar mistake has often been occasioned
by a series of faults.

If, for example, the dark line A H (Fig. 78.) represent the

Fig. 78.

Apparent alternations of strata caused by vertical faults.

surface of a country on which the strata a b c frequently crop
out, an observer, who is proceeding from H to A, might at first
imagine that at every step he was approaching new strata,
whereas the repetition of the same beds has been caused by ver-
tical faults, or downthrows. Thus, suppose the original mass,
A, B, C, D, to have been a set of uniformly inclined strata, and
that the different masses under E F, F G, and G D, sank down
successively, so as to leave vacant the spaces marked in the dia-
gram by dotted lines, and to occupy those marked by the conti-
nuous fainter lines, then let denudation take place along the line
A H, so that the protruding and triangular masses indicated by
the fainter lines are swept away, — a miner, who has not discov-
ered the faults, finding the mass a, which we will suppose to be
a bed of coal four times repeated, might hope to find four beds,
workable to an indefinite depth, but on arriving at the fault G he
is stopped suddenly in his workings, upon reaching the strata of
sandstone c, or on arriving at the line of fault F he comes partly
upon the shale 6, and partly on the sandstone c, and on reach-
ing E he is again stopped by a wall composed of the rock d.

i
78 LYELL'S ELEMENTS OF GEOLOGY.

Grooved surfaces of Faults.

The very different levels at which the separated parts of the
same strata are found on the different sides of the fissure, in
some faults, is truly astonishing. One of the most celebrated in
England is that called the " ninety-fathom dike," in the coal-field
of Newcastle. This name has been given to it, because the same
beds are ninety fathoms lower on the northern than they are on
the southern side. The fissure has been filled by a body of sand,
which is now in the state of sandstone, and is called the dike,
which is sometimes very narrow, but in other places more than
twenty yards wide.* The walls of the fissure are scored by
grooves, such as would have been produced if the broken ends
of the rock had been rubbed along the plane of the fault-f In
the Tynedale and Craven faults, in the north of England, the
vertical displacement is still greater, and the horizontal extent of
the movement is from twenty to forty miles. Some geologists
consider it necessary to imagine that the upward or downward
movement in these cases was accomplished at a single stroke,
and not by a series of sudden but interrupted movements. This
idea appears to have been derived from a notion that the grooved
walls have merely been rubbed in one direction. But this is so
far from being a constant phenomenon in faults, that it has often
been objected to the received theory respecting slickensides, that
the strisB are not always parallel, but often curved and irregular.
It has, moreover, been remarked, that not only the walls of the
fissure or fault, but its earthy contents, sometimes present the
same polished and striated faces. Now these facts seem to indi-
cate partial changes in the direction of the movement, and some
slidings subsequent to the first filling up of the fissure. Suppose
the mass of rock A, B, C, to overlie an extensive chasm d e,

formed at the depth
of several miles,
whether by the gra-
dual contraction in
bulk of a mass of
strata, baked by a
moderate heat, or by
the subtraction of matter by volcanic action, or any other cause.
Now, if this region be convulsed by earthquakes, the fissures f
g, and others at right angles to them, may sever the mass B
from A and from C, so that it may move freely, and begin to
sink into the chasm. A fracture may be conceived so clean and
perfect as to allow it to subside at once to the bottom of the sub-

* Conybeare and Phillips, Outlines, &c. p. 376.
t Phillips, Geology, Lardner'a Cyclop, p. 41.

PART I. CHAPTER VI.

Origin of Groat Faults Denudation of Rocks.

terranean cavity ; but it is far more probable that the sinking
will be effected at successive periods during different earthquakes,
the mass always continuing to slide in the same direction along
the planes of the fissures f g, and the edges of the falling mass
being continually more broken and triturated at each convulsion.
If, as is not improbable, the circumstances which have caused
the failure of support continue in operation, it may happen that
when the mass B has filled the cavity first formed, its foundations
will again give way under it, so that it will fall again in the same
direction. But, if the direction should change, the fact could not
be discovered by observing the slickensides, because the last scor-
ing would efface the lines of previous friction. In the present
state of our ignorance of the causes of subsidence, an hypothesis
which can explain the great amount of displacement in some
faults, on sound mechanical principles, by a succession of move-
ments, is far preferable to any theory which assumes each fault
to hwe been accomplished by a single upcast or downthrow of
several thousand feet. For we know that there are operations
now in progress, at great depths in the interior of the earth, by
which both large and small tracts of ground are made to rise
above and sink below their former level, some slowly and insen-
sibly, others suddenly and by starts, a few feet or yards at a
time ; whereas there are no grounds for believing that, during
the last 3000 years at least, any regions have been either up-
heaved or depressed, at a single stroke, to the amount of several
hundred, much less several thousand feet.

CHAPTER VI.

DENUDATION, AND THE PRODUCTION OF ALLUVIUM.

Denudation defined— Its amount equal to the entire mass of stratified deposits
in the earth's crust — Horizontal sandstone denuded in Ross-shire — Level surface
of countries in which great faults occur — Connexion of denudation and alluvial
formations — Alluvium, how distinguished from rocks in situ — Ancient alluviums
called diluvium— Origin of these— Erratic blocks and accompanying gravel-
Theory of their transportation by ice.

BEFORE we take leave of the aqueous or fossiliferous rocks,
we have still to consider the alluvial formations. Denudation,
which has been occasionally spoken of in the preceding chapters,
is the removal of mineral matter by running water, whether by

80 LYELL'S ELEMENTS OF GEOLOGY.

Denudation of Stratified Rocks.

a river or marine current, and the consequent laying bare of
some inferior rock. Geologists are, perhaps, seldom in the habit
of reflecting that this operation is the inseparable accompaniment
of the production of all new strata of mechanical origin. The
transport of sediment and pebbles, to form a new deposit, neces-
sarily implies that there has been, somewhere else, a grinding
down of rock into rounded fragments, sand, or mud, equal in
quantity to the new strata. The gain at one point has merely
been sufficient to balance the loss at some other. A ravine, per-
haps, has been excavated, or a valley deepened, or the bed of the
sea has, by successive upheaval, been exposed to the power of
the waves, so that part of the superior covering of the earth's
crust has been stripped off, and thus rocks, previously hidden,
have been denuded.

When we see a stone building, we know that somewhere, far
or near, a quarry has been opened. The courses of stone in the
building may be compared to successive strata, the quarrjj.to a
ravine or valley which has suffered denudation. As the strata,
like the courses of hewn stone, have been laid one upon another
gradually, so the excavation both of the valley and quarry have
been gradual. To pursue the comparison still farther, the super-
ficial heaps of mud, sand, and gravel usually called alluvium,
may be likened to the rubbish of a quarry which has been re-
jected as useless by the workmen, or has fallen upon the road
between the quarry and the building, so as to lie scattered at ran-
dom over the ground.

If, then, the entire mass of stratified deposits in the earth's
crust is at once the monument and measure of the denudation
which has taken place, on how stupendous a scale ought we to
find the signs of this removal of transported materials in past
ages ! Accordingly, there are different classes of phenomena,
which attest in a most striking manner the vast spaces left vacant
by the erosive power of water. I may allude first, to those val-
leys on both sides of which the same strata are seen following
each other in the same order, and having the same mineral com-
position and fossil contents. We may observe for example,
several formations, as No. 1, 2, 3, 4, in the accompanying dia-

gram (Fig 80.); No. 1. conglome-
rate, No. 2. clay, No. 3. grit, and
No. 4, limestone, each repeated in a
series of hills separated by valleys
varying in depth. When we exa-
mine the subordinate parts of these
four formations, we find, in like

Valleys of denudation. j. ,. , , ,

a. alluvium. manner, distinct beds in each, cor-

PART I. CHAPTER VI. 81

Denudation.

responding, on the opposite sides of the valleys, both in compo-
sition and order of position. No one can doubt that the strata
were originally continuous, and that some cause has swept away
the portions which once connected the whole series. A torrent
on the side of a mountain produces similar interruptions, and
when we make artificial cuts in lowering roads, we expose, in
like manner, corresponding beds on either side. But in nature,
these appearances occur in mountains several thousand feet high,
and separated by intervals of many miles or leagues in extent,
of which a grand exemplification is described by Dr. MacCul-
loch, on the north-western coast of Ross-shire, in Scotland.*

Fig: 81.

SuilVeinn. CoulUg, Coutmore.

Denudation of red sandstone on north-west coast of JRoss-skire.

The fundamental rock of that country is gneiss, in disturbed
strata, on which beds of nearly horizontal red sandstone rest
unconformably. The latter are often very thin, forming mere
flags, with their surface distinctly ripple-marked. They end ab-
ruptly on the declivities of many insulated mountains, which rise
up at once to the height of about 2000 feet above the gneiss of
the surrounding plain or table-land, and to an average elevation
of about 3000 feet above the sea, which all their summits gene-
rally attain. The base of gneiss varies in height, so that the
lower portions of the sandstone occupy different levels, and the
thickness of the mass is various, sometimes exceeding 3000 feet.
It is impossible to compare these scattered portions without ima-
gining that the whole country has once been covered with a great
body of sandstone, and that masses from 1000 to more than 3000
feet in thickness have been removed.

But perhaps the most convincing evidence of denudation on a
magnificent scale is derived from the levelled surface of many
districts in which large faults occur. I have already shown, in
Fig. 78, p. 77, and in Fig. 82, how angular and protruding
masses of rock might naturally have been looked for on the sur-
face immediately above great faults, although in fact they rarely
exist. This phenomenon may be well studied in those districts
where coal has been extensively worked, for there the former
relation of the beds which have shifted their position may be de-
termined with great accuracy. Thus in the coal field of Ashby
de la Zouch, in Leicestershire (see Fig. 82.), a fault occurs, on

* Western Islands, vol. ii. p. 89. pi. 31. fig. 4.

82 LYELL'S ELEMENTS OF GEOLOGY.

Denudation.

Fig. 82.

Faults and denuded coal strata, Jlshby de la Zouch.

one side of which the coal beds abed rise to the height of 500
feet above the corresponding beds on the other side. But the
uplifted strata do not stand up 500 feet above the general surface;
on the contrary, the outline of the country, as expressed by the
line % z, is uniform and unbroken, and the mass indicated by the
dotted outline must have been washed away.* There are proofs
of this kind in some level countries, where dense masses of strata
have been cleared away from areas several hundred square miles
in extent.

In the Newcastle coal district, it is ascertained that faults occur
in which the upward or downward movement could not have been
less than 140 fathoms, which, had they affected equally the con-
figuration of the surface to that amount, would produce mountains
with precipitous escarpments near 1000 feet high, or chasms of
the like depth ; yet is the actual level of the country absolutely uni-
form, affording no trace whatever of subterraneous disturbance.^

The ground from which these materials have been removed,
is usually overspread with heaps of sand and gravel, formed out of
the ruins of the very rocks which have disappeared. Thus, in the
districts above alluded to, rounded and angular fragments occur
of hard sandstone, limestone, and ironstone, with a small quan-
tity of the more destructible shale, and even rounded pieces of
coal, the form of these relics pointing to water as. the denuding
agent.

In geological descriptions we often read of "alluvium" and
" diluvium," as opposed to " regular strata," or " fixed rocks,"
or " rocks in situ." It will be useful, therefore, to explain these
terms. At the surface there is commonly a layer of vegetable
mould, derived partly from decayed plants, and partly caused by
the castings of earth-worms, which are continually sifting the

* See Mammal's Geological Facts, &c. p. 90. and plate,
t Cony beare's Report to Brit. Assoc. 1832. p. 381.

PART I. CHAPTER VI.

Alluvium.

fine from the coarse soil.*" Immediately beneath this mould the
regular or fundamental stratified or unstratified rocks of the dis-
trict may appear ; but there usually intervenes, if not an alluvial
mass, at least a quantity of broken and angular fragments of
the subjacent rock, provincially called rubble, or brash, in many
parts of England. This last may be referred partly to the wea-
thering or disintegration of stone on the spot, the effects of air
and water, sun and frost, and chemical decomposition, and partly
to the expanding force of the roots of trees, which may have
grown in small crevices, at former geological periods, though
they may now be wanting. Sometimes the vibratidhs and undu-
lations of earthquakes may have had power, at some former era,
to shatter a surface previously rent and weathered. Thus in Ca-
labria, subterranean movements have been known to throw up
into the air the slabs of a stone pavement ;f and Mr. Darwin
mentions, that in the Island of Quinquina, in Chili, some narrow
ridges of hard primary slate, which js there the fundamental rock,
were as completely shivered by the vibrations of the great earth-
quake of February, 1835, as if they had been blasted by gun-
powder. The effect was merely superficial, and had caused
fresh fractures and displacement of the soil, the slate below re-
maining solid and uninjured.:}:

Alluvium differs from the rubble or brash, just described, as
being composed of sand and gravel, more or less rolled, in part
local, but often in great part formed of materials transported from
a distance. The term is derived from alluvio, an inundation,
or alluo, to wash. The gravel is rarely consolidated, often un-
stratified, like heaps of rubbish shot from a cart, but occasion-
ally divided into wavy and oblique layers, marking successive
deposition from water. Such alluvium is strewed alike over in-
clined and horizontal strata, and unstratified rocks ; is most abun-
dant in valleys, but also occurs in high platforms, and even on
lofty mountains, that of the higher grounds usually differing from
that found at lower levels.

The inferior surface of an alluvial deposit is often very irregular,
conforming to all the inequalities of the subjacent rock. (Fig. 83.)
Occasionally a small mass, as at c, appears detached, and as if in-
cluded in the subjacent formation. Such isolated portions are usu-
ally sections of winding subterranean hollows filled up with allu-
vium. They may have been the courses of springs or subterra-

* See Proceedings of Geol. Soc. No. 52. p. 574. Darwin on Formation of
Mould.

t See Principles of Geology, Index, " Calabria."

t Darwin, Journal of Travels in South America, &c., 1832 to 1836, in Voyage
of H. M. S. Beagle, p. 370.

84

LYELL'S ELEMENTS OF GEOLOGY.

Relation of Alluvium to regular Strata.

Fig. 83.

nean streamlets, which have flowed
through and enlarged natural
rents ; or, when on a small scale,
they may "be spaces which the
roots of large trees have once oc-
cupied, gravel and sand having
been introduced after their decay.
It is not so easy as may at first
appear to draw a clear line of dis-
tinction between the fixed rocks, or

a. vegetable soil. regular strata, (rocks in situ^ or in

b. alluvium. » \ j i • n • i

c. mass of same, apparently detach- pZace,) and their alluvial covering

of travelled materials. If the bed

of a torrent or river be dried up, we call the gravel, sand, and
mud, left in their channels, or whatever, during floods, they may
have scattered over the neighbouring plains, alluvium. The
very same materials carried into a lake or sea, where they be-
come sorted by water, and arranged in more distinct layers, are
termed regular strata.

In the same manner we may contrast the gravel, sand, and
broken shells, strewed along the path of a marine current, with
strata formed by the discharge of similar materials, year after
year, into a deeper and more tranquil part of the sea.

If any fossils occur, the mass may still be called alluvial, pro-
vided the fossils appear to have been drifted to the spot. If any
of them, as, for.gxample, freshwater or marine shells, seem to
have lived and died where they are entombed, then the deposit,
though mainly consisting of drift materials, should not be termed
alluvial, but a regular marine or freshwater formation. It is,
however, easy to perceive that passages must occur from such
alluvial to regular deposits, both in the sea and the estuaries of
rivers ; and it is often most difficult to distinguish between them,
because organic remains have been often obliterated in forma-
tions of porous sand, gravel, and loam, which allow rainwater
to percolate freely through them.

After what has been said of the connexion of denudation and
alluvium, the student will expect to find alluviums of various
ages, and at all heights above the sea, formed both before and
during the emergence of land, but always most copiously at
periods when the level of a country has undergone changes by
subterranean movements ; for then the course of running water,
whether marine or fluviatile, has been most frequently deranged,
and the power of the waves of the ocean has been brought to
bear with the greatest effect against the land.

Before tfie doctrine of the rising and sinking of large conti-

PART I. CHAPTER VI. 85

Ancient Alluviums called Diluvium.

nental areas, whether insensibly or by a repetition of sudden
shocks, was admitted as part of the actual course of nature, all
ancient alluviums were classed by some authors under the com-
mon title of " diluvium," and were said not to be due to existing
causes. To establish this proposition, it was thought sufficient to
demonstrate that the rivers which may now happen to drain a
given district, could never, in the course of thousands of ages,
have given rise to the valleys of denudation in which they now
flow, and that these same rivers could never have washed into
their present situations (often the summits of hills, and high table-
lands,) all the gravel and boulders evidently connected with for-
mer denuding operations. It was therefore usual to refer the
" diluvium" to a deluge, or succession of deluges, which rolled
with ti'emendous violence over the land, after it had acquired its
present configuration, and its present height above the sea. Not
only small gravel, but large blocks of stone, were supposed to
have been transported from a distance by these devastating floods
or waves, and lodged upon the hill-tops.

But rivers, as we have seen, are not the only existing causes,
nor even the most energetic agents, by which denudation may be
effected. If the upward movement of land be very slow, the
waves may easily clear away a stratum of yielding materials as
fast as they rise, and before they reach the surface. Thus, a
wide uninterrupted expanse of denudation may take place, and
masses, many hundreds of feet or yards in thickness, may waste
away by inches in the course of thousands of centuries. But if
reefs composed of a more refractory stone should at length rise
up, the breakers, as they foam over them, may still tear off frag-
ments, and roll them along until the bottom of the sea becomes
strewed over with blocks and pebbles. This alluvium of marine
origin will be uplifted when the reefs are ultimately converted
into land, and may then constitute the covering of the summits
of hills, or of elevated terraces, or table-lands. At the same
time, this gravel may be wanting in all valleys excavated either
during the rise of the land, by currents of the sea running be-
tween islands, or eaten out or deepened by rivers after the emer-
gence of the land. At the bottom of such more modern valleys
a distinct alluvium will be found, containing, perhaps, some peb-
bles washed out of the older or upland gravel, but principally
composed of the ruins of rocks removed during the erosion of
the newer valleys.

It must be remembered, that when we introduce such an hy-
pothesis, and take for granted the rise of the land out of the sea,
we are merely supposing what we know, from the discovery of
marine fossils, to have happened again and again, at former periods.
H

86 LYELL'S ELEMENTS OF GEOLOGY. •

Erratic Blocks drifted by Ice.

Erratic blocks. — The great size of the boulders sometimes
found associated with ancient alluviums, in places between which
and the parent rock deep valleys, and even seas, now intervene,
has been thought by some to offer insurmountable objections to
any theory which does not introduce causes of great violence to
account for their removal. These blocks, called erratic, are
some of them a few feet, others several yards, in diameter.
They are strewed by myriads over the sandy countries of the
north of Germany, and parts of Sweden, Denmark, Finland,
and Russia. Some of them at least, must have been carried into
their present position since the commencement of a very modern
geological period, for they rest, near Stockholm, and elsewhere,
on layers of sand and marl containing shells of the species now
inhabiting the Baltic.

Although these erratics are far more numerous in northern
countries, some are met with as far south as the Swiss Jura, hav-
ing evidently been carried thither from the Alps, a chain which
is now separated from the Jura by one of the broadest and deepest
valleys in the world.

Now it is inconceivable how any velocity of water could con-
vey some of these huge masses, over seas and valleys, to the
places where they are now found; but there is no real difficulty
in supposing them to have been carried by ice, when the lands
over which they lie scattered were submerged beneath the sea.

As the reader may perhaps be incredulous respecting the ade-
quacy of the cause here alluded to, I shall enumerate many facts
recently brought to light, which incontestably prove how impor-
tant a part ice plays in the transfer of alluvium from place to
place, and especially of that containing large masses of rock. I
must confine myself, however, to a brief description of a few
examples, as it is not the object of the present work to treat at
large of the changes illustrative of geological phenomena, now
known to be in progress on the earth.

First, in regard to the distribution of erratics ; they occur,
both in the northern and southern hemispheres, between the for-
tieth parallels of latitude and the poles, but are not met with in
the intermediate equatorial and warmer regions.* This fact at
once raises a presumption that the greater warmth of parts of
Asia, Africa, and America, nearer the line, has proved unfavoura-
ble to the transport of such blocks. On the other hand, they
abound in the colder regions of North America, from Canada
northwards, as well as in northern Europe ; and when we travel
southwards, and cross the Line in South America, we fall in

k See Darwin, p. 289, on some supposed exceptions to this general ruh

PART I. CHAPTER VI. 87

Erratic Blocks drifted by Ice.

with them again in Chili and Patagonia, between lat. 41° S. and
Cape Horn.* Here, then, we have grounds for suspecting that
a cold climate is favourable to the production of erratics.

Now it is well known, that, annually, in the Baltic, stones are
moved by ice ; and, very recently, on the shores of the Gulf of
Finland, some large fragments were ascertained to have been
carried to some distance. In spring, when the fringe of ice
which has encircled the coast of the Gulf of Bothnia, and many
parts of Sweden, Norway, and Denmark, during winter, breaks
up, large stones, with small gravel and ice, which have been
firmly frozen into a solid mass on the beach, are floated off to a
distance. In Canada similar operations, but on a grander scale,
have been noticed by Captain Dayfield. In the river St. Law-
rence, the loose ice accumulates on the shoals during winter, at
which season the water is low. The separate fragments of ice
are readily frozen together in a climate where the temperature is
sometimes 30o below zero, and boulders become entangled with
them, so that in the spring, when the river rises, on the melting
of the snow, the packs are floated off, frequently conveying away
the boulders to great distances. A single block of granite, 15
feet long, by 10 feet both in width and height, and which could
not contain less than 1500 cubic feet of stone, was in this way
moved down the river several hundred yards, during the late
survey in 1837. Heavy anchors of ships, lying on the shore,
have in like manner been closed in and removed. In October,
1836, wooden stakes were driven several feet into the ground, at
one point on the banks of the St. Lawrence, at high water mark,
and over them were piled many boulders, as large as the united
force of six men jcould roll. The year after, all the boulders had
disappeared, and others had arrived, and the stakes had been
drawn out and carried away by the ice.

It has also been observed, that ice-islands, detached far to the
north, perhaps in Baffin's Bay, are brought by the current, in
great numbers, down the coast of Labrador every year, and are
often carried through the straits of Belle Isle, between New-
foundland and the continent of America, which, after passing
through the straits, sometimes float for several hundred miles to
the southwest, up the Gulf of St. Lawrence, between the 40th
and 50th degrees of N. latitude. In one of these icebergs,
heaps of boulders, gravel, and stones were seen.

A similar agency of ice extends in the southern hemisphere to
still lower latitudes. Thus, for example, we learn from Mr.
Darwin, that glaciers reaching down to the sea, occur at the

* Darwin, ibid.

88 LYELL'S ELEMENTS OF GEOLOGY.

Erratic Blocks drifted by Ice.

head of all the sounds along the western coast of the southern
extremity of South America, in latitudes as low as 46°, and still
farther, the ice being covered with great fragments of rock. Al-
though these glaciers come down to the sea, the mountains from
which they descend have only half the altitude of the Alps, and
yet are equidistant from the equator. Portions of this South
American ice, charged with large blocks of granite, were seen in
Sir George Eyre's sound, in the same parallel of latitude as
Paris, floating outwards to the ocean.*

It is therefore natural to suppose that masses of rock may fre-
quently be carried by icebergs from the foot of the Andes, in this
quarter of South America, across deep channels, and stranded
on adjacent islands in the Pacific, such as Chiloe, on which large
erratics from the Andes are actually seen ; and a general eleva-
tion of the mainland, together with the islands, accompanied by
the laying dry of the intervening sounds, might present to a
future geologist a problem respecting the transport of blocks, as
enigmatical as any which are now encountered in Europe.f

Icebergs then, detached from glaciers together with coast ice,
may convey, for hundreds of miles, pebbles, boulders, sand, and
mud, and let these fall wherever they may chance to melt, on
submarine hills and valleys. These, when the land emerges
from the deep, may constitute some of the far-transported allu-
vium which has been ascribed to diluvial agency.^

* Darwin, p. 283. t Ibid, p. 286.

I For speculations on the causes of a local and general change of climate, de-
pendent on fluctuations in physical geography, and proofs of the wide conversion
of sea into land in Europe, at periods comparatively modern, see Princ. of Geol.
booki.

PART I. CHAPTER VII. 89

Trap Rocks, whence named.

CHAPTER VII.

VOLCANIC ROCKS.

Trap rocks — Name, whence derived — Their igneous origin at first doubted —
Their general appearance and character — Volcanic cones and craters, how
formed — Mineral composition and texture of volcanic rocks — Varieties of felspar
— Hornblende and augite — Isomorphism — Rocks, how to be studied — Basalt,
greenstone, trachyte, porphyry, scoria, amygdaloid, lava, tuff— Alphabetical list,
and explanation of names and synonyms, of volcanic rocks — Table of the analy-
ses of minerals most abundant in the volcanic and hypogene rocks.

THE aqueous or fossiliferous rocks having now been described,
we have next to examine those which may be called volcanic, in
the most extended sense of that term. Suppose a a in the an-

Fig. 84.

a. Hypogene formations, stratified and unstratified.

b. Aqueous formations. c. Volcanic rocks.

nexed diagram, to represent the crystalline formations, such as
the granitic and metamorphic, b b the fossiliferous strata, and
c c the volcanic rocks. These last are sometimes found, as was
explained in the first chapter and Frontispiece, breaking through
a and &, sometimes overlying both, and occasionally alternating
with the strata b b. They also are seen, in some instances, to
pass insensibly into the unstratified division of a, or the Plutonic
rocks.

When geologists first began to examine attentively the struc-
ture of the northern and western parts of Europe, they were
almost entirely ignorant of the phenomena of existing volcanos.
They found certain rocks, for the most part without stratification,
and of a peculiar mineral composition, to which they gave differ-
ent names, such as basalt, greenstone, porphyry, and amygda-
loid. All these, which were recognized as belonging to one
family, were called "trap" by Bergmann (from trappa, Swe-
dish, for a flight of steps) — a name since adopted very generally
into the nomenclature of the science ; for it was observed that
many rocks of this class occurred in great tabular masses of

H*

90 LYELL'S ELEMENTS OF GEOLOGY.

General Appearance of Volcanic Rocks.

unequal extent, so as to form a succession of terraces, or steps,
on the sides of hills. This configuration appears to be derived
from two causes, first, the abrupt original terminations of sheets
of melted matter, which have spread, whether on the land or
bottom of the sea, over a level surface. For we know, in the
case of lava flowing from a volcano, that a stream, when it has
ceased to flow, and grown solid, very commonly ends in a steep
slope, as at o, Fig. 85. But, secondly, the step-like appearance
arises more frequently from the mode
Fig. 85. m which horizontal masses of igne-

ous rock, such as b c, intercalated
between aqueous strata, have, sub-
sequently to their origin, been ex-
posed, at different heights, by denu-
dation. Such an outline, it is true,
is not peculiar to trap rocks : great

Step-Me appearance of trap. ^ other

kinds of stone, often presenting similar terraces and precipices ;
but these are usually on a smaller scale or less numerous, than
the volcanic steps, or form less decided features in the landscape,
as being less distinct in structure and composition from the asso-
ciated rocks.

Although the characters of trap rocks are greatly diversified,
the beginner will easily learn to distinguish them as a class from
the aqueous formations. Sometimes they present themselves, as
already stated, in tabular masses, which are not divided into
strata, sometimes in shapeless lumps and irregular cones, form-
ing small chains of hills. Often they are seen in dikes or wall-
like masses, intersecting fossiliferous beds. The rock is occa-
sionally found divided into columns, often decomposing into balls
of various sizes, from a few inches to several feet in diameter.
The decomposing surface very commonly assumes a coating of
a rusty iron colour, from the oxidation of ferruginous matter, so
abundant in the traps in which augite or hornblende occur ; or,
in the felspathic varieties of trap, it acquires a white opaque coat-
ing, from the bleaching of the mineral called felspar. On exam-
ining any of these volcanic rocks, where they have not suffered
disintegration, we rarely fail to detect a crystalline arrangement
in one or more of the component minerals. Sometimes the tex-
ture of the mass is cellular or porous, or has been porous, and
the cells have become filled with carbonate of lime, or other in-
filtrated mineral, which has thus taken the globular form of the
cells.

Most of the volcanic rocks produce a fertile soil by their dis-
integration. It seems that their component ingredients, silica,

PART I. CHAPTER VII. 91

Cones and Craters.

alumina, lime, potash, iron, and the rest, are in proportions well
fitted for vegetation. As they do not effervesce with acids, a
deficiency of calcareous matter might at first have been appre-
hended ; but although carbonate of lime is rare, except in the
nodules of amygdaloids, yet it will be seen that lime sometimes
enters largely in the composition of augite and hornblende. (See
Table, p. 102.)

In regions where the eruption of volcanic matter has taken
place in the open air, and where the surface has never since been
subjected to great aqueous denudation, cones and craters are
strikingly characteristic. Many hundreds of these cones are
seen in central France, in the ancient provinces of Auvergne,
Velay, and Vivarais, where they observe, for the most part, a
linear arrangement, and form chains of hills. Although none

Fig. 86.

Part of the chain of extinct volcanos called theMonts Dome, Jluvergne.
(Scrope.)

of the eruptions have happened within the historical era, the
streams of lava may still be traced distinctly descending from
many of the craters, and following the lowest levels of the exist-
ing valleys. The origin of the cone and crater-shaped hill is
well understood, the growth of many having been watched during
volcanic eruptions. A chasm or fissure first opens in the earth,
from which great volumes of steam and other gases are evolved.
The explosions are so violent as to hurl up into the air fragments
of broken stone, parts of which are shivered into minute atoms.
At the same time melted stone or lava usually ascends through
the chimney or vent by which the gases make their escape. Al-
though extremely heavy, this lava is forced up by the expansive
power of entangled gaseous fluids, chiefly steam or aqueous va-
pour, exactly in the same manner as water is made to boil over
the edge of a vessel when steam has been generated at the bot-
tom by heat. Large quantities of the lava are also shot up into
the air, where it separates into fragments, and acquires a spongy
texture by the sudden enlargement of the included gases, and
thus forms scoria, other portions being reduced to an impalpable
powder or dust. The showering down of the various ejected

92 LYELL'S ELEMENTS OF GEOLOGY.

Volcanic Rocks.

materials round the orifice of eruption, gives rise to a conical
mound, in which the successive envelopes of sand and scoriae
form layers, dipping on all sides from a central axis. In the
mean time a hollow, called a crater, has been kept open in the
middle of the mound by the continued passage upwards of steam
and other gaseous fluids. The lava sometimes flows over the
edge of the crater, and thus thickens and strengthens the sides
of the cone ; but sometimes it breaks it down on one side, and
often it flows out from a fissure at the base of the hill. (See
Fig. 86.)

I have given a full history and description of the phenomena
of recent volcanos in the Principles of Geology, and cannot re-
peat them here, but shall merely consider the characters of the
igneous rocks as they appear to a geologist in the earth's crust.
The subject may be treated of in the following order ; first, the
mineral composition, internal texture, and nomenclature of vol-
canic rocks ; secondly, the manner and position in which they
occur in the earth's crust, and their external forms ; arid, lastly,
the connexion between the products of modern volcanos and the
rocks usually styled trappean.

Mineral composition and texture. — First, in regard to the
composition of volcanic rocks, the varieties most frequently
spoken of, are basalt, greenstone, syenitic greenstone, clink-
stone, claystone, and trachyte ; while those founded chiefly on
peculiarities of texture, are porphyry, amygdaloid, lava, tuff,
scoria?, and pumice. It may be stated generally, that all these
are mainly composed of two minerals, or families of simple
minerals, felspar and hornblende, some almost entirely of
hornblende, others of felspar.

These two minerals may be regarded as two groups, rather
than species. Felspar, for example, may be, first, common
felspar, that is to say, potash-felspar, in which the alkali is pot-
ash (see Table, p. 102.) ; or, secondly, albite, that is to say,
soda-felspar, where the alkali is soda instead of potash ; or,
thirdly, Labrador-felspar (Labradorite), which differs not only
in its iridescent hues, but also in its angle of fracture or cleav-
age, and its composition. We also read much of two other
kinds, called glassy felspar and compact felspar, which, how-
ever, cannot rank as varieties of equal importance, for both the
albitic and common felspar appear sometimes in transparent or
glassy crystals ; and as to compact felspar, it is probably a
compound of a less definite nature, sometimes containing, ac-
cording to Dr. MacCulloch, both soda and potash.

The other group, or hornblende, consists principally of two
varieties ; first, hornblende, and, secondly, augite, which were

PART I. CHAPTER VII. 93

Hornblende and Augite.

once regarded as very distinct, although now some eminent
mineralogists are in doubt whether they are not one and the
same mineral, differing only as one crystalline form of native
sulphur differs from another.

The history of the changes of opinion on this point is curious
and instructive. Werner first distinguished augite from horn-
blende ; and his proposal to separate them obtained afterwards
the sanction of Haiiy, Mohs, and other celebrated mineralogists.
It was agreed that the form of the crystals of the two species
was different, and their structure, as shown by cleavage, that is
to say, by breaking or cleaving the mineral with a chisel, or a
blow of the hammer, in the direction in which it yields most
readily. It was also found by analysis that augite usually con-
tained more lime, less alumina, and no fluoric acid ; which last,
though not always found in hornblende, often enters into its
composition, in minute quantity. In addition to these charac-
ters, it was remarked as a geological fact, that augite and horn-
blende are very rarely associated together in the same rock ; and
that when this happened, as in some lavas of modern date, the
hornblende occurs in the mass of the rock, where crystallization
may have taken place more slowly, while the augite merely lines
cavities where the crystals may have been produced rapidly. It
was also remarked, that in the crystalline slags of furnaces,
augitic forms were frequent, the hornblendic entirely absent;
hence it was conjectured that hornblende might be the result of
slow, and augite of rapid cooling. This view was confirmed
by the fact, that Mitscherlich and Berthier were able to make
augite artificially, but could never succeed in forming horn-
blende. Lastly, Gustavus Rose fused a mass of hornblende in
a porcelain furnace, and found that it did not, on cooling, assume
its previous shape, but invariably took that of augite. The same
mineralogist observed certain crystals in rocks from Siberia
which presented a hornblende cleavage, while they had the
external form of augite.

If, from these data, it is inferred that the same substance may
assume the crystalline forms of hornblende or augite indiffe-
rently, according to the more or less rapid cooling of the melted
mass, it is nevertheless certain that the variety commonly called
augite, and recognized by a peculiar crystalline form, has usually
more lime in it, and less alumina, than that called hornblende,
although the quantities of these elements do not seem to be
always the same. Unquestionably the facts and experiments
above mentioned show the very near affinity of hornblende and
augite ; but even the convertibility of one into the other by melt-
ing and recrystallizing, does not perhaps demonstrate their abso-

94 LYELL'S ELEMENTS OF GEOLOGY.

Theory of Isomorphism.

lute identity. For there is often some portion of the materials
in a crystal which are not in perfect chemical combination with
the rest. Carbonate of lime, for example, sometimes carries
with it a considerable quantity of silex into its own form of crys-
tal, the silex being mechanically mixed as sand, and yet not pre-
venting the carbonate of lime from assuming the form proper to
it. This is an extreme case, but in many others some one or
more of the ingredients in a crystal may be excluded from per-
fect chemical union ; and after fusion, when the mass recrys-
tallizes, the same elements may combine perfectly or in new
proportions, and thus a new mineral may be produced. Or some
one of the gaseous elements of the atmosphere, the oxygen, for
example, may, when the melted matter reconsolidates, combine
with some one of the component elements.

The different quantity of the impurities or refuse above
alluded to, which may occur in all but the most transparent and
perfect crystals, may partly explain the discordant results at
which experienced chemists have arrived in their analysis of the
same mineral. For the reader will find that a mineral deter-
mined to be the same by its physical characters, crystalline
form, and optical properties, has often been declared by skilful
analysers to be composed of distinct elements. (See the Table
at p. 102.) This disagreement seemed at first subversive of the
doctrine, that there is a fixed and constant relation between the
crystalline form and structure of a mineral, and its chemical
composition. The apparent anomaly, however, which threat-
ened to throw the whole science of mineralogy into confusion,
was in a great degree reconciled to fixed principles by the dis-
coveries of Professor Mitscherlich at Berlin, who ascertained
that the composition of the minerals which had appeared so va-
riable, was governed by a general law, to which he gave the
name of isomorphism (from KK>J, isos, and ^op^, morphe, form).
According to this law, the ingredients of a given species of
mineral are not absolutely fixed as to their kind and quality ;
but one ingredient may be replaced by an equivalent portion of
some analogous ingredient. Thus, in augite, the lime may be in
part replaced by portions of protoxide of iron, or of manganese,
while the form of the crystal, and the angle of its cleavage
planes, remain the same. These vicarious substitutions, how-
ever, of particular elements cannot exceed certain defined limits.

Having been led into this digression on the recent progress of
mineralogy, I may here observe that the geological student must
endeavour as soon as possible to familiarize himself with the
characters of five at least of the most abundant simple minerals
of which rocks are composed. These are, felspar, quartz, mica,

PART I. CHAPTER VII. 95

How to distinguish Rocks Basalt.

hornblende, and carbonate of lime. This knowledge cannot be
acquired from books, but requires personal inspection, and the
aid of a teacher. It is well to accustom the eye to know the
appearance of rocks under the lens. To learn to distinguish
felspar from quartz is the most important step to be first aimed
at ; when these occur in a granular and uncrystallized state, the
young geologist must not be ' discouraged if, after considerable
practice, he often fails to distinguish them by the eye alone. If
the felspar is in crystals, it is easily recognized by its cleavage ;
but when in grains the blow-pipe must be used, for the edges of
the grains can be rounded in the flame, whereas those of quartz
are infusible. If the geologist is desirous of distinguishing the
three varieties of felspar above enumerated, or hornblende from
augite, it will often be necessary to use the reflecting goniometer
as a test of the angle of cleavage, and shape of the crystal.
The use of this instrument will not be found difficult.

The external characters and composition of the felspars are
extremely different from those of augite or hornblende ; so that
the volcanic rocks in which either of these minerals decidedly
predominate, are easily recognized. But there are mixtures of
the two elements in every possible proportion, the mass being
sometimes exclusively composed of felspar, at other times solely
of augite, or, again, of both in equal quantities. Occasionally,
the two extremes, and all the intermediate gradations, may be
detected in one continuous mass. Nevertheless there are certain
varieties or compounds which prevail so largely in nature, and
preserve so much uniformity of aspect and composition, that it
is useful in geology to regard them as distinct rocks, and to as-
sign names to them, such as basalt, greenstone, trachyte, and
others, already mentioned.

Basalt. — As an example of rocks in which augite greatly
prevails, basalt may first be mentioned. Although we are more
familiar with this term than with that of any other kind of trap,
it is difficult to define it, the name having been used so vaguely.
It has been very generally applied to any trap rock of a black,
bluish, or leaden-grey colour, having a uniform and compact
texture. Most strictly, it consists of an intimate mixture of
augite, felspar, and iron, to which a mineral of an olive green
colour, called olivine, is often superadded, in distinct grains or
nodular masses. The iron is usually magnetic, and is often ac-
companied by another metal, titanium. Augite is the predo-
minant mineral, the felspar being in much smaller proportions.
There is no doubt that many of the fine-grained and dark-
coloured trap rocks, called basalt, contain hornblende in the
place of augite ; but this will be deemed of small importance

96 LYELL'S ELEMENTS OF GEOLOGY.

Greenstone Trachyte Porphyry Amygdaloid.

after the remarks above made. Other minerals are occasionally
found in basalt ; and this rock may pass insensibly into almost
any variety of trap, especially into greenstone, clinkstone, and
wacke, which will be presently described.

Greenstone, or Dolerite, is usually defined as a granular rock,
the constituent parts of which are hornblende and imperfectly
crystallized felspar ; the felspar "being more abundant than in
basalt, and the grains or crystals of the two minerals more dis-
tinct from each other. This name may also be applied when
augite is substituted for hornblende (the dolerite of some au-
thors), or when albite replaces common felspar, forming the
rock sometimes called Andesite.

Syenitic greenstone. — The highly crystalline compounds of
the same two minerals, felspar and hornblende, having a graniti-
form texture, and with occasionally some quartz accompanying,
may be called Syenitic greenstone, a rock which frequently
passes into ordinary trap, and as frequently into granite.

Trachyte. — A porphyritic rock of a whitish or greyish colour,
composed principally of glassy felspar, with crystals of the
same, generally with some hornblende and some titaniferous
iron. In composition it is extremely different from basalt, this
being a felspathic, as the other is an augitic, rock. It has a pe-
culiar rough feel, whence the name tfpa^uj, tracKus, rough. Some
varieties of trachyte contain crystals of quartz.

_. Porphyry is merely a certain

form of rock, very characteristic
of the volcanic formations. When
distinct crystals of one or more
minerals are scattered through an
earthy or compact base, the rock is
termed a porphyry. (See Fig. 87.)
Thus trachyte is porphyritic ; for
in it, as in many modern lavas,
there are crystals of felspar ; but in
some porphyries the crystals are ol
augite, olivine, or other minerals.
If the base be greenstone, basalt, or
Porphyry. pitchstone, the rock may be denomi-

whitt crystals of felspar in a dark nated greenstone-porphyry, pitch-

base of hornblende and felspar. & , , r ,

stone-porphyry, and so forth.

Amygdaloid. — This is also another form of igneous rock, ad-
mitting of every variety of composition. It comprehends any
rock in which round or almond-shaped nodules of some mineral,
such as agate, calcedony, calcareous spar, or zeolite, are scat-
tered through a base of wacke, basalt, greenstone, or other kind

PART 1. CHAPTER VII.

97

Sconce Pumice

Lava.

of trap. It derives its name from the Greek word amygdala,
an almond. The origin of this structure cannot be doubted, for
we may trace the process of its formation in modern lavas.
Small pores or cells are caused by bubbles of steam and gas
confined in the melted matter. After or during consolidation
these empty spaces are gradually filled up by matter separating
from the mass, or infiltered by water permeating the rock. As
these bubbles have been sometimes lengthened by the flow of the
lava before it finally cooled, the contents of such cavities have
the form of almonds. In some of the amygdaloidal traps of
Scotland, where the nodules have decomposed, the empty cells
are seen to have a glazed or vitreous coating, and in this respect
exactly resemble scoriaceous lavas, or the slags of furnaces.

Fig. 88. The annexed figure re-

presents a fragment of
stone taken from the upper
part of a sheet of basaltic
lava in Auvergne. One
half is scoriaceous, the
pores being altogether
empty, the other part is
amygdaloidal, the pores or
cells being mostly filled
up with carbonate of lime,
forming white kernels.

Scoriceand Pumice may
next be mentioned as po-
rous rocks, produced by

Swriaceous lava in part converted into an amyg- flip n^finn nf rrncpq rm ma.
datad-Montasne de la Veille, Department of me. *C

Puy de Dome, France. tenals melted by volcanic

heat. Rcoricz are usually of a reddish brown and black colour,
and are the cinders and slags of basaltic or augitic lavas. Pumice
is a light, spongy, fibrous substance, produced by the action of
gases on trachytic and other lavas ; the relation, however, of its
origin to the composition of lava is not yet well understood. Von
Buch says it does not occur where only Labrador-felspar is
present.

Lava. — This term has a somewhat vague m signification, hav-
ing been applied to all melted matter observed to flow in streams
from volcanic vents. When this matter consolidates in the open
air, the upper part is usually scoriaceous, and the mass becomes
more and more stony as we descend, or in proportion as it has
consolidated more slowly and under greater pressure. At the
bottom, however, of a stream of lava, a small portion of scoria-
ceous rock very frequently occurs, formed by the first thin sheet
i

98 LYELL'S ELEMENTS OF GEOLOGY.

Trap Tuff Volcanic Tuff.

of liquid matter, which often precedes the main current, or by
contact with water in or upon the damp soil.

The more compact lavas are often porphyritic, but even the
scoriaceous part sometimes contains imperfect crystals, which
have been derived from some older rocks, in which the crystals
pre-existed, but were not melted, as being more infusible in their
nature.

Although melted matter rising in a crater, and even that which
enters rents on the side of a crater, is called lava, yet this term
belongs more properly to that which has flowed either in the open
air or on the bed of a lake or sea. If the same fluid has not
reached the surface, but has been merely injected into fissures
below ground, it is called trap.

There is every variety of composition in lavas ; some are
trachytic, as in the Peak of Teneriffe ; a great number are ba-
saltic, as in Vesuvius and Auvergne ; others are Andesitic, as
those of Chili ; some of the most modern in Vesuvius consist of
green augite, and many of those of Etna of augite and Labra-
dor-felspar.*

Trap-tuff, volcanic-tuff. — Small angular fragments of the
scorice and pumice, above mentioned, and the dust of the same,
produced by volcanic explosions, form the tuffs which abound in
all regions of active volcanos, where showers of these materials,
together with small pieces of other rocks ejected from the crater,
fall down upon the land or into the sea. Here they often become
mingled with shells, and are stratified. Such tuffs are some-
times bound together by a calcareous cement, and form a stone
susceptible of a beautiful polish. But even when little or no lime
is present, there is a great tendency in the materials of ordinary
tuffs to cohere together.

Besides the peculiarity of their composition, some tuffs, or
volcanic grits, as they have been termed, differ from ordinary
sandstones by the angularity of their grains. When the frag-
ments are coarse, the rock is styled a volcanic breccia. Tufa-
ceous conglomerates result from the intermixture of rolled frag-
ments or pebbles of volcanic and other rocks with tuff.

According to Mr. Scrope, the Italian geologists confine the
term tuff, or tufa, to felspathose mixtures, and those composed
principally of pumice, using the term peperino for the basaltic
tuffs.f

We meet occasionally with extremely compact beds of volca-
nic materials, interstratified with fossiliferous rocks, much re-

*G. Rose, Ann. des Mines, torn. 8. p. 32.
t Geol. Trans, vol. ii. p. 211. Seoend Series.

PART I. CHAPTER VII. 99

Volcanic Rocks.

sembling the trap which may be found in a dike. These may
sometimes be tuffs, notwithstanding their density or compactness.
The chocolate-coloured mud, which was poured for weeks out of
the crater of Graham's Island, in the Mediterranean, in 1831,
must, when unmixed with other materials, have constituted a
stone heavier than granite. Each cubic inch of. the impalpable
powder which has fallen for days through the atmosphere during
some modern eruptions, has been found to weigh, without being
compressed, as much as ordinary trap rocks, which are often
identical in mineral composition.

The fusibility of the igneous rocks generally exceeds that of
other rocks, for there is much alkaline matter and lime in their
composition, which serves as a flux to the large quantity of silica,
which would be otherwise so refractory an ingredient.

It is remarkable, that notwithstanding the abundance of this
silica, quartz is wanting in the volcanic rocks, or is present only
as an occasional mineral, like mica. The elements of mica, as
of quartz, occur in lava and trap, but the circumstances under
which these rocks are formed, are evidently unfavourable to the
development of mica and quartz, minerals so characteristic of the
hypogene formations.

It would be tedious to enumerate all the varieties of trap and
lava which have been regarded by different observers as suffi-
ciently abundant to deserve distinct names, especially as each
investigator is too apt to exaggerate the importance of local va-
rieties which happen to prevail in districts best known to him. It
will be useful, however, to subjoin here, in the form of a glos-
sary, an alphabetical list of the names and synonyms most com-
monly in use, with brief explanations, to which I have added a
table of the analysis of the simple minerals most abundant in
the volcanic and hypogene rocks. f

Explanation of the names, synonyms, and mineral composition
of the more abundant volcanic rocks.

AMPHIBOLITE. See Hornblende rock,amphibole being Haiiy's name for horn-
blende.

AMYGDALOID. A particular form of volcanic rock; see p. 96.

AUGITE ROCK. A kind of basalt or greenstone, composed wholly or principally
of granular augite. (Leonhord's Mineralreichs, 2d edition, p. 85.)

AUGITIOPORPHYRY. Crystals of Labrador-felspar and of augite, in a green or
dark grey base. (Rose, Ann. des Mines, torn. 8. p. 22. 1835.)

BASALT. Chiefly augite — an intimate mixture of augite and felspar with mag-
netic iron, olivine, &c. See p. 95. The yellowish green mineral called

100 LYELL'S ELEMENTS OF GEOLOGY.

Mineral Composition of Volcanic Rocks.

oiivine, can easily be distinguished from yellowish felspar by its infusibility
and having no cleavage. The edges turn brown in the flame of the blow-
pipe.

CL.AYSTO.NE and CLAYSTONE-PORPHYRY. An earthy and compact stone, usually
of a purplish colour, like an indurated clay; passes into hornstone ; gene-
rally contains scattered crystals of felspar and sometimes of quartz.

CLINKSTONE. Syn. Phonolite, fissile Petrosilex; a greenish or greyish rock,
having a tendency to divide into slabs and columns ; hard, with clean frac-
ture, ringing under the hammer ; principally composed of compact felspar,
and, according to Gmelin, of felspar and mesot5'pe. (Leonhard, Mineral-
reichs, p. 102.) A rock much resembling clinkstone, and called by some
Petrosilex, contains a considerable percentage of quartz and felspar. As
both trachyte and basalt pass into clinkstone, the rock so called must be very
various in composition.

COMPACT FELSPAR, which has also been called Petrosilex ; the rock so called
includes the hornstone of some mineralogists, is allied to clinkstone, but is
harder, more compact, and translucent. It is a varying rock, of which the
chemical composition is not well defined, and is perhaps the same as that of
clay. (MacCulloch's Classification of Rocks, p. 481.) Mr. MacCulloch says,
that it contains both potash and soda.

CORNEAN. A variety of claystone allied to hornstone. A fine homogeneous
paste, supposed to consist of an aggregate of felspar, quartz, and hornblende,
with occasionally epidote, and perhaps chlorite ; it passes into compact fels-
par and hornstone. (De la Beche, Geol. Trans, second series, vol. 2. p. 3.)

DIALLAGE ROCK. Sun. Euphatide, Gabbro, and some Ophiolites. " Compounded
of felspar, and diallage, sometimes with the addition of serpentine, or mica,
or quartz. (MacCulloch, ibid. p. 648.)

DIORITE. A kind of greenstone, which see. Components, felspar and horn-
blende in grains. According to Rose, Ann. des Mines, tern. 8. p. 4.,diorile
consists of albite and hornblende.

DIORITIC-PORPHYRY. A porphyritic greenstone, composed of crystals of albite
and hornblende, in a greenish or blackish base. (Rose, ibid. p. 10.)

DOLERITE. Formerly defined as a synonym of greenstone, which see. But
according to Rose (ibid. p. 32.), its composition is black augite and Labrador-
felspar ; according to Leonhard (Mineralreichs, &c. p. 77,), augite, Labrador-
felspar, and magnetic iron.

DOMITE. An earthy condition of trachyte, found in the Puy de Dome, in Au
vergne. ,

EUPHOTIDE. A mixture of grains of Labrador-felspar and diallage. (Rose, ibid.
p. 19.) According to some, this rock is defined to be a mixture of augite or
hornblende, and Saussurite, a mineral allied to jade. (Allan's Mineralogy,
p. 158.) See Diallage rock.

FELSPAR-PORPHYRY. Syn. Hornstone-porphyry ; a base of felspar, with crys-
tals of felspar, and crystals and grains of quartz. See also Hornstone.

GABBRO, see Diallage rock.

GREENSTONE ; Syn. Dolerite and diorite ; components, hornblende and felspar,

or augite and felspar in grains. See above, p. 96.
GREYSTONE. (Graustein of Werner.) Lead grey and greenish rock, composed

of felspar and augite, the felspar being more than seventy-five per cent.

(Scrope, Journ. of Sci. No. 42. p. 221.) Greystone lavas are intermediate in

composition between basaltic and trachytic lavas.

HORNBLENDE ROCK. A greenstone, composed wholly or principally of granular
hornblende, or augite. (Leonhard, Mineralreichs, &c., p. 85.)

HORNSTONE, HORNSTONE-PORPHYRY. A kind of felspar- porphyry, (Leonhard,
ibid.) with a base of hornstone, a mineral approaching near to flint, differing
from compact felspar in being infusible.

PART I. CHAPTER VII. 101

Mineral Composition of Volcanic Rocks.

HYPERSTHENE ROCK, a mixture of grains of Labrador-felspar and hypersthene,
(Hose, Ann. des Mines, torn. 8. p. 13.) having the structure of syenite or
granite ; abundant among the rocks of Sky. In a geological view, it has
been called a greenstone, in which hypersthene takes the place of horn-
blende.

MELAPHYRE. A variety of black porphyry, the base being black augite with
crystals of felspar ; from ne\as, melas, black.

OBSIDIAN. Vitreous lava like melted glass, nearly allied to pitchstone.

OPHIOLITE, sometimes same as Diallage rock (Leonhard, p. 77.) ; sometimes a
kind of serpentine.

OPHITE. A green porphyritic rock, composed chiefly of hornblende, with crys-
tals of that mineral in a base of the same, mixed with some felspar. It
passes into serpentine by a mixture of talc. (BuraCs D'Aubuisson, torn. 2.
p. 63.

PEARLSTONE. A volcanic rock haying the lustre of mother of pearl ; usually
having a nodular structure ; intimately related to obsidian, but less glassy.

PEPERINO. A form of volcanic tuff! composed of basaltic scorias. See p. 98.

PETROSILEX. See Clinkstone and Compact Felspar.

PHONOLITE. Syn. of Clinkstone, which see.

PITCHSTONE ; vitreous lava, less glassy than obsidian ; a blackish green rock
resembling glass, having a resinousjustre and appearance of pitch ; compo-
sition various, usually felspar and augite ; passes into basalt ; occurs in
veins, and in Arran forms a dike thirty feet wide, cutting through sand-
stone ; forms the outer walls of some basaltic dikes.

PORPHYRY. Any rock in which detached crystals of felspar, or of one or more
minerals, are diffused through a base. See p. 96.

POZZOLANA. A kind of tuff. See p. 53.

PUMICE. A light, spongy, fibrous form of trachyte. See p. 97.

PYROXENIC-PORPHYRY, same as augitic-porphyry, pyroxene being Haiiy's name
for augite.

Syn.
See p. 97.

SERPENTINE. A greenish rock, in which there is much magnesia ; usually con-
tains diallage, which is nearly allied to the simple mineral called serpentine.
Occurs sometimes, though rarely, in dikes, altering the contiguous strata ; is
indifferently a member of the trappean or hyppgene series.

SYENITIC-GREENSTONE ; composition, crystals or grains of felspar and hornblende.

TEPHRINE, synonymous with lava.

TOADSTONE. A local name in Derbyshire for a kind of wacke, which see.

TRACHYTE, chiefly composed of glassy felspar, with crystals of glassy felspar.

See p. 96.

TRAP TUFF. See p. 98.
TRASS. A kind of tuff or mud poured out by lake-craters during eruptions :

common in the Eifel, in Germany.

TUFACEOUS CONGLOMERATE. See p. 98.

TUFF. Syn. Trap-tuff, volcanic tuff See p. 98.

VITREOUS LAVA. See Pitchslone and Obsidian.
VOLCANIC TUFF. See p. 98.

WACKE. A soft and earthy variety of trap, having an argillaceous aspect. It
resembles indurated clay, and when scratched, exhibits a shining streak.

WHINSTONE. A Scotch provincial term for greenstone and other hard trap
rocks.

102

LYELL'S ELEMENTS OF GEOLOGY.

Analysis of Minerals..

-ss ^sssx^

PART I. CHAPTER VIII.

103

Volcanic Dikes.

CHAPTER VIII.
VOLCANIC ROCKS — continued.

Trap dikes — sometimes project — sometimes leave fissures vacant by decom-
position— Branches and veins of trap— Dikes more crystalline in the centre —
Foreign fragments of rock imbedded — Strata altered at or near the contact — Ob-
literation of organic remains — Conversion of chalk into marble — and of coal into
coke — Inequality in the modifying influence of dikes — Trap interposed between
strata — Columnar and globular structure — Relation of trappean rocks to the pro-
ducts of active volcanos — Submarine lava and ejected matter corresponds gene-
rally to ancient trap.

HAVING in the last chapter spoken of the composition and min-
eral characters of volcanic rocks, I shall next describe the man-
ner and position in which they occur in the earth's crust, and
their external forms. Now the leading varieties, such as basalt,
greenstone, trachyte, porphyry, and th« rest are found some-
times in dikes penetrating stratified and unstratified formations,
sometimes in shapeless masses protruding through or overlying
them, or in horizontal sheets intercalated between strata.

Volcanic dikes. — Fissures have already been spoken of as
occurring in all kinds of rocks, some a few feet, others many
yards in width, and often filled up with earth or angular pieces
of stone, or with sand and pebbles. Instead of such materials,
suppose a quantity of melted stone to be driven or injected into
an open rent, and there consolidated, we have then a tabular

Fig. 89.

mass resembling a
wall, and called a
trap dike. It is not
uncommon to find
such dikes passing
through strata of soft
materials, such as tuff
or shale, which, being
more perishable than
the trap, are often
washed away by the
sea, rivers, or rain,
in which case the dike

Z»*e ,n tnlana valley, near tU Brmen Heaa, Madeira. gtandg prominentlv

out in the face of precipices, or on the level surface of a country.
(See the annexed figure.)

L04

LYELL'S ELEMENTS OF GEOLOGY.

Trap Dikes and Veins.

Fig. 90.

In the islands of Arran, Sky, and other parts of Scotland,
where sandstone, conglomerate, and other hard rocks are tra-
versed by dikes of trap, the converse of the above phenomenon
-is seen. The dike having decomposed more rapidly than the
containing rock, has once more left open the original fissure,
often for a distance of many yards inland from the sea-coast, as
represented in the annexed view. (Fig. 90.) In these instances

the greenstone of the dike is usu-
ally more tough and hard than the
sandstone; but chemical action,
and chiefly the oxidation of the
iron, has given rise to the more ra-
pid decay.

There is yet another case, by no
means uncommon in Arran and
other parts of Scotland, where the
strata in contact with the dike, and
for a certain distance from it, have
been hardened, so as to resist the
action of the weather more than
the dike itself, or the surrounding
rocks. When this happens, two
parallel walls of indurated strata are

Fig. 91.

Fissures left vacant by decomposed trap, seen protruding above the general
Strathaird, Sky. (MacCuiioch.) level of the country, and following

the course of the dike.

As fissures sometimes send off branches, or divide into two
or more fissures of equal size, so also we find trap dikes bifur-
cating and ramifying, and sometimes they are so tortuous as to
be called veins, though this is more common in granite than in
trap. The accompanying sketch
(Fig. 91.) by Dr. MacCuiioch repre-
sents part of a sea-cliff in Argyleshire,
where an overlying mass of trap, 6,
sends out some veins which terminate
downwards. Another trap vein, a a,
cuts through both the limestone, c,
and the trap, b.

In Fig. 92. a ground plan is given
of a ramifying dike of greenstone,
which I observed cutting through
sandstone on the beach near Kildonan Castle, in Arran. The
larger branch varies from five to seven feet in width, which will
afford a scale of measurement for the whole.

In the Hebrides and other countries the same masses of trap

Trap veins in Airdnamurchan.

PART I. CHAPTER VIII.

105

Various forms of Trap Dikes and Veins.

Fig. 92.

Ground plan of greenstone dike traversing sandstone. Arran.

which occupy the surface of the country far and wide, conceal-
ing the subjacent stratified rocks, are seen also in the sea-cliffs,

Fig. 93.

Trap dividing and covering sandstone near Suishnish in Sky. (MacCulloch.)

prolonged downwards in veins or dikes, which probably unite
with other masses of igneous rock at a greater depth. The
largest of the dikes represented in the annexed diagram, and
which are seen in part of the coast of Sky, is no less than 100
feet in width.

Every variety of trap rock is sometimes found in these dikes,
as basalt, greenstone, felspar, porphyry, and more rarely tra-
chyte. The amygdaloidal traps also occur, and even tuff and
breccia, for the materials of these last may be washed down into
open fissures at the bottom of the sea, or during eruptions on the
land may be showered into them from the air.

Some dikes of trap may be followed for leagues uninterrupt-
edly in nearly a straight direction, as in the north of England,
showing that the fissures which they fill must have been of ex-
traordinary length.

Dikes more crystalline in the centre. — In many cases trap at
the edges or sides of a dike is less crystalline or more earthy
than in the centre, in consequence of the melted matter having
cooled more rapidly by coming in contact with the cold sides of
the fissure ; whereas, in the centre, the matter of the dike being
kept long in a fluid or soft state, the crystals are slowly formed.
In the ancient part of Vesuvius a thin band of half-vitreous lava
is found at the edge of some dikes. At the junction of green-
stone dikes with limestone, a sahlband, or selvage, of serpentine
is occasionally observed.

On the left shore of the fiord of Christiania, in Norway, a re-
markable dike of syenitic greenstone is traced through transition
strata, until at length, in the promontory of Nsesodden, it enters

106

LYELL'S ELEMENTS OF GEOLOGY.

Foreign Fragments in Dikes.

Green-
stone.

Fig. 94. mica-schist. Fig. 94. repre-

Sy enitic greenstone dike of Nassodden, Christiania- sents a ground plan, where

the dike appears eight paces
in width. In the middle it is
highly crystalline and gra-
nitiform, of a purplish co-
lour, and containing a few
crystals of mica, and strong-
ly contrasted with the whitish
mica-schist, between which
and the syenitic rock there is
usually on each side a dis-
tinct black band, 18 inches wide, of dark greenstone. When
first seen, these bands have the appearance of two accompanying
dikes ; yet they are, in fact, only the different form which the
syenitic materials have assumed where near to or in contact with
the mica-schist. At one point, a, one of the sahlbands termi-
nates for a space ; but near this there is a large detached block
&, having a gneiss-like structure, consisting of hornblende and
felspar, which is included in the midst of the dike. Round this
a smaller encircling zone, is seen, of dark basalt, or fine-grained
greenstone, nearly corresponding to the larger ones which border
the dike, but only one inch wide.*

The fact above alluded to, of a foreign fragment, such as b
(Fig. 94.), included in the midst of the trap, as if torn off from
some subjacent rock or the walls of a fissure, is by no means

uncommon. A fine illus-
tration is seen in a dike of
greenstone, ten feet wide,
in the northern suburbs of
Christiania, in Norway, of
which the annexed figure
is a ground plan. The
dike passes through shale,
known by its fossils to be-
long to the transition, or
Silurian series. In the
black base of greenstone
are angular and roundish
pieces of gneiss, some white, others of a light flesh-colour, some
without lamination, like granite, others with laminse, which, by
their various and often opposite directions., show that they have

Fi

Greenstone dike, with fragments of gneiss ;
Sorgenfri, Christiania.

*This dike has been described by Professor Keilhau, of Chrisiiania, in whose
company I examined it.

PART I. CHAPTER VIII. 107

Rocks altered by Dikes.

Deen scattered at random through the matrix. These imbedded
pieces of gneiss measure from one to about eight inches in
diameter.

Rocks altered by volcanic dikes. — After these remarks on
the form and composition of dikes themselves, I shall describe
the alterations which they sometimes produce in the rocks in
contact with them. The changes are usually such as the intense
heat of the melted matter and the entangled gases might be
expected to cause.

Plas-Newydd. — A striking example, near Plas-Newydd, in
Anglesea, has been described by Professor Henslow.* The dike
is 1 34 feet wide, and consists of a rock which is a compound of
felspar and augite (dolerite of some authors.) Strata of shale
and argillaceous limestone, through which it cuts perpendicu-
larly, are altered to a distance of thirty, or even, in some places,
to thirty-five feet from the edge of the dike. The shale, as it
approaches the trap, becomes gradually more compact, and is
most indurated where nearest the junction. Here it loses part
of its schistose structure, but the separation into parallel layers
is still discernible. In several places the shale is converted into
hard porcellanous jasper. In the most hardened part of the mass
the fossil shells, principally Productce, are nearly obliterated ;
yet even here their impressions may frequently be traced. The
argillaceous limestone undergoes analogous mutations, losing its
earthy texture as it approaches the dike, and becoming granular
and crystalline. But the most extraordinary phenomenon is the
appearance in the shale of numerous crystals of analcime and
garnet, which are distinctly confined to those portions of the
rock affected by the dike.f Garnets have been observed, under
very analogous circumstances, in High Teesdale, by Professor
Sedgwick, where they occur in shale and limestone, altered by
basalt.f

Antrim — In several parts of the county of Antrim, in the
north of Ireland, chalk with flints is traversed by basaltic dikes.
The chalk is there converted into granular marble near the
basalt, the change sometimes extending eight or ten feet from the
wall of the dike, being greatest near the point of contact, and
thence gradually decreasing till it becomes evanescent. " The
extreme effect," says Dr. Berger, " presents a dark brown crys-
talline limestone, the crystals running in flakes as large as those
of coarse primitive (metamorphic) limestone ; the next state is
saccharine, then fine-grained and arenaceous ; a compact variety,

* Cambridge Transactions, vol. i. p. 410. t Ibid. vol. ii. p. 175.

108 LYELL'S ELEMENTS OF GEOLOGY.

Rocks altered by Trap Dikes.

having a porcellanous aspect and a bluish-grey colour, suc-
ceeds : this, towards the outer edge, becomes yellowish-white,
and insensibly graduates into the unaltered chalk. The flints in
the altered chalk usually assume a grey yellowish colour." * All
traces of organic remains are effaced in that part of the lime-
stone which is most crystalline.

The annexed drawing (Fig. 96.) represents three basaltic
dikes traversing the chalk, all within the distance of ninety feet.

Fig. 96.

Clialk

Dike 35 ft. Dike Dike 20 ft.

1 foot.

Basaltic dikes in chalk in island of Rathlin, Antrim. — Ground plan as seen on the
bead). (Conybeare and Buckland.)f

The chalk contiguous to the two outer dikes is converted into a
finely granular marble, m m, as are the whole of the masses
between the outer dikes and the central one. The entire con-
trast in the composition and colour of the intrusive and invaded
rocks, in these cases, renders the phenomena peculiarly clear
and interesting.

Another of the dikes of the north-east of Ireland has con-
verted a mass of red sandstone into hornstone4 By another,
the slate elay of the coal measures has been indurated, and has
assumed the character of flinty slate ; § and in another place the
slate clay of the lias has been changed into flinty slate, which
still retains numerous impressions of ammonites.||

It might have been anticipated that beds of coal would, from
their combustible nature, be affected in an extraordinary degree
•by the contact of melted rock. Accordingly, one of the green-
stone dikes of Antrim, on passing through a bed of coal, re-
duces it to a cinder for the space of nine feet on each side.lT

At Cockfield Fell, in the north of England, a similar change
is observed. Specimens taken at the distance of about thirty
yards from the trap are not distinguishable from ordinary pit
coal ; those nearer the dike are like cinders, and have all the
* . . .

* Dr. Berger, Geol. Trans., First Series, vol. iii. p. 172.

t Geol. Trans., First Series, vol. iii. p. 210. and plate 10.

t Ibid. p. 201. $ Ibid. p. 205.

II Ibid. p. 213. ; and Playfair, Illust of Hutt Theory, s. 253. T Ibid. p. 206.

PART I. CHAPTER VIII. 109

Rocks altered by Trap Dikes.

character of coke ; while those close to it are converted into a
substance resembling soot.*

As examples might be multiplied without end, I shall merely
select one or two others, and then conclude. The rock of Stir-
ling Castle is a calcareous sandstone, fractured, and forcibly dis-
placed by a mass of greenstone, which has evidently invaded
the strata in a melted state. The sandstone has been indurated,
and has assumed a texture approaching to hornstone near the
junction. In Arthur's Seat and Salisbury Craig, near Edin-
burgh, a sandstone which comes in contact with greenstone, is
converted into a jaspideous rock.f

The secondary sandstones in Sky are converted into solid
quartz in several places, where they come in contact with veins
or masses of trap ; and a bed of quartz, says Dr. MacCulloch,
found near a mass of trap, among the coal strata of Fife, was
in all probability a stratum of ordinary sandstone, having been
subsequently indurated and turned into quartzite by the action
of heat4

But although strata in the neighbourhood of dikes are thus
altered in a variety of cases, shale being turned into flinty slate
or jasper, limestone into crystalline marble, sandstone into
quartz, coal into coke, and the fossil remains of all such strata
wholly or in part obliterated, it is by no means uncommon to
meet with the same rocks, even in the same districts, absolutely
unchanged in the proximity of volcanic dikes.

This great inequality in the effects of the igneous rocks may
often arise from an original difference in their temperature, and
in that of the entangled gases, such as is ascertained to prevail
in different lavas, or in the same lava near its source and at a
distance from it. The power also of the invaded rocks to con-
duct heat may vary, according to their composition, structure,
and the fractures which they may have experienced, and per-
haps, also, according to the quantity of water (so capable of
being heated) which they contain. It must happen in some
cases that the component materials are mixed in such propor-
tions as prepare them readily to enter into chemical union, and
form new minerals ; while in other cases the mass may be more
homogeneous, or the proportions less adapted for such union.

We must also take into consideration, that one fissure may be
simply filled with lava, which may begin to cool from the first ;

* Sedgwick, Camb. Trans., vol. ii. p. 37.

t Illust. of Hutt. Theory, § 253. and 261. Dr. MacCulloch, Geol. Trans., First
Series, vol. ii. p. 305.

J Syst. of Geol., vol. i. p. 206.
K

110 LYELL'S ELEMENTS OF GEOLOGY.

Forcibly intruded Trap.

whereas in other cases the fissure may give passage to a current
of melted matter, which may ascend for days or months, feed-
ing streams which are overflowing the country above, or are
ejected in the shape of scoriae from some crater. If the walls
of a rent, moreover, are heated by hot vapour before the lava
rises, as we know may happen on the flanks of a volcano, the
additional caloric supplied by the dike and its gases will act
more powerfully.

Intrusion of trap between strata.— In proof of the mechanical
force which the fluid trap has sometimes exerted on the rocks
into which it has intruded itself, I may 'refer to the Whin-Sill,
where a mass of basalt, from sixty to eighty feet in height, re-
presented by a, Fig. 97., is in part wedged in between the rocks
of limestone, b, and shale, c, which have been separated from
the great mass of limestone and shale, d, with which they were
united.

Fig. 97.

Trap interposed between displaced beds of limestone and shale, at White, Force, High
Teesdale, Durham. (Sedgwick.)*

The shale in this place is indurated ; and the limestone, which
at a distance from the trap is blue, and contains fossil corals, is
here converted into granular marble without fossils.

Masses of trap are not unfrequently met with intercalated be-
tween strata, and maintaining their parallelism to the planes of
stratification throughout large areas. They must in some places
have forced their way laterally between the divisions of the strata,
a direction in which there would be the least resistance to an ad-
vancing fluid, if no vertical rents communicated with the surface,
and a powerful hydrostatic pressure was caused by gases pro-
pelling the lava upwards.

Columnar and globular structure. — One of the characteristic
forms of volcanic rocks, especially of basalt, is the columnar,
where large masses are divided into regular prisms, sometimes
separable, but in other cases adhering firmly together. The
columns vary in the number of angles, from three to twelve ;

*Gamb. Trans., vol. ii. p. 180.

PART I. CHAPTER VIII.

Ill

Columnar Structure of Volcanic Rocks.

but they have most commonly from five to seven sides. They
are often divided transversely, at nearly equal distances, like the
joints in a vertebral column, as in the Giant's Causeway, in Ire-
land. They vary exceedingly in respect to length and diameter.
Dr. MacCulloch mentions some in Sky which are about 400 feet
long ; others, in Morven, not exceeding an inch. In regard to
diameter, those of Ailsa measure nine feet, and those of Morven
an inch or less.* They are usually straight, but sometimes
curved ; and examples of both these occur in the island of StafFa.
In a horizontal bed or sheet of trap the columns are vertical ; in
a vertical dike they are horizontal. Among other examples of
the last-mentioned phenomenon is the mass of basalt, called the
Chimney, in St. Helena (See Fig.
98.), a pile of hexagonal prisms, 64
feet high, evidently the remainder
of a narrow dike, the walls of rock
which the dike originally traversed
having been removed down to the
level of the sea. In Fig. 99. a small
portion of this dike is represented on
a less reduced scale, f

It being assumed that columnar
trap has consolidated from a fluid
state, the prisms are said to be al-
ways at right angles to the cooling
surfaces. If these surfaces, there-

Fig. 98.

Volcanic dike composed of horizontal fore, instead of being either perpen-
prisms. St. Helena. dicular or horizontal, are curved, tho

Fig. 99.

columns ought to be inclined at every angle
to the horizon ; and there is a beautiful ex-
emplification of this phenomenon in one of
the valleys of the Vivarais, a mountainous
district in the South of France, where, in the
midst of a region of gneiss, a geologist en-
counters unexpectedly several volcanic cones
of loose sand and scorise. From the crater
of one of these cones, called La Coupe
d'Ayzac, a stream of lava descends and occupies the bottom of
a narrow valley, except at those points where the river Volant,
or the torrents which join it, have cut away portions of the solid
lava. The accompanying sketch (Fig. 100.) represents the rem-
nant of the lava at one of the points where a lateral torrent joins

Small portion of the dike
Fig. 98.

* MacCulloch, Syst. of Geol. vol. ii. p. 137.
t Scale's Geognosy of St. Helena, plate 9.

112

LYELL'S ELEMENTS OF GEOLOGY.

Columnar Structure of Volcanic Rocks.

Fig. 100.

Lava of La Coupe d'rfyzac, near dntraigue, in the Province of Jirdechc.

the main valley of the Volant. It is clear that the lava once
filled the whole valley up to the dotted line da,' but the river
has gradually swept away all below that line, while the tributary
torrent has laid open a transverse section ; by which we per-
ceive, in the first place, that the lava is composed, as usual in
this country, of three parts ; the uppermost at a, being scoriace-
ous ; the second, 6, presenting irregular prisms ; and the third,
c, with regular columns, which are vertical on the banks of the
Volant, where they rest on a horizontal base of gneiss, but which
are inclined at an angle of 45° at g, and then horizontal at f,
their position having been every where determined, according to
the law before mentioned, by the concave form of the original
valley.

In the annexed figure (101.) a
view is given of some of the in-
clined and curved columns which
present themselves on the sides of
the valleys in the hilly region north
of Vicenza, in Italy, and at the foot
of the higher Alps.* Unlike those
of the Vivarais, last mentioned, the
basalt of this country was evidently
submarine, and the present val-
leys have since been hollowed out
by denudation.

The columnar structure is by no
means peculiar to the trap rocks in
which hornblende or augite predo-
minate ; it is also observed in clink-
stone, trachyte, and other felspathic rocks of the igneous class,

Fig. 101.

Columnar basalt in the Vicentin.
(Fortis.)

* Fortia, Mem. sur 1'Hist Nat. de 1'Italie, torn. i. p. 233. plate 7.

PART I. CHAPTER VIII.

113

Columnar and Globular Structure.

although in these it is rarely exhibited in such regular polygonal
forms.

It has been already stated that basaltic columns are often di-
vided by cross joints. Sometimes each segment, instead of an
angular, assumes a spheroidal form, so that a pillar is made up
of a pile of balls, usually flattened, as in the Cheese-grotto at Ber-
trich-Baden, in the Eifel, near the Moselle. (Fig. 102.) The

Fig. 102.

Basaltic pillars of the Kiisegrotte, Bertrich- Baden, halfway between Trevet and
Coblentz. Height of grotto from 7 to 8 feet.

basalt, there, is part of a small stream of lava, from 30 to 40
feet thick, which has proceeded from one of several volcanic cra-
ters, still extant, on the neighbouring heights. The position of
the lava bordering the river in this valley, might be represented
by a section like that already given (Fig. 100. p. 112.), if we
merely suppose inclined strata of slate and the argillaceous sand-
stone called greywacke to be substituted for gneiss.

In some masses of decomposing greenstone, basalt, and other
trap rocks, the globular structure is so conspicuous that the rock
has the appearance of a heap of large cannon-balls.

A striking example of this structure occurs in a resinous tra-
chyte or pitchstone-porphyry in one of the Ponza islands, which
rise from the Mediterranean, off the coast of Terracina and
Gaieta. The globes vary from a few inches to three feet in di-
ameter, and are of an ellipsoidal form. (See Fig. 103.) The
whole rock is in a state of decomposition, " and when the balls,"
says Mr. Scrope, " have been exposed a short time to the wea-
ther, they scale off at a touch into numerous concentric coats,
like those of a bulbous root, inclosing a compact nucleus. The

114

LYELL'S ELEMENTS OF GEOLOGY.

Relation of Trap, Lava, and Scoriae.

Fig. 103.

Globiform pitchstone. Chiaja di Luna,
Isle of Ponza. (Scrope.)

laminee of this nucleus have not been
so much loosened by decomposition ;
but the application of a ruder blow will
produce a still further exfoliation.*
A fissile texture is occasionally
assumed by clinkstone and other
trap rocks, so that they have been
used for roofing houses. Some-
times the prismatic and slaty struc-
ture is found in the same mass.
The causes which give rise to such
arrangements are very obscure, but
are supposed to be connected with
changes of temperature during the
cooling of the mass, as will be
pointed out in the sequel. (See
Chap. X.)

Relation of trappean rocks to the
products of active volcanos.

I When we reflect on the changes
above described in the strata near their contact with trap dikes,
and consider how great is the analogy in composition and struc-
ture of the rocks called trappean and the lavas of active volca-
nos, it seems difficult at first to understand how so much doubt
could have prevailed for half a century as to whether trap was
of igneous or aqueous origin. To a certain extent, however,
there was a real distinction between the trappean formations and
those to which the term volcanic was almost exclusively con-
fined. The trappean rocks first studied in the north of Germany,
and in Norway, France, Scotland, and other countries, were
either such as had been formed entirely under deep water, or had
been injected into fissures and intruded between strata, and which
had never flowed out in the air, or over the bottom of a shallow
sea. When these products, therefore, of submarine or subterra-
nean igneous action were contrasted with loose cones of scoriae,
tuff, and lava, or with narrow streams of lava in great part sco-
riaceous and porous, such as were observed to have proceeded
from Vesuvius and Etna, the resemblance seemed remote and
equivocal. It was, in truth, like comparing the roots of a tree
with its leaves and branches, which, although they belong to the
same plant, differ in form, texture, colour, mode of growth, and
position. The external cone, with its loose ashes and porous

* Scrope, Gcol. Trans, vol. ii. p. 205. Second Series.

PART I. CHAPTER VIII. 115

Relation of Trap, Lava, and Scorise.

lava, may be likened to the light foliage and branches, and the
rocks concealed far below, to the roots. But it is not enough to
say of the volcano,

" quantum vertice in auras
" ^Etherias, tantum radice in Tartara tendit,"

for its roots do literally reach downwards to Tartarus, or to the
regions of subterranean fire ; and what is concealed far below,
is probably always more important in volume and extent than
what is visible above ground.

We have already stated how frequently dense masses of strata
have been removed by denudation from wide areas- (see Chap.
VI.) ; and this fact prepares us to expect a similar destruction of
whatever may once have formed the uppermost part of ancient
submarine or subaerial volcanos, more especially as those super-
ficial parts are always of the lightest and most perishable mate-
rials. The abrupt manner in which dikes of trap usually termi-

nate at the surface (see Fig. 104.),
and the water-worn pebbles of trap
m the alluvium which covers the
dike, prove incontestably that what-
ever was uppermost in these forma-
tions has been swept away. It is
easy, therefore to conceive that
what is gone in regions of trap may
have corresponded to what is now

Strata intersected by a trap dike, and visible in active VOlcattOS. .

covered with alluvium. It will be shown in the second part

of this volume, that in the earth's crust there are volcanic tuffs
of all ages, containing marine shells, which bear witness to erup-
tions at many successive geological periods. These tuffs, and the
associated trappean rocks, must not be compared to lava and sco-
ria3 which had cooled in the open air. Their counterparts must
be sought in the products of modern submarine volcanic eruptions.
If it be objected that we have no opportunity of studying these last,
it may be answered, that subterranean movements have caused,
almost everywhere in regions of active volcanos, great changes in
the relative level of land and sea, in times comparatively modern,
so as to expose to view the effects of volcanic operations at the
bottom of the sea.

Thus, for example, the recent examination of the igneous
rocks of Sicily, especially those of the Val di Noto, has proved
that all the more ordinary varieties of European trap have been
there produced under the waters of the sea, at a modern period ;

116 LYELL'S ELEMENTS OF GEOLOGY.

Relation of Trap, Lava, and Scoriae.

that is to say, since the Mediterranean has been inhabited by a
great proportion of the existing species of testacea.

These igneous rocks of the Val di Noto, and the more ancient
trappean rocks of Scotland and other countries, differ from sub-
aerial volcanic formations in being more compact and heavy,
and in forming sometimes extensive sheets of matter intercalated
between marine strata, and sometimes stratified conglomerates,
of which the rounded pebbles are all trap. They differ also in
the absence of regular cones and craters, and in the want of con-
formity of the lava to the lowest levels of existing valleys.

It is highly probable, however, that insular cones did exist in
some parts of the Val di Noto ; and that they were removed by
the waves, in the same manner as the cone of Graham Island,
in the Mediterranean, was swept away in 1831, and that of
Nyoe, off Iceland, in 1783. All that would remain in such
cases, after the bed of the sea has been upheaved and laid
dry, would be dikes and shapeless masses of igneous rock, cut-
ting through sheets of lava, which may have spread over the
level bottom of the sea, and strata of tuff, formed of materials
first scattered far and wide by the winds and waves, and then
deposited. Trap conglomerates also, to which the action of the
waves must give rise during the denudation of such volcanic
islands, will emerge from the deep whenever the bottom of the
sea becomes land.*

The proportion of volcanic matter which is originally subma-
rine must always be very great, as those volcanic vents which
are not entirely beneath the sea, are almost all of them in
islands, or, if on continents, near the shore. This may explain
why extended sheets of trap so often occur, instead of narrow
threads, like lava streams. For, a multitude of causes tend, near
the land, to reduce the bottom of the sea to a nearly uniform
level, — the sediment of rivers, — materials transported by the
waves and currents of the sea from wasting cliffs, — showers of
sand and scoriae ejected by volcanos, and scattered by the wind
and waves. When, therefore, lava is poured out on such a sur-
face, it will spread far and wide in every direction in a liquid
sheet, which may afterwards, when raised up, form the tabular
capping of the land.

As to the absence of porosity in the trappean formations, the
appearances are in a great degree deceptive, for all amygdaloids

* See Principlesof Geology, Index, " Graham Island," " Nyb'e," " Conglome-
rates, volcanic." &c.

PART I. CHAPTER VIII. 117

Volcanic Rocks.

are, as already explained, porous rocks, into the cells of which
mineral matter, such as silex, carbonate of lime, and other in-
gredients, have been subsequently introduced. (See p. 97.)

In the little Cumbray, one of the Western Islands, near Arran,
the amygdaloid sometimes contains elongated cavities filled with
brown spar ; and when the nodules have been washed out, the
interior of the cavities is glazed with the vitreous varnish so cha-
racteristic of the pores of slaggy lavas. Even in some parts of
this rock which are excluded from air and water, the cells are
empty, and seem to have always remained in this state, and are
therefore undistinguishable from some modern lavas.*

Dr. MacCulloch, after examining with great attention these
and the other igneous rocks of Scotland, observes, " that it is a
mere dispute about terms, to refuse to the ancient eruptions of
trap the name of submarine volcanos ; for they are such in every
essential point, although they no longer eject fire and smoke."")"
The same author also considers it not improbable that some of
the volcanic rocks of the same country may have been poured
out in the open air.:}:

Although the principal component minerals of subaerial lavas
are the same as those of intrusive trap, and both the columnar
and globular structure are common to both, there are, neverthe-
less, some volcanic rocks which never occur as lava, such as
greenstone, clinkstone, the more crystalline porphyries, and all
those traps in which quartz and mica frequently appear as constitu-
ent parts. In short the intrusive trap rocks, forming the intermedi-
ate step between lava and the plutonic rocks, depart in their char-
acters from lava in proportion as they approximate to granite.

These views respecting the relations of the volcanic and trap
rocks will be better understood, when the reader has studied, in
the next chapter, what is said of the plutonic formations.

* MacCulloch, West. Isl., vol. ii. p. 487.

t Syst. of Geol., vol. ii. p. 114. t Ibid.

118 LYELL'S ELEMENTS OF GEOLOGY.

Plutonic Rocks.

CHAPTER IX.

PLUTONIC ROCKS — GRANITE.

General aspect of granite — Decomposing into spherical masses — Rude colum-
nar structure — Analogy and difference of volcanic and plutonic formations —
Minerals in granite, and their arrangement — Graphic and porphyritic granite —
Occasional minerals — Syenite — Syenitic, talcose, and schorly granites — Eurite —
Passage of granite into trap — Examples near Christiania and in Aberdeenshire —
Analogy in composition of trachyte and granite — Granite veins in Glen Tilt,
Cornwall, the Valorsine, and other countries — Different composition of veins
from main body of granite — Metalliferous veins in strata near their junction with
granite — Apparent isolation of nodules of granite — Quartz veins — Whether plu-
tonic rocks are ever overlying — Their exposure at the surface due to denuda-
tion.

THE plutonic rocks may be treated of next in order, as they
are most nearly allied to the volcanic class, already considered.
I have described, in the first chapter, these plutonic rocks as the
unstratified division of the crystalline or hypogerie formations,
and have endeavoured to point out in the Frontispiece, at D, the
position which they occupy, when first formed, relatively to the
volcanic formations, B.

By some writers all the rocks now under consideration have
been comprehended under the name of granite, which is, then,
understood to embrace a large family of crystalline and corn-
Fig. 105.

Mass of granite near the Sharp Tor, Cornwall.

pound rocks, usually found underlying all other formations;
whereas we have seen that trap very commonly overlies strata
of different ages. Granite often preserves a very uniform char-
acter throughout a wide range of territory, forming hills of a pe-

PART I. CHAPTER IX.

119

General Aspect of Granite.

culiar rounded form, usually clad with a scanty vegetation. The
surface of the rock is for the most part in a crumbling state, and
the hills are often surmounted by piles of stones like the remains
of a stratified mass, as in the annexed figure, and sometimes like
heaps of boulders, for which they have been mistaken. The ex-
terior of these stones, originally quadrangular, acquires a round-
ed form by the action of air and water, for the edges and angles
waste away more rapidly than the sides. A similar spherical
structure has already been described as characteristic of basalt,
and other volcanic formations, and it must be referred to analo-
gous causes, as yet but imperfectly understood.

Although it is the general peculiarity of granite to assume no
definite shapes, it is nevertheless occasionally subdivided by fis-
sures, so as to assume a cuboidal, and even a columnar, struc-
ture. Examples of these appearances may be seen near the
Land's End, in Cornwall, (see figure.)

Fig. 106.

Granite having a cuboidal and rude columnar structure.
Land's End, Cornwall.

The plutonic formations also agree with the volcanic, in hav-
ing veins or ramifications proceeding from central masses into
the adjoining rocks, and causing alterations in these last, which
will be presently described. They also resemble trap in con-
taining no organic remains ; but they differ in being more uni-

120

LYELL'S ELEMENTS OF GEOLOGY.

Mineral Composition of Granitic Rocks.

form in texture, whole mountain masses of indefinite extent ap-
pearing to have originated under conditions precisely similar.
But they differ in never being scoriaceous or amygdaloidal, in
never forming a porphyry with an uncrystalline base, and never
alternating with tuffs. Nor do they form conglomerates, al-
though there is sometimes an insensible passage from a fine to a
coarse grained granite, and occasionally patches of a fine tex-
ture are imbedded in a coarser variety.

Felspar, quartz, and mica are usually considered as the mine-
rals essential to granite, the felspar being most abundant in quan-
tity, and the proportion of quartz exceeding that of mica. These
minerals are united in what is termed a confused crystallization ;
that is to say, there is no regular arrangement of the crystals in
granite as in gneiss (see Fig. 107.), except in the variety termed

Fig. 107.

Gneiss. (See description, ji. 132.)

graphic granite, which occurs mostly in granitic veins. This
variety is a compound of felspar and quartz, so arranged as to
produce an imperfect laminar structure. The crystals of felspar

Fig. 108.

Fig. 109.

OrapJiic granite.

• Fig. 108. Section parallel to the laminae.
Fig. 109. Section transverse to the laming.

appear to have been first formed, leaving between them the space
now occupied by the darker coloured quartz. This mineral,
when a section is made at right angles to the alternate plates of

PART I. CHAPTER IX. 121

Porphyritic Granite Syenite.

felspar and quartz, presents broken lines, which have been com-
pared to Hebrew characters.

Porphyritic granite. — This name has been sometimes given
to that variety in which large crystals of felspar, sometimes more
than an inch in length, are scattered through an ordinary base
of granite. An example of this texture may be seen in the gra-
nite of the Land's End, in Cornwall. (Fig. 110.) The two

Fig. 110.

Porphyritic granite, Land's End, Cornwall.

larger prismatic crystals in this drawing represent felspar,
smaller crystals of which are also seen, similar in form, scattered
through the base. In this base also appear black specks of mica,
the crystals of which have a more or less perfect hexagonal out-
line. The remainder of the mass is quartz, the translucency of
which is strongly contrasted to the opaqueness of the white fel-
spar and black mica. But neither this transparency of the
quartz, nor the silvery lustre of the mica, can be expressed in
the engraving.

The uniform mineral character of large masses of granite
seems to indicate that large quantities of the component elements
were thoroughly mixed up together, and then crystallized under
precisely similar conditions. There are, however, many acci-
dental, or " occasional," minerals, as they are termed, which be-
long to granite. Among these black schorl or tourmaline, acti-
nolite, zircon, garnet, and fluor spar, are not uncommon ; but
they are too sparingly dispersed to modify the general aspect of
the rock. They show, nevertheless, that the ingredients were
not every where exactly the same ; and a still greater variation
may be traced in the ever-varying proportions of the felspar,
quartz, and mica.

Syenite. — When hornblende is the substitute for mica, which
is very commonly the case, the rock becomes Syenite : so called
from the celebrated ancient quarries of Syene in Egypt. It has
all the appearance of ordinary granite, except when mineralogi-

122 LYELL'S ELEMENTS OF GEOLOGY.

Syenitic Granite .... Takose Granite .... Schorl Rock .... Eurite .... Pegmatite.

cally examined in hand specimens, and being fully entitled to
rank as a geological member of the same plutonic family as
granite. Syenite, however, after maintaining the granitic cha-
racter throughout extensive regions, is not uncommonly found
to lose its quartz, and to pass insensibly into syenitic-greenstone,
a rock of the trap family.

Syenitic-granite. — The quadruple compound of quartz, fel-
spar, mica, and hornblende, may be so termed. This rock occurs
in Scotland and in Guernsey.

Talcose granite, or Protogine of the French, is a mixture of
felspar, quartz, and talc. It abounds in the Alps, and in some
parts of Cornwall, producing by its decomposition the china clay,
more than 12,000 tons of which are annually exported from that
county for the potteries.*

Schorl rock, and schorly granite. — The former of these is
an aggregate of schorl, or tourmaline, and quartz. When fel-
spar and mica are also present, it may be called schorly granite.
This kind of granite is comparatively rare.

Eurite. — A rock in which all the ingredients of granite are
blended into a finely granular mass. Crystals of quartz and
mica are sometimes scattered through the base of Eurite.

Pegmatite. — A name given by the French writers to a variety
of granite ; a granular mixture of quartz and felspar ; frequent
in granite veins ; passes into graphic granite.

All these granites pass into certain kinds of trap, a circum-
stance which affords one of many arguments in favour of what
is now the prevailing opinion, that the granites are also of igneous
origin. The contrast of the most crystalline form of granite, to
that of the most common and earthy trap, is undoubtedly great ;
but each member of the volcanic class is capable of becoming
porphyritic, and the base of the porphyry may be more and
more crystalline, until the mass passes to the kind of granite
most nearly allied in mineral composition.

The minerals which constitute alike the granitic and volcanic
rocks, consist, almost exclusively, of seven elements, namely,
silica, alumina, magnesia, lime, soda, potash, and iron ; and these
may sometimes exist in about the same proportions in a porous
lava, a compact trap, or a crystalline granite. It may perhaps
be found, on farther examination, for on this subject we have yet
much to learn, that the presence of these elements in certain pro-
portions is more favourable than in others to their assuming a
crystalline or true granitic structure; but it is also ascertained
by experiment, that the same materials may, under different cir-

* Boase on Primary Geology, p. 16.

PART I. CHAPTER IX. 123

Passage of Granite into Trap Granite Veins.

cumstances, form very different rocks. The same lava, for
example, may be glassy, or scoriaceous, or stony, or porphyritic,
according to the more or less rapid rate at which it cools ; and
some trachytes and syenitic-greenstones may doubtless form gra-
nite and syenite, if the crystallization take place slowly.

It would be easy to multiply examples and authorities to prove
the gradation of the granitic into the trap rocks. On the western
side of the fiord of Christiania, in Norway, there is a large dis-
trict of trap, chiefly greenstone-porphyry, and syenitic-green-
stone, resting on fossiliferous strata. To this, on its southern
limit, succeeds a region equally extensive of syenite, the passage
from the volcanic to the plutonic rock being so gradual that it is
impossible to draw a line of demarcation between them.

" The ordinary granite of Aberdeenshire," says Dr. MacCul-
loch, " is the usual ternary compound of quartz, felspar, and
mica ; but sometimes hornblende is substituted for the mica. But
in many places a variety occurs which is composed simply of
felspar and hornblende ; and in examining more minutely this
duplicate compound, it is observed in some places to assume a
fine grain, and at length to become undistinguishable -from the
greenstones of the trap family. It also passes in the same unin-
terrupted manner into a basalt, and at length into a soft clay-
stone, with a schistose tendency on exposure, in no respect
differing from those of the trap islands of the western coast."*
The same author mentions, that in Shetland, a granite composed
of hornblende, mica, felspar, and quartz, graduates in an equally
perfect manner into basalt, f

In Hungary there are varieties of trachyte, which, geologi-
cally speaking, are of modern origin, in which crystals, not only
of mica, but of quartz, are common, together with felspar and
hornblende. It is easy to conceive how such volcanic masses
may, at a certain depth from the surface, pass downwards into
granite.

I have already hinted at the close analogy in the forms of cer-
tain granitic and trappean veins ; and it will be found that strata
penetrated by plutonic rocks have suffered changes very similar
to those exhibited near the contact of volcanic dikes. Thus, in
Glen Tilt, in Scotland, alternating strata of limestone and argil-
laceous schist come in contact with a .mass of granite. The
contact does not take place as might have been looked for, if the
granite had been formed there before the strata were deposited,
in which case, the section would have appeared as in Fig. 111.;
but the union is as represented in Fig. 112., the undulating .out-
line of the granite intersecting different strata, and occasionally

* Sy«t. of Geol., vol. i. p. 157. t Ibid., p. 158.

124

LYELL'S ELEMENTS OF GEOLOGY.

Rocks altered by Granite Veins.

Fig. 111.

Junction of granite and argillaceous schist in Glen
Tilt. (MacCulloch.)*

intruding itself in tortuous veins into the beds of clay-slate and
limestone, from which it differs so remarkably in composition.
The limestone is sometimes changed in character by the prox-

Fig. 113.

Junction of granite and limestone in Glen Tilt.
a. Granite. b. Limestone.

c. Blue argillaceous schist.

imity of the granitic mass or its veins, and acquires a more com-
pact texture, like that of hornstone or chert, with a splintery frac-
ture, effervescing feebly with acids.

* Geol. Trans., First Series, vol. iii. pi. 21.

PART I. CHAPTER IX.

125

Rocks altered by Granite Veins.

The annexed diagram (Fig. 113.) represents another junction,
in the same district, where the granite sends forth so many veins
as to reticulate the limestone and schist, the veins diminishing
towards their termination to the thickness of a leaf of paper or a
thread. In some places fragments of granite appear entangled,
as it were, in the limestone, and are not visibly connected with
any larger mass ; while sometimes, on the other hand, a lump
of the limestone is found in the midst of the granite. The ordi-
nary colour of the limestone of Glen Tilt is lead blue, and its
texture large-grained and highly crystalline ; but where it ap-
proximates to the -granite, particularly where it is penetrated by
the smaller veins, the crystalline texture disappears, and it as-
sumes an appearance exactly resembling that of hornstone. The
associated argillaceous schist often passes into hornblende-slate,
where it approaches very near to the granite.*

The conversion of the limestone in these and many other
instances into a siliceous rock, effervescing slowly with acids,
would be difficult of explanation, were it not ascertained that such
limestones are always impure, containing grains of quartz, mica,
or felspar disseminated through them. The elements of these
minerals, when the rock has been subjected to great heat, may
have been fused, and so spread more uniformly through the
whole mass.

In the plutonic, as in the volcanic rocks, there is every grada-
tion from a tortuous vein to the most regular form of a dike, such
as intersect the tuffs and lavas of Ve-
suvius and Etna. Dikes of granite may
be seen, among other places, on the
southern flank of Mount Battock, one
of the Grampians, the opposite walls
sometimes preserving an exact paral-
lelism for a considerable distance.

As a general rule, however, granite
veins in all quarters of the globe are
more sinuous in their course than those
of trap. They present similar shapes
at the most northern point of Scotland,
and the southernmost extremity of Af-
^ rica, as the annexed drawing will show.

Granite veins traversing clay It is not uncommon for One set of

Cape°f granite veins to intersect another; and
sometimes -there are three sets, as in

* MacCulloch, Geol. Trans., vol. iii. p. 259.

t Capt. B. Hall, Trans. Roy. Soc. Edin., vol. vii.

126

LYELL'S ELEMENTS OF GEOLOGY.

Granite Veins.

the environs of Heidelberg, where the granite on the hanks of
the river Necker is seen to consist of three varieties, differing in
colour, grain, and various peculiarities of mineral composition.
One of these, which is evidently the second in age, is seen to cut
through an older granite ; and another, still newer, traverses
both the second and the first.

In Shetland there are two kinds of granite. One of them,
composed of hornblende, mica, felspar, and quartz, is of a dark
colour, and is seen underlying gneiss. The other is a red gra-
nite, which penetrates the dark variety every where in veins.*

The accompanying sketches will explain the manner in which
granite veins often ramify and cut each other. (Figs. 115. and

116.) They represent the
manner in which the gneiss
at Cape Wrath, in Suther-
landshire, is intersected by
veins. Their light colour,
strongly contrasted with that
of the hornblende-schist,
here associated with the
gneiss, renders them very

Fig. 115.

conspicuous.

Granite very generally
assumes a finer grain, and
undergoes a change in min-
eral composition, in the veins
which it sends into contigu-
ous rocks. Thus, according to Professor Sedgwick, the main
body of the Cornish granite is an aggregate of mica, quartz, and

Fig. 116.

Granite veins traversing gneiss, Cape Wrath.
(MacCultoch.)t

Granite veins traversing gneiss at Cape Wrath, in Scotland. (MacCulloch.)

felspar ; but the veins are sometimes without mica, being a gra-

* MacCulloch, Syst. of Geol., vol. i. p. 58.
t Western Islands, pi. 31.

PART I. CHAPTER IX.

127

Mineral Structure of Granite in Veins.

nular aggregate of quartz and felspar. In other varieties quartz
prevails to the almost entire exclusion both of felspar and mica ;
in others, the mica and quartz both disappear, and the vein is
simply composed of white granular felspar.*

Fig. 117.

Granite veins passing through hornblende slate, Carnsilver Cove, Cornwall.

Fig. 117. is a sketch of a group of granite veins in Cornwall,
given by Messrs. Von Oeynhausen and Von Dechen.f The
main body of the granite here is of a porphyritic appearance,
with large crystals of felspar ; but in the veins it is fine-grained,
and without these large crystals. The general height of the

veins is from six-
Fig. 118. ,,////////////!& teen to twenty feet,

but some are much
higher.

In the Valorsine,
a valley not far
from Mont Blanc,
in Switzerland, an
ordinary granite,
consisting of fel-

reins of granite talcose gneiss. (L. A. Necker.) sparj quartZ) and

mica, sends forth veins into a talcose gneiss, (or stratified proto-
gine,) and in some places lateral ramifications are thrown off
from the principal veins at right angles (see Fig. 118.) the veins,
especially the minuter ones, being finer grained than the granite
in mass.

It is here remarked, that the schist and granite, as they approach,
seem to exercise a reciprocal influence on each other, for both

* On Geol. of Cornwall, Trans, of Cambridge Soc. vol. i. p. 124.
t Phil. Mag. and Annals, No. 27. New Series, March, 1829.

128

LYELL'S ELEMENTS OF GEOLOGY.

Metals near Granite Isolated Masses of Granite.

undergo a modification of mineral character. The granite, still
remaining unstratified, becomes charged with green particles ;
and the talcose gneiss assumes a granitiform structure, without
losing its stratification.*

Professor Keilhau drew my attention to several localities in the
country near Christiania, where the mineral character of gneiss
appears to have been affected by a granite of much newer origin,
for some distance from the point of contact. The gneiss, with-
out losing its laminated structure, seems to have become charged
with a larger quantity of felspar, and that of a redder colour,
than the felspar usually belonging to the gneiss of Norway.

Granite, syenite, and those porphyries which have a graniti-
form structure, in short all plutonic rocks, are frequently observed
to contain metals, at or near their junction with stratified forma-
tions. On the other hand, the veins which traverse stratified
rocks are, as a general law, more metalliferous near such junc-
tions than in other positions. Hence it has been inferred that
these metals may have been spread in a gaseous form through
the fused mass, and that the contact of another rock, in a
different state of temperature, or sometimes the existence of rents
in other rocks in the vicinity, may have caused the sublimation
of the metals.f

There are many instances, as at Markerud, near Christiania,
in Norway, where the strike of the beds has not been deranged
throughout a large area by the intrusion of granite, both in large
masses and in veins. This fact is considered by some geologists
to militate against the theory of the forcible injection of granite in a

fluid state. But it may
Fig. 119. be stated in reply, that

ramifying dikes of
trap, which almost all
now admit to have
been once fluid, pass
through the same fos-
siliferous strata, near
Christiania, without
deranging their strike

ralorsine. (L. A. Necker.) or dip.}

The real or apparent isolation of large or small masses of
granite detached from the main body, as at ab, Fig. 119., and

General vieio of junction of granite and schist oftkc
(L.

* Necker, sur la Val. de Valorsine, Mem. de la Soc. de Phys. de Geneve, 1828.
—I visited, in 1832, the spot referred to in Fig. 118.
t Necker, Proceedings of Geol. Soc., No. 26. p. 392.
I See Keilhau's Gaea Norvegica ; Christiania, 1838.

PART I. CHAPTER IX.

129

Fig. 120.

gneiss.

gneiss.

quartz Veins Conformable Porphyries.

above, Fig. 113., and a, Fig. 118., has been thought by some
writers to be irreconcilable with the doctrine usually taught re-
specting veins ; but many of them may, in fact, be sections of
root-shaped prolongations of granite ; while, in other cases, they
may in reality be detached portions of rock having the plutonic
structure. For there may have been spots in the midst of the
invaded strata, in which there was an assemblage of materials
more fusible than the rest, or more fitted to combine readily into
some form of granite.

Veins of pure quartz are often found in granite, as in many
stratified rocks, but they are not traceable, like veins of granite
or trap, to large bodies of rock of similar composition. They
appear to have been cracks, into which siliceous matter was infil-
tered. Such segregation, as it is called, can sometimes be shown
to have clearly taken place long subsequently to the original con-
solidation of the containing rock. .Thus, for example, in the
gneiss of Tronstad Strand, near Drammen, in Norway, the an-
nexed section is seen on the beach. It appears that the alter-
nating strata of whitish
granitiform gneiss, and
black hornblende-schist,
were first cut through by
a greenstone dike, about
2 ^ feet wide; then the
crack a b passed through
all these rocks, and was
filled up with quartz. The
opposite walls of the vein
are in some parts incrust-
ed with transparent crys-
tals of quartz, the middle
of the vein being filled up
with common opaque white quartz.

We have seen that the volcanic formations have been called
overlying, because they not only penetrate others, but spread
over them. Mr. Necker has proposed to call the granites the
underlying igneous rocks, and the distinction here indicated is
highly characteristic. It was indeed supposed by Von Buch, at
the commencement of his geological career, that the granite of
Christiania, in Norway, was sometimes intercalated in mountain
masses between the transition strata of that country, overlying
fossiliferous shale and limestone. But although the granite sends
veins into these fossiliferous rocks, and is decidedly posterior in
origin, the opinion expressed of its actual superposition in mass
has been disproved by Professor Keilhau, some of whose ob-

a. b. Quartz vein passing through gneiss and green-
stone^ Tronstad Strand, near Christiania.

130

LYELL'S ELEMENTS OF GEOLOGY.

Conformable Porphyries .' Granitic Rocks.

servations respecting localities referred to by Von Buch, I have
lately had opportunities of verifying. There are, however, on
a smaller scale, certain beds of euritic porphyry > some a few
feet, others many yards in thickness, which pass into granite,
and deserve perhaps to be classed as plutonic rather than trap-
pean rocks, which may truly be described as interposed con-
formably between fossiliferous strata, as the porphyries (a c,
Fig. 121.), which divide the bituminous shales and argillaceous
limestones, ff. But some of these same porphyries are par-
Fig. 121.

Euritic porphyry alternating with fossiliferous transition strata, near*Christiania.

tially unconformable, as &, and may lead us to suspect that the
others also, notwithstanding their appearance of interstratifica-
tion, have been forcibly injected. Some of the porphyritic rocks
above mentioned are highly quartzose, others very felspathic.
In proportion as the masses are more voluminous, they become
more granitic in their texture, less conformable, and even begin
to send forth veins into contiguous strata. In a word, we have
here a beautiful illustration of the intermediate gradations be-
tween volcanic and plutonic rocks, not only in their mineralogical
composition and structure, but also in their relations of position
to associated formations. If the term overlying can in this in-
stance be applied to a plutonic rock, it is cnly in proportion as
that rock begins to acquire a trappean aspect.

It has been already hinted that the heat, which in every ac-
tive volcano extends downwards to indefinite depths, must pro-
duce simultaneously very different effects near the surface, and
far below it ; and we cannot suppose that rocks resulting from
the crystallizing of fused matter under a pressure of several
miles of the earth's crust can resemble those formed at or near
the surface. Hence the production at great depths of a class
of rocks analogous to the volcanic, and yet differing m many
particulars, might almost have been predicted, even had we no
plutonic formations to account for. How well these agree, both
in their positive and negative characters with the theory of their
deep subterranean origin, the student will be able to judge by
considering the descriptions already given.

It has, however, been objected, that if the granitic and vol-

PART I. CHAPTER X. 131

Granite Metamorphic Rocks.

canic rocks were simply'different parts of one great series, we
ought to find in mountain chains volcanic dikes passing upwards
into lava, and downwards into granite. But we may answer,
that our vertical sections are usually of small extent ; and if we
find in certain places a transition from trap to porous lava, and
in others a passage from granite to trap, it is as much as could
be expected of this evidence.

The prodigious extent of denudation which has been already
demonstrated to have occurred at former periods, will reconcile
the student to the belief, that crystalline rocks of high antiquity,
although deep in the earth's crust when originally formed, may
have become uncovered and exposed at the surface. Their ac-
tual elevation above the sea may be referred to the same causes
to which we have attributed the upheaval of marine strata, even
to the summits of some mountain chains. But to these and
other topics, I shall revert when speaking, in the second part,
of the relative ages of different masses of granite.

CHAPTER X.

METAMORPHIC ROCKS.

General character of metamorphic rocks — Gneiss — Hornblende-schist — Mica-
schist — Clay-slate — Quarlzite — Chlorite-schist — Metamorphic limestone — Alpha-
betical list and explanation of other rocks of this family — Origin of the meta-
morphic strata — Their stratification is real and distinct from cleavage — On joints
and slaty cleavage — Supposed causes of these structures — how far connected
with crystalline action.

WE have now considered three distinct classes of rocks : first,
the aqueous, or fossiliferous ; secondly, the volcanic; and,
thirdly, the plutonic, or granitic ; and we have now lastly to exa-
mine those crystalline strata to which the name of metamorphic
has been assigned. The last-mentioned term expresses, as before
explained, a theoretical opinion that such strata, after having
been dejBsited from water, acquired by the influence of heat and
other causes a highly crystalline texture.

These rocks, when in their most characteristic or normal state,
are wholly devoid of organic remains, and contain no distinct
fragments of other rocks whether rounded or angular. They
sometimes break out in the central parts of narrow mountain
chains, but in other cases extend over areas of vast dimensions,

132 LYELL'S ELEMENTS OF GEOLOGY.

Metamorphic Rocks Gneiss.

occupying, for example, nearly the whole of Norway and Swe-
den, where, as in Brazil, they appear alike in the lower and
higher grounds. In Great Britain those members of the series
which approach most nearly to granite in their composition* as
gneiss, mica-schist and hornblende-schist, are confined to the
country north of the rivers Forth and Clyde.

Many attempts have been made to trace a general order of
succession or superposition in the members of this family ; gneiss,
for example, having been often supposed to hold invariably a
lower geological position than mica-schist. But although such
an order may prevail throughout limited districts, it is by no
means universal, nor even general throughout the globe. To
this subject, however, we shall again revert in the second part of
this volume, when the chronological relations of the metamor-
phic rocks are pointed out.

The following may be enumerated as the principal members
of the metamorphic class, gneiss, mica-schist, hornblende-schist,
clay-slate, chlorite-schist, hypogene or metamorphic limestone,
and certain kinds of quartz rock or quartzite.

Gneiss. — The first of these, gneiss, may be called stratified
granite, being formed of the same materials as granite, namely
felspar, quartz, and mica. In the specimen here figured, the

Fig. 122.

Fragment of gneiss, natural size, section at right angles to planes
of stratification.

white layers consist almost exclusively of granular felspar, with
here and there a speck of mica and grain of quartz. The dark
layers are composed of grey quartz and black mica^vith oc-
casionally a grain of felspar intermixed. The rock sJR.ts most
easily in the plane of these darker layers, and the surface thus
exposed is almost entirely covered with shining spangles of mica.
The accompanying quartz however greatly predominates in
quantity, but the most ready cleavage is determined by the
abundance of mica in certain parts of the dark layer.

Instead of these thin laminae, gneiss is sometimes simply

PART I. CHAPTER X. 133

Hornblende-Schist Mica-Schist Clay-Slate Quartzite.

divided into thick beds, in which the mica has only a slight degree
of parallelism to the planes of stratification.

The term " gneiss," however, in geology is commonly used in
a wider sense to designate a formation in which the above men-
tioned rock prevails, but with which any one of the other meta-
morphic rocks, and more especially hornblende-schist, may alter-
nate. These other members of the metamorphic series are, in
this case, considered as subordinate to the true gneiss. In some
rare instances fragments of pre-existing rocks may be detected
in gneiss.

The different varieties of rock allied to gneiss, into which fel-
spar enters as an essential ingredient, will be understood by re-
ferring to what was said of granite. Thus, for example, horn-
blende may be superadded to mica, quartz, and felspar, forming
a syenitic gneiss ; or talc may be substituted for mica, consti-
tuting talcose gneiss, a rock composed of felspar, quartz, and
talc, in distinct crystals or grains (stratified protogine of the
French.)

Hornblende-schist is usually black, and composed principally
of hornblende, with a variable quantity of felspar, and some-
times grains of quartz. When the hornblende and felspar are
nearly -in equal quantities, and the rock is not slaty, it corre-
sponds in character with the greenstones of the trap family, and
has been called " primitive greenstone." Some of these horn-
blendic masses may really have been volcanic rocks, which
have since assumed a more crystalline or metamorphic texture.

Mica^schist, or micaceous schist, is, next to gneiss, one of the
most abundant rocks of the metamorphic series. It is slaty,
essentially composed of mica and quartz, the mica sometimes
appearing to constitute the whole mass. Beds of pure quartz
also occur in this formation. In some districts garnets in regu-
lar twelve-sided crystals form an integrant part of mica-schist.
This rock passes by insensible gradations into clay-slate.

Clay-slate, or Argillaceous schist. — This rock resembles an
indurated clay or shale, is for the most part extremely fissile,
often affording good roofing slate. It may consist of the ingre-
dients of gneiss, or of an extremely fine mixture of mica and
quartz, or talc and quartz. Occasionally it derives a shining
and sill^ lustre from the minute particles of mica or talc which
it contains. It varies from greenish or bluish-grey to a lead
colour. It may be said of this, more than of any other schist,
that it is common to the metamorphic and fossiliferous series,
for some clay-slates taken from each division would not be dis-
tinguishable by mineralogical characters.

Quartzite, or Quartz rock, is an aggregate of grains of

M

134 LYELL'S ELEMENTS OF GEOLOGY.

Chlorite-schist Hypogene Limestone Metamorphic Rocks.

quartz, which are either in minute crystals, or in many cases
slightly rounded, occurring in regular strata associated with
gneiss or other metamorphic rocks. Compact quartz, like that so
frequently found in veins, is also found together with granular
quartzite. Both of these alternate with gneiss or mica-schist, or
pass into those rocks by the addition of mica, or of felspar and
mica.

Chlorite-schist is a green slaty rock in which chlorite is
abundant in foliated plates, usually blended with minute grains
of quartz, or sometimes with felspar or mica. Often associated
with, and graduating into, gneiss and clay-slate.

Hypogene or metamorphic limestone. — This rock, commonly
called primary limestone, is sometimes a thick bedded white
crystalline granular marble used in sculpture ; but more fre-
quently it occurs in thin beds, forming a foliated schist much
resembling in colour and appearance certain varieties of gneiss
and mica-schist. It alternates with both these rocks, and in like
manner with argillaceous schist. It then usually contains some
crystals of mica, and occasionally quartz, felspar, hornblende,
and talc. This member of the metamorphic series enters
sparingly into the structure of the hypogene districts of Nor-
way, Sweden, and Scotland, but is largely developed in the Alps.

Before offering any further observations on the probable origin
of the metamorphic rocks, I subjoin in the form of a glossary,
a brief explanation of some of the principal varieties and their
synonymes.

ACTINOLJTE-SCHIST. A slaty foliated rock, composed chiefly of actinolite, (an
emerald-green mineral, allied to hornblende,) with some admixture of fel-
spar, or quartz, or mica.

AMPELITE. Aluminous slate (Brongniart) ; occurs both in the metamorphic and
fossiliferous series.

AMPHIBOLITE. Hornblende rock, which see.

ARGILLACEOUS-SCHIST, or CLAY-SLATE. See p. 133.

CHIASTOLITE-SLATE scarcely differs from clay-slate, but includes numerous crys-
tals of Chiastolite ; in considerable thickness in Cumberland. Chiastolite
occurs in long slender rhomboidai crystals. For composition, see Table, p.
102.

CHLORITE-SCHIST. A green slaty rock, in which chlorite, a green scaly mineral,
is abundant See p. 134.

CLAY-SLATE, or ARGILLACEOUS-SCHIST. See p. 133.

EURITE and EURITIC PORPHYRY. A base of compact felspar, with grains of
laminar felspar, and often mica and other minerals disseminated (Brong-
niart.) M. D'Aubuisson regards eurite as an extremely fine grained granite,
in which felspar predominates, the whole forming an apparently homogene-
ous rock. Eurite has been already mentioned as a plutonic rock, but occurs
also in beds subordinate to gneiss or mica-slate.

GNEISS. A stratified or laminated rock, same composition as granite. See p. 132.

PART I. CHAPTER

Metamorphic Rocks, and their Origin.

HORNBLENDE ROCK, or AMPHIBOLITE. The same composition as hornblende-
schist, stratified, but not fissile. See p. 100.

HORNBLENDE-SCHIST, or SLATE. Composed chiefly of hornblende, with occa-
sionally some felspar. See p. 133.

HORNBLENDIC or SvENiTic GNEISS. Composed of felspar, quartz, and horn-
blende.

HYPOGENE LIMESTONE. See p. 134.

MARBLE. See p. 134.

MICA-SCHIST, or MICACEOUS-SCHIST. A slaty rock, composed of mica and quartz

in variable proportions. See p. 133.
MICA-SLATE. See MICA-SCHIST, p. 133.

PHYLLADE. D'Aubuisson's term for clay-slate, from 0uAAa?, a heap of leaves.
PRIMARY LIMESTONE. See HYPOGENE LIMESTONE, p. 134.

PROTOGINE. See TALCOSE-GNEISS, p. 133.: when unstratified it is Talcose-gra-
nite.

QUARTZ ROCK, or QUARTZITE. A stratified rock ; an aggregate of grains of
quartz. See p. 133.

SERPENTINE occurs in both divisions of the hyppgene series, as a stratified or
unstratified rock; contains much magnesia; is chiefly composed of the min-
eral called serpentine, mixed with diallage, talc, and steatite. The pure
varieties of this rock, called noble serpentine, consist of a hydrated silicate
of magnesia, generally of a greenish colour ; this base is commonly mixed
with oxide of iron.

TALCOSE-GNEISS. Same composition as talcose granite or protogine, but either
stratified or laminated.

TALCOSE-SCHIST consists chiefly of talc, or of talc and quartz, or of talc and fel-
spar, and has a texture something like that of clay-slate.

WHITESTONE. Same as Eurite.

Origin of the Metamorphic Strata.

Having said thus much of the mineral composition of the met-
amorphic rocks, I may combine what remains to be said of their
structure and history, with an account of the opinions entertained
of their probable origin. At the same time it may be well to fore-
warn the reader that we are here entering upon ground of con-
troversy, and soon reach the limits where positive induction ends,
and beyond which we can only indulge in speculations. It was
once a favourite doctrine, and is still maintained by many, that
these rocks owe their crystalline texture, their want of all signs
of a mechanical origin, or of fossil contents, to a peculiar and
nascent condition of the planet at the period of their formation.
The arguments in refutation of this hypothesis will be more fully
considered when I show, in the second part of this volume, to
how many different ages the metamorphic formations are refer-
able, and how gneiss, mica-schist, clay-slate, and hypogene lime-
stone (that of Carrara for example,) have been formed, not only

136 LYELL'S ELEMENTS OF GEOLOGY.

Origin of tlie Metamorphic Rocks.

since the first introduction of organic beings into this planet, but
even long after many distinct races of plants and animals' had
passed away in succession.

The doctrine respecting the crystalline strata, implied in the
name metamorphic, may properly be treated of in this place ;
and we must first inquire whether these rocks are really entitled
to be called stratified in the strict sense of having been origin-
ally deposited as sediment from water. The general adoption
by geologists of the term stratified, as applied to these rocks,
sufficiently attests their division into beds very analogous, at
least in form, to ordinary fossiliferous strata. This resemblance
is by no means confined to the existence in both of an .occa-
sional slaty structure, but extends to every kind of arrangement
which is compatible with the absence of fossils, and of sand,
pebbles, ripple mark, and other characters which the meta-
morphic theory supposes to have been obliterated by plutonic
action. Thus, for example, we behold alike in the crystalline
and fossiliferous formations an alternation of beds varying
greatly in composition, colour, and thickness. We observe, for
instance, gneiss alternating with layers of black hornblende-
schist, or with granular quartz, or limestone; and the inter-
change of these different strata may be repeated for an indefi-
nite number of times. In the like manner, mica-schist alter-
nates with chlorite-schist, and with granular limestone in thin
layers.

As in fossiliferous formations strata of pure siliceous sand
alternate with micaceous sand and with layers of clay, so in
the crystalline or metamorphic rocks we have beds of pure
quartzite alternating with mica-schist and clay-slate. As in the
secondary and tertiary series we meet with limestone alternating
again and again with micaceous or argillaceous sand, so we
find in the hypogene, gneiss and mica-schist alternating with
pure and impure granular limestones.

It has also been shown that the ripple mark is very com-
monly repeated throughout a considerable thickness of fossil-
iferous strata, so in mica-schist and gneiss, there is sometimes
an undulation of the laminae on a minute scale, which may,
perhaps, be a modification of similar inequalities in the original
deposit.

In the crystalline formations also, as in many of the sedi-
mentary before described, single strata are sometimes made up
of laminae placed diagonally, such laminae not being regularly
parallel to the planes of cleavage.

PART I. CHAPTER X; 137

Slaty Cleavage.

Fig. 123. ™s disposition of the

layers is illustrated in the
accompanying diagram, in
which I have represented
carefully the stratification
of a coarse argillaceous
schist, which I examined in
the Pyrenees, part of which
approaches in character to
a green and blue roofing

Lamination of clay-slate, Montague de Seguinat, slate, while part is extremely
near Gavarnie, in tke Pyrenees. quartzose, the whole mass

passing downwards into micaceous schist. The vertical section
here exhibited is about three feet in height, and the layers are
sometimes so thin that fifty may be counted in the thickness of
an inch. Some of them consist of pure quartz.

The inference drawn from the phenomena above described,
in favour of the aqueous origin of clay-slate and other crys-
talline strata, is greatly strengthened by the fact that many of
these metamorphic rocks occasionally alternate with, and some-
times pass, by intermediate gradations, into rocks of a decidedly
mechanical origin, and exhibiting traces of organic remains.
The fossiliferous formations, moreover, into which this passage
is effected, are by no means invariably of the same age nor of
the highest antiquity, as will be afterwards explained. (See
Part II.)

Stratification of the metamorphic rocks distinct from cleav-
age.— The beds into which gneiss, mica-schist, and hypogene
limestone divide, exhibit most commonly, like ordinary strata, a
want of perfect geometrical parallelism. For this reason, there-
fore, in addition to the alternate recurrence of layers of distinct
materials, the stratified arrangement of the crystalline rocks
cannot be explained away by supposing it to be simply a divi-
sional structure like that to which we owe some of the slates
used for writing and roofing. Slaty cleavage, as it has been
called, has in many cases been produced by the regular depo-
sition of thin plates of fine sediment one upon another ; but
there are many instances where it is decidedly unconnected with
such a mode of origin, and where it is not even confined to the
aqueous formations. Some kinds of trap, for example, as clink-
stone, split into laminae, and are used for roofing.

There are, says Professor Sedgwick, three distinct forms of
structure exhibited in certain rocks throughout large districts :
viz. — First, stratification; secondly, joints; and thirdly, slaty
cleavage ; the two last having no connection with true bedding,

138 LYELL'S ELEMENTS OF GEOLOGY.

Jointed Structure Cleavage and Joints.

and having been superinduced by causes absolutely independent
of gravitation. All these different structures must have different
names, even though there be some cases where it is impossible,
after carefully studying the appearances, to decide upon the class
to which they belong.*

Joints. — Now in regard to the second of these forms of
structure or joints, they are natural fissures which often tra-
verse rocks in straight and well determined lines. They afford
to the quarryman, as Mr. Murchison observes, when speaking
of the phenomena, as exhibited in Shropshire and the neigh-
bouring counties, the greatest aid in the extraction of blocks of
stone, and, if a sufficient number cross each other, the wht>le
mass of rock is split into symmetrical blocks, f The faces of
the joints are for the most part smoother and more regular than
the surfaces of true strata. The joints are straight-cut chinks,
often slightly open, often passing, not only through layers of
successive deposition, but also through balls of limestone or
other matter which have been formed by concretionary action,
since the original accumulation of the strata. Such joints,
therefore, must often have resulted from one of the last changes
superinduced upon sedimentary deposits.:}:

In the annexed diagram the flat surfaces of rock A, B, C, re-
present exposed faces of joints, to which the walls of other
joints, J J, are parallel. S S are the lines of stratification ; C C
are lines of slaty cleavage, which intersect the rock at a con-
siderable angle to the planes of stratification.

J J

Stratification, joints, and cleavage.

Joints according to Professor Sedgwick are distinguishable
from lines of slaty cleavage in this, that the rock intervening

* Geol. Trans., Second Series, vol. iii. p. 480.

t The Silurian System of Rocks, as developed in Salop, Hereford, &c., p. 845.

tlbid. p.246.

PART I. CHAPTER X. 139

Slaty Cleavage.

between two joints has no tendency to cleave in a direction
parallel to the planes of the joints, whereas a rock is capable
of indefinite subdivision in the direction of its slaty cleavage.
In some cases where the strata are curved, the planes of cleav-
age are still perfectly parallel. This has been observed in the
slate rocks of part of Wales (See Fig. 125.), which consist of a

Fig. 125.

Parallel planes of cleavage intersecting' curved strata. (Sedgwick.)

hard greenish slate. The true bedding is there indicated by a
number of parallel stripes, some of a lighter and some of a
darker colour than the general mass. Such stripes are found to
be parallel to the true planes of stratification, wherever these
are manifested by ripple-mark, or by beds containing peculiar
organic remains. Some of the contorted strata are of a coarse
mechanical structure, alternating with fine-grained crystalline
chloritic slates, in which case the same slaty cleavage extends
through the coarser and finer beds, though it. is brought out in
greater perfection in proportion as the materials of the rock are
fine and homogeneous. It is only when these are very coarse
that the cleavage planes entirely vanish. These planes are usu-
ally inclined at- a very considerable angle to the planes of the
strata. In the Welsh chains, for example, the average angle is
as much as from 30° to 40°. Sometimes the cleavage planes
clip towards the same point of the compass as those of stratifi-
cation, but more frequently to opposite points. It may be stated
as a general rule, that when beds of coarser materials alternate
with those composed of finer particles, the slaty cleavage is
either entirely confined to the fine-grained rock, or is very im-
perfectly exhibited in that of coarser texture. This rule holds,
whether the cleavage is parallel to the planes of stratification
or not.

In the Swiss and Savoy Alps, as Mr. Bakewell has remarked,
enormous masses of limestone are cut through so regularly by
nearly vertical partings, and these are often so much more con-
spicuous than the seams of stratification, that an unexperienced
observer will almost inevitably confound them, and suppose the
strata to be perpendicular in places where in fact they are almost
horizontal.*

* Introduction to Geology, chap. iv.

140 LYELL'S ELEMENTS OF GEOLOGY.

Jointed Structure in Rocks.

Now these joints are supposed to be analogous to those part-
ings which have been already observed to separate volcanic and
plutonic rocks into cuboidal and prismatic masses. On a small
scale we see clay and starch when dry split into similar shapes,
which is often caused by simple contraction, whether the shrink-
ing be due to the evaporation of water, or to a change of tem-
perature. It is well known that many sandstones and other
rocks expand by the application of moderate degrees of heat,
and then contract again on cooling ; and there can be no doubt
that large portions of the earth's crust have, in the course of
past ages, been subjected again and again to very different de-
grees of heat and cold. These alternations of temperature have
probably contributed largely to the production of joints in
rocks.

In some countries, as in Saxony, where masses of basalt rest
on sandstone, the aqueous rock has for the distance of several
feet from the point of junction assumed a columnar structure sim-
ilar to that of the trap. In like manner some hearthstones, after
exposure to the heat of a furnace without being melted, have be-
come prismatic. Certain crystals also acquire by the application
of heat a new internal arrangement, so as to break in a new di-
rection, their external form remaining unaltered.

Scoresby, when speaking of the icebergs of Spitzbergen, states
that " they are full of rents, extending perpendicularly down-
wards, and dividing them into innumerable columns." Colonel
Jackson, who has lately investigated this subject more attentively,
found that the ice on the Neva, at St. Petersburg, at the begin-
ning of a thaw, when two feet in thickness, is traversed by rows
of very minute air-bubbles extending in straight lines, sometimes
a little inflected, from the upper surface of the ice towards the
lower, within from two to five inches of which they terminate.
'* Other blocks presented these bubbles united, so as to form cy-
lindrical canals, a little thicker than a horsehair. Observing still
further," he says, " I found blocks in which the process was
more advanced, and two, three, or more clefts, struck off in dif-
ferent directions from the vertical veins, so that a section perpen-
dicular to the vein would represent in miniature the star-formed
cracks of timber. Finally, in some pieces, these cracks united
from top to bottom of the veins, separating the whole mass into
vertical prisms, having a greater or less number of sides. In
this state a slight shock was sufficient to detach them ; and the
block with its scattered fragments was in all respects the exact
miniature resemblance, in crystal, of a Giant's Causeway. The
surface was like a tessellated pavement, and the columns rose
close, adhering and parallel, from the compact mass of a few

PART I. CHAPTER X. 141

Jointed Structure and Cleavage.

inches at the under surface. More or less -time is required for
the process, which I have since seen in all its different stages."*
Here again we find the columnar or jointed structure in a solid
mass, which had been subjected to great changes of temperature.
It seems, therefore, that the fissures called joints may have
been the result of different causes, as of some modification of
crystalline action, or simple contraction during consolidation, or
during a change of temperature. And there are cases where
joints may have been due to mechanical violence, and the strain
exerted on strataMuring their upheaval, or when they have sunk
down below their former level. Professor Phillips has suggested
that the previous existence of divisional planes may often have
determined, and must greatly have modified, the lines and points
of fracture caused in rocks by those forces to which they owe
their elevation or dislocations. These lines and points being
those of least resistance, cannot fail to have influenced the direc-
tion in which the solid mass would give way on the application
of external force.

It has been observed by Mr. Murchison, that in referring both
joints and slaty cleavage to crystalline action, we are borne out
by a well-known analogy in which crystallization has in like
'manner given rise to two distinct kinds of structure in the same
body. Thus for example, in a six-sided prism of quartz, the
planes of cleavage are distinct from those ~of the prism. It is
impossible to cleave the crystals parallel to the plane of the prism,
just as slaty rocks cannot be cleaved parallel to the joints, but
the quartz crystal, like the older schists, may be cleaved ad infi-
nitum in the direction of the cleavage planes.f

I have already stated that extremely fine slates, like those of
the Niesen, near the Lake of Thun, in Switzerland, are perfectly
parallel to the planes of stratification, and are, therefore, proba-
bly due to successive aqueous deposition. Even when the slates
are oblique to the general planes of the strata, it by no means
follows as a matter of course, that they have been caused by
crystalline action, for they may be the result of that diagonal
lamination which I have before described (p. 32.) In this case,
however, there is usually much irregularity, whereas those clea-
vage planes oblique to the true stratification, which are referred
to a crystalline action, are often perfectly symmetrical, and ob-
serve a strict geometrical parallelism, even when the strata are
contorted, as already described (p. 139.)

In regard to the origin of slaty cleavage, where it is uncon-

* Journ. of Roy. Geograph. Soc., vol. v. p. 19.
t Silurian System of Rocks, &c., p. 246.

142 LYELL'S ELEMENTS OF GEOLOGY.

Slaty Cleavage.

nected with sedimentary deposition, Professor Sedgwick is of
opinion that no retreat of parts, no contraction in dimensions, in
passing to a solid state, can account for the phenomenon. It
must be referred to crystalline or polar forces acting simultane-
ously and somewhat uniformly, in given directions, on large
masses having a homogeneous composition.

A fact recorded by Mr. Darwin affords confirmation to this
theory. The ore of the gold mines of Yaquil, in Chili, is ground
in a mill into an impalpable powder. After this powder has Been
washed, and nearly all the metal separated, the mud which passes
from the mills is collected into pools, where it subsides, and is
cleared out and thrown into a common heap. A great deal of
chemical action then commences, salts of various kinds effloresce
on the surface, and the mass becomes hard, and divides into con-
cretionary fragments. These fragments were observed to pos-
sess an even and well defined slaty structure ; but the laminae
were not inclined at any uniform angle.*

Mr. R. W. Fox lately submitted a mass of moist clay, worked
up with acidulated water, to weak voltaic action for some months,
and it was found when dry to be rudely laminated, the planes of
the slightly undulating laminae being at right angles to the direc-
tion of the electrical forces.f

Sir John Herschel, in allusion to slaty cleavage, has suggested,
" that if rocks have been so heated as to allow a commencement
of crystallization ; that is to say, if they have been heated to a
point at which the particles can begin to move amongst them-
selves, or at least on their own axes, some general law must then
determine the position in which these particles will rest on cool-
ing. Probably, that position will have some relation to the di-
rection in which the heat escapes. Now, when all, or a majority
of particles of the same nature have a general tendency to one
position, that must of course determine a cleavage plane. Thus
we see the infinitesimal crystals of fresh precipitated sulphate of
baryte, and some other such bodies, arrange themselves alike in
the fluid in which they float ; so as, when stirred, all to glance
with one light, and give the appearance of silky filaments. Some
sorts of soap, in which insoluble margarates:}: exist, exhibit the
same phenomenon when mixed with water ; and what occurs in
our experiments on a minute scale may occur in nature on a
great one."§

* Journal, p. 324.

t Although the lamination in the specimen shown to me was very imperfect,
it was sufficiently evident to encourage farther experiments.

t Margaric acid is an oleaginous acid, formed from different animal and vege-
table fatty substances. A margarate is a compound of this acid with soda, pot-
ash, or some other base, and is so named from its pearly lustre.

$ Letter to the author, dated Cape of Good Hope, Feb. 20, 1836.

PART I. CHAPTER XL 143

Alterations of Strata in contact with Granite.

CHAPTER XL

METAMORPHIC ROCKS — continued.

Strata near some intrusive masses of granite converted into rocks identical
with different members of the metamorphic series — Arguments hence derived as
to the nature of plutonic action — Time may enable this action to pervade denser
masses — From what kinds of sedimentary rock each variety of the metamorphic
class may be derived — Certain objections to the metamorphic theory considered.

IT has been seen that geologists have been very generally led
to infer, from the phenomena of joints and slaty cleavage, that
mountain masses, of which the sedimentary origin is unquestion-
able, have been acted upon simultaneously by vast crystalline
forces. That the structure of fossiliferous strata has often been
modified by some general cause since their original deposition,
and even subsequently to their consolidation and dislocation, is
undeniable. These facts prepare us to believe, that still greater
changes may have been worked out by a greater intensity, or
more prolonged development of the same agency, combined, per-
haps, with other causes. Now we have seen that, near the im-
mediate contact of granite veins and volcanic dikes, very extra-
ordinary alterations in rocks have taken place, more especially
in the neighbourhood of granite. It will be useful here to add
other illustrations, showing that a texture undistinguishable from
that which characterizes the more crystalline metamorphic form-
ations, has actually been superinduced in strata once fossilife-
rous.

In the southern extremity of Norway, there is a large district,
on the west side of the fiord of Christiana, in which granite or
syenite protrudes in mountain masses through fossiliferous strata,
and usually sends veins into them at the point of contact. The
stratified rocks, replete with shells and zoophytes, consist chiefly
of shale, limestone, and some sandstone, and all these are inva-
riably altered near the granite for a distance of from 50 to 400
yards. The aluminous shale are hardened and have become
flinty. Sometimes they resemble jasper. Ribboned jasper is
produced by the hardening of alternate layers of green and cho-
colate-coloured schist, each stripe faithfully representing the ori-
ginal lines of stratification. Nearer the granite the schist often
contains crystals of hornblende, which are even met with in seme

144

LYELL'S ELEMENTS OF GEOLOGY.

Alterations of Strata in contact with Granite.

places for a distance of several hundred yards from the junction ;
and this black hornblende is so abundant, that eminent geologists,
when passing through the country, have confounded it with the
ancient hornblende-schist, subordinate to the great gneiss forma-
tion of Norway. Frequently, between the granite and the horn-
blendic slate, above mentioned, grains of mica and crystalline
felspar appear in the schist, so that rocks resembling gneiss and
mica-schist are produced. Fossils can rarely be detected in these
schists, and they are more completely effaced in proportion to
the more crystalline texture of the beds, and their vicinity to the
granite. In some places the siliceous matter of the schist be-
comes a granular quartz, and when hornblende and mica are
added, the altered rock loses its stratification, and passes into a
kind of granite. The limestone, which at points remote from
the granite is of an earthy texture, blue colour, and often abounds
in corals, becomes a white granular marble near the granite,
sometimes siliceous, the granular structure extending occasion-

Fig. 126.

Altered zone offossiliferous slate and limestone near

Christiania.

The arrows indicate the dip, and the straight lines the strike, of
the beds.

ally upwards of 400 yards from the junction ; and the corals be-
ing for the most part obliterated, though sometimes preserved,
even in the white marble. Both the altered limestone and hard-
ened slate contain garnets in many places, also ores of iron,
lead, and copper, with some silver. These alterations occur
equally, whether the granite invades the strata in a line parallel
to the general strike of the fossiliferous beds, or in a line at right
angles to their strike, as will be seen by the accompanying ground
plan.*

The indurated and ribboned schists above mentioned, bear a

* Keilhau, Gcea Norvegica, pp. 61 — 63.

PART I. CHAPTER XI. 145

Alterations of Strata in contact with Granite.

strong resemblance to certain shales of the coal found at Rus-
sell's Hall, near Dudley, where coal mines have been on fire for
ages. Beds of shale of considerable thickness, lying over the
burning, have been baked and hardened so as to acquire a flinty
fracture, the layers being alternately green and brick-coloured.

The granite of Cornwall, in like manner, sends forth veins
into a coarse argillaceous-schist, provincially termed killas. This
killas is converted into hornblende-schist near the contact with
the veins. These appearances are well seen at the junction of
the granite and killas, in St. Michael's Mount, a small island
nearly 300 feet high, situated in the bay, at a distance of about
three miles from Penzance.

The granite of Dartmoor, in Devonshire, says Mr. De la
Beche, has intruded itself into the slate and slaty sandstone
called greywacke, twisting and contorting the strata, and send-
ing veins into them. Hence some of the slate rocks have be-
come " micaceous, others more indurated, and with the charac-
ters of mica-slate and gneiss, while others again appear convert-
ed into a hard-zoned rock strongly impregnated with felspar."*

We learn from the investigations of M. Dufrenoy, that in the
eastern Pyrenees there are mountain masses of granite poste-
rior in date to the formation called lias and chalk of that dis-
trict, and that these fossiliferous rocks are greatly altered in
texture, and often charged with iron-ore, in the neighbourhood
of the granite. Thus in the environs of St. Martin, near St.
Paul de Fenouillet, the chalky limestone becomes more crystal-
line and saccharoid as it approaches the granite, and loses all
traces of the fossils which it previously contained in abundance.
At some points also it becomes dolomitic, and filled with small
veins of carbonate of iron, and spots of red iron-ore. At Ran-
cie the lias nearest the granite is not only filled with iron-ore,
but charged with pyrites, tremolite, garnet, and a new mineral
somewhat allied to felspar, called, from the place in the Py-
renees where it occurs, " couzeranite."

Now the alterations above described as superinduced in rocks
by volcanic dikes and granite veins, prove incontestably that
powers exist in nature capable of transforming fossiliferous into
crystalline strata, — powers capable of generating in them a new
mineral character, similar, nay, often absolutely identical with
that of gneiss, mica-schist, and other stratified members of the
hypogene series. The precise nature of these altering causes,
which may provisionally be termed plutonic, is in a great degree
obscure and doubtful ; but their reality is no less clear, and we

* Geol. Manual, p. 479.

146 LYELL'S ELEMENTS OF GEOLOGY.

Rocks altered by Subterranean Gases.

must suppose the influence of heat to be in some way connected
with the transmutation, if, for reasons before explained, we con-
cede the igneous origin of granite.

The experiments of Gregory Watt, in fusing rocks in the
laboratory, and allowing them to consolidate by slow cooling,
prove distinctly that a rock need not be perfectly melted in order
that a re-arrangement of its component particles should take
place, and a partial crystallization ensue.* We may easily sup-
pose, therefore, that all traces of shells and other organic re-
mains may be destroyed ; and that new chemical combinations
may arise, without the mass being so fused as that the lines of
stratification should be wholly obliterated.

We must not, however, imagine that heat alone, such as may
be applied to a stone in the open air, can constitute all that is
comprised in plutonic action. We know that volcanos in erup-
tion not only emit fluid lava, but give off steam and other heated
gases, which rush out in enormous volume, for days, weeks, or
years continuously, and are even disengaged from lava during
its consolidation. When the materials of granite, therefore,
came in contact with the fossiliferous stratum in the bowels of
the earth under great pressure, the contained gases might be un-
able to escape ; yet when brought into contact with rocks, might
pass through their pores with greater facility than water is
known to do. (See p. 52.) These aeriform fluids, such as sul-
phuretted hydrogen, muriatic acid, and carbonic acid, issue in
many places from rents in rocks, which they have discoloured
and corroded, softening some and hardening others. If the
rocks are charged with water, they would pass through more
readily ; for, according to the experiments of Henry, water,
under an hydrostatic pressure of ninety-six feet, will absorb
three times as much carbonic acid gas as it can under the ordi-
nary pressure of the atmosphere. Although this increased
power of absorption would be diminished, in consequence of the
higher temperature found to exist as we descend in the earth,
yet Professor Bischoff has shown that the heat by no means
augments in such a proportion as to counteract the effect of aug-
mented pressure."!" There are other gases, as well as the car-
bonic acid, which water absorbs, and more rapidly in proportion
to the amount of pressure. Now even the most compact rocks
may be regarded, before they have been exposed to the air and
dried, in the light of sponges filled with water ; and it is con-
ceivable that heated gases brought into contact with them, at
great depths, may be absorbed readily, and transfused through

* Phil. Trans. 1804. t Poggendorf's Annalen, No. XVI., Second Series, vol. Hi.

PART I. CHAPTER XL 147

Rocks altered by Subterranean Gases.

their pores. Although the gaseous matter first absorbed would
soon be condensed, and part with its heat, yet the continued arri-
val of fresh supplies from below, might, in the course of ages,
cause the temperature of the water, and with it that of the con-
taining rock, to be materially raised.

M. Fournet, in his description of the metalliferous gneiss near
Clermont, in Auvergne, states that all the minute fissures of the
rock are quite saturated with free carbonic acid gas, which rises
plentifully from the soil there and in many parts of the surround-
ing country. The various elements of the gneiss, with the ex-
ception of the quartz, are all softened ; and new combinations
of the acid, with lime, iron, arid manganese, are continually in
progress.*

Another illustration of the power of subterranean gases is af-
forded by the stufas of St. Calogero, situated in the largest of the
Lipari Islands. Here, according to the description lately pub-
lished by Hoffmann, horizontal strata of tuff, extending for four
miles along the coast, and forming cliffs more than 200 feet high,
have been discoloured in various places, and strangely altered
by the " all-penetrating vapours." Dark clays have become yel-
low, or often snow-white ; or have assumed a chequered and
brecciated appearance, being crossed with ferruginous red stripes.
In some places the fumeroles have been found by analysis to con-
sist partly of sublimations of oxide of iron ; but it also appears
that veins of calcedony and opal, and others of fibrous gypsum,
have resulted from these volcanic exhalations.f

The reader may also refer to M. Virlet's account of the corro-
sion of hard, flinty, and jaspideous rocks near Corinth, by the
prolonged agency of subterranean gases ; J and to Dr. Daubeny's
description of the decomposition of trachyte rocks in the Solfa-
tara, near Naples, by sulphuretted hydrogen and muriatic acid

*•§

Although in all these instances we can only study the phe-
nomena as exhibited at the surface, it is clear that the gaseous
fluids must have made their way through the whole thickness of
porous or fissured rocks, which intervene between the subterra-
nean reservoirs of gas and the external air. The extent, there-
fore, of the earth's crust, which the vapours have permeated and
are now permeating, may be thousands of fathoms in thickness,

* See Principles of Geology, Index, " Auvergne," &c.
t Hoffmann's Liparischen Inseln, p. 38. Leipzig, 1832.

I See Principles of Geolgy ; and Bulletin de la Soc. Geol. de France, torn. ii.
p. 330.
$ See Principles of Geology ; and Daubeny's Volcanos, p. 167.

148 LYELL'S ELEMENTS OF GEOLOGY.

Origin of Metamorphic Structure.

and their heating and modifying influence may be spread through-
out the whole of the solid mass.

The above observations are calculated to meet some of the
objections which have been urged against the metamorphic the-
ory on the ground of the small power of rocks to conduct heat;
for it is well known that rocks, when dry and in the air, differ
remarkably from metals in this respect. It has been asked how
the changes which extend merely for a few feet from the con-
tact of a dike could have penetrated through mountain masses
of crystalline strata several miles in thickness. Now it has been
stated that the plutonic influence of the syenite of Norway, has
sometimes altered fossiliferous strata for a distance of a quarter
of a mile, both in the direction of their dip and of their strike.
(See Fig. 126. p. 144.) This is undoubtedly an extreme case;
but is it not far more philosophical to suppose that this influence
may, under favourable circumstances, affect denser masses, than
to invent an entirely new cause to account for effects merely dif-
fering in quantity, and not in kind ? The metamorphic theory
does not require us to affirm that some contiguous mass of gra-
nite has been the altering power ; but merely that an action, ex-
isting in the interior of the earth at an unknown depth, whether
thermal, electrical, or other, analogous to that exerted near in-
truding masses of granite, has, in the course of vast and indefi-
nite periods, and when rising perhaps from a large heated sur-
face, reduced strata thousands of yards thick to a state of semi-
fusion, so that on cooling they have become crystalline, like
gneiss. Granite may have been another result of the same ac-
tion in a higher state of intensity, by which a thorough fusion
has been produced ; and in this manner the passage from granite
into gneiss may be explained.

Some geologists are of opinion, that the alternate layers of
mica and quartz, or mica and felspar, or lime and felspar, are
so much more distinct in certain metamorphic rocks, than the
ingredients composing alternate layers in many sedimentary de-
posits, that the similar particles must be supposed to have exert-
ed a molecular attraction for each other, and to have thus con-
gregated together in layers, more distinct in mineral composition
than before they were crystallized.

In considering, then, the various data already enumerated, the
forms of stratification in metamorphic rocks, their passage on the
one hand into the fossiliferous. and on the other into the plutonic
formations, and the conversions which can be ascertained to have
occurred in the vicinity of granite, we may conclude that gneiss
and mica-schist may be nothing more than altered micaceous
and argillaceous sandstones, that granular quartz may have been

PART I. CHAPTER XL 149

Origin of Metamorphic Rocks.

derived from siliceous sandstone, and compact quartz from the
same materials. Clay-slate may be altered shale, and granular
marble may have originated in the form of ordinary limestone,
replete with shells and corals, which have since been obliterated;
and, lastly, calcareous sands and marls may have been changed
into impure crystalline limestones.

" Hornblende-schist," says Dr. MacCulloch, " may at first
have been mere clay ; for clay or shale is found altered by trap
into Lydian stone, a substance differing from hornblende-schist
almost solely in compactness and uniformity of texture."* " In
Shetland," remarks the same author, "argillaceous-schist (or
clay-slate,) when in contact with granite, is sometimes converted
into hornblende-schist, the schist becoming first siliceous, and
ultimately, at the contact, hornblende-schist."f

The anthracite found associated with hypogene rocks may
have been coal ; for we know that, in the vicinity of some trap
dikes, coal is converted into anthracite.

The total absence of any trace of fossils has inclined many
geologists to attribute the origin of crystalline strata to a period
antecedent to the existence of organic beings. Admitting, they
say, the obliteration, in some cases, of fossils by plutonic action,
we might still expect that traces of them would oftener occur in
certain ancient systems of slate, in which, as in Cumberland,
some conglomerates occur. But in urging this argument, it seems
to have been forgotten, that there are stratified formations of
enormous thickness, and of various ages, and some of them very
modern, all formed after the earth had become the abode of liv-
ing creatures, which are nevertheless in certain districts entirely
destitute of all vestiges of organic bodies. In some, the traces
of fossils may have been effaced by water and acids, at many
successive periods ; and it is clear, that the older the stratum,
the greater is the chance of its being non-fossiliferous, even if it
has escaped all metamorphic action.

It has been also objected to the metamorphic theory, that the
chemical composition of the secondary strata differs essentially
from that of the crystalline schists, into which they are supposed
to be convertible.^: The "primary" schists, it is said, usually
contain a considerable proportion of potash or of soda, which the
secondary clays, shales, and slates do not, these last being the
result of the decomposition of felspathic rocks, from which the
alkaline matter has been abstracted during the process of decom=
position. But this reasoning proceeds on insufficient and appa-

* Syst. of GeoL, vol. i. p. 210. t Ibid., p. 211.

t Dr. Boase, Primary Geology, p. 319.

N*

150 LYELL'S ELEMENTS OF GEOLOGY.

Objections to the Metamorphic Theory.

rently mistaken data ; for a large portion of what is usually called
clay, marl, shale, and slate does actually contain a certain and
often a considerable proportion of alkali ; so that it is difficult in
many countries to obtain clay or shale sufficiently free from
alkaline ingredients to allow of their being burnt into bricks or
used for pottery.

Thus the argillaceous shales, as they are called, and slates
of the old red sandstone, in Forfarshire and other parts of Scot-
land, are so much charged with alkali, derived from triturated
felspar, that, instead of hardening when exposed to fire, they
melt readily into a glass. They contain no lime, but appear to
consist of extremely minute grains of the various ingredients of
granite, which are distinctly visible in the coarser-grained vari-
eties, and in almost all the interposed sandstones. These lami-
nated clays, marls, and shales might certainly, if crystallized,
resemble in composition many of the primary strata.

There is also potash in the vegetable remains included in strata,
and soda in the salts by which they are sometimes so largely
impregnated, as in Patagonia.

Another objection has been derived from the alternation of
highly crystalline strata with others having a less crystalline tex-
ture. The heat, it is said, in its ascent from below, must have
traversed the less altered schists before it reached a higher and
more crystalline bed. In answer to this, it may be observed,
that if a number of strata differing greatly in composition from
each other be subjected to equal quantities of heat, there is every
probability that some will be more fusible than others. Some,
for example, will contain soda, potash, lime, or some other ingre-
dient capable of acting as a flux ; while others may be destitute
of the same elements, and so refractory as to be very slightly
affected by a degree of heat capable of reducing others to semi-
fusion. Nor should it be forgotten that, as a general rule, the
less crystalline rocks do really occur in the upper, and the more
crystalline in the lower part of each metamorphic series.

But it will be impossible for the reader duly to appreciate the
propriety of the term metamorphic, as applied to the strata hith-
erto called primary, until I have shown in the second part of this
work, that these crystalline strata have been formed at a great
variety of distinct periods.

ELEMENTS OF GEOLOGY.

PART II.

CHAPTER XII.

ON THE DIFFERENT AGES OF THE FOUR GREAT CLASSES OF ROCKS.

Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologi-
cally— Lehman's division into primitive and secondary — Werner's addition of a
transition class — Neptunian theory — Hutton on igneous origin of granite — How
the name of primary was still retained for granite — The term "transition," why
faulty — The adherence to the old chronological nomenclature retarded the pro-
gress of geology — New hypothesis invented to reconcile the igneous origin of
granite to the notion of its high antiquity — Explanation of the chronological no-
menclature adopted in this work, so far as regards primary, secondary, and ter-
tiary periods.

IN the first part of this work the four great classes of rocks,
the aqueous, the volcanic, the plutonic and the metamorphic,
have been considered with reference to their external characters,
their mineral composition, and mode of origin ; and it now re-
mains to treat of the same classes with reference to the different
periods at which they were formed. In speaking of the aqueous
rocks, for example, it has been shown that they are stratified,
that some are calcareous, others argillaceous, some made up of
sand, others of pebbles ; that some contain freshwater, others
marine fossils, and so forth ; but the student has still to learn
which rocks, exhibiting some or all of these characters, have ori-
ginated at one "period of the earth's history, and which at an-
other.

So in regard to the volcanic and plutonic formations, we have
hitherto examined their mineral peculiarities, forms, and mode
of origin, but have still to inquire into their chronological his-
tory.

Lastly, a more curious question will demand our attention,
when we endeavour to ascertain the relative ages of the meta-
morphic rocks, the chronology of which may be said to be two-
fold, each formation having been deposited at one period, and
having assumed a crystalline texture at another.

* (151)

152 LYELL'S ELEMENTS OF GEOLOGY.

Age of Rocks Lehman's and Werner's Classification of Rocks.

It was for many years a received opinion, that the formation
of whole classes of rocks, such as the plutonic and metamorphic,
began and ended before any members of the aqueous and volca-
nic orders were produced ; and although this idea has long been
modified, and is nearly exploded, it will be necessary to give
some account of the ancient doctrine, in order that beginners
may understand whence part of the nomenclature of geology still
partially in use was derived.

About the middle of the last century, Lehman, a German mi-
ner, proposed to divide rocks into three classes, the first and old-
est to be called primitive, comprising the plutonic and metamor-
phic rocks ; the next to be termed secondary, comprehending the
aqueous or fossiliferous strata ; and the remainder or third class,
the supposed effect of " local floods, and the deluge of Noah,"
corresponding to our alluvium, ancient and mordern. In the
primitive class, he said, such as granite and gneiss, there are no
organic remains, nor any signs of materials derived from the
ruins of pre-existing rocks. Their origin, therefore, may have
been purely chemical, antecedent to the creation of living beings,
and probably coeval with the birth of the world itself. The se-
condary formations, on the contrary, which often contain sand,
pebbles, and organic remains, must have been mechanical de-
posits, produced after the planet had become the habitation of
animals and plants- This bold generalization, although antici-
pated in some measure by Steno, a century before, in Italy,
formed at the time an importanPstep in the progress of geology,
and sketched out correctly some of the leading divisions into
which rocks may be separated. About half a century later,
Werner, so justly celebrated for his improved methods of dis-
criminating the mineralogical characters of rocks, attempted to
improve Lehman's classification, and with this view intercalated
a class, called by him " the transition formations," between the
primitive and secondary. Between these last he had discovered,
in northern Germany, a series of strata, which in their mineral
peculiarities were of an intermediate character, partaking in some
degree of the crystalline nature of micaceous and clay-slate, and
yet exhibiting here and there signs of a mechanical origin and
organic remains. For this group, therefore, forming a passage
between Lehman's primitive and secondary rocks, the name of
transition was proposed. They consisted principally of clay-
slate and an argillaceous sandstone, called greywacke and partly
of calcareous beds. It happened in the district which Werner
first investigated, that both the primitive and transition strata
were highly inclined, while the beds of the newer and fossilife-
rous rocks were horizontal. To these latter, therefore, he gave

PART II. CHAPTER XII. 153

Neptunian Theory Button on Granite.

the name of/?ote, or flat ; and every deposit more modern than
the chalk, or uppermost of the flotz series, was designated " the
overflowed land," an expression which may be regarded as
equivalent to alluvium. As the followers of Werner soon dis-
covered that the inclined position of the " transition beds," and
the horizontality of the flotz, or newer fossiliferous strata, were
mere local accidents, they soon abandoned the term flotz ; and
the four divisions of the Wernerian school were then named pri-
mitive, transition, secondary, and alluvium.

As to the trappean rocks, although their igneous origin had
been already demonstrated by Arduino, Fortis, Faujas, and
others, and especially by Desmarest, they were all regarded by
Werner as aqueous, and as mere subordinate members of the
secondary formations.*

This theory of Werner's was called the " Neptunian," and for
many years enjoyed much popularity. It assumed that the
globe had been at first invested by an universal chaotic ocean,
holding the materials of all rocks in solution. From the waters
of this ocean, granite, gneiss, and other crystalline formations,
were first precipitated ; and afterwards, when the waters were
purged of these ingredients, and more nearly resembled those of
our actual seas, the transition strata were deposited. These were
of a mixed character, not purely chemical, because the waves
and currents had already begun to wear down solid land, and
to give rise to pebbles, sand, and mud ; nor entirely without fos-
sils, because a few of the first marine animals had begun to ex-
ist. After this period, the secondary formations were accumu-
lated in waters resembling those of the present ocean, except at
certain intervals, when, from causes wholly unexplained, a par-
tial recurrence of the "chaotic fluid" took place, during which
various trap rocks, some highly crystalline, were formed. This
arbitrary hypothesis rejected all intervention of igneous agency,
volcanos being regarded as partial and superficial accidents, of
trifling account among the great causes which have modified the
external structure of the globe.

Meanwhile Hutton, a contemporary of Werner, began to teach,
in Scotland, that granite as well as trap was of igneous origin,
and had at various periods intruded itself in a fluid state into dif-
ferent parts of the earth's crust. He recognized and faithfully
described many of the phenomena of granitic veins, and the al-
terations produced by them on the invaded strata, which have
been treated of in the ninth chapter. He, moreover, advanced
the opinion, that the crystalline strata called primitive had not

* See Principles of Geology, vol. i. chap. iv.

154 LYELL'S ELEMENTS OF GEOLOGY.

Term Transition, why objectionable.

been precipitated from a primeval ocean, but were sedimentary
strata altered by heat. In his writings, therefore, and in those
of his illustrator, Playfair, we find the germ of that metamorphic
theory which has been already expounded.*

At length, after much controversy, the doctrine of the igneous
origin of trap and granite made its way into general favour ;
but although it was, in consequence, admitted that both granite
and trap had been produced at many successive periods, the term
primitive or primary still continued to be applied to the crystal-
line formations in general, whether stratified, like gneiss, or un-
stratified, like granite. The pupil was told that granite was a
primary rock, but that some granites were newer than certain
secondary formations ; and in conformity with the spirit of the
ancient language, to which the teacher was still determined to
adhere, a desire was naturally engendered of extenuating the
importance of those more modern granites which new observa-
tions were continually bringing to light.

A no less decided inclination was shown to persist in the use
of the term "transition," after it had been proved to be almost
as faulty in its original application as that of flotz. The name
of transition, as already stated, was first given by Werner, to
designate a mineral character, intermediate between the meta-
morphic state and that of an ordinary fossiliferous rock. But
the term acquired also from the first a chronological import, be-
cause it had been appropriated to sedimentary formations, which,
in the Hartz and other parts of Germany, were more ancient
than the oldest of the secondary series, and were characterized
by peculiar fossil zoophytes and shells. When, therefore, geol-
ogists found in other districts stratified rocks occupying the same
position, and inclosing similar fossils, they gave to them also the
name of transition, according to rules which will be explained
in the next chapter ; yet, in many cases, such rocks were found
not to exhibit the same mineral texture which Werner had called
transition. On the contrary, many of them were not more crys-
talline than dhTerent members of the secondary class ; while, on
the other hand, these last were sometimes found to assume a
semi-crystalline and almost metamorphic aspect, and thus, on
lithological grounds, to deserve equally the name of transition.
So remarkably was this the case in the Swiss Alps, that certain
rocks, which had for years been regarded by some of the most
skilful disciples of Werner to be transition, were at last acknow-
ledged, when their relative position and fossils were better un-
derstood, to belong to the newest of the secondary groups ! If

» j.

* See chapters X. and XL

PART II. CHAPTER XII. 155

Neptunian Theory of the Origin of Granite.

under such circumstances the name of transition was retained, it
is clear that it ought to have been applied without reference to
the age of strata, and simply as expressive of a mineral pecu-
liarity. The continued appropriation of the term to formations
of a given date, induced geologists to go on believing that the
ancient strata so designated bore a less resemblance to the se-
condary than is really the case, and to imagine that these last
never pass, as they frequently do, into metamorphic rocks.

The poet Waller, when lamenting over the antiquated style of
Chaucer, complains that —

We write in sand, our language grows,
And, like the tide, our work o'erflows ;

But the reverse is true in geology ; for here it is our work
which continually outgrows the language. The tide of observation
advances with such speed, that improvements in theory outrun
the changes of nomenclature ; and the attempt to inculcate new
truths by words invented to express a different or opposite opin-
ion, tends constantly, by the force of association, to perpetuate
error ; so that dogmas renounced by the reason still retain a
strong hold upon the imagination.

In order to reconcile the old chronological views with the new
doctrine of the igneous origin of granite, the following hypothe-
sis was substituted for that of the Neptunists. Instead of begin-
ning with an aqueous menstruum or chaotic fluid, the materials
of the present crust of the earth were supposed to have been at
first in a state of igneous fusion, until part of the heat having
been diffused into surrounding space, the surface of the fluid con-
solidated, and formed a crust of granite. This covering of crys-
talline stone, which afterwards grew thicker and thicker as it
cooled, was so hot, at first, that no water could exist upon it ; but
as the refrigeration proceeded, the aqueous vapour in the atmo-
sphere was condensed, and, falling in rain, gave rise to the first
thermal ocean. So high was the temperature of this boiling sea,
that no aquatic beings could inhabit its waters, and its deposits
were not only devoid of fossils, but, like those of some hot springs,
were highly crystalline. Hence the origin of the primary or
crystalline strata.

Afterwards, when the granitic crust had been partially broken
up, land and mountains began to rise above the waters, and rains
and torrents ground down rock, so that sediment was spread over
the bottom of the seas. Yet the heat still remaining in the solid
supporting substances was sufficient "to increase the chemical
action exerted by the water, although not so intense as to pre-
vent the introduction and increase of some living beings. During

156 LYELL'S ELEMENTS OF GEOLOGY.

Simultaneous Origin of the four Classes of Rocks.

this state of things some of the residuary mineral ingredients of
the primeval ocean were precipitated, and formed deposits (the
transition strata of Werner), half chemical and half mechanical,
and containing a few fossils.

By this new theory, which was in part a revival of the doc-
trine of Leibnitz, published in 1680, on the igneous origin of the
planet, the old ideas respecting the priority of all crystalline rocks
to the creation of organic beings, were still preserved ; and the
notion, that all the semi-crystalline and partially fossiliferous
rocks belonged to one period, while all the earthy and uncrys-
talline formations originated at a subsequent epoch, was also
perpetuated.

It may or may not be true, as the great Leibnitz imagined,
that the whole planet was once in a state of liquefaction by heat ;
but there are certainly no geological proofs that the granite which
constitutes the foundation of so much of the earth's crust was
ever in a state of universal fusion. On the contrary, all our
evidence tends to show that the formation of granite, like the
deposition of the stratified rocks, has been successive, and that
different portions of granite have been in a melted state at dis-
tinct and often distant periods. One mass was solid, and had
been fractured, before another body of granitic matter was in-
jected into it, or through it, in the form of veins. In short, the
universal fluidity of the crystalline foundations of the earth's
crust, can only be understood in the same sense as the univer-
sality of the ancient ocean. All the land has been under water,
but not all at one time ; so all the subterranean unstratified rocks
to which man can obtain access have been melted, but not simul-
taneously.

In the present work the four great classes of rocks, the aqueous,
plutonic, volcanic, and metamorphic, will form four parallel, or
nearly parallel, columns in one chronological table. They will be
considered as four sets of monuments relating to four contem-
poraneous, or nearly contemporaneous, series of events. I have
endeavoured, in the Frontispiece, to express the manner in which
members of each of the four classes may have originated simul-
taneously at every geological period. According to this view, the
earth's crust may have been continually remodelled, above and
below, by aqueous and igneous causes, from times indefinitely re-
mote. In the same manner as aqueous and fossiliferous strata
are now formed in certain seas or lakes, while in other places
volcanic rocks break out a,t the surface, and are connected with
reservoirs of melted matter at vast depths in the bowels of the
earth, — so, at every era of the past, fossiliferous deposits and
superficial igneous rocks were in progress contemporaneously

PART II. CHAPTER XII. 157

, Chronological Arrangement of the four Classes of Rocks.

with others of subterranean and plutonic origin, and some sedi-
mentary strata were exposed to heat and made to assume a
crystalline or metamorphic structure.

It can by no means be taken for granted, that during all these
changes the solid crust of the earth has been increasing in
thickness. It has been shown, that so far as aqueous action is
concerned, the gain by fresh deposits, and the loss by denuda-
tion, must at each period have been equal ; and in like manner,
in the inferior portion of the earth's crust, the acquisition of new
crystalline rocks, at each successive era, may merely have coun-
terbalanced the loss sustained by the melting of materials pre-
viously consolidated. As to the relative antiquity of the crys-
talline foundations of the earth's crust, when compared to the
fossiliferous and volcanic rocks which they support, I have al-
ready stated, in the first chapter, that to pronounce an opinion
on this matter is as difficult as at once to decide which of the
two, whether the foundations or superstructure of an ancient city
built on wooden piles may be the oldest. We have seen that to
answer this question, we must first be prepared to say whether
the work of decay and restoration had gone on most rapidly
above or below, whether the average duration of the piles has
exceeded that of the stone buildings, or the contrary. So also
in regard to the relative age of the superior and inferior portions
of the earth's crust ; we cannot hazard even a conjecture on this
point, until we know whether, upon an average, the power of
water above, or that of fire below, is most efficacious in giving
new forms to solid matter.

After the observations which have now been made, the reader
will perceive that the term primary must either be entirely re-
nounced, or, if retained, must be differently defined, and not
made to designate a set of crystalline rocks, some of which
may be newer than the secondary formations. In this work I
shall follow most nearly the method proposed by Mr. Boue, who
has called all fossiliferous rocks older than the secondary by the
name of primary, which thus becomes a substitute for the term
transition, so far as regards the aqueous strata. To prevent
confusion, however, I shall always speak of these as the primary
fossiliferous formations, because the word primary has hitherto
been almost inseparably connected with the idea of a non-fossili-
ferous rock.

If we can prove any plutonic, volcanic, or metamorphic rocks
to be older than the secondary formations, such rocks will also
be primary, according to this system. Mr. Boue having with great
propriety excluded the metamorphic rocks, as a class, from the
primary formations, proposed to call them all "crystalline

158 LYELL'S ELEMENTS OF GEOLOGY.

Ages of Rocks.

schists," restricting the name of primary to the older fossilifer-
ous or transition strata.

As there are secondary fossiliferous strata, so we shall find
that there are plutonic, volcanic, and metamorphic rocks of con-
temporaneous origin, which I shall also term secondary.

In the next chapter it will be shown that the strata above the
chalk have been called tertiary. If, therefore, we discover any
volcanic, plutonic, or metamorphic rocks, which have originated
since the deposition of the chalk, these also will rank as tertiary
formations.

It may perhaps be suggested that some metamorphic strata,
and some granites, may be anterior in date to the oldest of the
primary fossiliferous rocks. The opinion is certainly not impro-
bable, and will be discussed in future chapters ; but I may here
observe, that when we arrange the four classes of rocks in four
parallel columns in one table of chronology, it is by no means
assumed that these columns are all of equal length ; one may
begin at an earlier period than the rest, and another may come
down to a later point of time. In the small part of the globe
hitherto examined, it is hardly to be expected that we should
have discovered either the oldest or the newest of all the four
classes of rocks. Thus, if there be primary, secondary, and
tertiary rocks of the fossiliferous class, and in like manner pri-
mary, secondary, and tertiary plutonic formations, we may not
be yet acquainted with the most ancient of the primary fossili-
ferous beds, or with the newest of the plutonic, and so of the
rest.

PART II. CHAPTER XIII. 159

Tests of the different Agea of Aqueous Rocks.

CHAPTER XIII.

ON THE DIFFERENT AGES OF THE AQUEOUS ROCKS.

On the three principal tests of relative age — superposition, mineral character,
and fossils — Change of mineral character and fossils in the same continuous
formation — Proofs that distinct species of animals and plants have lived at suc-
cessive periods — Test of age by included fragments — Frequent absence of strata
of intervening periods — Principal groups of strata in western Europe — Tertiary
strata separable into four groups, the fossil shells of which approach nearer to
those now living in proportion as the formation is more modern — Terms Eocene,
Miocene, and Pliocene — Identifications of fossil and recent shells by M. De-
shayes — Opinions of Dr. Beck.

IN the last chapter I spoke generally of the chronological re-
lations of the four great classes of rocks, and I shall now treat
of the aqueous rocks in particular, or of the successive periods
at which the different fossiliferous formations have been deposited.

Now there are three principal tests by which we determine
the age of a given set of strata ; first, superposition ; secondly,
mineral character; and, thirdly, organic remains. Some aid
can occasionally be derived from a fourth kind of proof, namely,
the fact of one deposit including in it fragments of a preexisting
rock, which last may thus be shown, even in the absence of all
other evidence, to be the older of the two.

Superposition. — The first and principal test of the age of one
aqueous deposit, as compared to another, is relative position. It
has been already stated, that where the strata are horizontal, the
bed which lies uppermost is the newest of the whole, and that
which lies at the bottom the most ancient. So, of a series of
sedimentary formations, they are like volumes of history, in
which each writer has recorded the annals of his own times,
and then laid down the book, with the last written page upper-
most, upon the volume in which the events of the era immedi-
ately preceding were commemorated. In this manner a lofty
pile of chronicles is at length accumulated; and they are so
arranged as to indicate, by their position alone, the order in
which the events recorded in them have occurred.

In regard to the crust of the earth, however, there are some
regions where, as the student has already been informed, the
beds have been disturbed, and sometimes reversed. (See pp. 73,
74.) But the experienced geologist will not be deceived by

160 LYELL'S ELEMENTS OF GEOLOGY.

Tests of the different Ages of Aqueous Rocks.

these exceptional cases. When he finds that the strata are frac-
tured, curved, inclined, or vertical, he knows that the original
order of superposition must be doubtful, and he will endeavour
to find sections in some neighbouring district where the strata
are horizontal, or only slightly inclined. Here it is impossible
that they can have been extensively thrown over and turned
upside down, for such a derangement cannot have taken place
throughout a wide area without leaving manifest signs of dis-
placement and dislocation.

Mineral character. — The same rocks may often be observed
to retain for miles, or even hundreds of miles, the same mineral
peculiarities, if we follow them in the direction of the planes of
stratification. But this uniformity ceases almost immediately, if
we pursue them in an opposite direction. In that case we can
scarcely ever penetrate a stratified mass for a few hundred yards,
much less several miles, without beholding a succession of ex-
tremely dissimilar calcareous, argillaceous, and siliceous rocks.
These phenomena lead to the conclusion, that rivers and currents
have dispersed the same sediment over wide areas at one period,
but at successive periods have been charged, in the same region,
with very different kinds of matter. The first observers were
so astonished at the vast -spaces over which they were able to
follow the same homogeneous rocks in a horizontal direction,
that they came hastily to the opinion, that the whole globe had
been environed by a succession of distinct aqueous formations,
disposed round the nucleus of the planet, like the concentric
coats of an onion. But although, in fact, some formations may
be continuous over districts as large as half of Europe, or even
more, yet most of them either terminate wholly within narrower
limits, or soon change their lithological character. Sometimes
they thin out gradually, as if the supply of sediment had failed
in that direction, or they come abruptly to an end, as if we had
arrived at the borders of the ancient sea or lake which served
as their receptacle. It no less frequently happens that they vary
in mineral aspect and composition, as we pursue them horizon-
tally. For example,' we trace a limestone for a hundred miles,
until it becomes more arenaceous, and finally passes into sand,
or sandstone. We may then follow this sandstone, already proved
by its continuity to be of the same age, throughout another dis-
trict a hundred miles or more in length.

Organic remains. — This character must be used as a crite-
rion of the age of a formation, or of the contemporaneous origin
of two deposits in distant places, under very much the same
restrictions as the test of mineral composition.

First, the same fossils may be traced over wide regions, if

PART II. CHAPTER XIII. 161

Tests of the different Ages of Aqueous Roctfs.

we examine strata in the direction of their planes, although by
no means for indefinite distances. This might have been ex-
pected ; for although many species of animals and plants have
a wide geographical range, yet each species generally inhabits a
small part only of the entire globe, and is often incapable of
existing in other regions. But, in those cases where the fossils
vary, the mineral character of the rock often remains constant ;
and, on the other hand, the fossils are sometimes uniform through-
out spaces where the lithological nature of the rock is variable.
In this manner we are frequently enabled to prove the contem-
poraneous origin of the same formation by one test, when the
other fails.

Secondly, while the same fossils prevail in a particular set of
strata for hundreds of miles in a horizontal direction, we seldom
meet with the same remains for many fathoms, and scarcely
ever for several hundred yards, in a vertical line, or a line trans-
verse to the strata. This fact has now been verified in almost
all parts of the globe, and has led to a conviction, that at suc-
cessive periods of the past, the same area of land and water has
been inhabited by species of animals and plants as distinct as
those which now people the antipodes, or which now coexist in
the arctic, temperate, and tropical zones. It appears, that from
the remotest periods there has been ever a coming in of new
organic forms, and an extinction of those which pre-existed on
the earth ; some species having endured for a longer, others for
a shorter time ; but none having ever re-appeared after once dy-
ing out. The law which has governed the creation and extinc-
tion of species seems to be expressed in the verse of the poet,

Natura il fece e poi ruppe la starapa.— Ariosto.
Nature made it, and then broke the die.

And this circumstance it is, which confers on fossils their highest
value as chronological tests, giving to each of them, in the eyes
of the geologist, that authority which belongs to contemporary
medals in history.

The same cannot be said of each peculiar variety of rock ; —
for some of these, as red marl and red sandstone, for example,
may occur at once at the top, bottom, and middle of the entire
sedimentary series ; exhibiting in each position so perfect an
identity of mineral aspect as to be undistinguishable. Such
exact "repetitions, however, of the same mixtures of sediment
have not often occurred, at distant periods, in precisely the same
parts of the globe ; and even where this has happened, we may
usually avoid confounding together the monuments of remote
eras, by the aid of fossils and relative position.

162

LYELL'S ELEMENTS OF GEOLOGY.

Chronological Classification of Aqueous Rocks.

Test by included fragments of old rocks. — It was stated,
that independent proof may sometimes be obtained of the rela-
tive date of two formations, by fragments of an older rock being
included in a newer one. This evidence may sometimes be of
great use, where a geologist is at a loss to determine the relative
age of two formations, from want of clear sections exhibiting
their true order of position, or because the strata of each group
are vertical. In such cases we sometimes discover that the
more modern rock has been in part derived from the degradation
of the older. Thus, for example, we may find chalk with flints ;
and, in another part of the same country, a distinct series, con-
sisting of alternations of clay, sand, and pebbles. If some of
these pebbles consist of flints, with fossil shells of the same spe-
cies as those in the chalk, we may confidently infer that the chalk
is the oldest of the two formations.

The number of groups into which the fossiliferous strata may
be separated, is more or less, according to the views of classifi-
cation which different geologists entertain; but when we have
adopted a certain system of arrangement, we immediately find
that a few only of the entire series of groups occur one upon
the other in any single section or district.

The thinning out of individual strata was before described
(p. 31.). But let the annexed diagram represent seven fossili-

Fig. 127.

ferous groups, instead of as many strata. It will then be seen
that in the middle all the superimposed formations are present ;
but in consequence of some of them thinning out, No. 2. and
No. 5. are absent at one extremity of the section, and No. 4. at
the other.

If the reader consults the Frontispiece, he will see, that as the
strata A rest unconformably upon the older groups, a, ft, c, e,f,
g, we should meet with a very different succession in a vertical
section exposed at different places ; in one spot A lying imme-
diately on c, in another on g, and so forth. Now here the dif-
ference has been partly occasioned by denudation ; the forma-
tions a, ft, for instance, once extended much farther to the left,
and but for denudation would have been everywhere interposed
between A and the rocks e, f, g. In many instances the entire
absence of one or more formations of intervening periods be-
tween two groups, such as A and c, (see Frontispiece,) arises,

PART II. CHAPTER XIII. 163

Chronological Classification of Aqueous Rocks.

not from the destruction of what once existed, by denudation, but
because no strata of an intermediate age were ever deposited on c.
They were not formed at that place, either because the region
was dry land during the interval, or because it was part of a sea
or lake to which no sediment was carried.

In order, therefore, to establish a chronological succession of
fossil iferous groups, a geologist must begin with a single section,
in which several sets of strata lie one upon the other. He must
then trace these formations, by attention to their mineral char-
acter and fossils, continuously, as far as possible, from the start-
ing point. As often as he meets with new groups, he must ascer-
tain by superposition their age relatively to those first examined,
and thus learn how to intercalate them in a tabular arrangement
of the whole.

By this means the German, French, and English geologists
have determined the succession of strata throughout a great part
of Europe, and have adopted pretty generally the following
groups, almost all of which have their representatives in the
British Islands.

Groups of Fossiliferous Strata observed in Western Europe,
arranged in what is termed a descending series, or beginning
with the newest.

I.

Newer Pliocene.

2.
3.

Older Pliocene.
Miocene.

Tertiary or Supracretaceous.*

4.

Eocene.

5.

Chalk.

§

6.

Greensand.

7.

Wealden.

8.

Upper Oolite.

9.

Middle Oolite.

10.

Lower Oolite.

11.

Lias.

> Secondary.

12.

Upper New Red sandstone and

Muschelkalk.

13.

Lower New Red and Magnesian

limestone.

14.

Coal.

15.

Old Red sandstone.

16.
17.

Upper Silurian. "1
Lower Silurian. (Primary fossiliferous (or transition of

18.

Cambrian and older fossiliferous j some authors).

strata. J

A glance at the above table will show that the three great
sections called primary fossiliferous, secondary, and tertiary, are
by no means of equivalent importance, if the eighteen subordi-
nate groups comprise monuments relating to equal portions of

* For tertiary, Mr. De la Beche has used the term " supracretaceous," a name
implying that the strata so called are superior in position to the chalk.

164 LYELL'S ELEMENTS OF GEOLOGY.

Chronological Classification of Aqueous Rocks.

past time, or of the earth's history. But this we cannot assert ;
but merely know that they each relate to successive periods,
during which certain animals and plants, for the most part pecu-
liar to that era, flourished, and during which different kinds of
sediment were deposited in the space now occupied by Europe.

If we were disposed, on palseontologieal grounds, to divide the
entire fossiliferous series into a few groups, less numerous than
those in the above table, and more nearly co-ordinate in value
than the sections called primary, secondary, and tertiary, we
might, perhaps, adopt the six following groups or periods.* At
the same time I may observe, that in the present state of the
science, when we have not yet compared the evidence derivable
from all classes of fossils, not even those most generally distri-
buted, such as shells, corals, and fish, such generalizations are
premature, and can only be regarded as conjectural schemes for
the founding of large natural groups.

1 . Tertiary from the Newer Pliocene to the Eocene inclusive.

2. Cretaceous from the Chalk to the Wealden inclusive.

3. Oolitic from the Oolite to the Lias inclusive.

A TT AT o A $ including the Keuper, Muschelkalk, and Bunter

4. Upper New Red J Sandstein of the Germans.

5. Lower New Red and ( including Magnesian Limestone (Zechstein), Coal,

Carboniferous .....'.. ( and Old Red sandstone.

6. Primary fossiliferous ... j ^S^SS^?^ tOthe °WeSt fossiliferous

The limits of this volume will not allow of a full description,
even of the leading features of all the formations enumerated in
the above tables ; but I shall briefly advert to each of them in
chronological order, as they will afford illustrations of the rules
of classification, the tests of relative age, and the mode of deriving
information from geological monuments respecting the former
history of the earth and its inhabitants.

Tertiary formations. — TJiese strata, as we have seen, were
so called because, when first discovered, they were observed to
be of a date posterior to the chalk, which had long been regarded
as the last or uppermost of the secondary formations. It was
remarked, that in France, Italy, Germany, and England, the
tertiary deposits occupied a position, in reference to all older
rocks, like that of the waters of lakes, inland seas, and gulfs in
relation to a continent, being often, like such waters, of great
depth, though of limited area, and frequently occurring in de-
tached and isolated patches. The strata were for the most part

* Palaeontology is the science which treats of fossil remains, both animal and
vegetable. Elym. ira\aios, palaios, ancient, oira, onto, beings, and Xoyoj, logos,
a discourse.

PART II. CHAPTER XIII. 105

Classification of the Tertiary Formations.

horizontal, but usually surrounded by older rocks, of which the
beds were highly inclined or vertical.

On comparing together the fossils of the aqueous formations
in general, especially the testacea, which are the most abundant
and best preserved of all, it appears that those of the primary
fossiliferous rocks depart most widely in form and structure from
the type of the living creation, those of the secondary less wide-
ly, and the tertiary least of all. In like manner, if we divide
the tertiary deposits into four principal groups, and then compare
the fossil shells which they contain with the testacea now living
in the nearest seas in the same latitudes, we find that the shells
of the oldest strata have much less resemblance, on the whole,
to the fauna of the neighbouring seas, than those of the newest
group. In a word, in proportion as the age of a tertiary forma-
tion is more modern, so also is the resemblance greater of its
fossil shells to the testaceous fauna of the actual seas.

Having observed the prevalence of this change of character
in the tertiary strata of France and Italy, in 1828, I conceived
the idea of classing the whole series of tertiary strata into four
groups, endeavouring to find characters for each expressive of
their different degrees of affinity to the living fauna. I hoped
that an estimate of this varying relation to the fauna of the
existing seas might be obtained by determining the proportional
number of shells identical with living species which belonged to
each group. With this view, I obtained information respecting
the specific identity of many tertiary and recent shells from seve-
ral Italian naturalists ; and among others, from Professors Bo-
nelli, Guidotti, and Costa.

I have explained at length, in the Principles of Geology, the
opinions which were at that time generally entertained respecting
the classification of tertiary formations, and the observations
which led me, in 1828, to divide them into four groups, by refer-
ence not only to their geological position, but also to the propor-
tional number of recent species found fossil in each. I have also
there stated, that having, in 1829, become acquainted with M.
Deshayes, of Paris, I learnt from him that he had arrived, by in-
dependent researches, and by the study of a large collection of
fossil and recent shells, at very similar views. At my request
he drew up, in a tabular form, lists of all the shells known to
him to occur both in some tertiary formation and in a living state,
for the express purpose of ascertaining the proportional number
of fossil species identical with the recent which characterized the
successive group ; and this table was published by me in 1833.*

* See Principles of Geology, vol. iii. 1st ed.

166 LYELI/S- ELEMENTS OF GEOLOGY. ^

Cfossification of the Tertiary Formations.

The number of tertiary fossil shells examined by M. Deshayes
was about 3000 ; and the recent species with which they had
been compared, about 5000. The result at which that naturalist
arrived was, that in the oldest tertiary deposits, such as those
found near London and Paris, there were about 3^ per cent, of
species of fossil shells identical with recent species ; in the next,
or middle tertiary period, to which certain strata on the Loire
and Gironde, in France, belonged, about 17 per cent. ; and in
the deposits of a third, or newer era, embracing those of the
Subapennine hills, from 35 to 50 per cent. In formations still
more modern, some of which I had particularly studied in Sicily,
where they attain a vast thickness and elevation above the sea,
the number of species identical with those now living was from
90 to 95 per cent. For the sake of clearness and brevity, I pro-
posed to give short technical names to these four groups, or the
periods to which they respectively belonged. I called the first or
oldest of them Eocene, the second Miocene, the third Older Plio-
cene, and the last or fourth, Newer Pliocene. The first of the
above terms, Eocene, is derived from »;wj, eos, dawn, and seawoj,
cainos, recent, because the fossil shells of this period contain an
extremely small proportion of living species, which may be look-
ed upon as indicating the dawn of the recent or existing state
of the testaceous fauna.

The other terms, Miocene and Pliocene, are comparative ; the
first meaning less recent, (from pt tw, meion, less, and xawo$, cainos,
recent,) and the other more recent, (from ttteiov, pleion, more, and
jcaw/05, cainos, recent,) they express the more or less near ap-
proach which the deposits of these eras, when contrasted with
each other, make to the existing creation, at least so far as the
mollusca are concerned. It may assist the memory of students
to remind them, that the Miocene contain a minor proportion,
and PZiocene a comparative pZurality of recent species ; and
that the greater number of recent species always implies the
more modern origin of the strata.

Two subjects of discussion have arisen respecting the tables
above alluded to ; first, whether the fossil shells were, upon the
whole, correctly identified with recent species by M. Deshayes ;
secondly, whether such a per-centage of recent species occurring
fossil in particular groups, affords the best criterion for estimat-
ing the relation of each fossil fauna to the living creation.

Now in regard to the per-centage test, its application must
evidently depend on the extent to which conchologists are agreed
in their determination of species. In every branch of natural
history there is always some difference of opinion as to certain
species which are variable in their characters, and seem to pass

PART II. CHAPTE

Classification of the Tertiary Forma

by imperceptible gradations into other forms, considered! fty many
zoologists and botanists as entitled to rank as distinct species. —
The difficulty of defining the limits in such cases is not greater,
perhaps, in conchology than in other departments ; but it happens
that this science has advanced very rapidly since the year 1830,
when M. Deshayes drew up the tables published in the Principles
of Geology. In that year he had it in his power to refer to no
more than 5000 species of recent shells then in Paris ; but the
number of species now in the public and private collections of
Europe has increased to between 8000 and 9000 ; and, what is
of no less consequence, individuals of species which before that
time were extremely rare, have been supplied in abundance.
Fossil shells also have been collected with equal zeal and suc-
cess ; and thus the facility of discriminating nice distinctions in
closely allied species, or of deciding which characters are con-
stant and which variable, has been greatly promoted ; and the
study of these more ample data has led all conchologists to
separate many species, both fossil and recent shells, which before
they had confounded together.

In consequence of the changes of opinion brought about by
these additions to our knowledge, it has become necessary not
only to examine all the newly discovered fossil and recent tes-
tacea, but also to reconsider all the species previously known.
As this laborious task has not yet been executed by M. Deshayes,
engaged as he is in other scientific Jabours, I am unable at present
to offer to the reader the improved results which the revision of
the tables drawn up in 1830 would afford. In the mean time I
have obtained the aid of several eminent conchologists, arid in
particular of Dr. Beck, of Copenhagen, in comparing a great
number of the recent and fossil shells which had been identified.
By this investigation I have come to the conclusion* that the per-
centage of recent species in a fossil state is decidedly less, espe-
cially in the older tertiary strata, than was indicated in the list
published in 1833. A large number, in particular, of the forty-
two species of ^Eocene testacea, to which the names of recent
shells were given in the tables, cannot be considered as identical,
if we adopt the same standard of specific distinctions as is recog-
nized in the new edition of Lamarck's conchology, edited by M.
Deshayes himself, in 1836.

But although many corrections are indispensable, and the
proportion of recent species found fossil in the Eocene, Miocene,
and older Pliocene strata may be considerably less than was at
first supposed, we have no reason on this account to feel dis-
couraged in an attempt to found the classification and nomen-
clature of the tertiary periods on the great principle before

168 LYELL'S ELEMENTS OF GEOLOGY.

Classification of the Tertiary Formations.

explained ; namely, the comparative resemblance of the testa-
ceous fauna of each period to that of the neighbouring seas.
There can be no cabalistic virtue in such numbers as 3. 17. or
40., which were at first imagined to express correctly the pro-
portional number of identical species in three of the tertiary
periods ; but until the time arrives when we can obtain the gene-
ral acquiescence of conchologists as to the real proportional
numbers, we must endeavour to find some readier method of
estimating the relation of one fauna to another ; a method not
involving the question of the identity or non-identity of every
fossil with some known recent species.

Now, it has been suggested by Dr. Beck that, in order to form
such an estimate of the comparative resemblance of the faunas
of different eras, we may follow the same plan as would enable
us to appreciate the amount of agreement or discrepancy between
the faunas now existing in two distinct geographical regions.

It is well known that, although nearly all the species of mol-
lusca inhabiting the temperate zones on each side of the equator
are distinct, yet the whole assemblage of species in one of these
zones bears a striking analogy to that in the other, and differs in
a corresponding manner from the tropical and arctic faunas. By
what language can the zoologist express such points of agree-
ment or disagreement, where the species are admitted to be dis-
tinct?

In such cases it is necessary to mark the relative abundance
in the two regions compared of certain families, genera, and
sections of genera ; the entire absence of some of these, the
comparative strength of others, this strength being sometimes
represented by the numbers of species, sometimes by the great
abundance and size of the individuals of certain species. It is,
moreover, important to estimate the total number of species
inhabiting a given area; and also the average proportion of
species to genera, as this differs materially according to climate.
Thus, if we adopt comprehensive genera like those of Lamarck,
we shall find, according to Dr. Beck, that, upon^an average, there
are in arctic latitudes nearly as many genera as species ; in the
temperate regions, about three or four species to a genus ; in the
tropical, five or six species to a genus.

The method of which the above sketch conveys but a faint
outline, is the more easy of application to the tertiary deposits
of Europe, because the conchological fauna of the Eocene period
indicates a tropical climate; that of the Miocene strata, a climate
bordering on the tropics; and that of the Older and Newer
Pliocene deposits, a climate much more closely approaching to,
if not the same as, that of the seas in corresponding latitudes.

PART II. CHAPTER XIV. 169

Recent and Newer Pliocene Strata.

Although I cannot enter in this work into farther details, it
may be stated that, if we compare tertiary formations on this
principle, the nomenclature above proposed will not be inappro-
priate ; for the fauna of the older, or Eocene tertiary formations
is still the first in the order of time in which there is an assem-
blage of testacea like that of the present ocean between the tro-
pics ; and in this period a small proportion of mollusca are un-
distinguishable from living species ; whereas at the opposite ex-
treme of the series, or in the Newer Pliocene deposits, ail con-
chologists agree that the marine shells are all, or nearly all,
identical with those now inhabiting the nearest seas. As to the
Miocene and Older Pliocene groups, the terms less and more will
always express correctly the different degrees of analogy which
their fossils bear to the assemblage of living species in similar
latitudes.

But it should never be forgotten, that, as the extinct species
preponderate in all groups, with the exception of the Newer Pli-
ocene, it is from their characters that we derive the distinguish-
ing feature in the palaeontology of each period. The relative
approach which the shells may make to the living fauna affords
a useful and interesting term of comparison ; but it is one fea-
ture only, and by no means the most prominent one, in the or-
ganic remains of successive periods.

CHAPTER XIV.

RECENT AND TERTIARY FORMATIONS.

How to distinguish Recent from Tertiary strata — Recent and Newer Pliocene
strata near Naples — near Stockholm and Christiania — in South America, on
coasts of Chili and Peru — Rocks of recent period, with human skeleton, in
Guadaloupe— Shells of living species, with extinct mammalia, in loess of the
Rhine — Recent and Newer Pliocene deposits in England— Older Pliocene strata
in England — Crag — Red and Coralline crag — their fossils in part distinct —
their strata uncomfortable — belong to the same period — London clay — Its
shells and fish imply a tropical climate — Tertiary mammalia — Fossil quadru-
mana.

Recent and Newer Pliocene strata.- — If we begin with the his-
tory of the more modern aqueous formations, and then pass on
to the more ancient, the first strata which present themselves are
those termed, in the last chapter, the Newer Pliocene. But in

170 LYELL'S ELEMENTS OF GEOLOGY.

Recent and Newer Pliocene Strata.

what manner shall we define the limits between this group and
those fossiliferous deposits which are now in progress, or which
have accumulated under water since the globe was inhabited by
man? The strata last mentioned, namely, those of the human
period, I shall call Recent, distinguishing them from the most
modern tertiary formations. Strata may be proved to belong to
the Recent period by our finding in them the bones of man in a
fossil state, that is to say, imbedded in them by natural causes ;
or we may recognize them by their containing articles fabricated
by the hands of man, or by showing that such deposits did not
exist in the place where we now observe them at a given period
of the past when man existed, so that they must be of subsequent
origin. In general all recent formations lie hidden from our sight
beneath the waters of lakes and seas ; but we may examine them
wherever these lakes or seas have been partially converted into
land, as in the deltas of rivers, or where the submerged ground
has been heaved up by subterranean movements, and laid dry.

Thus at Puzzuoli, near Naples, marine strata are seen contain-
ing fragments of sculpture, pottery, and the remains of buildings,
together with innumerable shells retaining in part their colour,
and of the same species as those now inhabiting the Mediterranean.
The uppermost of these beds is about twenty feet above the level
of the sea. Their emergence can be proved to have taken place
since the beginning of the sixteenth century.* But the hills at
the base of which these strata have been deposited, and those of
the interior of the adjacent country round Naples, some of which
rise to the height of 1500 feet above the sea, are formed of hori-
zontal strata of the Newer Pliocene period ; that is to say, the
marine shells observed in them are of living species, and yet are
not accompanied by any remains of man or his works. Had
such been discovered, it would have afforded to the antiquary
and geologist matter of great surprise, since it would have shown
that man was an inhabitant of that" part of the globe, while the
materials composing the present hills and plains of Campania
were still in the progress of deposition at the bottom of the sea ;
whereas we know that for nearly 3000 years, or from the times
of the earliest Greek colonists, no extensive revolution in the
physical geography of that part of Italy has occurred.

In Sweden, analogous phenomena have been observed. Near
Stockholm, for example, when the canal of Sodertelje was dug,
horizontal beds of sand, loam, and marl were passed through, in
some of which the same peculiar assemblage of testacea which
now live in the Baltic were found. Mingled with these, at different

* See Principles of Geology, Index, " Serapis."

PART II. CHAPTER XIV. 171

Recent and Newer Pliocene Strata.

depths, were detected various works of art implying a rude state
of civilization, and some vessels built before the introduction of
iron. These vessels and implements must have sunk to the
bottom of an arm of the sea, afterwards filled up with sand and
loam including marine shells, and the whole must then have been
upraised ; so that the upper beds became sixty feet higher than *
the surface of the Baltic. There are, however, in the neighbour-
hood of these formations, others precisely similar in mineral
composition and testaceous remains, which ascend to the height
of between 100 and 200 feet, in which no vestige of human art
has been seen. Similar deposits reach an elevation of 500 and
even 600 feet in Norway, as in the neighbourhood of Christiania,
where they have usually been described as raised beaches, but
are, in fact, strata of clay, sand, and marl, often many hundred
feet thick, which cover the inland country far ancj wide, filling
valleys and deep depressions in the granite gneiss, and primary
fossiliferous rocks, just as the tertiary formations of England and
France rest upon the chalk, or fill depressions in it.

All conchologists are agreed that the shells of the deposits
above mentioned are nearly all, perhaps all, absolutely identical
with those now peopling the contiguous ocean ; so that, in the
absence of any evidence of their being Recent, we must regard
them as Newer Pliocene formations.

Along the western shores of South America, RecenJ and
Newer Pliocene strata have in like manner been brought to light.
These often consist of enormous masses of shells, similar to those
now swarming in the Pacific. In one bed of this kind, in the
island of San Lorenzo, near Lima, Mr. Darwin found, at the
altitude of eighty-five feet above the sea, pieces of cotton-thread,
plaited rush, and the head of a stalk of Indian corn, all of which
had evidently been imbedded with the shells. At the same
height on the neighbouring mainland, he found other signs cor-
roborating the opinion that the ancient bed of the sea had there
also been uplifted eighty-five feet, since the region was first peo-
pled by the Peruvian race.* But similar shells, or strata, con-
taining them, have been found much higher, almost everywhere
between the Andes and the sea coasts of Chili and Peru, in which
no human remains were ever, or in all probability ever will be,
discovered. These strata, therefore, may provisionally, at least,
be designated Newer Pliocene.

In the West Indies, also, rocks both of the Recent and Newer
Pliocene periods abound. Thus, a solid limestone occurs at the
level of the sea-beach in the island of Guadaloupe, enveloping

* Journal, p. 451.

172 LYELL'S ELEMENTS OF GEOLOGY.

Recent and Newer Pliocene Strata.

human skeletons. The stone is extremely hard, arid chiefly com-
posed of comminuted shell and coral, with here and there some
entire corals and shells, of species now living in the adjacent
sea. With them are included arrow-heads, fragments of pottery,
and other fabricated articles. A limestone with similar contents
has been formed, and is still forming, in St. Domingo and other
islands. But there are also more ancient rocks in the West
Indian Archipelago, as in Cuba, near the Havanna, and in other
islands in which are shells identical with those now living in
corresponding latitudes ; some well preserved, others in casts, all
referable to a period which, if we can depend on negative evi-
dence, was anterior to the introduction of man into the New
World.

The history of Holland, during the last 2000 years, makes
us acquainted with a vast accession of Recent strata, by which
parts of the sea near the mouths of the Rhine have been filled
up and converted into dry land. But, if we ascend the Rhine,
we find throughout its course, from Cologne to the frontiers of
Switzerland, a yellow calcareous loam, called loess by the Ger-
mans, in which are fossil shells, both freshwater and terrestrial,
of common European species. The entire thickness of this loam,
amounts in some places to 200 or 300 feet, and it rises from the
height of 300 to 1200 feet above the sea. Bones of the mam-
moth»or extinct elephant, together with those of the horse, and
some other quadrupeds, have been met with in this Newer Plio-
cene formation, but no remains or signs of man ; and it can be
proved that the physical geography of the whole valley of the
Rhine has undergone enormous changes since the deposition of
this loam.

No marine strata of the Recent period have yet been brought
to light in England which rise to such a height above the level
of the sea as the highest tides may not once have reached.
Buried ships have been found in the former channels of the
Rother in Sussex, of the Mersey in Kent, and Thames near
London. Canoes and stone-hatches have been dug up, in almost
all parts of the kingdom, in peat and shell-marl ; but there is no
evidence, as in Sweden, Italy, Peru, Chili, and other parts of the
world, of the bed of the sea, and the adjoining coast, having
been uplifted bodily in modern times, so that Recent formations
have become land. There are, however, in various parts of
Great Britain and Ireland, Newer Pliocene deposits of marine
origin, consisting of sand and clay, usually of small thickness ;
as, for example, in Cornwall, and near the borders of the great
estuaries of the Clyde and Forth, in Scotland, and ;n that of
the Shannon, in Ireland. These are found usually near the

PART II. CHAPTER XIV. 173

Recent and Newer Pliocene Strata.

coast, but in some rare instances they penetrate inland to a dis-
tance of sixty miles from the sea, as at Bridgnorth, in Shrop-
shire/* They also rise occasionally to great heights, as at
Preston, in Lancashire, where they are 350 feet above the sea ;
and, what is still more remarkable, on a mountain called Moel
Tryfane, in Wales, near the Menai Straits, they attain an eleva-
tion of about 1400 feet.f In all these places they contain shells
indisputably of the same species as those which now people the
British seas ; and although, perhaps, on more accurate examina-
tion some slight intermixture of extinct testacea will appear, yet
the geologist will always refer them to the most modern tertiary
era.

There are, moreover, a great many freshwater deposits scatter-
ed over England, which belong to the Newer Pliocene period, as
at North Cliff, in the county of York, where thirteen species of
British land and freshwater shells were found imbedded in the
same strata with the remains of the bison and mammoth.:]: In
like manner, at Cropthorne, in Worcestershire, on the banks of
the Avon, a tributary of the Severn, Mr. Strickland observed flu-
viatile and land shells, nearly all of recent species, with the bones
of an extinct kind of hippopotamus. Recent freshwater shells
also appear in beds of loam, together with bones of the deer
and mammoth, in the cliffs of the estuary of the Stour, in Suffolk.
Some writers have confounded these and similar fluviatile and
lacustrine strata, with the ancient alluviums which they term
diluvial.

Older Pliocene strata in England — Crag. — There are some
few countries in Europe, as in the district between the Gironde
and the Pyrenees, in the south of France, or that between the
Alps, north of Vicenza, and the hills near Turin, in the north of
Italy, where fossiliferous strata representing all the three periods,
the Eocene, Miocene, and Older Pliocene, are present. But the
tertiary deposits of England are limited to the Eocene and the
Older and Newer Pliocene groups, the Miocene being wanting.

It is chiefly in the eastern part of the county of Suffolk that a
deposit provincially named crag is seen in its most characteristic
form. This crag consists chiefly of a series of thin layers of
quartzose sand and comminuted shell, which rest sometimes on
chalk, sometimes on an Eocene tertiary formation, called the
" London Clay." Mr. Charlesworth, whose opinion I have lately
had opportunities of confirming, has correctly stated that the
crag, in part of Suffolk, may be divided into two distinct masses,

* See Murchison, Proceedings of Geol. Soc., vol. ii. p. 333.
t Proceedings of Geol. Soc., vol. i. p. 331., and vol. ii. p. 333.
t See Principles of Geology, Index, " Mammoth."

P*

174 LYELL'S ELEMENTS OF GEOLOGY.

Older Pliocene Strata English Crag.

the upper of which may be termed the red, and the lower the
coralline crag.* The inferior division, however, is of very
limited extent, ranging over an area about twenty miles in
length, and three or four miles in breadth, between the rivers
Aide and Stour.

The red crag is generally at once distinguishable from the
coralline, by the deep red ferruginous or ochreous-colour of its
sands and fossils. Its strata are also remarkable for the oblique
or diagonal position of the subordinate layers (see p. 32.) ; and
these often consist of small flat pieces of shell, which lie parallel
to the planes of the smaller strata, showing clearly that they
were so deposited, and that this structure has not been due to any
subsequent rearrangement of the mass after deposition. That
the ancient sandbanks in question had sometimes sides sloping in
all directions, is implied by the fact that the oblique layers some-
times slant towards all points of the compass in different parts
of the same quarry. They were probably shifting sands, and a
great proportion of the shells composing them have been ground
down to small pieces, while others have been rolled ; and the
two parts of the bivalves are almost invaricfbly disunited. The
red crag contains some peculiar fossils, and others which seem to
have been washed out of the lower or coralline crag. Some
few of the bivalves of the red crag are entire, with both valves
joined.

The coralline crag is usually free from ferruginous stains, and
consists of light greenish shelly marl, and white calcareous sand.
Sometimes it forms a soft building stone, in which entire shells,
echini, and many zoophytes are imbedded. Here and there the
softer mass is divided by thin flags of hard limestone, in which
are corals in a good state of preservation, which evidently grew
at the bottom of a tranquil sea, in the position in which we now
see them. Yet the sands in this formation, as in the red crag,
are often composed entirely of comminuted shell.

In some places, as at Tattingstone, in Suffolk, the lithological
distinction of the two divisions, though perceptible, is much less
marked ; the inferior crag being composed chiefly of greenish
marl, with only a few stony beds. The shells also are mostly
broken, and corals are almost as rare as in the red crag.

At some places, as near Orford, the coralline crag is exposed
at the surface, and the bottom of it has not been reached at the
depth of fifty feet. Yet not far from this town, the surface is
occupied exclusively with red crag, which rests immediately upon
the London clay. Wherever the two divisions are found toge-
ther, the coralline mass is the lower of the two, and is interposed

* London and Edin. Phil. Mag. No. 38. p. 81. Aug. 1836.

PART II. CHAPTER XIV.

175

Older Pliocene Strata English Crag.

between the red crag and London clay ; and the strata of the
upper and lower crag are unconformable one to the other, as in
the section represented in the annexed diagram, which I have
myself examined.

Fig. 128.

a. red craj

Section near Ipswich, in Suffolk,
b. coralline crag.

c. London clay.

In the places here referred to, the coralline crag varies in
thickness from fifteen to more than twenty feet, and the red crag
is often much thicker.

Amongst upwards of 400 species of testacea found in the
crag, there are many common to both divisions ; but some,
which are very abundant in the red, have never been met with
in the coralline crag ; as, for example, the Fusus contrarius
(Fig. 129.), and several species of Nassa and Murex, (see Figs.

Fig. 129.

Fossils characteristic of the Red Crag.

Fig. 130.

Fig. 131.

Nassa granulata.

Fig. 132.

Fusus contrarius.

Murex alveolatus.

Cyprcea coccinelloides.

130, 131.), which two genera seem never to have been disco-
vered in the lower crag. On the other hand, scarcely any corals
have been found in the red crag. These abound in the inferior
division, and some of them are of a globular form, and belong
to the genera Theonoa (Lamoroux), Cellepora, and a third to
Fascicularia, which is of very peculiar structure, unknown in
the living creation. (See Fig. 133.)

176

LYELL'S ELEMENTS OF GEOLOGY.

Older Pliocene Strata English Crag.

Fig. 133.

Fascicularia auruntium, Milne Edwards. Family, Tubuliporidce, of same author-
A coral of an extinct genus, from the inferior or coralline crag, Suffolk.

a. exterior. b. vertical section of interior.

e. portion of exterior magnified.

d. portion of interior magnified, showing that it is made up of long, thin,
straight tubes, united in conical bundles.

The general analogy of the crag shells to those now living in
the neighbouring seas, between the latitudes 50° and 60° north,
is so striking that we cannot hesitate to refer the formation to the
Pliocene period ; but, as all conchologists are agreed that more
than half the species are extinct or unknown, it is to the Older
and not the Newer Pliocene period that they belong. Dr. Beck,
after examining 260 species of shells, informs me that the average
number of species to genera is such as indicates a temperate cli-
mate, a result which is also confirmed by the large development of
certain northern forms, such as the genus Astarte (see Fig. 134.),

Fig. 134.

dstarte, (Crassina, Lam.); species common to upper and lower crag:

of which there are about fourteen species, many of them being
rich in individuals ; and there is an absence of genera peculiar
to hot climates, such as Conus, Oliva, Mitra, Fasciolaria, and
others. The cowries (Cypraa) (Fig. 132.) also are small,

PART II. CHAPTER XIV. 177

Older Pliocene Strata English Crag.

as in the colder regions. A large volute, called Valuta Lam-
berti, (Fig. 135.) may seem an exception ; but it differs in form
from the volutes of the torrid zone, and may, like
Fig. 135. the large Voluta Magellanica, have been extra-
tropical.

When I first submitted the shells of the crag to
M. Deshayes, in 1829, he recognized their general
resemblance to the fauna of the German ocean, and
determined that out of 1 1 1 species there were 45
identical with those now living. Dr. Beck, on the
other hand, who has since seen much larger collec-
tions, considers that almost all the species are dis-
tinguishable from those now living, and this subject
is still under discussion.

Lambent, It has been asked whether, as the upper and
ld' lower crag of Suffolk differs greatly in mineral
composition and fossils, they may not belong to two different ter-
tiary periods. To this I may reply, that the general character
of the shells is the same, and by no means leads to such a con-
clusion. The two deposits may have been going on contempo-
raneously under different geographical conditions in the same
sea. One region of deep and clear water, far from the shore,
may have been fitted for the growth of certain corals, echini,
and testacea ; while, another shallower part nearer the shore, and
more frequently turbid, or where sand and shingle were occa-
sionally drifted along, may have been favourable to other species.
After this, the region of deep and tranquil water becoming shal-
low, or exposed to the action of waves and currents, a formation
like the coralline crag may have been covered over with sandy
deposits, such as the red crag, and many fossils of the older beds
may have been washed into the newer strata. If a considerable
lapse of time intervened in a particular spot between the conver-
sion of a deep sea into a shoal, some small change in organic
life may have taken place, and consequently the distinctness in
character of the fossils of the two formations may be derived
from two causes, first and principally the difference of geo-
graphical conditions, and, secondly, that law of the coming in
and going out of species which was alluded to in the last chap-
ter, (p. 161.)

The area over which both divisions of the crag can be traced
is too small to enable us to arrive at satisfactory conclusions on
a question of such magnitude ; but the section given above (p.
175.) shows distinctly that, near Sutton, the lower crag had suf-
fered much denudation before the deposition of the red crag. At
D (Fig. 128.) there is not only a distinct cliff, eight or ten feet

178 LYELL'S ELEMENTS OF GEOLOGY.

Eocene Strata London Clay.

high, of coralline crag, running in a direction N. E. and S. W.,
against which the red crag abuts with its horizontal layers, but
this cliff occasionally overhangs. The rock composing it is
drilled everywhere by Pholades belonging to the period of the
red crag. The cliff may have been caused by submarine denu-
dation, in a shallow sea ; and had the red crag been equally
solid, it would probably have presented many similar perpendicu-
lar cliffs ; for beds, ten or twelve feet thick, of loam or sand, in
this formation, are often seen to be unconformable to older beds,
which have been in part cut away. Similar excavations are now
made, even on a larger scale, by the sea, in the great sandbanks
off Yarmouth, in part of which Captain Hewett, R. N., found, in
1836, a broad channel, sixty-five feet deep, where there had been
only a depth of four feet in 1822. This remarkable change was
ascertained during two hydrographical surveys, in the years
above mentioned, and shows how denudation, amounting to sixty
feet in vertical depth-, can take place under water in the course
of fourteen years. The new channel thus formed, serves now
(1838) for the entrance.of ships into Yarmouth Roads.

Eocene formations in England — London Clay. — In the sec-
tion already given of the tertiary strata of Suffolk (p. 175.), it
will be seen that the crag rests on a formation called the London
clay, which there consists of alternating beds of blue and brown
clay, with many nodules of calcareous stone, used for Roman
cement. This formation is well seen in the neighbouring cliffs
of Harwich, where the nodules contain many marine shells, and
sometimes the bones of turtles. The relative position of the
chalk, London clay, and crag, between the coast of Essex and
the interior, may be understood by reference to the annexed dia-

Fig. 136.

Crag. London clay. Chalk.

gram. The London clay has been so named, because it occurs
in the neighbourhood of the metropolis, in a trough or basin of
the chalk. (See section, p. 182.) We know, by numerous
borings made for water, that the chalk exists everywhere below,
after we have penetrated through clay and sand to the depth of
from 200 to 600 feet ; and, if we proceed to the south of Lon-
don, we find the chalk rising up to the surface and forming the
Surrey hills ; while if we proceed northwards, into Hertford-
shire, or, westward, by the Thames, into Oxfordshire, we again
meet with the same chalk.

PART II. CHAPTER XIV.

179

Eocene Strata Fossil Shells.

The overlying Eocene deposit consists of two portions ; the
upper of blue clay, with occasional cement stones, as before
mentioned ; the lower of various coloured sands and clays ; the
fossils throughout all the beds being very different from those of
the crag. Scarcely any one of the shells can be identified with
species now living; and the whole assemblage is such as to
resemble the testaceous fauna of the tropics. This opinion is
favoured by the occurrence of many species of Mitra and Voluta,
a large Cyprae, a very large Rostellaria, and shells of the genera

FOSSIL SHELLS OF THE LONDON CLAY.
Fig. 137. Fig. 138.

Mitra scalra.

Fig. 139.

Orassatella sulcata.

Rostellaria macroptera, Sow.
one third of nal. size.

Fig. 140.

Fig. 141.

Fig. 142.

Nautilus cenlralis.

Voluta athleta.

180 LYELL'S ELEMENTS OF GEOLOGY.

Fossil Mammalia.

Terebellum, Cancellaria, Crassatella, and others, with four or
more species of Nautilus. (See Figures, p. 179.) There are
fish, also, which indicate a warm climate ; among which may be
mentioned a sword-fish, (Tetrapterus prisons, Agassiz,) about
eight feet long, and a saw-fish, (Pristis bisvlcatus, Ag.) about
ten feet in length ; genera foreign to the British seas.

These last have been found in the island of Sheppey, which
is composed of London clay, where also, as I learn from M.
Agassiz, the remains of no less than fifty other species of fish
have been discovered. It does not appear that the fossil plants
and fruits, so numerous in this same island, or the fossil plants
of the corresponding Eocene formation of Paris, have by any
means so tropical an aspect as the shells, but rather indicate such
a flora as might be found on the borders of the Mediterranean.

Besides the marine formation called London clay, there are
freshwater strata of the Eocene period in the Isle of Wight, and
opposite coast of Hampshire. They contain shells, such as
Limnea, and Planorbis ; Gyrogonites, or the fossil seeds of
Chara (see p. 47.) ; and the bones of several quadrupeds of ex-
tinct genera, such as Palasotherium, Anoplotherium, and Choero-
potamus, which were lately found by the Rev. W. D. Fox, near
Binstead.

It has already been remarked that fossil mammalia of extinct
species have been met with in the Newer Pliocene deposits of
England and other countries. Different species characterize the
Miocene, and others are proper to the Eocene formations ; and
among them nearly every order and family of the herbivorous
and carnivorous tribes are represented ; but those which inhabit
trees are most rare ; and it was not until very lately, namely in
1837, that any remains of quadrumana, or of the ape and mon-
key tribe, were discovered. These were obtained about the same
time in France and India ; in France, by M. Lartet, near Auch,
in the department of Gers, about forty miles west of Toulouse,
where the bones of an ape, or gibbon, accompanied those of the
rhinoceros, dinotherium, mastodon, and others; in India, by
Captain Cautley and Dr. Falconer, who found the remains of a
monkey, with the bones of many extinct quadrupeds, in the
Sewalik hills, a lower range of the Himalaya mountains, near
Saharuripore.

* The frequent occurrence in the tertiary strata of fossils refera-
ble to the highest class of vertebrata is a fact the more worthy
of notice, as we shall find in the sequel how great is their rarity
in the secondary formations.

PART II. CHAPTER XV.

Cretaceous Group.

CHAPTER XV.

CRETACEOUS GROUP.

White chalk — Its marine origin shown by fossil shells — Extinct genera of
cephalopoda — Sponges and corals in the chalk— No terrestrial or fluviatile shells,
no land plants — Supposed origin of white chalk from decomposed corals —
Single pebbles, whence derived — Cretaceous coral-reef in Denmark — Maes-
tricht beds and fossils — Origin of flint in chalk — Wide area covered by chalk
— Green-sand formation and fossils — Origin of — External configuration of
chalk — Outstanding columns or needles — Period of emergence from the sea —
Difference of the chalk of the north and south of Europe — Hippurites — Num-
mulites — Altered lithological character of cretaceous formation in Spain and
Greece — Terminology.

THE next group which succeeds to the tertiary strata in the
descending order has been called Cretaceous or chalky, because
it consists in part of that remarkable white earthy limestone called
chalk (creta). With this limestone however are usually asso-
ciated other deposits of sand, marl, and clay, called the Green-
sand formation, because some of its sands are remarkable for
their bright green colour.

The following are the subdivisions into which the Cretaceous
Strata have been divided in the south of England : —

a. soft white chalk, with
flints

b. hard white chalk, with
few or no flints

c. chalk marl

a. upper green-sand -

b. Gault, or blue marl, 10 to

c. lower green-sand and }
iron-sand, with occa- > 250.t
sional limestone - )

The accompanying section (Fig. 143.) will show the manner
in which the tertiary strata of the London and Paris basins, as
they are called, rest upon the chalk, and how the white chalk in
its turn reposes throughout this region upon the green-sand form-
ation.

1. Chalk

formation. 1

Cretaceous

group.

2. Green-sand

formation.

* Conybeare, Outline, &c., p. 85.

t Fitton, Geol. Trans., Second Series, vol. iv. p. 319.

182

LYELL'S ELEMENTS OF GEOLOGY.

Fossils of the Chalk.

I shall now speak first of the chalk,
its fossils, and probable origin ; and then
say something of the green-sand ; after
which I shall pou*t out the probable rela-
tions of the chalk and green-sand to each
other.

White Chalk. — The white chalk used
in writing consists almost purely of car-
bonate of lime. Although usually soft,
this substance passes in some districts by
a gradual change into a solid stone used
for building. The stratification is often
| obscure, except where rendered distinct
by alternating layers of flint. These
« layers are from two to four feet distant
g from each other, and from three to six
inches in thickness, occasionally in con-
J tinuous beds, but more frequently in
nodules.

The annexed figures represent some
s few of the fossil shells which are abun
dant in the white chalk, and these alone
| are sufficient to prove its marine origin.
1 Some of them, such as the Terebratula3,
I4 (see Figs. 148. 150, 151, 152.) are
jj known to live at the bottom of the sea,
£ where the water is tranquil, and of some
§ depth. The Crania and Catillus (Figs.
145. and 144.) may be pointed out as
forms which, so far as our present infor-
mation extends, became extinct at the
close of the cretaceous period, and are
therefore never met with in any tertiary
stratum, or in a living state. Among
other forms, equally conspicuous among
the fossil mollusca of the cretaceous
group, and foreign to the tertiary and
recent periods, may be mentioned the
Belemnite, Ammonite, Baculite, and Tur-
rilite of the family Cephalopoda, to which
the living cuttle-fish and nautilus belong.
One of these, the Belemnite, like the bone of the common
cuttle-fish, was an internal shell. Besides these there are other
fossils in the chalk, such as sea-urchins, corals, and sponges,

PART II. CHAPTER XV.

183

Fossils of the White Chalk.

FOSSILS OF THE WHITE CHALK.
Fig. 144.

Catillus Cuvicri. (Syn. Inoceramus Cuvieri, Sow.)

Fig. 147.

Fig. 145.

Crania Parisiensis,
inferior or attached
valve.

Fig. 148.

Fig. 150.

Fig. 146.

Plagiostoma
Hoperi.

Plagiostoma spinosum.

Fig. 149.

Ostrea carinata,
also in Upper Green-sand.

Fig. 151.

Fig. 152.

Terebratula

octoplicata,

(Var. of T. plicatilis.)

Torebratula pumilus.
(Magas pumilua, Sow.)

184

LYELL'S ELEMENTS OF GEOLOGY.

Fossil Cephalopoda of Extinct Genera.

Fig. 153.

Ostrea vesicularis (Gryphaa globosa, Sow.)'
also in Upper Green-sand.

FOSSIL CEPHALOPODA OF EXTINCT GENERA.

Cretaceous Period.

Fig. 154.

a. Turrilites costatus, Gault.

b. Same, showing the indented border of the partition of the chambtra.

Fig. 155.

^.e Chalk and Upper Greenland.

Pig. 166.

Portion of Baculites Faujasii,
White Chalk and Upper Green -sand.

Fig. 157.

Portion of Baculites anceps,
White Chalk and Upper Green-sand.

PART II. CHAPTER XV.

185

Fossils of the Chalk.

(see Figures,) which are alike marine. They are dispersed
indifferently through the soft chalk and the hard flint.

Fig. 158.

rfnanchytes ovatus.

a. side view.

b. bottom of the shell on which both the oral and anal apertures are placed ;

the anal being to the right, and more oval.

Fig. 159.

Eschara disticha.
a. natural size. b. portion magnified.

To some of these inclosed zoophytes many flints owe their
irregular forms, as in the flint represented in Fig. 161., where
the hollows on the exterior are caused by the branches of a
sponge, which is seen on breaking open the flint. (See Fig, 160.)

Fig. 160.

Fig. 161.

A branching sponge in a flint from the chalk.

186 LYELL'S ELEMENTS OF GEOLOGY.

Origin of White Chalk. **

With these fossils the remains of fish and Crustacea are not
uncommon ; but we meet with no bones of land animals, nor
any terrestrial or fluviatile shells, nor any plants, except pieces
of drift-wood and sea-weed, nor any sand or pebbles ; all the
appearances concur in leading us to believe that this deposit was
formed in a deep sea, far from land, and at a time when the
European fauna was perfectly distinct from that of the tertiary
period, from which its numerous species of plants and animals
entirely differ.

Origin of the White Chalk. — Having then come to the con-
clusion, that the chalk was formed in an open sea of some depth,
we may next inquire, in what manner so large a quantity of this
peculiar white substance could have accumulated over an area
many hundred miles in diameter, and some of the extreme
points of which are distant, as we shall see in the sequel, more
than 1000 geographical miles from each other.

It was remarked in an early part of this volume, that some
even of that chalk which appears to an ordinary observer quite
destitute of organic remains, is nevertheless seen under the
microscope to be full of fragments of corals and sponges ; the
valves of Cytherina, the shells of foraminifera, and still more
minute infusoria. (See p. 41.)

Now it had been often suspected before these discoveries, that
white chalk might be of animal origin, even where every trace
of organic structure has vanished. This bold idea was partly
founded on the fact, that the chalk consisted of pure carbonate
of lime, such as would result from the decomposition of testacea,
echini, and corals, and in the passage observable between these
fossils when half decomposed into chalk. But this conjecture
seemed to many naturalists quite vague and visionary, until its
probability was strengthened by new evidence -brought to light
by modern geologists.

We learn from Lieutenant Nelson, that, in the Bermuda
islands, there are several basins or lagoons almost surrounded
and inclosed by reefs of coral. At the bottom of these lagoons
a soft white calcareous mud is formed by the decomposition of
Eschara, Flustra, Cellepora, and other soft corallines. This
mud, when driedj is undistinguishable from common white earthy
chalk ; and some portions of it, presented to the Museum of the
Geological Society of London, might, after full examination, be
mistaken for ancient chalk, but for the labels attached to them.
About the same time Mr. C. Darwin observed similar facts in the
coral islands of the Pacific ; and came also to the opinion, that
much of the soft white mud found at the bottom of the sea near
coral reefs has passed through the bodies of worms, by which

JPART II. CHAPTER XV. 187

Pebbles in Chalk, whence derived.

the stony masses of coral are everywhere bored ; and other por-
tions through the intestines of fish ; for certain gregarious fish
of the genus Spams are visible through the clear water, brows-
ing quietly, in great numbers, on living cor-
Fig. 162. Fig. 163. ais> \{\^e grazing herds of graminivorous
quadrupeds. On opening their bodies, Mr.
Darwin found their intestines filled with
impure chalk. This circumstance is the
more in point, when we recollect how the
fossilist was formerly puzzled by meeting
with certain bodies, called cones of the
larch, in chalk, which were afterwards
offish called fuio- recognized by Dr. Buckland to be the ex-
n, from the chalk. crement of fish.* These spiral coprolites
(see Figures) like the scales and bones of fossil fish in the chalk,
are composed chiefly of phosphate of lime.

Single pebbles in chalk. — The general absence of sarj.d
and pebbles in the white chalk has been already mentioned ;
but the occurrence here and there of a few isolated pebbles
of quartz and green-schist, some of them two or three inches
in diameter, in the south-east of England, has justly excited
much wonder. If these had been carried to the spots where
we now find them by waves or currents from the lands once
bordering the cretaceous sea, how happened it that no sand
or mud were transported thither at the same time ? We cannot
conceive such rounded stones to have been drifted like erratic
blocks by ice, for that would imply a cold climate in the cre-
taceous period ; a supposition inconsistent with the luxuriant
growth of large chambered univalves, numerous corals, and
many fish, and other fossils of tropical forms. (See p. 86.)

Now in Keeling Island, one of those detached masses of coral
which rise up in the wide Pacific, Captain Ross found a single
fragment of green-stone, where every other particle of matter
was calcareous ; and Mr. Darwin concludes that it must have
come there entangled in the roots of a large tree. He reminds
us that Chamisso, a distinguished naturalist who accompanied
Kotzebue, affirms, that the inhabitants of the Radack archipelago,
a group of lagoon islands, in the midst of the Pacific, obtained
stones for sharpening their instruments by searching the roots
of trees which are cast up on the beach, f

It may perhaps be objected, that a similar mode of transport
cannot have happened in the cretaceous sea, because fossil wood

* Geol. Trans. Second Series, vol. iii. p. 232. plate 31. figs. 3. and 11.
t Darwin, p. 549. Kotzebue's First Voyage, vol. iii. p. 155.

188 LYELL'S ELEMENTS OF GEOLOGY.

Cretaceous Coral Reef.

is very rare in the chalk. Nevertheless wood is sometimes met
with, and in the same parts of the chalk where the pebbles are
found, beth in soft stone and in a silicified state in flints. In
these cases it has often every appearance of having been floated
from a distance, being usually perforated by boring-shells, such
as the Teredo and Fistulana.*

The only other mode of transport which suggests itself is sea-
weed. Dr. Beck informs me, that in the Lym-Fiord, in Jutland,
the Fucus vesiculosus sometimes grows to the height of ten
feet, and the branches rising from a single root, form a cluster
several feet in diameter. When the bladders are distended, the
plant becomes so buoyant as to float up loose stones several
inches in diameter, and these are often thrown by the waves
high up on the beach. The Fucus giganteus, of Solander, so
common in Terra del Fuego, is said by Captain Cook to obtain
the length of 360 feet, although the stem is not much thicker
than a man's thumb. It is often met with floating at sea, with
shells attached, several hundred "miles from the spots where it
grew. Some of these plants, says Mr. Darwin, were found ad-
hering to large loose stones in the inland channels of Terra del
Fuego, during the voyage of the Beagle in 1834; and that so
firmly, that the stones were drawn up from the bottom into the
boat, although so heavy that they could scarcely be lifted in by
one person.f Some fossil sea-weeds have been found in the
cretaceous formation, but none, as yet, of large size.

Cretaceous coral reef in Denmark. — Having said so much
on the probable derivation of chalk from the decay of corals
and shells, I may add, that in the island of Seeland, in Denmark,
there is a yellow limestone intimately connected with the chalk,
and containing a vast number of the same fossils, which consists
of an aggregate of corals, retaining their forms as distinctly as
the dead zoophytes which enter into the structure of reefs now
growing in the sea. The thickness of this rock is unknown, but
it has been quarried at Faxoe to the depth of forty feet. At
Stevensklint, in Seeland, it is seen to rest on white chalk with
flints, from which it differs greatly in appearance, and where it
is covered again by another limestone, which although of later
date, agrees more nearly with the white chalk, both in fossils and
mineral character. Out of 104 species of sponges, corals, and
other zoophytes, collected from the limestone of Faxoe, and from
the ordinary white chalk of Denmark, which agrees with that of
England, no less than forty-two are common to both formations ;
and many of the same species of bivalve shells and echinoder-

* Mantell, Geol. of S. E. of England, p. 96.' t Darwin, p. 303.

PART U. CHAPTER XV. 189

Cretaceous Coral Reef ...................... Maastricht Beds.

mata have been found in both. The Faxoe formation, however,
is not only remarkable for the number and good preservation of
its fossil corals, but also from the generic resemblance of many
of its univalve shells to forms . usually supposed to appertain
chiefly or exclusively to the tertiary period. Thus among the
patelliform univalves, we find Patella and Emarginula, and among
the spiral, the following genera, Cyprsea, Oliva, Mitra, Cerithium,
Fusus, Trochus, Triton, Nassa, and Bulla.

The species however do not agree with those of the tertiary
strata, and are associated with cephalopoda of those extinct
families before mentioned as characteristic of the cretaceous,
and foreign to the tertiary epoch ; as, for example, the ammonite,
belemnite, and baculite. Two species, the Belemnites mucronatus
(Fig. 155.), and the Baculites Faujasii (Fig. 156.), being
common to the Faxoe beds and white chalk.

From these facts, we may conclude that the Faxoe limestone
was formed in the cretaceous sea, in a spot favourable for the
multiplication of stony corals and univalve shells ; and as some
small portions of the rock consist of white earthy chalk, this
latter substance must have been produced simultaneously, and
some of it may have been washed away, in the form of mud,
from the coral reef of Faxoe, and dispersed over the deeper parts
of the same ocean, just as the white mud, swept out of the la-
goons of the Bermudas or coral islets of the Pacific, must form
deposits of white chalk, covering much wider spaces than those
occupied by the reefs.

The same remarks apply to a rock, which reposes on the
Upper Chalk with flints, at St. Peter's Mount, Maestricht, and at
Ciply, near Mons. It is a soft yellowish stone, not very unlike
chalk, and "includes siliceous masses, which are much more
rare than those of the chalk, of greater bulk, and not composed
of black flint, but of chert and calce-
dony."* Like the Faxoestone, it is
characterized by a peculiar assemblage
of organic remains which are specifi-
cally distinct from those of the tertiary
period, but many of them common to
the white chalk.

As these Maestricht beds have been
thought to be intermediate in character
Rkotomagevsis, Between the secondary and tertiary for-

,
Maestricht ; found by Count mations, it may be proper to mention,

as opposed to this opinion, that the Am-
monite (Fig. 164.), Baculite, Hamite, and Hippurite, have been

* Fitton, Geol. Proceedings, 1830.

190 LYELL'S ELEMENTS OF GEOLOGY.

Chalk Flints.

found in the Maestricht limestone, genera which have not yet
been detected in strata newer than the chalk. In the same
formation, also, large turtles have been found, and a gigantic
reptile, the Mosasaurus, or fossil Monitor, some of the vertebrce
of which appear also in the English chalk.* The osteological
characters of this oviparous quadruped prove it to have been
intermediate between the living Monitors and Iguanas ; and, from
the size of the head, vertebras, and other bones, it is supposed
to have been twenty-four feet in length.

The existence of such turtles and saurians seems to imply
some neighbouring land, on the sandy shores of which these
creatures may have laid their eggs. But a few small islets in
mid ocean, like Ascension, so much frequented by turtles, may
perhaps have afforded the required retreat to these cretaceous
reptiles.

Origin of the Jlint in chalk. — It is difficult to give a satis-
factory explanation of the origin of the flint in chalk, whether
it occurs in nodules or continuous layers. It seems that there
was originally siliceous as well as calcareous earth in the muddy
bottom of the cretaceous sea, at least when the upper chalk was
deposited. Whether both these earths could have been alike
supplied by the decay of organic bodies may be matter of spe-
culation ; but what was said of the origin of Tripoli (see p. 39.)
shows how microscopic infusoria can give rise to dense masses
of pure flint. The skeletons of many living sponges consist of
needles or spicula of flint, and these are found very abundantly
in the flints of the chalk. There are also other living zoophytes,
which have the power of secreting siliceous matters from the
waters of the sea, just as mollusca secrete calcareous particles.

From whatever source the mud derived its silex, we may at-
tribute the parallel disposition of the flinty layers to successive
deposition. The distances between the layers, says Dr. Buck-
land, must have been regulated by the intervals of precipitation,
each new mass forming at the bottom of the ocean a bed of
pulpy fluid, which did not penetrate the preceding bed on which
it rested, because the consolidation of this last was so far advanc-
ed as to prevent such intermixture, f Nevertheless the separa-
tion of the flint into layers, so distinct from the chalk, is a sin-
gular phenomenon, and not yet accounted for. Perhaps, as the
specific gravity of the siliceous exceeds that of the calcareous
particles, the heavier flint may have sunk to the bottom of each
stratum of soft mud ?

* See Mantell's Geol. of S. E. of England.
t Geol. Trans., First Series, vol. iv. p. 420.

PART II. CHAPTER XV. 191

Area covered by Chalk Green Sand Formation.

Geographical extent of White Chalk. — The area over
which the white chalk preserves a nearly homogeneous aspect is
so great that geologists have often despaired of finding any ana-
logous deposits of recent date ; for chalk is met with in a north-
west and south-east direction, from the north of Ireland to the
Crimea, a distance of about 1140 geographical miles, and in an
opposite direction it extends from the south of Sweden to the
south of Bordeaux, a distance of about 840 geographical miles.
But we must not conclude that it was ever spread out uniformly
over the whole of this vast space, but merely that there were
patches of it, of various sizes, throughout this area. ' Now, if
we turn to those regions of the Pacific over which coral reefs
are scattered, we find some archipelagoes of lagoon islands, such
as that of the Dangerous archipelago, for instance, and that of
Radack, with some adjoining groups, which are from 1100 to
1200 miles in length, and 300 or 400 miles broad; and the
space to which Flinders proposed to give the name of the Coral-
lian sea is still larger ; for it is bounded on the east by the Aus-
tralian barrier, on the west by New Caledonia, and on the north
by the reefs of Louisiade. Although the islands in these spaces
may be thinly sown, the mud of the decomposing zoophytes
may be scattered far and wide by oceanic currents.

Green-sand formation. — The lower division of the Cretace-
ous group in England is divisible, as we have already seen, into
Upper Green-sand, Gault, and Lower Green-sand. The green
grains have been found, by analysis, to consist chiefly of silicate
of iron, and they agree in composition with chlorite. The infe-
rior white marly chalk becomes more and more charged with
these grains until it passes into the upper green-sand, a forma-
tion of sand and sandy marl, frequently mixed with chert, and
this again passes downwards into the clay and marl, provincially
called Gault. Both of these subdivisions, although often dimin-
ishing in volume to a thickness of two or three yards, form dis-
tinct and continuous bands of sand and clay between the chalk,
and lower green-sand throughout considerable tracts in England,
France, and Belgium ; and each preserves throughout this space
certain mineral peculiarities and characteristic fossils.

The lower green-sand below the gault is formed partly of
green and partly of ferruginous sand and sandstone, with some
limestone. These rocks succeed each other in the following
descending order in Kent : —

No. 1. Sand, white, yellowish, or ferruginous, with concretions of limestone and
chert, ! 70 feet.

2. Sand with green matter, 70 to 100 feet.

3. Calcareous stone, called Kentish rag 60 to 80 feet *

* Fitton, Geol. Trans., Second Series, vol. iv. p. 319.

192

LYELL'S ELEMENTS OF GEOLOGY.

Origin of the Green-sand 'Formation.

FOSSILS OF THE GREEN-SAND FORMATION.

Fig. 166.
Fig. 165.

a. Terebratula lyra. Upper Green-sand.

b. Same, seen in profile. France.

Fig. 167.

Pecten 5 costatus.

Upper and lower

green-sand.

Hamites spiniger (Fitton,) near Folkstone.*

The fossils of the green-sand are marine, and some of them,
like the Pecten quinquecostatus, (Fig. 166.) range through all
the members of the series. Several forms of cephalopoda, such
as the Hamite, (Fig. 167.) Scaphite, and others distinguish the
Green-sand formation in England from the White Chalk.

Origin of the Green-sand formation. — Unlike the white
chalk, this deposit consists of a succession of ordinary beds of
sand, clay, marl, and impure limestone, the materials of which
might result from the wearing down of pre-existing rocks. The
nature of these derivative rocks we learn, from finding in the
green-sand pebbles of quartz, quartzose sandstone, jasper, and
flinty slate, together with grains of chlorite and mica."f" But we
naturally inquire, how it could happen that, throughout a large
submarine area, there should be formed, first, a set of mechani-
cal strata, such as the green-sand, and then over the same space
a pure zoophytic and shelly limestone, such as the white chalk.
Certain causes, which during the first period gave rise to depo-

* .Fitton, Geol. Trans;, Second Series, vol. iv. pi. 12.

t Ibid. p. 116.

PART II. CHAPTER XV. 193

Green-sand Formation External Configuration of Chalk.

sits of mud, sand, and pebbles, must subsequently have ceased
to act ; for it is evident that no similar sediment disturbed the
clear waters of the sea in which the white chalk accumulated,
The only hypothesis which seems capable of explaining such
changes is the gradual submergence of land which had been pre-
viously exposed to aqueous denudation. This operation may
have gone on with such slowness as to allow time for consider-
able fluctuations in the state of the organic world, so that differ-
ent sets of strata, beginning with the lower green-sand, and end-
ing with the upper white chalk, may each contain some peculiar
remains of animals which lived successively in the sea ; while
some species may have continued to exist throughout, the whole
period, arid are therefore common to all these formations.

It will be seen in the next chapter, when we treat of the strata
called the Wealden, that such a general subsidence of land as is
here supposed to explain the manner in which the chalk succeeds
the green-sand, may be inferred from other independent proofs
to have taken place throughout large areas.

It cannot however be assumed, that all the green-sand in
Europe had ceased to be deposited before any chalk began to
accumulate. Such indeed was the order of events in parts of
England, France, Belgium, and Denmark ; but if we compare
different countries, and some of these not far distant from each
other, we find reason to believe that sand and clay continued to
be thrown down in one place, while pure chalk was forming in
another. In Westphalia, for example, strata containing the same
fossils as the white chalk of England, consist of sand and marl
with green grains like the upper green-sand. Similar facts have
been observed in Hungary in the Carpathian mountain chain.
Such variations would occur if the supposed sinking down of
land did not take place simultaneously everywhere ; and for this
reason the minor subdivisions of the cretaceous group, however
persistent and uniform in their mineral characters in some
regions, vary rapidly, and change entirely in other directions.

External configuration of Chalk. — The smooth rounded out-
line of the hills composed of white chalk is well known to all
who have travelled in the south-east of England. The chalk
downs, being free from trees or hedge-rows, afford us an oppor-
tunity of observing the manner in which the upper valleys unite
with larger ones, and how these become wider and deeper as
they descend. For the most part they are dry, yet occasionally
they afford a perfect system of drainage, when a sudden flood is
caused by heavy rains or the melting of snow. We may con-
ceive their excavation to have been caused by the action of the
waves and currents while the chalk was gradually emerging from

194

LYELL'S ELEMENTS OF GEOLOGY.

Chalk Needles.

the sea. To the same action we may ascribe the escarpments,
as they are called, or those long lines of precipitous cliffs in
which the chalk often terminates abruptly, and which, though
now inland, have been undermined by the waves when the chalk
was upheaved from the sea.

Many examples occur in England ; but there are no precipices
of chalk more striking than those which bound the lower part
of the great valley or gorge through which the Seine flows in
Normandy, At various heights on the steep sides of these hills
are outstanding pillars and pinnacles of a very hard and com-
pact chalk, as at Tournedos and Elbeuf, near Rouen, which evi-
dently owe their shape to the power of the waves. (See Figures.)

Fig. 168.

Fig. 169.

Tournedos, above Rouen.

Fig. 170.

Elbeuf. Elbeuf.

Needles and grooved pillars of chalk.

Some small columns near Elbeuf exhibit parallel and horizontal
grooves scooped out of the columns at different heights. (See
Fig. 170.) These greatly resemble certain limestone pillars,
described by Captain Bayfield, in the Mingan islands in the gulf
of St. Lawrence. There is evidence there of the coast having
been upheaved at successive periods, so that parallel ranges of
sea beaches, with recent shells, have been laid dry, terrace
above terrace. At heights corresponding to the beaches the iso-
lated masses of calcareous rock retain the marks worn by the
waves. These marks probably indicate pauses in the upheaving
process, during which the sea had a considerable time to wear

PART II. CHAPTER XV.

195

Chalk Needles

Cretaceous Group.

away the stone, as well as to throw up a beach at the same
level.*

The needles of the Isle of Wight, and the Old Harry Rocks
of the coast of Dorsetshire, are well known to those who have
examined the chalk cliffs of the South of England. Besides the
inland columns in Normandy, above described, there are others
more recently formed on the sea coast of that same country.

Fig. 171.

Needle and Arch of Etretat, in the chalk cliffs of Normandy,
Height of Arch 100 feet. (Passy.)f

If we inquire at what period the emergence and denudation
'of the cretaceous rocks took place, we shall find that it occurred
in great part after the deposition of various rnarine tertiary forma-
tions, so that both the cretaceous and tertiary beds were upraised
together. The greatest elevation which the chalk reaches in
England, is the summit of Inkpen Beacon, in Berkshire, which
is 1011 feet above the sea; but marine deposits of the same age
attain an elevation of 8000 feet in the Alps and Pyrenees.
These may have partly emerged during the cretaceous period,
just as the coral reefs in some regions of the Pacific are grow-
ing in one spot, while other portions of. the same have been
uplifted by subterranean forces, and converted into land.

Difference between the chalk of the north and south of
Europe. — By the aid of the three tests of relative age, namely,
superposition, mineral character, and fossils, the geologist has
been enabled to refer to the same cretaceous period certain rocks
in the north and south of Europe, which differ greatly both in
their fossil contents, and in their mineral composition and struc-
ture.

If we attempt to trace the cretaceous deposits from England

* Captain Bayfield, Geol. Trans., Second Series, vol. v. p. 94. Also Principles
of Geology, Index, " Niapisca island."

t Seine-Inferieure, p. 142. and plate 6. fig. 1,

196

LYELL'S ELEMENTS OF GEOLOGY.

Different Character of Chalk in South of Europe.

Fig. 172.

and France to the countries bordering the Mediterranean, we
perceive, in the first place, that the chalk and green-sand in the
neighbourhood of London and Paris, form one great continuous
mass, the strait of Dover being a trifling interruption, a mere
valley with chalk cliffs on both sides. We then observe that the
main body of the chalk which surrounds Paris stretches from
Tours to near Poitiers, (see the annexed map (Fig. 172.), in
which the shaded part represents chalk.)

Between Poitiers and La Ro-
chelle, the space marked A on the
map separates two regions of
chalk. This space is occupied by
the oolite and certain other forma-
tions older than the chalk, and has
been supposed by M. E. de Beau-
mont to have formed an island in
the cretaceous sea. South of this
space we again meet with a for-
mation which we at once recog-
nize by its mineral character to be
chalk, although there are some
places where the rock becomes
oolitic. The fossils also are upon
the whole very similar, although"
some new forms now begin to ap-
pear in abundance, which are rare
or wholly unknown further to the
north. Among these may be men-
tioned many Hippurites, Sphasru-
lites, and other members of that

great family of mollusca called Rudistes by Lamarck, to which
nothing analogous has been discovered in the living creation.
Although very uncommon in England, one species of this family
has been discovered in our chalk.

Fiff. 174.

Fig. 175.

Hippurites Mortoni, Mantell. Maidstone, Kent.
Diameter one-seventh of nat. size.

PART II. CHAPTER XV.

197

Different Character of Chalk in South of Europe.

Fig. 173. Two individuals deprived of their opercula, adhering together

174. Same seen from above.

175. Transverse section of part of the wall of the shell, magnified to show

the structure.

176. Vertical section of the same.

On the side where the shell is thinnest, there is one external furrow and cor-
responding internal ridge, a. 6. Figs. 173, 174. ; but they are usually less promi-
nent than in these figures. This species has been referred to Hippurites, but
does not, I believe, fully agree in character with that genus. I have never seen
the opercular piece, or valve, as it is called by those conchologists who regard the
Rudisles as bivalve mollusca.

But this family, which is so feebly represented in England
and the north of France, becomes quite characteristic of rocks
of the cretaceous era in the south of France, Spain, Greece, and
other countries bordering the Mediterranean.

Fig. 177.

'. O^Sar%alve of H. radios*. \ Lower chalk> South of France"

Fig. 178.

Fig. 179.

Spharulites agariciformis.

Hippurites organisans. Desm. Pyrenees.

Between the region of chalk last mentioned in which Perigeux
is situated, and the Pyrenees, the space B intervenes. (See Map.)

Here the tertiary strata cover, and for the most part conceal,
the cretaceous rocks, except in some spots where they have
been laid open to view by the denudation of the newer forma-
tions. In these places they are seen still preserving the form of
a white chalky rock, which is filled in part with grains of green-
sand. Even as far south as Tercis, on the Adour, near Dax, it
retains this character where I have examined it, and where M.
Grateloup has found in it Ananchytes ovata (Fig. 158.), and
other fossils of the English chalk, together with Hippurites.
When we arrive at Bayonne and the Pyrenees, the cretaceous

198

LYELL'S ELEMENTS OF GEOLOGY.

Different character of Chalk in South of Europe.

formation, although still exhibiting some of the same mineralo-
gical peculiarities, is nevertheless greatly changed. Its calca-
reous division consists for the most part of compact crystalline
marble, often full of nummulites (see Fig. 180.), and those por-

Fig. 180.

Nummulite limestone ; Peyrehorade, Pyrenees.

a External surface of one of the nummulites, of which longitudinal sections are

seen in the limestone.
b Transverse section of same. %

tions which may be imagined to represent the green-sand, are
composed of shales, grits, and micaceous sand-stone, containing
impressions of marine plants, together with lignite and coal.
There are also beds of red sandstone and conglomerate, belong-
ing to the same group. These rocks ascend gradually into the
highest part of the Pyrenees, and cross over into Spain, where
the cretaceous system assumes a character still more unlike that
of northern Europe.

Here, as on the north side of the Pyrenees, the most conspicu-
ous fossils are .hippurites, sphaBrulites, and nummulites. The
last-mentioned fossil, so called from its resemblance to a piece
of money, is a genus of mollusca, very abundant in the tertiary
strata of Northern Europe ; but only met with in chalk in the
South of Europe.

So many species and genera of shells now wanting in our
northern seas, are frequent in the Mediterranean, that we need
not be surprised, when following from north to south the deposits
of the old cretaceous sea, at finding similar modifications in or-
ganic forms.

The cretaceous rocks in the Alps, Italy, Greece, and Asia
Minor, are distinct in like manner from the type of that forma-
tion in the North of Europe ; yet their age in most of these
countries can be clearly ascertained, partly by following them
continuously from the north in the manner above described ; and
partly by their position below the tertiary, and above the oolitic
strata.

PART II. CHAPTER XV. 199

Ancient Submergence of South of Europe Terminology.

We learn from the researches of M. M. Boblaye, and Virlet,
(hat the cretaceous system in the Morea, is composed of compact
and lithographic limestones of great thickness ; also of granular
limestones, with jasper ; and in some districts, as in Messenia,
a puddingstone, with a siliceous cement more than 1600 feet in
thickness, belongs to the same group.f

It is evident, observe these geologists, from the great range
of the hippurite and nummulite limestone, that the South of
Europe was occupied at the cretaceous period by an immense
sea, which extended from the Atlantic Ocean into Asia, and com-
prehended the southernmost part of France, together with Spain,
Sicily, part of Italy, and the Austrian Alps, Dalmatia, Albania,
a portion of Syria, the isles of the ./Egean, coasts of Thrace, and
the Troad.

In proportion, therefore, as we enlarge the sphere of our re-
searches, we may find in the strata of one era, the mineralogical
counterparts of the rocks, which, in a single country like Eng-
land, may characterise successive periods. Thus, the grits,
sandstone, and shale with coal, of the Pyrenees have actually
been mistaken by skilful miners for the ancient carboniferous
group of England and France. In like manner the cretaceous
red marl and salt of northern Spain have been regarded as the
same as our new red and saliferous sandstone ; and the litho-
graphic limestone of the Morea might be confounded with the
oolite of Solenhofen in Germany.

The beginner, perhaps, on hearing these facts, may object to
the term cretaceous, as applied to the rocks of the southern re-
gion in which there is no chalk. But the term green-sand
would have been equally inappropriate as a general name for
this group ; and that of hippurite and nummulite limestone, how-
ever well suited to the Mediterranean region, would be inappli-
cable to the chalk of the north. Scarcely any designation would
remain unexceptionable as we enlarge the bounds of our know-
ledge, and we must therefore be content to retain many ancient
names, as simply expressing the mineral or palseontological cha-
racters of rocks in the country where they were first studied.

*Bull. de la Soc. Geol. de France, torn. iii. p. 149.

200 LYELL'S ELEMENTS OF GEOLOGY.

Wealdeii Group.

CHAPTER XVI.

WEALDEN GROUP.

The Wealden, including the Weald clay, Hastings sand, and Purbeck beds —
Intercalated between two marine formations — Fossil shells freshwater, with a
few marine — Cypris — Fish — Reptiles — Birds — Plants — Section showing passage
of Wealden beneath chalk — Junction of Wealden and Oolite — Dirt-bed — Theory
of gradual subsidence — Proofs that the Wealden-strata, notwithstanding their
thickness, may have been formed in shallow water — Geographical extent of
Wealden — Bray near Beauvais — Relation of the Wealden to the Lower Green-
sand and Oolite.

BENEATH the cretaceous rocks in the S. E. of England, a
freshwater formation is found, called the Wealden, which, al-
though it occupies a small area in Europe, as compared to the
chalk, is nevertheless of great interest, as being intercalated be-
tween two marine formations. It is composed of three minor,
groups, of which the aggregate thickness in some places cannot
be less than 800 feet.* These subdivisions are,

Thickness.
1st. Weald clay, sometimes including thin beds of sand and

shelly limestone *. 140 to 280 ft.

2d. Hastings sand, in which occur some clays and calcareous

grits ;— between 400 and 500 ft.

3d. Purbeck beds, consisting of various kinds of limestones and

marls about 250 ft.

To all these subdivisions, the common name of the Wealden
has been given, because they may be best studied in part of
Kent, Surrey, and Sussex, called the Weald.

We have seen that the fossils of the chalk and green-sands
which repose upon the Wealden are all marine, and the species
numerous ; and the same remark applies to the Portland stone
and other members of the Oolitic series which lie immediately
beneath (see Fig. 181.). But in the Wealden itself, although
the fossils are abundant as to quantity, the number of different
species is comparatively small, and by far the greater part of
them show that they were deposited in a freshwater lake, or es-
tuary communicating with the sea.f

Dr.Fitton, Geol. Trans, vol. iv. p. 320. Second Series. tlbid. p. 104.

PART II. CHAPTER XVI.

201

Wealden Group Fossils.

Fig. 181.

marine

freshwater

Weald Clay, )
Hastings sand, > Wealden.
Purbeck beds, ;

Oolite.

Position of the Wealden between two marine formations.

Fossils of the Wealden. — The shells of this formation are
almost exclusively of fluviatile or lacustrine genera, such as Me-
lanopsis, Paludina, Neritina, Cyclas, Unio, and others. The indi-
viduals are sometimes in such profusion, that the surface of each
thin layer of marl or clay is covered with the valves of Cyclas, and
whole beds of limestone are almost entirely composed of Palu-
dinse. Intermixed with these freshwater shells, there are a few
which seem to mark the occasional presence of salt water, as for
example, a species of Bulla, together with an Oyster, and the
Exogyra, a genus of unimuscular bivalves allied to the oyster (see
Fig. 182.). The conclusion to be drawn from the presence of a
Corbula (see Fig. 183.) and Mytilus is more doubtful; for al-

Fig. 182.

Fig. 183.

Exogyra bulla. Fitton.

Corbula alata. Fitton. Magnified.

though these genera are for the most part marine, still there is a
Mytilus living in the Danube, and one species of Corbula in-
habits the river La Plata, in South America, as well as the ad-
joining sea, while another is common to the Caspian, and the
rivers Don and Wolga. But admitting all these to have been
marine, they by no means outweigh the evidence, both of a posi-
tive and negative kind, derived from shells in favour of the fresh-
water origin of the Wealden. In no part of this deposit do we
meet with ammonites, belemnites, terebratulse, corals, sea-urchins,
or other testacea and zoophytes so characteristic of the chalk
above, or the oolite below the Wealden.

202

LYELL'S ELEMENTS OF GEOLOGY.

Wealden Group Fossils.

Fig. 184.

Fig. 185.

Fig. 186.

Cypris spini-
gera, Fitton.

Cypris Valdensis, Fitton. (C.faba,
Min. Con. 485.)

Ci/pris tuberculata,
Fitton.

Fig. 187.

Shells of the Cypris, an animal allied to the Crustacea, and
before mentioned (p. 47.) as abounding in lakes and ponds, are
also plentifully scattered through the. clays
of the Wealden, sometimes producing like
plates of mica, a thin lamination (see Fig.
187.). Similar cypriferous marls are found
in the lacustrine tertiary beds of Auvergne,
and in recent deposits of shell marl.

The fishes of the Wealden belong partly
to the genera Pyenodus and Hybodus, forms
common to the Wealden and Oolite (see Fig.
225.); but the teeth and scales of a species of Lepidotus are
most widely diffused (see Fig. 188.). The general form of these
fish was that of the carp tribe, although perfectly distinct in ana-
tomical character, and more allied to the pike. The whole body
was covered with large rhomboidal scales very thick, and having
the exposed part covered with enamel. Most of the species of

Fig. 188.

a. palate and teeth.

Lepidotus Mantelli. Agass. Wealden.
b. side view of teeth.

c. scale.

this genus are supposed to have been either river fish, or inhab-
itants of the coasts, having not sufficient powers of swimming to
advance into the deep sea.

Among the remains of vertebrata, those of reptiles form the
most remarkable feature. Some of them belong to tortoises,
such as the Trionyx and Emys, genera now occurring in fresh-

PART II. CHAPTER XVI. 203

Wealden Group Fossil Plants.

water in tropical regions. Of Saurian lizards there are at least
five genera ; the Crocodile, Plesiosaur, Megalosaur, Iguanodon,
and Hyteosaur. The Iguanodon, of which the remains were
first discovered by Mr. Mantell, was an herbivorous reptile, and
was regarded by Cuvier as more extraordinary than any with
which he was acquainted ; for the teeth, though bearing a great
analogy to the modern Iguanas which now frequent the tropical
woods of America and the West Indies, exhibit many striking
and important differences (see Fig. 190.). It appears that they
have been worn by mastication ; whereas the existing herbivo-
rous reptiles clip and gnaw off the vegetable productions on

which they feed, but do not chew

Teeth of Iguanodon. them. Their teeth, when worn,

Fig. 190. present an appearance of having
been chipped off, and never, like the
fossil teeth of the Iguanodon, have a

Fig". 189. §§llli}.) ^at grounc' surface, (see Fig. 189.),

resembling the grinders of herbivo-
rous mammalia. From the large
bones, found in great numbers near
these teeth, and fairly presumed to
belong to the same animal, it is
computed that the entire length of
Crown of tooth in Pointed tooth of this reptile could not have been less

adult, worn down, a young animal. ,1

Mantcii. Manteii. than seventy feet.

The bones of birds of the order

GrallaB or Waders have been discovered by Mr. Mantell in the
Wealden, and appear to be the oldest well authenticated exam-
ples of fossils of this class hitherto found in Great Britain.* But
no portion of the skeleton of a mammiferous quadruped has yet
been met with. •

The vegetable remains, which are numerous, exhibit many
characters of a tropical flora, some being allied to the living
genera Cycas and Zamia, (see Fig. 194.,) others to large Equi-
seta. There are also Coniferse allied to Araucaria, and other
genera of warm climates, (see Fig. 191.) besides numerous
ferns. (See Fig. 192.)

Passage of Wealden beneath Chalk. — It has been already
seen that the chalk and green-sand have an aggregate thickness
of 1000 or sometimes 1500 feet. It is therefore a wonderful
fact that after penetrating these rocks, we come down upon a
subjacent freshwater formation ' from 800 to 1000 feet in thick-
ness. The order of superposition is clear, for we see the weald

* Mantell. Proceedings Geol. Soc. vol. ii. p. 203.

204

LYELL'S ELEMENTS OF GEOLOGY.

Position of Wealden.

Fig. 191.

Cone from the Isle of Purbeck,
resembling the Dammara of
the Moluccas. Fitton.

Sphenopteris gracilis (Fitton), from

near Tunlridge Wells.
a. portion of the same magnified.

clay passing beneath the green-sand in various parts of Surrey,
Kent, and Sussex ; and if we proceed from Sussex westward to
the Vale of Wardour, we there again observe the same forma-
tion occupying the same relative position, and resting on the

Fig. 193.

Vale of Wardour. Wilts.

Hants.

Sussex.

Wealden.

Wealden.

oolite. (See Fig. 193.) Or if we pass from the base of the
south downs in Sussex, and cross to the Isle of Wight, we there
again meet with the same series reappearing beneath the green-
sand, and we cannot doubt that the beds are prolonged subterra-
neously, as indicated by the dotted lines in Fig. 194.

Isle of Wight.

It has been already suggested that, during the accumulation
of the green-sand, there was a gradual sinking down and sub-
mersion of land, by which the wide open sea of the chalk was
produced. But the position of the Wealden points still more
forcibly to such a conclusion, and especially the appearances
exhibited at the point of junction of the wealden, and the oolitic

PART n. CHAPTER xvi. 205

Portland Dirt-Bed Fossil Forest in Isle of Portland.

formation on which it rests. First, in regard to its junction with
the superincumbent lower green-sand, the beds of this last, says
Dr. Fitton, repose in the south-east of England, conformably
upon those of the subjacent weald clay. There is no indication
of disturbance : " To all appearance the change from the depo-
sition of the freshwater remains to that of the marine shells,
may have been effected simply by a tranquil submersion of the
land to a greater depth beneath the surface of the waters."*

Portland dirt-bed and proofs of subsidence. — But when we
examine the contact of the Purbeck beds, or inferior division of
the wealden, with the Portland stone, or upper member of the
oolite, some very singular phenomena are observed. Between
the two formations, the marine and the freshwater, there inter-
venes in Portland a layer of dark matter, called by the quarry-
men the " Dirt," or " Black dirt," which appears evidently to
have been an ancient vegetable soil. It is from twelve to eighteen
inches thick, is of a dark brown or black colour, and contains a
large proportion of earthy lignite. Through it are dispersed
rounded fragments of stone, from three to nine inches in diame-
ter, in such numbers that it almost deserves the name of gravel.
Many silicified trunks of coniferous trees, and the remains of
plants allied to the Zamia and Cycas are buried in this dirt-bed.
(See figure of living Zamia.)

Fig. 195.

Zamia spiralis; Southern Australia-!

These plants must have become fossil on the spots where they
grew. The stumps of the trees stand erect for a height of from
one to three feet, and even in one instance to six feet, with their
roots attached to the soil at about the same distances from one

* Geol. of Hastings, p. 28. t See Flinder's Voyage.

S

206 LYELL'S ELEMENTS OF GEOLOGY.

Fossil Forest in Isle of Portland.

another as the trees in a modern forest.* The carbonaceous
matter is most abundant immediately around the stumps, and
round the remains of fossil Cycadete.^

. Besides the upright stumps above mentioned, the dirt-bed con-
tains the stems of silicified trees laid prostrate. These are partly
sunk into the black earth, and partly enveloped by a calcareo-
siliceous slate which covers the dirt-bed. The fragments of the
prostrate trees are rarely more than three or four feet in length ;
but by joining many of them together, trunks have been restored
having a length from the root to the branches of from 20 to 23
feet, the stems being undivided for^l? or 20 feet, and then forked.
The diameter of these near the roots is about one foot.ij: Root-
shaped cavities were observed by Professor Henslow to descend
from the bottom of the dirt-bed into the subjacent Portland stone,
so that the uppermost beds of the Portland limestone, though
now solid, were in a soft and penetrable state when the trees
giew.§

The thin layers of calcareous slate (Fig. 196.) were evidently

Fig. 196.

freshwater calcareous
slate.

dirt-bed and ancient
forest.

marine Portland
stone.

V s

Section in Isle of Portland, Dorset. (Buckland and De la Beche.)

deposited tranquilly, and would have been horizontal but for the
protrusion of the stumps of the trees, around the top of each of
which they form hemispherical concretions.

The dirt-bed is by no means confined to the island of Port-
land, but is seen in the same relative position in a cliff east of
Lul worth Cove, in Dorsetshire, where, as the strata have been
disturbed, and are now inclined at an angle of 45°, the stumps
of the trees are also inclined at the same angle in an opposite
direction — a beautiful illustration of a change in the position of

* Mr. Webster first noticed the erect position of the trees and described the
Dirt-bed. The account here given is drawn from Dr. Buckland and Mr. De la
Beche, Geol. Trans., Second Series, vol. iv. p. 1.; Mantell, Geol. of S. E. of
England, p. 336. ; and Dr. Fitton, Geol. Trans., Second Series, vol. iv. p. 220.

t Fitton, ibid. pp. 220, 221. $ Ibid.

$ Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. p. 16.

PART II. CHAPTER XVI.

207

Fossil Forest in Isle of Portland.

Fig. 197.

freshwater calcareous

slate,
dirt-bed.

Portland stone of ma-
rine formation.

Section in cliff east of Lulworth Cove. (Buckland and De la Beche.)

beds originally horizontal. (See Fig. 197.) Traces of the dirt-
bed have also been observed by Dr. Buckland, about two miles
north of Thame, in Oxfordshire ; and by Dr. Fitton, in the cliffs
of the Boulonnois, on the French coast: but, as might be
expected, this freshwater deposit is of limited extent when com-
pared to most marine formations.

From the facts above described, we may infer, first, that the
superior beds of the oolite, which are full of marine shells,
became dry land, and covered by a forest, throughout a portion
of the space now occupied by the south of England, the climate
being such as to admit the growth of the zamia and cycas.
2dly. This land at length sank down and was submerged with
its forests beneath a body of freshwater, from which sediment
enveloping fluviatile shells was deposited. 3dly. " The regular
and uniform preservation of this thin bed of black earth over a
distance of many miles, shows that the change from dry land to
the state of a freshwater lake or estuary, was not accompanied
by any violent denudation, or rush of water, since the loose
black earth, together with the trees which lay prostrate on its
surface, must inevitably have been swept away had any such
violent catastrophe then taken place."*

The dirt-bed has been described above in its most simple form,
but in some sections the appearances are more complicated.
The forest of the dirt-bed was not everywhere the first vegeta-
tion which grew in this region. Tw^j other beds of carbona-
ceous clay, one of them containing CycadetB in an upright posi-
tion, have been found below it,f which implies other oscillations
in the level of the same ground, and its alternate occupation by
land and water more than once. There must have been, first,

* Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. p. 16.
t Fitton, Geol. Trans., Second Series, vol. iv. p. S23.

208 LYELL'S ELEMENTS OF GEOLOGY.

Wealden Strata formed in Shallow Water.

the sea in which the corals and shells of the oolite grew ; then,
land, which supported a vegetable soil with Cycadese ; then, a
lake or estuary, in which freshwater strata were deposited ; then,
again, land, on which other Cycadese and a forest of dicotyledo-
nous trees flourished ; then, a second submergence under fresh-
water, in which the wealden strata were gradually formed ; and,
finally, in the cretaceous period, a return over the same space
of the ocean.

To imagine such a series of events will appear extravagant
and visionary to some who are not aware that similar changes
occur in the ordinary course of nature ; and that large areas
near the sea are now subject to be laid dry, and then submerged,
after remaining years covered with houses and trees.*

In some of these modern revolutions, such as have been wit-
nessed in the delta of the Indus, in Cutch, we have instances of
land being permanently laid under the waters, both of the river
and the sea, without the soil and its shrubs being swept away ;
but such preservation of an ancient soil must be a rare exception
to the general rule, for it would be destroyed by denuding waves
and currents, unless the land sank suddenly down to a great
depth, or unless its form was such as to exclude the free ingress
of the sea.

Notwithstanding the enormous thickness of the wealden, ex-
ceeding in some places perhaps 1000 feet, there are many
grounds for believing that the whole of it was a deposit in water
of moderate depth, and often extremely shallow. This idea may
seem startling at first, yet such would be the natural consequence
of a gradual and continuous sinking of the ground in an estuary
or bay, into which a great river discharged its turbid waters.
By each foot of subsidence, the fundamental rock, such as the
Portland oolite, would be depressed one foot farther from the sur-
face of the ocean ; but the bay would not be deepened if new
strata of mud and sand should raise the bottom one foot. On
the contrary, such sand and mud might be frequently laid dry
at low water, or overgrown for a season by a vegetation proper
to marshes. At different heights in the Hastings Sand in the
middle of the Wealden, we find again and again slabs of sand-
stone with a strong ripple-mark, and between these slabs beds
of clay many yards thic£. In some places, as at Stammerham,
near Horsham, there are indications of this clay having been
exposed so as to dry and crack before the next layer was thrown
down upon it. The open cracks .in the clay have served as

* For an account of recent movements of land attended by such conse-
quences, see Principles of Geology, Index, " Cutch," " Sindree," &c.

PART II. CHAPTER XVI.

209

Wealden Group Its Geographical Extent.

198.

Underside of slab of sandstone, about one yard in diame
ter ; Stammerham, Sussex.

moulds, of which
casts have been taken
in relief, and which
are, therefore, seen
on the lower surface
of the sandstone* (see
Fig. 198.).

Near the same
place a reddish sand-
stone occurs in which
are innumerable tra-
ces of a fossil vegeta-
ble, apparently Sphe-
nopteris, the stems
and branches of which
are disposed as if the
plants were standing
erect on the spot where they originally grew, the sand having
been gently deposited upon and around them ; and similar ap-
pearances have been remarked in other places in this formation.!
In the same division also of the wealden, at Cuckfield, is a bed
of gravel or conglomerate, consisting of water-worn pebbles of
quartz and jasper, with rolled bones of reptiles. These must
have been drifted by a current, probably in water of no great
depth.

The occasional presence of oysters in the Purbeck limestone,
and throughout the Hastings sand and Weald clay, proves that
the waters of the sea sometimes found access into the estuary ,:{:
whether in consequence of subsidence, or in seasons when the
body of freshwater was lessened in volume.

Geographical extent. — The Wealden strata have been traced
about 200 English miles from west to east, from Lul worth Cove
to near Boulogne, in France, and about 220 miles from north-
west to south-east, from Whitchurch, in Buckinghamshire, to
Beauvais, in France. If the formation be continuous throughout
this space, which is very doubtful, it does not follow that the
whole was contemporaneous ; because in all likelihood the phy-
sical geography of the region underwent frequent change through-
out the whole period, and the estuary may have altered its form,
and even shifted its place. Yet some modern deltas are of vast
size, as for example, that of the newly discovered Quorra, or

* Observed by Mr. Mantell and myself, in 1831.
t Mantell, Geol. of S.E. of England, p. 244.
\ Fitton, Geol. Trans., 2d Ser., vol. iv, p. 321.

210 LYELL'S ELEMENTS OF GEOLOGY.

Origin of Wealden Group.

Niger, in Africa, which stretches into the interior for more than
170 miles, and occupies, it is supposed, a space of more than 300
miles along the coast; thus forming a surface of more than
25,000 square miles, or equal to about one half of England.*

I have stated that the Wealden has been observed near Beau-
vais, in France ; and the locality is marked in the section, at p.
196. It is called "the country of Bray;" and resembles in
structure the English Weald beneath the north and south downs.
In a similar manner the green-sand crops out from beneath the
chalk, and fresh- water strata from beneath the green-sand. One
member of the series, a fine whitish sand, contains impressions
of ferns, considered by M. Adolphe Brongniart as identical with
Lonchopteris Mantelli, a plant frequently found in the Wealden.
I examined part of the valley of Bray in company with M.
Graves, in 1833, and I observed that the sand last mentioned,
with its vegetable remains, was intercalated between two sets of
marine strata, containing trigonise, and referred by French geol-
ogists to the lower green-sand. In the same country of Bray,
and associated with the same formation, is a limestone resem-
bling the Purbeck marble, and containing a Paludina which
seems specifically identical with that of Purbeck.

If it be asked where the continent was placed from the ruins
of which the Wealden strata were derived, and by the drainage
of which a great river was fed, we are half tempted to speculate
on the former existence of the Atlantis of Plato. The story of
the submergence of an ancient continent, however fabulous in
history, may be true as a geological- event. Its disappearance
may have been gradual ; and we need not suppose that the rate
of subsidence was hastened at the period when the displacement
of a great body of freshwater by the cretaceous sea took place.
Suppose the mean height of the land drained by the river of the
Wealden estuary to have been no more than 800 or 1000 feet;
in that case, all except the tops of the mountains would be
covered as soon as the fundamental oolite and the dirt-bed were
sunk down about 1000 feet below the level which they occupied
when the forest before-mentioned was growing. Towards the
close of the period of this subsidence, both the sea would en-
croach and the river diminish in volume more rapidly ; yet in
such a manner, that we may easily conceive the sediment at
first washed into the advancing sea to have resembled that pre-
viously deposited by the river in the estuary. In fact, the
upper beds of the Wealden, and the inferior strata of the lower

*Fitton, Geoi. of Hastings, p. 58. ; who cites Lander's Travels.

PART II. CHAPTER XVI. 211

Relative Age of the Wealden Group.

green-sand, are not only conformable, but of similar mineral
composition.

It is also a remarkable fact, that the same Iguanodon Man-
telli which is so conspicuous a fossil in the Wealden, has recently
been discovered near Maidstone, in the overlying Kentish rag, or
marine limestone of the lower green-sand. Hence we may infer
that some of the saurians which inhabited the country of the
great river, continued to live when part of the country had
become submerged beneath the sea. Thus, in our own times,
we may suppose the bones of large alligators to be frequently
entombed in recent freshwater strata in the delta of the Ganges.
But if part of that delta should sink down so as to be covered
by the sea, marine formations might begin to accumulate in the
same space where freshwater beds had previously been formed;
and yet the Ganges might still pour down its turbid waters in
the same direction, and carry the carcasses of the same species
of alligator to the sea, in which case their bones might be in-
cluded in marine as well as in subjacent freshwater strata.

Age of the Wealden. — Some geologists have classed the
Wealden as a member of the cretaceous group, while others have
considered it as more nearly connected with the antecedent
oolitic deposits ; nor is it easy to decide which opinion is prefer-
able, because the organic remains of the cretaceous and oolitic
groups are marine, while those of the interposed Wealden are
almost all freshwater. The testacea and plants of the latter
appear as yet to be specifically distinct from those of any other
formation ; but if we examine the reptiles, it appears that the
Megalosaurus Bucklandi is common to the Oolite and Wealden,
the teeth and bones of this great Saurian occurring both in the
limestone of Stonesfield and in the Hastings sand.

There are also some generic forms, both of reptiles and fish,
common to the Oolite and Wealden, and not yet discovered in
the Chalk. VertebraB, for example, of the Plesiosaurus are not
confined to the oolite and lias, but have been also found in the
Wealden ; and the Lepidotus, a gejius of fish very characteristic
of the Wealden, is unknown in the cretaceous group, while it is
abundant in the oolitic series.

On the other hand, the same species of Iguanodon has been
already mentioned as decidedly common to the Wealden and
green-sand.

In Scotland, and in different parts of the Continent, marine
deposits have been found which are supposed to have been coe-
val with the Wealden, and which are intermediate in fossil cha-
racters as in position between the Cretaceous and Oolitic sys-

212 LYELL'S ELEMENTS OF GEOLOGY.

Absence of Mammalia in the Wealden.

terns.* They may have been contemporaneous deltas of other
rivers flowing from the same ancient continent.
~- Absence of mammalia. — Among the numerous fossils of the
Wealden, no remains of mammalia have been hitherto detected ,
whereas we should naturally expect, on examining the deposits
recently formed at the mouths of the Quorra, Indus, or Ganges,
to find, not only the bones of birds, and of amphibious and land
reptiles, but also those of such warm-blooded quadrupeds as fre-
quent the banks of rivers, or, like the hippopotamus, inhabit
their waters. Would not the same current of water which drifted
down and rolled the bones of the lizards, tortoises, and fish of
the Wealden, have also swept down into the delta some frag-
ments at least of mammiferous bones, had any animals of the
highest class been then in existence ? As a general rule, indeed,
we cannot lay much stress on mere negative evidence ; and it
may be well to notice, that although so many teeth of the Iguan-
odon have been collected, it is only of late that a single small
portion of a jaw of one of these gigantic lizards was obtained.
Perhaps, in like manner, some bone or tooth of a fossil quadru-
ped will one day be found. We may at least say, that we have
at present no example of a continent covered with a luxuriant
vegetation, and forests inhabited by large saurians, both aquatic
and terrestrial, and by birds, yet at the same time entirely desti-
tute of warm-blooded quadrupeds. The nearest analogy to this
state of things is that of New Zealand ; and this fact will be
more particularly alluded to in the sequel. (See p. 256.)

In conclusion I may remark, that from the time of the com-
mencement of the Wealden, to far on in the cretaceous period,
we have signs of subsidence, and consequent diminution of land.
But after the chalk was formed, or during the tertiary periods,
we have, on the contrary, proofs of an increase of land in
Europe. But we must not extend these generalizations to the
whole surface of the globe ; for other large areas may have been
growing more and more continental during the cretaceous, and
more and more oceanic during the tertiary periods, the direction
of the prevailing subterranean movement being reversed.

* See Fitton, Geol. Trans., Second Series, vol. iv. p. 328., and his references.

PART II. CHAPTER XVII. 213

Divisions of the Oolite.

CHAPTER XVII.

OOLITE AND LIAS.

Subdivisions of the Oolitic group — Fossil shells — Corals in the calcareous divi-
sions only — Buried forest of Encrinites in Bradford clay— Changes in organic
life during accumulation of Oolites — Characteristic fossils — Signs of neighbour-
ing land and shoals — Supposed cetacea in Oolite — Oolite of Yorkshire and Scot-
land.

OOLITE. — Below the freshwater group last described, or,
where this is wanting, immediately beneath the Cretaceous for-
mation, a great series of marine strata, commonly called " the
Oolite," occurs in many parts of Europe. This group has been
so named, because, in England and other places where it was
first examined, the limestones belonging to it had an oolitic
structure. (See p. 27.) These rocks occupy in England a zone
which is nearly thirty miles in average breadth, and extends
across the island, from Yorkshire on the north-east, to Dorset-
shire on the south-west.* Their mineral characters are not uni-
form throughout this region ; but the following are the names
of the principal subdivisions observed in the central and south-
eastern parts of England : —

OOLITE.

TT__D1. f a. Portland stone and sand.
Upper{ 6. Kimmeridge clay.
Tiyfj^i { c" Coral rag.
Mlddle U Oxford clay.

( e. Cornbrash and Forest Marble.
Lower <f. Great Oolite and base of Fullers' earth.

£ g. Inferior Oolite.
The Lias then succeeds to the Inferior Oolite.

The upper oolitic system of the above Table has usually the
Kimmeridge clay for its base, and the middle oolitic system the
Oxford clay. The lower system reposes on the Lias, an argillo-
calcareous formation, which some include in the lower oolite, 'but
which will be treated of separately in the next chapter. Many
of these subdivisions are distinguished by peculiar organic

* For details respecting this formation in England, see Conybeare and Phil-
lips's Geology, chap. iii.

214 LYELL'S ELEMENTS OF GEOLOGY.

Oolite Group Fossil Corals.

remains ; and though varying in thickness, may be traced in
certain directions for great distances, especially if we compare
the part of England to which the above-mentioned type refers
with the north-west of France, and the Jura mountains, which
separate that country from Switzerland, and in which, though
distant above 400 geographical miles, the analogy to the English
type above mentioned is more perfect than in Yorkshire or Nor-
mandy.

To enter upon a systematic description of this complicated
series of strata would require many chapters ; the following
facts, therefore, are selected from a multitude of others, with a
view of illustrating the origin of the oolitic rocks, and of show-
ing the state of organic life arid geographical condition of part
of the globe when they were formed.

In almost all the minor divisions enumerated in the above
Table, Ammonites and Belemnites are found, (see Figs. 213.
215.) but of species different from those of the cretaceous period.
The ammonites are of various sizes, from the size of a small
carriage-wheel to less than an inch diameter.

It is not uncommon to find belemnites in different members of
the series, with full-grown serpulae attached to them. As these
shells, like the bone of the cuttle-fish, so often thrown on our
shores, were internal, it is clear, that after the death of the
cephalopod the belemnite remained for some time unburied at the
bottom of the sea, so that the serpulse grew upon it.

These cephalopoda, swimming about in the open sea, left their
shells to be imbedded indifferently in whatever sediment was
then in the course of deposition, whether calcareous or argilla-
ceous. But the corals are almost entirely confined to the lime-
stones, and are wanting in the dense formations of interposed
clay, as also in the lias, these zoophytes requiring, not only car-
bonate of lime for their support, and clear water, but a bottom
remaining Tor years unchanged, either by the shifting of sand or
the accession of fresh sediment.

In the Upper Oolite of England, corals are rare, although one
species is found plentifully at Tisbury, in Wiltshire, in the Port-
land sand, converted into flint and chert, the original calcareous
matter being replaced by silex. (Fig. 199.) One of the lime-
stones of the Middle Oolite has been called the " Coral Rag,"
because it consists, in part, of continuous beds of petrified corals,
for the most part retaining the position in which they grew at the
bottom of the sea. They belong chiefly to the genera Caryo-
phyllia (Fig. 200.), Agaricia, and Astrea, and sometimes form
masses of coral fifteen feet thick. These coralline strata extend
through the calcareous hills of the N. W. of Berkshire, and

PART II. CHAPTER XVII.

215

Corals of Oolite.

Eig. 199.

Fig. 200.

Columnaria oblonga, Blainv.
Upper Oolite, Tisbury.

Caryophyllia annularis, Parkin.
Coral rag, Steeple Ashton.

north of Wilts, and again recur in Yorkshire, near Scarborough.
Although the name of coral rag has been thus appropriated,
there are portions of the lower oolite, as for example the Great
and Inferior Oolite (/. g. Table, p. 213.), which are equally en-
titled in many places to be called coralline limestones. Thus
the Great Oolite near Bath contains various corals, among which
the Eunomia radiata (Fig. 201.) is very conspicuous, single

Fig. 201.

Eunomia radiata, Lamouroux.

a. section transverse to the tubes.

6. vertical section, showing the radiation of the tubes.

c. portion of interior of tubes magnified, showing striated surface.

individuals forming masses several feet in diameter ; and having
probably required, like the large existing brain-coral (Meandrina)
of the tropics, many centuries before their growth was completed.
Different species of Crinoideans, or stone-lilies, are also com-
mon in the same rocks with corals ; and, like them, must have

216

LYELL'S ELEMENTS OF GEOLOGY.

Bradford Encrinites.

enjoyed a firm bottom, where their root, or base of Attachment,
remained undisturbed for years (c. Fig. 202.) Such fossils,

Fig. 202.

Jlpiocrinites rotundas, or Pear Encrinite ; Miller. Fossil at Bradford, Wilts.
a. Stem of Apiocrinitea, and one of the articulations, natural size.
5. Section at Bradford of great oolite and overlying clay, containing the fossil
encrinites. See text.

c. Three perfect individuals of the^ Apiocrinite, represented as they grew on

the surface of the Great Oolite.

d. Body of the Apiocriniles rotundus.

therefore, are almost confined to the limestones ; but an excep-
tion occurs at Bradford, near Bath, where they are enveloped in
clay. In this case, however, it appears that the solid upper sur-
face of the " Great Oolite" had supported, for a time, a thick
submarine forest of these beautiful zoophytes, until the clear and
still water was invaded by a current charged with mud, which
threw down the stone-lilies, and broke most of their stems short
off near the point of attachment. The stumps still remain in
their original position ; but the numerous articulations once com-
posing the stem, arms, and body of the zoophyte, were scattered
at random through the argillaceous deposit in which some of
them now lie prostrate. These appearances are represented in
the section £, Fig. 202., where the darker strata represent the
Bradford clay, a member of the Forest marble (e. Table, p. 213.).
The upper surface of the calcareous stone below is completely
incrusted over with a continuous pavement, formed by the stony
roots or attachments of the Crinoidea ; and besides this evidence
of the length of time they had lived on the spot, we find great
numbers of single vertebra?, or circular plates of the stem and
body of the encrinite, covered over with serpulse. Now these
serpulse could only have begun to grow after the death of some
of the stone-lilies, parts of whose skeletons had been strewed
over the floor of the ocean before the irruption of argillaceous

PART II. CHAPTER XVH. 217

Peculiar Fossils.

mud. In some instances we find that, after the parasitic serpu-
\3d were full grown, they had become incrusted over with a coral,
called Berenicea diluviana ; and many generations of these
polyps had succeeded each other in the pure water before they
became fossil.

Fig. 203.

a. Single vertebra, or articulation of an Encrinite overgrown with serpulae and

corals. Natural size. Bradford clay.
I. Portion of the same magnified, showing the coral Berenicea diluviana

covering one of the serpulse.

We may, therefore, perceive distinctly that, as the pines and
cycadeous plants of the ancient Portland Forest were killed by
submergence under fresh-water, and soon buried under muddy
sediment, so an invasion of argillaceous matter put a sudden stop
to the growth of the Bradford Encrinites, and led to their preser-
vation in marine strata.*

Such differences in the fossils as distinguish the calcareous
and argillaceous deposits from each other, would be described by
naturalists as arising out of a difference in the stations of spe-
cies ; but besides these, there are variations in the fossils of the
higher, middle, and lower part of the oolitic series, which must
be ascribed to that great law of change in organic life by which
distinct assemblages of species have been adapted, at successive
geological periods, to the varying conditions of the habitable sur-
face. In a single district it is difficult to decide how far the
limitation of species to certain minor formations has been due to
the local influence of stations, or how far it has been caused by
time, or the creative and destroying law above alluded to. v But
we recognize the reality of the last-mentioned influence, when
we contrast the whole oolitic series of England with that of parts

* For a fuller account, of these Encrinites, see Buckland's Bridgewater Trea-
tise, vol. i. p. 429.
T

218

LYELL'S ELEMENTS OF GEOLOGY.

Fossils of Oolite Group.

of the Jura, Alps, and other distant regions, where there is
scarcely any lithological resemblance ; and yet some of the same
fossils remain peculiar in each country to the Upper, Middle, and
Lower Oolite formations respectively. Mr. Thurmann has shown
how remarkably this fact holds true in the Bernese Jura, although
the argillaceous divisions, so conspicuous in England, are feebly
represented, and some entirely wanting.

Amongst the characteristic fossils of the Upper Oolite, may
be mentioned the Ostrea deltoidea (Fig. 205.), found in the Kim-
meridge clay throughout England and the north of France, and

FOSSILS OF THE OOLITE.
Fig. 205.

Fig. 204.

r

Fig. 206.

GrypJuea virgula. Ostrea deltoidea.

Upper Oolite.

Fig. 207.

Trigonia gibbosa.

a the binge.
Portland Oolite, Tisbury.

Fig. 208.

hieroglyphica.
Coral rag.

Jferinaa Goodhallii, Fit ton.
Coral rag, Weymoutli.

also in Scotland, near Brora. The Gryphcsa virgula (Fig.
204.), also met with in the same clay near Oxford, and so abun-
dant in the upper oolite of parts of France as to have caused the
deposit to be termed " marnes a gryphees virgule." Near Cler-
rnont, in Argonne, a few leagues from St. Menehould, these indu-

PART II. CHAPTER XVII.

219

Fossils of Oolite Group.

Fig. 209.

Fig. 210.

Cast of Diceras arietina.
Coral rag.

Cidaris coronata.
Coral rag.

rated marls crop out from beneath the gault; and, on decomposing,
leave the surface of every ploughed field literally strewed over
with fossil oysters.

One of the limestones of the Jura, referred to the age of the
English coral rag, has been called " Nerinsean limestone" (Cal-
caire a Nerinees) by M. Thirria; Nerincea being an extinct
genus of univalve shells, much resembling the Cerithium in
external form, and peculiar to the oolitic period. The annexed
section (Fig. 207.) shows the curious form of the hollow part of
each whorl, and also the perforation which passes up the middle
of the columella. N. Goodhallii (Fig. 208.) is another English
species of the same genus, from a formation which seems to form
a passage from the Kimmeridge clay to the coral rag.*

A division of the oolite in the Alps, regarded by most geolo-
gists as coeval with the English coral rag, has been often named
" Calcaire a Dicerates," or " Diceras limestone," from its con-
taining abundantly a bivalve shell (see Fig. 209.) of a genus
allied to the Chama.

Among the characteristic shells of the Inferior Oolite, I may
instance Terebratula spinosa (Fig. 211.), Pholadomya fdi-

Fig. 211.

Fig. 212.

Terebratula spinosa.
Inferior Oolite.

a Pholadomya fidicula. Inferior Oolite.

b Heart-shaped anterior termination of the same.

* Fitton, Geol. Trans., Second Series, vol. iv. pi. 23, fig. 12.

220

LYELL'S ELEMENTS OF GEOLOGY.

Fossils of Oolite Group.

Fisr. 213.

JBelemnites hastatus. Inferior Oolite.

Fig. 214. Fig. 215.

Orbicula reflexa, Sow.

a. upper valve.

b. lower or attached valve, and

showing part of the upper.

Fig. 216.

Ammonites slriatulus, Sow.
Inferior Oolite and Lias.

Fig. 217.

Terebratula digona,
Inferior Oolite.

Ostrea Marshii.
Middle and Lower Oolite.

cula (Fig. 212.), Belemnites hastatus (Fig. 213.), and Terebra-
tula digona (Fig 216.)

As illustrations of shells having a great vertical range, I may
allude to Trigonia gibbosa (Fig. 205.), which abounds in the
Portland stone of Wiltshire, and the Inferior Oolite of York-
shire.* Also Ostrea Marshii (Fig. 217.), common to the
Cornbrash of Wilts and the Inferior Oolite of Yorkshire ; and,
lastly, Orbicula reflexa (Fig. 214.) and Ammonites striatulus
(Fig. 215.), fossils common to the Inferior Oolite and Lias.

Such facts by no means invalidate the general rule, that cer-
tain fossils are good chronological tests of geological periods ;
but they serve to caution us against attaching too much import-
ance to single species, some of which may have a wider, others
a more confined vertical range. We have before seen that, in

* See Williamson, Proceedings Geol. Soc. No. 47.

PART II. CHAPTER XVII. 221

Oolite Group Signs of Land.

some of the tertiary formations, some species occur both in the
older and newer groups, yet these groups may be distinguishable
from one another by a comparison of the whole assemblage of
fossil shells proper to each.

Signs of neighbouring land and shoals, — The corals and
shells above alluded to, and the fish, Crustacea, and other accom-
panying fossils, sufficiently attest the marine origin of the oolitic
strata in general. Yet there are frequent signs of shallow water
and of neighbouring land ; and these are the more worthy of
attention, as they by no means diminish as we proceed down-
wards to the inferior parts of the oolitic series. Had the bottom
of the sea in Europe been unmoved during the entire oolitic
period, the first, or oldest beds of the oolite, must have been
accumulated in the deepest water, the middle oolite in water of
less depth, and the upper in the shallowest of all. The appear-
ances about to be described militate against this conclusion. The
Kimmeridge clay, in the Upper Oolite, consists, in great part, of
a bituminous shale, sometimes forming an impure coal several
hundred feet in thickness. In some places in Wiltshire it much
resembles peat ; and the bituminous matter may have been, in
part at least, derived from the decomposition of vegetables. But
as impressions of plants are rare in these shales, which contain
ammonites, oysters, and other marine shells, the bitumen may
perhaps be of animal origin. The occurrence, however, of fossil
wood in the Upper Oolite shows that there were then lands from
which plants were drifted into the sea.

The celebrated lithographic stone of Solenhofen, in Bavaria,
belongs to one of the upper divisions of the oolite, and affords a
remarkable example of the variety of fossils which may be pre-
served under favourable circumstances, and what delicate impres-
sions of the tender parts of certain animals and plants may be
retained where the sediment is of extreme fineness. Although
the number of testacea in this slate is small, and the plants few,
and those all marine, Count Munster had determined no less than
237 species of fossils when I saw his collection in 1833; and
among them no less than seven species of flying lizards, or
pterodactyls, six saurians, three tortoises, sixty species of fish,
forty-six of Crustacea, and twenty-six of insects. These insects,
among which is a libellula, or dragon-fly, must have been blown
out to sea, probably from the same land to which the flying
lizards, and other contemporaneous reptiles, resorted.

In one of the upper members of the Inferior Oolite of Eng-
land the ripple-mark is distinctly seen throughout a considerable
thickness of thin fissile beds of a coarsely oolitic limestone. The
rippled slabs are used for roofing, and have been traced over a

T*

222 LYELL'S ELEMENTS OF GEOLOGY.

Oolite Group Stonesfield Slate.

broad band of country from Bradford, in Wilts, to Tetbury, in
Gloucestershire. These calcareous slabs, or tile-stones, are sepa-
rated from each other by thin seams of clay, which have been
deposited upon them, and have taken their form, preserving the
undulating ridges and furrows of the sand in such complete
integrity, that the impressions of small footsteps, apparently of
crabs, which walked over the soft wet sands, are still visible. In
the same stone the claws of crabs, fragments of echini, broken
shells, pieces of drift wood, and other signs of a neighbouring
beach, are observed.

The slate of Stonesfield has lately been shown by Mr. Lons-
dale to lie at the base of the Inferior Oolite. It is an oolitic
shelly limestone, only six feet thick, but very rich in organic
remains. It contains some pebbles of a rock very similar to
itself, and with them the fossil remains of belemnites, trigonia?,
and other marine shells. Besides fragments of wood,
Fig. 218. which occur in all parts of the oolitic group, there are
many impressions of ferns, cycadese, and other ter-
restrial plants. Several insects also, and among the
rest, the wing-covers of beetles, are perfectly pre-
served, (see Fig. 218.) some of them approaching
nearly to the genus Buprestis.* The remains, also,
of many genera of reptiles, such as Plesiosaurus,
Crocodile, and Pterodactyl, have been discovered in
the same limestone ; and, what is still more remark-
able, the jaws of at least two species of mammiferous
Buprestis; quadrupeds, allied to the Didelphys, or opossum.

Stonesfield. «l J. . , «, , Al , i A i /•

1 hese fossils afford the only example yet known of
terrestrial mammalia in rocks of a date anterior to the Eocene
period.

This exception is the more deserving of notice, because even
no cetacea have as yet been observed in any secondary strata,
although certain bones, from the great oolite of Enstone, near
Woodstock, in Oxfordshire, have been cited, on the authority of
Cuvier, as referable to this class. Dr. Buckland, who has stated
this in his late Bridgewater Treatise,f has had the kindness to
send me the supposed ulna of a whale, in order that Mr. Owen
might examine into its claims to be considered as cetaceous. It
is the opinion of that eminent comparative anatomist, that it can-
not have belonged to the cetacea, because the fore-arm in these
marine mammalia is invariably much flatter, and devoid of all
muscular depressions and ridges, one of which is so prominent in
the middle of this bone. (See Fig. 219.) In saurians, on the

* See Buckland's Bridgewater Treatise. t Vol. i. p. 115.

PART II. CHAPTER XVII.

223

No Fossil Cetacea in Oolite Group.

Fig. 219.

Bone of a reptile, formerly supposed to be the ulna of a Cetacean ; from the
Oolite of Enstone, near Woodstock.

contrary, such ridges exist for the attachment of muscles ; and
to some animal of that class the bone is probably referable.

Oolite of Yorkshire and Scotland. — North of the Humber,
in Yorkshire, the Inferior Oolite assumes a form very different
from that which distinguishes it in the south. It may there be
called a coal formation, as it contains much vegetable matter,
and coal, interstratified with sand and sandstones. The high
state of preservation and number of the plants render it probable
that land was not far distant. The same may be said of the
oolitic coal of Brora, on the south-east coast of Sutherland-
shire, in Scotland, where the Inferior Oolite contains coal, one
bed of which is 3£ feet in thickness. The plants resemble those
in the Yorkshire oolite, and a great number of the associated
marine shells and other fossils are the same ;* but the mineral
characters of the sandstone, shale, and calcareous grit, differ
considerably.

* Murchison, Geol. Trans,, vol. ii. Second Series.

- \

224 LYELts ELEMENTS OF GEOLOGY.

. ,x ">

'.Xj-1' Lias.

CHAPTER

OOLITE AND LIAS — continued.

Mineral character of Lias — Name of Gryphite limestone — Fossil fish — Ich-
thyodorulites — Reptiles of the Lias — Ichthyosaur and Plesiosaur — Newly disco-
vered marine Reptile of the Galapagos Islands — Sudden death and burial of
fossil animals in Lias — Origin of the Oolite and Lias, and of alternating calcare-
ous and argillaceous formations — ^Physical geography — Vales of clay — Hills and
escarpments of limestone.

LIAS. — The English provincial name of Lias has been very
generally adopted for a formation of argillaceous limestone, marl,
and clay, which forms the base of the oolite, and is classed by
many geologists as part of that group. They pass, indeed, into
each other in some places, as near Bath, a sandy marl called
the marlstone of the Lias being interposed, and partaking of the
mineral characters of the upper lias and inferior oolite. These
last mentioned divisions have also some fossils in common, such
-p- 220 as *ne Avicula intsquivalvis (Fig. 220.)

Nevertheless the Lias may be traced through-
out a great part of Europe as a separate and
independent group, of considerable thickness,
varying from 500 to 1000 feet, containing
many peculiar fossils, and having a very
uniform lithological aspect. Although usu-
a^7 conformable to the oolite, it is sometimes,
Sow. as in the Jura, unconformable. Thus, in

the environs of Lons-le-Saulnier, for instance, the strata of lias
are inclined at an angle of about 45°, while the incumbent ooli-
tic marls are horizontal.

The peculiar aspect which is most characteristic of the Lias
in England, France, and Germany, is an alternation of thin
beds of limestone, with a light brown weathered surface, sepa-
rated by dark-coloured narrow argillaceous partings, so that the
quarries of this rock, at a distance, assume a striped and riband-
like appearance.*

Although the prevailing colour of the limestone of this form-
ation is blue, yet some beds of the lower lias are of a .yellowish

* Conyb. and Phil p. 261.

PART II. CHAPTER

Fossils of the Lias.

white colour, and have been called white lias. In some parts
of France, near the Vosges mountains, and in Luxembourg, M.
E. de Beaumont has shown that the lias containing Gryph&a
arcuata, Plagiostoma giganteum, and other characteristic
fossils, becomes arenaceous ; and around the Hartz, in West-
phalia and Bavaria, the inferior parts of the lias are sandy, and
sometimes afford a building stone called by the Germans qua-
dersandstein.

The name of Gryphite limestone has sometimes been applied
to the lias, in consequence of the great number of shells which
it contains of a species of oyster, or Gryphsea (Fig. 221.).
Many cephalopoda, also, such as Ammonite, Belemnite, and
Nautilus (Fig. 222.), prove the marine origin of the formation.

Fig. 221.

Fig. 222.

Oryphcea incurva, Sow.
(G.arcuata, Lam.)

Nautilus truncatus, Lias.

The fossil fish resemble generically those of the oolite, belong-
ing all, according to M. Agassiz, to extinct genera, and differing
remarkably from the ichthyolites of the cretaceous period.
Among them is a species of Lepidotus (L. gigas, Ag.) Fig.
202.), which is found in the lias of England, France, and Ger-

a Fig. 223.

Scales of Lepidotus gigas, Agas.
a. two of the scales detached.

many.* This genus was before mentioned (p. 348.) -as occur-
ing in the Wealden, and is supposed to have frequented both

* Agassiz, Pois. Fos. vol. ii. tab. 28, 29.

226

LYELL'S ELEMENTS OF GEOLOGY.

Ichthyodorulites.

rivers and coasts. The teeth of a species of Acrodus, also, are
very abundant in the lias (Fig. 224.)

Fig. 224.

Acrodus nobilis, Agas. tooth ; commonly called fossil leach.
Lias, Lyme Regis, and Germany.

But the remains of fish which have excited more attention
than any others, are those large bony spines called ichthyodo-
rulites (a. Fig. 225.), which were once supposed by some natu-

Fig. 225.

Hybodus reticulatus, Agas. Lias, Lyme Regis.
a. Part of fin, commonly called Ichthyodorulite.
6- Tooth.

ralists to be jaws, and by others weapons, resembling those of

the living Balistes andSilurus; but which M. Agassiz has shown

to be neither the one nor the other. The spines, in the genera

Fig. 226. ^ast metinoned, articu-

late with the back-
bone, whereas there
are no signs of any
such articulation in
the ichthyodorulites.
These last appear to
have been bony spines
which formed the an-
terior part of the dor-

Sal. fin> ^ke tnat °f the
living genera Cestra-

CMmara monstrosa*
a. Spine forming anterior part of the dorsal fin.

* Agassiz, Poissons Fossiles, vol. iii. tab. C. fig. 1.

PART II. CHAPTER XVIII. 227

Reptiles of the Lias.

cion.and Chimsera (see a. Fig. 226.). In both of these genera,
the posterior concave face is armed with small spines like that
of the fossil Hybodus (Fig. 225.), one of the shark family found
fossil at Lyme Regis. Such spines are simply imbedded in
the flesh, and attached to strong muscles. " They serve," says
Dr. Buckland, " as in the Chimcera (Fig. 226.), to raise and
depress the fin, their action resembling that of a moveable mast,
raising and lowering backwards the sail of a barge."*

Reptiles of the Lias. — It is not, however, the fossil fish
which form the most striking feature in the organic remains of
the Lias ; but the reptiles, which are extraordinary for their
number, size, and structure. Among the most singular of these
are several species of Ichthyosaurus and Plesiosaurus. The
genus Ichthyosaurus, or fish-lizard, is not confined to this form-
ation, but has been found in strata as high as the chalk-marl and
gault of England, and as low as the muschelkalk, a formation
which immediately succeeds the lias in the descending order.f
It is evident from their fish-like vertebrae, their paddles, resem-
bling those of a porpoise or whale, the length of their tail, and
other parts of their structure, that the habits of the Ichthyosaurs
were aquatic. Their jaws and teeth show that they were carni-
vorous ; and the half-digested remains of fishes and reptiles,
found within their skeletons, indicate the precise nature of their
food4 Mr. Conybeare was enabled, in 1824, after examining
many skeletons nearly perfect, to give an ideal restoration of
the osteology of this genus, and of that of the Plesiosaurus. §
The latter animal had an extremely long neck and small head,
with teeth like those of the crocodile, and paddles analogous to
those of the Ichthyosaurus, but larger. It is supposed to have
lived in shallow seas and estuaries, and to have breathed air like
the Ichthyosaur, and our modern cetacea.|| Some of the rep-
tiles above mentioned were of formidable dimensions. One
specimen of Ichthyosaurus playtyodon, from the lias at Lyme,
now in the British Museum, must have belonged to an animal
more than twenty-four feet in length, and another of the Plesio-
saurus, in the same collection, is eleven feet long. The form of
the Ichthyosaurus may have fitted it to cut through the waves
like the porpoise ; but it is supposed that the Plesiosaurus, at
least the long-necked species (Fig. 228.), was better suited to
fish in shallow creeks and bays, defended from heavy breakers.

* Bridgewater Treatise, p. 290. t Buckland, Bridgew. Treat., p. 168.

t Ibid. p. 187. § Geol. Trans., Second Series, vol. i. pi. 49.

II Conybeare and De la Beche, Geol. Trans.; and Buckland, Bridgew.
Treat., p. 203.

238

LYELL'S ELEMENTS OF GEOLOGY.

Reptiles of the Lias.

•a .SP

For the last twenty years, anatomists have agreed that these
extinct saurians must have inhabited the sea, although no living
marine reptile was known. They argued that, as there are now
Chelonians, like the tortoise, living in fresh water, and others,
as the turtle, frequenting the ocean, so there may have been

PART II. CHAPTER XVIII. 229

Living Marine Saurian of the Galapagos Islands.

formerly some saurians proper to salt, others to fresh water.
The recent discovery, however, of a maritime saurian, has now
rendered it unnecessary to speculate on such possibilities. This
creature was found in the Galapagos islands, during the visit of
H. M. S. Beagle to that archipelago, in 1835, and its habits were
then observed by Mr. Darwin. The islands alluded to are situ-
ated under the equator, nearly 600 miles to the westward of the
coast of South America. They are volcanic, some of them
being 3000 or 4000 feet high ; and one of them, Albemarle
Island, 75 miles long. The climate is mild, very little rain falls ;
and, in the whole archipelago, there is only one rill of fresh
water that reaches the coast. The soil is for the most part dry
and harsh, and the vegetation scanty. The birds, reptiles, plants,
and insects are, with very few exceptions, of species found no-
where else in the world, although all partake, in their general
form, of an American character. Of the mammalia, says Mr.
Darwin, one species alone appears to be indigenous, namely, a
large and peculiar kind of mouse ; but the number of lizards,
tortoises, and snakes is so great that it may be called a land of
reptiles. The variety, indeed, of species is small ; but the indi-
viduals of each are in wonderful abundance. There is a turtle,
a large tortoise (Testudo Indicus), four lizards, and about the
same number of snakes, but no frogs or toads. Two of the
lizards belong to the family Iguanidce of Bell, and to a peculiar
genus (Amblyrhynchus) established by that naturalist ; and so

Pig. 229.

Amblyrhynchus cristatus, Bell, Length varying from 3 to 4ft. The only

existing marine lizard now known.
a Tooth of same of natural size, and magnified.

named from their obtusely truncated head and short snout.* Of
these lizards, one is terrestrial in its habits, and burrows in the

* apBXvs, amblys, blunt, and py^o?, rhynchus, snout.

230 LYELL'S ELEMENTS OF GEOLOGY.

Living Marine Saurian.

ground, swarming everywhere on the land, having a round tail,
and a mouth somewhat resembling in form that of the tortoise.
The other is aquatic, and has its tail flattened laterally for swim-
ming (see Fig. 229.). " This marine saurian," says Mr. Darwin,
" is extremely common on all the islands throughout the archi-
pelago. It lives exclusively on the rocky sea-beaches, and I
never saw one even ten yards inshore. The usual length is
about a yard, but there are some even four feet long. It is of a
dirty black colour, sluggish in its movements on the land; but,
when in the water, it swims with perfect ease and quickness by
a serpentine movement of its body and flattened tail, the legs
during this time being motionless, and closely collapsed on its
sides. Their limbs and strong claws are admirably adapted for
crawling over the rugged and fissured masses of lava which
everywhere form the coast. In such situations a group of six
or seven of these hideous reptiles may oftentimes be seen on the
black rocks, a few feet above the surf, basking in the sun with
outstretched legs. Their stomachs, on being opened, were found
to be largely distended with minced sea- weed, of a kind which
grows at the bottom of the sea, at some little distance from the
coast. To obtain this, the lizards are seen occasionally going
out to sea in shoals. One of these animals was sunk in salt
water, from the ship, with a heavy weight attached to it, and
drawn up again after an hour ; it was quite active and unharmed.
It is not yet known by the inhabitants where this animal lays
its eggs ; a singular fact, considering its abundance, and that
the natives are well acquainted with the eggs of the terrestrial
Amblyrhynchvs, which last is also herbivorous, although feed-
ing on a very different kind of vegetation."*

In those deposits now forming by the sediment washed away
from the wasting shores of the Galapagos islands, the remains
of saurians, both of the land and sea, as well as of chelonians
and fish, may be mingled with marine shells without any bones
of land quadrupeds or batrachian reptiles ; yet even here we
should expect the remains of marine mammalia to be imbedded
in the new strata, for there are seals, besides several kinds of
cetacea, on the Galapagian shores; and, in this respect, the
parallel between the modern fauna, above described, and the
ancient one of the lias, would not hold good.

Sudden destruction of saurians, fyc. — It has been remarked,
and truly, that many of the fish and saurians, found fossil in the
lias, must have met with sudden death and immediate burial ; and

* Danvin's Journal, chap. xix.

PART II. CHAPTER XVIII. 231

Sudden Destruction of Marine Animals.

.hat the destructive operation, whatever may have been its nature,
was often repeated.

" Sometimes," says Dr. Buckland, " scarcely a single bone or
scale has been removed from the place it occupied during life ;
which could not have happened had the uncovered bodies of these
saurians been left, even for a few hours, exposed to putrefaction,
and to the attacks of fishes and other smaller animals at the
bottom of the sea."* Not only are the skeletons of the Ichthyo-
sauri entire, but sometimes the contents of their stomachs still
remain between their ribs, so that we can discover the particular
species offish on which they lived, and the form of their excre-
ments. Not unfrequently there are layers of these coprolites at
different depths in the lias, at a distance from any entire skele-
tons of the marine lizards, from which they were derived, " as
if," says M. De la Beche, " the muddy bottom of the sea received
small sudden accessions of matter from time to time, covering
up the coprolites and other exuviae which had accumulated during
the intervals."! It is further stated that, at Lyme Regis, those
surfaces only of the coprolites which lay uppermost at the bottom
of the sea have suffered partial decay, from the action of water
before they were covered and protected by the muddy sediment
that has afterwards permanently enveloped them.ij;

Numerous specimens of the pen-and-ink fish (Sepia loligo,
Lin., Loligo vulgaris, Lam.) have also been met with in the
lias at Lyme, with the ink-bags still distended, containing the
ink in a dried state, chiefly composed of carbon, and but slightly
impregnated with carbonate of lime. These cephalopoda, there-
fore, must, like the saurians, have died suddenly, and have been
instantly buried in sediment; for, if exposed after death, the
membrane containing the ink would have decay ed.§

As we know that river fish are sometimes stifled, even in their
own element, by muddy water during floods, it cannot be doubt-
ed that the periodical discharge of large bodies of turbid fresh
water into the sea may be still more fatal to marine tribes. In
the Principles of Geology, I have shown how large quantities of
mud and drowned animals are swept down into the sea by rivers
during earthquakes, as in Java, in 1699; and how undescriba-
ble multitudes of dead fish have been seen floating on the sea
after a discharge of noxious vapours from similar convulsions. ||
But, in the intervals between such catastrophes, strata may have

* Bridgew. Treat., p. 125. t Geological Researches, p. 334.

t Buckland, Bridgew. Treat., p. 307. " $ Ibid.

|| See Principles of Geology, Index, " Lancerote," "Graham Island," "Cala-
bria,"

232 LYELL'S ELEMENTS OF GEOLOGY.

Fossils of the Lias Origin of the Oolite and Lias.

accumulated slowly in the sea of the lias, some being formed
chiefly of one description of shell, such as ammonites, others of
gryphites.

Fossil plants. — Among the vegetable remains of the Lias,
several species of Zamia have been
Fig. 230. found at Lyme Regis, and the re-

mains of coniferous plants at Whitby.
Fragments of wood are common, and
often converted into argillaceous lime-
stone. That some of this wood,
though now petrified, was soft, when
it first lay at the bottom of the sea,
is shown by a specimen now in the museum of the Geological
Society, (see Fig. 230.) which has the form of an ammonite
indented on its surface.

Origin of the Oolite and Lias. — If we now endeavour to
restore, in imagination, the ancient condition of the European
area at the period of the Oolite and Lias, we must conceive a
sea in which the growth of coral reefs and shelly limestones,
after proceeding without interruption for ages, was liable to be
stopped suddenly by the deposition of clayey sediment. Then,
again, the argillaceous matter, devoid of corals, was deposited
for ages, and attained a thickness of hundreds of feet, until ano-
ther period arrived when the same space was again occupied by
calcareous sand, or solid rocks of shell and coral, to be again
succeeded by the recurrence of another period of argillaceous
deposition. Mr. Conybeare has remarked of the entire group of
Oolite and Lias, that it consists of repeated alternations of clay,
sandstone, and limestone, following each other in the same order.
Thus the clays of the lias are followed by the sands of the infe-
rior oolite, and these again by shelly and coralline limestone,
(Bath oolite, &c.;) so, in the middle oolite, the Oxford clay is
followed by calcareous grit and " coral rag ;" lastly, in the
upper oolite the Kimmeridge clay is followed by the Wey mouth
sands and the Portland limestone.* The clay beds, however, as
Mr. De la Beche remarks, can be followed over larger areas
than the sands or sandstones. f It should also be remembered,
that while the oolitic system becomes arenaceous, and resembles
a coal-field in Yorkshire, it assumes, in the Alps, an almost
purely calcareous form, the sands and clays being omitted ; and
even in the intervening tracts, it is more complicated and vari-
able than appears in ordinary descriptions. Nevertheless, some
of the clays and intervening limestones do, in reality, retain a

* Con. and Phil. p. 166. t Geol. Researches, p. 337.

PART II. CHAPTER XVIil. 233

Origin of the Oolite and Lias.

pretty uniform character, for distances of from 400 to 600 miles
from east to west and north to south.

According to M. Thirria, the entire oolitic group in the depart-
ment of the Haute Saone, in France, may be equal in thickness
to that of England ; but the importance of the argillaceous divi-
sions is in the inverse ratio to that which they exhibit in Eng-
land, where they are about equal to twice the thickness of the
limestones, whereas, in the part of France alluded to, they reach
only about a third of that thickness.* In the Jura the clays are
still thinner ; and in the Alps they thin out and almost vanish.

In order to account for such a succession of events, we may
imagine, first, the bed of the ocean to be the receptacle for ages
of fine argillaceous sediment, brought by oceanic currents,
which may have communicated with rivers, or with part of the
sea near a wasting coast. This mud ceases, at length, to be
conveyed to the same region, either because the land which had
previously suffered denudation is depressed and submerged, or
because the current is deflected in another direction by the
altered shape of the bed of the ocean and neighbouring dry
land. By such changes the water becomes once more clear and
fit for the growth of stony zoophytes. Calcareous sand is then
formed from comminuted shell and coral, or, in some cases, are-
naceous matter replaces the clay, because it commonly happens
that the finer sediment, being first drifted farthest from coasts, is
subsequently overspread by coarse sand, after the sea has grown
shallower, or when the land, increasing in extent, has approached
nearer to the spots first occupied by fine mud.

In order to account for another great formation, like the Ox-
ford clay, again covering one of coral limestone, we must sup-
pose a sinking down like that which is now taking place in some
existing regions of coral between Australia and South America. f
* The occurrence of subsidences, on so vast a scale, may again
have caused the bed of the ocean and the adjoining land through-
out the European area, to assume a shape favourable to the
deposition of another set of clayey^strata ; ~~and this change may
have been succeeded by a series of events analogous to that
already explained, and these again by a third series in similar
order. Both the ascending and descending movements may
have been extremely slow, like those now going on in the
Pacific ; and the growth of every stratum of coral, a few feet
in thickness, may have required centuries for its completion,
during which certain species of organic beings may have disap-
peared from the earth, and others have been introduced in their

* Burat's D'Aubuisson, torn. ii. p. 456. t See Darwin, chap. xxii.

234 LYELL'S ELEMENTS OF GEOLOGY.

Valleys and Escarpments of Chalk, Oolite, and Lias.

place ; so that, in each set of strata, from the Upper Oolite to
the Lias, some peculiar and characteristic fossils were imbedded.

Physical geography. — The alternation, on so large a scale,
of distinct formations of clay and limestone, has given rise to
some marked features in the physical outline of parts of Eng-
land and France. Wide valleys can usually be traced through-
out the long bands of country where the argillaceous strata
crop-out ; and between these valleys the limestones are observed,
composing ranges of hills, or more elevated grounds* These
ranges terminate abruptly on the side on which the several clays
crop-out from beneath the calcareous strata.

The annexed diagram will give the reader an idea of the con-
figuration of the surface now alluded to, such as may be seen in
passing from London to Cheltenham, or in other parallel lines,
from east to west, in the southern part of England. It has been

Fig. 231.

Middle Upper London

Oolite. Oolite. Chalk, clay.

Lias. Oxford clay. . Kim. clay. Gault.

necessary, however, in this drawing, greatly to exaggerate the
inclination of the beds, and the height of the several formations,
as compared to their horizontal extent. It will be remarked,
that the lines of cliff, or escarpment, face towards the west in
the great calcareous eminences formed by the Chalk and the
Upper, Middle, and Lower Oolites ; and at the base of each we
have respectively the Gault, Kimmeridge clay, Oxford clay, and
Lias. This last forms, generally, a broad veil at the foot of the
escarpment of Inferior Oolite; but a considerable portion of
that escarpment is sometimes occupied by lias. The external
outline of the country which the geologist observes in travelling
westward from Paris to Metz, is precisely analogous, and is
caused by a similar succession of rocks intervening between the
tertiary strata and the Lias ; with this difference, however, that
the escarpments of Chalk, Upper, Middle, and Inferior Oolites,
face towards the east instead of the west.

The Chalk crops-out from beneath the tertiary sands and
clays of the Paris basin, near Epernay, and the Gault from
beneath the Chalk and Upper Green-sand at Clermon* en Ar-
gonne ; and passing from this place by Verdun and Etain to
Metz, we find two limestone ranges, with intervening vales of
clay, precisely resembling those of southern and central Eng-

PART II. CHAPTER XIX. 235

New Red Sandstone.

land, until we reach the great plain of Lias at the base of the
Inferior Oolite at Metz.

It is evident, therefore, that the denuding causes have acted
similarly over an area several hundred miles in diameter, sweep-
ing away the softer clays more extensively than the limestones,
and undermining these last so as to cause them to form steep
cliffs wherever the harder calcareous rock was based upon a
more yielding and destructible clay. This denudation probably
occurred while the land was slowly rising out of the sea.*

CHAPTER XIX.

NEW RED SANDSTONE GROUP.

Distinction between New and Old Red sandstone — Between Upper and Lower
New Red — Muschelkalk in Germany — Fossil plants and shells of New Red
Group, entirely different from Lias and Magnesian limestone — Lower New Red
and Magnesian limestone — Zechstein in Germany of the same age — General
resemblance between the organic remains of the Magnesian limestone and Car-
boniferous strata— Origin of red sandstone and red marl.

BETWEEN the Lias and the Coal, or Carboniferous group,
there is interposed in the midland and western counties of Eng-
land a great series of red marls and sandstones, to which the
name of the New Red Sandstone formation was given, to dis-
tinguish it from the other marls and sandstones called the " Old
Red," (c. Fig. 232.) often identical in mineral character, which
lie immediately beneath the coal, b.

Fig. 232.

Coal. New Eed sandstone.

In some parts of the south-west of England, the entire " New
Red" group consists exclusively of red loam, clay, and sand-
stone, devoid of fossils, strongly contrasted in colour, and the
general absence of calcareous matter, with the Oolitic rocks and

* See Principles of Geology, Index, Weatden denudation.

236 LYELL'S ELEMENTS OF GEOLOGY.

Upper New Red Sandstone.

Lias before described. But when we extend our observations
over England and other countries, we no longer find this simpli-
city of structure ; but perceive that the strata between the Lias
and the Coal are divisible into two very distinct systems, which
will be understood from the accompanying Table, and the
description which follows.

NEW RED SANDSTONE GROUP.
Poikilitic group of Conybeare and Buckland.*

Synonyms.

German. French.

'a. Saliferous marls
and sandstone . . .
b. (wanting in Eng-

land)

c. Sandstone and
quartzose conglo-
merate

Keuper Marnes irisees.

A/r v 11 n ( Muschelkalk, ou cal-

Muschelkalk . . . . J caire CO(lui\liere>

Bunter sandstein > Gres bigarre.

£ frf. Magnesian lime- ^ Zechstein, and
stone (dolomiticS- Kupfer schie-

conglomerate) . . . ) fer

& ® i

o ^ \e. Lower New Red > T, ., ,• „„,, B

2 sandstone. 5 Roth-hegendes .

Zechstein, ou schiste
cuivreux — et Cal-
caire Magnesien.

Gres des Vosges,
couches inferi-
eures?

UPPER NEW RED SANDSTONE.
(Including the Muschelkalk of the Germans.)

The Lias is succeeded in England by strata of red and green
marl, or clay, which are conformable to the Lias, and pass into
it, as in Gloucestershire. It is in this upper New Red system
that rock-salt and salt springs occur in Cheshire and other parts
of England ; and to this, therefore, the term " Saliferous marl
and sandstone formation, is properly applicable."f It consists,
in Cheshire, of alternating beds of red and green clay, or marl,
gypsum, and rock-salt, upwards of 600 feet in thickness.

A few traces only of fossil shells, fish, and plants have been
detected in this formation in England ; but in a corresponding
position in Germany there occur similar strata of red sandstone
and marl, in which are many organic remains, and associated
with the same a great calcareous formation called the " Mus-

* From TToiKiXos, Poikilos, variegated, see Buckland, Bridgw. Treat., vol. ii.
p. 38., because some of the most characteristic strata of this group were called
variegated by Werner, from their exhibiting spots and streaks of light blue,
green, and buff colour, in a red base.

t Murchison, Silurian System, p. 32.

PART II, CHAPTER XIX. 237

Muschelkalk.

chelkalk," or " shelly-limestone." As the fossil fauna and flora
of these formations supply the chasm which exists in our British
series, I shall say a few words of the " Upper New Red," as it
appears in Bavaria and Wurtemberg. First in order beneath the
Lias come mottled marls and sandstones, red, green, purple, and
white, containing gypsum and salt ; then the Muschelkalk above
mentioned, and then another set of marls and sandstones much
resembling the first. That these three formations, the Keuper,
Muschelkalk, and Bunter Sandstein, (see Table,) may be
referred to one period, appears from the fact that Count Munster
has obtained the same plants from the Keuper and Bunter Sand-
stein ; and M. Agassiz the same species of fish from both of
them, and from the interposed Muschelkalk. It is also worthy
of remark, that the strata of the Muschelkalk alternate with
those of the Keuper and Bunter Sandstein at their junction.

The fossil Flora, above alluded to, consists of Cycadeas and
several genera of ferns, also extinct coniferre of the genus
Voltzia (Ad. Brongniart) peculiar to this period, in which even
the fructification has been preserved, (Fig. 233.) and a gigantic
species of Equisetum, (Fig. 234.) which is not uncommon in the
Keuper sandstone.

Fig. 233. Fig. 234.

Voltzia brevifolia, and portion magnified Equisetum columnare ; fragment

to show fructification ; Sulzbad. of stem, and small portion of

Keuper and Bunter Sandstein. same magnified. Keuper.

X

Among the shells, some of the Cephalopoda are peculiar, as,
for example, that form of Ammonite which is called Ceratite by
De Haan, in which the descending lobes, see a, b, c, Fig. 235.,
terminate in a few small denticulations pointing inwards. Among
the bivalve shells, the Posidonia keuperina, Voltz. (Posidonomya
minuta, Bronn, Fig. 236.) is abundant, ranging from the Keuper
to the Bunter Sandstein ; and the Avicula socialis, (Fig. 237.)
having a similar range, but most characteristic of the Muschel-
kalk in Germany, France, and Poland.

There are also some encrinites in the Muschelkalk, and some
teeth of cartilaginous fish, a few decapod Crustacea, and no less

238 L YELL'S ELEMENTS OF GEOLOGY.

Fossil Footsteps.

Fig. 235.

Ammonites nodosus. Muschelkalk.

a. Side view. b. Front view.

c. Partially denticulated outline of the septa dividing the chambers.

Fig. 236.

Fig. 237.

Posidonomya
ininuta, Bronn.

a. dvicula socialis. b. Side view of same.

Characteristic of the Muschelkalk.

than five genera of large extinct reptiles, all peculiar to the
Muschelkalk, as Phytosaurus, Dracosaurus, and others. Upon
the whole, Professor Bronn has enumerated, in his LethaBa
Geognestica, no less than forty-seven genera of fossil remains
from the three divisions of the " Upper New Red" system in
Germany ; and these fossils are the more important as being all
distinct in species, and many of them in genera, from those of
the incumbent Lias or more ancient Magnesian limestone.

In the Bunter Sandstein near Hildburghausen, some remark-
able fossil footsteps have lately been
discovered in quarries of a gray quart-
zose sandstone. On the upper surfaces
of the slabs of stone the steps form de-
pressions, while those on the lower sur-
faces are in relief. These last are
natural casts formed in the subjacent
footsteps, as in moulds. The larger
prints seem to be those of the hind foot,
and are about eight inches long and five
wide. Near to each, and at the regular
distance of an inch and a half before
is a smaller print of a fore-foot

Fig. 238.

Single footstep of Chirotherium. CUStan
one eighth of nat. size. them

PART II. CHAPTER XIX. 239

Lower New Red Sandstone.

(see Fig. 238.). In each pair of large and small steps, the
great toes are turned alternately both to the right or both to the
left.*

For this unknown animal, Professor Kaup has proposed the
provisional name of Chirotherium ; and he conjectures that it was
a mammiferous quadruped, allied to the marsupialia.f

Fig. 239.

Line of footsteps on slab of sandstone. Hildburghausen, in Saxony.

In the kangaroo, says Dr. Buck land, the first toe of the fore-
foot is, in a similar manner, set obliquely to the others, like a
thumb ; and the disproportion between the fore and hind-feet is
also very great. If it should be eventually proved that this ani-
mal was really marsupial, these fossil relics belong to the most
ancient mammiferous quadruped yet known to palaeontologists.

It would scarcely be possible to draw a distinct line of de-
marcation between the Keuper and Bunter Sandstein, in Germa-
ny, where they are not barren of fossils, if the Muschelkalk did
not intervene between them. In England, therefore, where this
calcareous formation is wanting, and where there are scarcely
any organic remains in the Upper New Red marl and sandstone,
we cannot feel assured that the divisions a. and c. of our Table,
p. 236., do really coincide with the German Keuper and Bunter
Sandstein. But it has been found convenient in the counties of
Salop, Stafford, and Worcester, to divide the saliferous marls
from the inferior quartzose conglomerate in the manner above
indicated.

LOWER NEW RED SANDSTONE AND MAGNESIAN LIMESTONE.

We now come to the Lower New Red system, the position of
which can best be determined in Germany, because it is there
interposed between the Coal and Bunter Sandstein, or oldest part
of the " Upper New Red," above described. In the south-west
of England the New Red sandstone formation is unconformable
to the Coal (see Fig. 232.) ; but in the north-east of England
Professor Sedgwick has shown that the same series is conform-
able to the carboniferous strata, and passes into them. In other
words, the movements which deranged " the Coal" in the

* One of these slabs is now in the British Museum,
t See Buckland's Bridgew., p. 2C3.

210 LYELL'S ELEMENTS OF GEOLOGY.

Lower New Red and Magnesian Limestone. *

south-west, previously to the origin of the New Red sandstone,
did not extend towards Durham and the more northern counties.

Near Bristol, in Somersetshire, and in other counties border-
ing the Severn, the unconformable beds of the Lower New Red,
resting immediately upon the Coal, consist of a conglomerate
called " dolomitic," because the pebbles of older rocks are ce-
mented together by a base of magnesian limestone. Among the
imbedded pebbles are many derived from the Coal, particularly
from the carboniferous limestone, the peculiar fossils of which
are still seen in many large rounded fragments. In the north-
east of England the dolomitic conglomerate is represented by a
yellow limestone, generally called the Magnesian Limestone ;
which passes upwards into marl slate, and downwards into red
marl and gypsum. In the intermediate counties of Worcester-
shire, Staffordshire, and Shropshire, are conglomerates referred
to the same age, but which are calcareous, with scarcely any
magnesia. Between these conglomerates and the Coal is a great
formation, called the Lower New Red sandstone (see Table, p.
236.), composed of sandstones, red shales, and marls, occasion-
ally spotted green.*

The country of Mansfeld, in Thuringia, may be called the
classic ground of the Lower New Red, or Magnesian Lime-
stone formation on the continent. It has there been long cele-
brated, because one of its members, a slaty marlstone, is richly
impregnated with copper pyrites, for which it is extensively
worked. The formation in that country is composed of an upper
calcareous division, called the Zechstein, and a lower red quart-
zose formation of sandstone and conglomerate, called the Roth-
liegendes. The upper of these systems is very complex,
consisting of marl, limestone, copper-slate, magnesian limestone,
gypsum, and rock-salt, in which numerous fossils occur, bearing
a striking generic resemblance to those of our English Magne-
sian Limestone. The Lower system, or Rothliegendes, is inter-
posed between the Zechstein and the Coal ; and is supposed to
correspond with the Lower New Red sandstone, above mentioned,
as occupying a similar place in England between our Magnesian
Limestone and Coal. Its local name of Rothliegendes, red-Iyer,
or " Roth-todt-liegendes," red-dead-lyer, was given by the work-
men in the German mines from its red colour, and because the
copper has died out when they reach this rock which is not
metalliferous. It is, in fact, a great deposit of red sandstone
and conglomerate, with associated porphyry, basaltic trap, and
amygdaloid.

* Murchison, Silurian System, p. 54.

PART II. CHAPTER XIX.

241

Fossils of the Magnesian Limestone.

When we consider the fossils of the Magnesian Limestone in
England, or corresponding Zechstein in Germany, we find that
they approach much nearer in their character to the organic
remains of the older carboniferous group than to those of the
Upper New Red. Thus, for example, the two genera of shells,
Producta and Spirifer, of the family Brachiopoda, are common

Fig. 240.

Fig. 241.

Magnesian Limestone.

Spirifer undulatus, Sow.
Magnesian Limestone.

to the Magnesian Limestone, Coal, and Primary fossiliferous
strata, but have never been met with in any rock above the
Magnesian Limestone. There are certain fish also found both
in England and Germany, in the Lower New Red System,
which occur in the carboniferous strata, but in no formation
higher in the series than the Magnesian limestone, not even in
the Muschelkalk.

The genus Palaoniscus, Agas. (Pal&othrissum, Blain.) is

Fig. 242.

Restored outline of a fish of the genus Paleeoniscus. Agass.* Magnesian
Limestone.

the most striking example, as three species have been found in
England in marl slate, immediately below the Magnesian Lime-
stone ; and three other different, but nearly allied species, in the
slate of the Zechstein of Germany, f

It was first pointed out by M. Agassiz, that all the bony fish
of the Magnesian Limestone, and of all the more ancient form-
ations, have the vertebral column continued into the upper lobe

* Poissons Fossiles, vol. i. tab. A. fig. 4.

t Sedgwick, Geol. Trans., Second Series, vol. iii. p. 117.

242 LYELL'S ELEMENTS OF GEOLOGY.

Origin of Red Sandstone and Marl.

of the tail, which is much longer than the lower lobe (see Fig.
242.), whereas, in strata newer than the Magnesian Limestone,
the tail-fin is divided into two equal lobes, as in almost all living
fishes, the vertebrae not being prolonged into either lobe.

The remains of at least two saurian animals of new genera,
Paloeosaurus and Thecodontosaurus have been lately discovered
in the dolomitic conglomerate near Bristol.* They are allied to
the Iguana and Monitor, and are the most ancient examples of
fossil reptiles yet found in Great Britain. The Zechstein of
Germany is also the oldest rock on the continent in which Sau-
rian remains have been found. They are referred to a genus
called Protorosaurus, also allied to the Monitor.

The resemblance above alluded to between the
Fig. 243. fossils of the Lower New Red system and those
iHHi 'iffifli °f tne Coal, is not confined to the mollusca, fish,
and reptiles, but extends to the Crinoidea, or
Stone-lilies. Thus one species, the Cyathocri-
nites planus (Fig. 243.) of the Magnesian Lime-
stone of Durham, has been identified by Mr.
Miller with a fossil of the Mountain limestone of
Bristol.t

Origin of the New Red Sandstone group. —
The red sandstone and red marl, which, in point
of thickness, form the most considerable part
both of the upper and lower New Red formation
Cyathocrinites in England and Germany, may have arisen in
MagiSiSi^Sd great Part fr°m tne disintegration of various
Mountain Lime- crystalline, or metamorphic schists ; and some-
times, as in parts of Saxony and Devonshire,
from porphyritic trap rocks containing much oxide of iron. In
some districts of the eastern Grampians in Scotland, as in the
north of Forfarshire, the sides of mountains composed of gneiss,
mica-schist, and clay-slate, are covered with alluvium, derived
from the disintegration of those rocks ; and the mass of detritus
is stained by oxide of iron, of precisely the same colour as the
Old Red sandstone of the adjoining Lowlands. Now this allu-
vium merely requires to be swept down to the sea, or into a lake,
to form strata of red sandstone and red marl, similar to those of
the " Old Red" or New Red system, or those of the cretaceous
era in Spain (see p. 199.), or those of tertiary origin, as at
Coudes and Champheix, in Auvergne, all of which are in litho-
logical characters quite undistinguishable from one another. The

* See paper by Messrs. Riley and Stuchbury, Proceedings Geol. Soc. No. 46.
t Sedgwick, Geol. Trans., Second Series, vol. iii. p. 120.

PART II. CHAPTER XX. 243

Carboniferous Group.

pebbles of gneiss in the tertiary red sandstone of Auvergne,
point clearly to the rocks from which it -has been derived. The
red colouring matter may have been furnished by the decomposi-
tion of hornblende, or mica, which contain oxide of iron in large
quantity (see p. 102.).

It is a general fact, and one not yet accounted for, that scarce-
ly any fossil remains are preserved in stratified rocks in which
this oxide of iron abounds ; and when we find fossils in the New
or Old Red sandstone in England, it is in the grey and usually
calcareous beds, that they occur.

CHAPTER XX.

THE COAL, OR CARBONIFEROUS GROUP.

Carboniferous strata in the south-west of England — Superposition of Coal-
measures to Mountain limestone — Departure from this type in north of England
and Scotland — Freshwater strata — Intermixture of freshwater and marine beds
— Sauroidal fish — Fossil plants — Ferns and Sigillarise — Lepidodendra — Calamites
— Coniferse — Stigmariae.

THE next group which we meet with in the descending order
is the Carboniferous, commonly called "The Coal," because*
many beds of that mineral, in a more or less pure state, are in-
terstratified with sandstone, shale, and limestone, of which the
bulk of the formation is made up. The combustible coal itself,
even in Great Britain and Belgium, where it is most abundant,
constitutes but a small proportion of the whole mass. In the
north of England, for example, the thickness of the coal-bearing
strata has been estimated at 3000 feet, while the various coal-
seams, 20 or 30 in number, do not exceed 60 feet.*

In the south-west of England, in Somersetshire, and in South
Wales, the Carboniferous series consists of,

( Strata of shale, sandstone, and grit, with occasional
1st. Coal-measures, < seams of coal, sometimes exceeding 600 feet in
;'J; "~ Q thickness.

f A coarse quartzose sandstone passing into a con-
2d. Millstone grit. < glomerate, sometimes used for millstones; de-
( void of coal ; occasionally above 600 feet thick.
3d. Mountain or C A calcareous rock containing marine shells and
Carboniferous < corals, devoid of coal ; thickness variable ; some-
limestone, r times 900 feet.

* Phillips; art. "Geology," Encyc. Britan.

244 LYELL'S ELEMENTS OF GEOLOGY.

Carboniferous Group Freshwater Strata.

Beneath all these is the Old Red sandstone, which was for-
merly considered as part of the Carboniferous series ; but which,
now that its organic remains are better known, appears entitled
to rank as a distinct formation.

As we proceed northwards from South Wales and Somerset-
shire to Yorkshire and the more northern counties, we find the
Carboniferous group beginning gradually to assume a new cha-
racter, there being first a slight intermixture of the Coal-measures
and Mountain limestone at their contact, and these alternations
taking place afterwards on a still greater scale. The Coal, in
Yorkshire, does not cease when we reach the Millstone-grit,
although it is there in diminished quantity ; and beneath that
grit is a complex deposit, 1000 feet thick, of limestones, alter-
nating with coal-bearing sandstones and shale, below which
comes the great mass of mountain limestone.* In Scotland we
observe a still wider departure from the type of the south of
England, the mixture of marine limestone with sandstone and
shale, containing coal, being more complete.

The importance of the coal in England, considered economi-
cally, is greatly enhanced by the rich beds of iron-ore which
occur in the associated shales, and the contiguity as a flux to re-
duce the iron-ore to a metallic state, f

It is now generally admitted, that all coal is of vegetable ori-
gin, the vegetable structure being still recognizable in many kinds
£>f coal, when slices thin enough to transmit light are obtained
and examined by the miscroscope. Impressions also of plants,
together with entire trunks of trees, are frequently met with in
the accompanying shale and sandstone ; leaves also, and small
branches, and fruits, occur in nodules of clay-ironstone, the in-
closed vegetable having served as a nucleus round which the
ferruginous matter, usually carbonate of iron, has concreted.
Some of the coal-measures are of freshwater origin, and many
have been formed in lakes, others seem to have been deposited
in estuaries, or at the mouths of rivers, in spaces alternately oc-
cupied by fresh and salt water.

Thus a freshwater deposit, near Shrewsbury, has been ascer-
tained by Mr. Murchison to be the youngest member of the car-
boniferous series of that district, at the point where the coal-mea-
sures pass into the lower New Red formation. It consists of
shales and sandstones about 150 feet thick, with coal, and traces

* Sedgwick, Geol. Trans., Second Series, vol. iv. ; and Phillips, Geol. of
Yorksh., part 2.
t Conybeare, Outlines, &c., p. 333.

PART II. CHAPTER XX.

245

Fossils of the Carboniferous Group.

Freshwater fossils. — CoaZ.

Fig. 244.

245.

a. Microconchus
carbonarius.

b. var. of same, nat.
size, and magnified.

of plants, including a bed of
limestone, varying from two
to nine feet in thickness,
which is cellular, and re-
sembles the lacustrine lime-
stone of France and Ger-
many. It has been traced
for 30 miles in a straight
line, and recognized at more
distant points. The charac-
teristic fossils are a small
bivalve, having the form of
cypris inflate, natural a cyclas, a small cypris,
size, and magnified. (Fig. 245.) and a miscrosco-

Murchison.* V & /

pic shell, (microconchus) of
an extinct genus.

But in the lower coal-measures of Coalbrook Dale, the strata,
according to Mr. Prestwich, often change completely within very
short distances, beds of sandstone passing horizontally into clay,
and clay into sandstone. The coal-seams often wedge out or
disappear; and sections, at places nearly contiguous, present
marked lithological distinctions. In this single field, in which
the strata are from 700 to 800 feet thick, between 40 and 50
species of terrestrial plants have been discovered, besides several
fishes and trilobites ; the latter distinct in form from those occur-
ring in the Silurian strata. Also upwards of 40 species of mol-
lusca, among which are two or three of the. fresh water genus
Unio, and others of marine forms such as Nautilus, Orthoceras,
Spirifer, and Productus. Mr. Prestwich suggests, that the inter-
mixture of beds containing freshwater shells with others full of
marine remains, and the alternation of coarse sandstone and
conglomerate with beds of fine clay or shale containing the re-
mains of plants, may be explained by supposing that the deposit
of Coalbrook Dale, originated in a bay of the sea or estuary into
which flowed a considerable river subject to occasional freshes.f

In the Edinburgh coal-field at Burdiehouse, fossil fishes, mol-
lusca and cypris, very similar to those in Shropshire and Staf-
fordshire, have been found by Dr. Hibbert.ij: In the coal-field
also of Yorkshire there are freshwater strata, some of which

* Silurian System, p. 84.

t Prestwich, Geol. Soc. Proceedings, No. 46. Murchison, Silurian System,
p. 150.

t Trans. Roy. Soc. Edin. vol: xiii. Homer, Edin. New Phil. Journ., April
1836.

246

LYELL'S ELEMENTS OF GEOLOGY.

Fossil Fish of the Coal.

contain shells referred to the genus Unio ; but in the midst of
the series there is one thin but very widely spread stratum,
abounding in marine shells, such as Ammonites Listeri. (Fig.
246.) Orthoceras, Pecten papyraceus, (Fig. 247.) and seve-
ral fishes.*

Fig. 246.

Fig. 247.

Ammonites Listeri, Sow.

Pecten papyraceus, Sow.

Fig. 248.

No similarly intercalated layer of marine shells has been
noticed in the neighbouring coal-field of Newcastle, where, as in
South Wales, and Somersetshire, the marine deposits are entirely
below those containing terrestrial and freshwater remains.f
No bones of mammalia or reptiles have as yet been discovered
in strata of the carboniferous group. The
fish are numerous, and for the most part
very remote in their organization from
those now living, as they belong chiefly to
the Sauroid family of Agassiz ; as Mega-
lichthys, Holoptychus, and others, which
were often of great size, and all predace-
ous. Their osteology, says M. Agassiz,
reminds us in many respects of the skele-
tons of saurian reptiles, both by the close
sutures of the bones of the skull, their
large conical teeth striated longitudinally,
(see Fig. 248.) the articulations of the spi-
nous processes with the vertebrae, and
other characters. Yet they do not form a
family intermediate between fish and rep-
tiles, but are true fish.^.

The annexed figure represents a large
tooth of the Megalichthys, found by Mr.
Horner in the Cannel coal of Fifeshire.
It probably inhabited an estuary, frequenting both the mouths of
rivers and the sea.

Megalichthys Hibberti, Ag.

Edinburgh coal-field ;

natural size.

* Phillips ; art " Geology," Encyc. Metrop., p. 590.
t Agassiz, Poiss. Foss., livr. 4. p. 62. and livr. 5. p. 88.

t Ibid., p. 592.

PART II. CHAPTER XX. 247

Fossil Plants of the Coal Strata.

Fossil Plants of the Coal. — But the flora of the coal forms
the most interesting feature in its palaeontology, and is far better
known to us than any other flora antecedent to the tertiary era.
About 300 species of terrestrial plants are enumerated by M.
Adolphe Brongniart as proper to the Coal, but botanists have
encountered the greatest difficulty in determining the natural
affinities of these fossils, it being rare to find in them any vestige
of flower, seed, or fruit, those organs which afford the most con-
venient characters for classifying living plants. They have been
obliged, therefore, first to study more minutely the different
forms of bark in existing trees, their various modes of branch-
ing, the tissue of their wood, nervures of the leaves, and other
peculiarities of vegetable structure which might enable them to
institute a direct comparison between the analogous parts of
recent and fossil plants.*

The most common of these vegetable remains may be provi-
sionally classed under the following heads : — First, Ferns and
Sigillarise ; secondly, Lepidodendra, allied to Lycopodiacea ?
thirdly, Calamites, allied to Equisetacece 1 fourthly, Coniferous
plants ; fifthly, Stigmarise, apparently an extinct family of plants.

Ferns and Sigillaria. — The leaves, or more properly speak-
ing, the fronds, of ferns, (see Figs. 249, 250.) for the most part
destitute of fructification, exceed in number all other plants in
the shale of the coal. They have been divided by M. Ad. Brong-
niart into genera, characterized chiefly by the branching of the
fronds, and the way in which the veins of the leaves are dis-
posed. These fronds are often accompanied by large fluted
stems or trunks of trees which have been squeezed down and
flattened as they lay prostrate in the shale, so that the opposite
sides meet, but which when they occur in the accompanying grit
or sandstone, and are placed obliquely or vertically to the planes
of stratification, are round and uncompressed. Their bark has
been converted into coal ; and they must have been hollow when
first deposited, for the interior became filled, not only with sand,
but with leaves and branches of ferns, introduced from above.
Impressions of these fronds are now frequent in the pillars of
sandstone, which may be regarded as casts of the interior of
those ancient trees. Most of the trunks or stems now alluded
to have been called Sigillariee. They vary from half a foot to
five feet in diameter, and must have been sometimes forty or fifty
feet high.

It is admitted by all botanists that some of these gigantic
stems, all of which are comprehended by Brongniart in his

* See the works of MM. Ad. Brongniart, Stern berg, and others, and the Fossil
Flora of Lmdiey and Hut ton.

248

LYELL'S ELEMENTS OF GEOLOGY.

Fossil Plants of the Coal Strata.

Fig. 249.

Fiff. 250.

Pecopteris lonchittca.
(Foss. Flo. 153.)

a. Sphenopteris crenata.

b. The same, magnified.

(Foss. Flo. 101.)

Fig. 251.

Fig. 252.

Sigillaria lAndleyi, Brong.
(Cavlopteris primaxa, Lindley.)

Sigillaria lavigata, Brong.

PART II. CHAPTER XX. 249

Recent Tree-Ferns.

genus Sigillaria, were true arborescent ferns, as for example,
that section which has been named Caulopteris by Lindley and
Hutton. (See Fig. 251.) But these are comparatively rare,
whereas of the other section (Fig. 252.) more than forty species
have been described. In these the scars on the stem are smaller
and more regularly arranged in parallel series on the fluted
bark. (Fig. 252.)

The recent tree-ferns belong to one tribe (Polypodiacece.} and
to a small number only of genera in that tribe, in all of which
the surface of the trunk is marked with scars, or cicatrices, left
after the fall of the fronds. These scars are precisely similar to
those of Caulopteris (Fig. 251); but Mr. Lindley objects to the
opinion that the remaining Sigillarise of Brongniart were Tree-

Fig. 253. Fig. 254. Fig. 255.

Living Tree-ferns of different genera. (Ad. Brong.)

Fig. 253. Tree-fern from Isle of Bourbon.
Fig. 254. Cylhea glauca, Mauritius.
Fig. 255. Tree-fern from Brazil.

ferns, because the scars in these are smaller, dissimilar in form, and
more regularly arranged in parallel lines ; also, because the stems
are fluted, (see Fig. 252.) and sometimes bifurcating. M. Brong-
niart has replied, that the forking of the stems of some of the
fossil trees is no more than might have been expected from their
large size ; and as to the forms of the discs or scars from which
the fronds have fallen, their individual variations are not greater

250 LYELL'S ELEMENTS OF GEOLOGY.

Fossil Plants of the Coal Strata.

than those which we find in the fronds of different genera of
living ferns, which do not in the present state of the globe attain
the size of trees.

Lepidodendra. — Another class of fossils, very common in
the coal -shales, have been named Lepidodendra. Some of these
are of small size, and approach very near in form to the modern

Fig. 256. Fig. 257. Fig. 258.

Lepidodendron Sternbergii. Coal-measures, near Newcastle.

Fig. 256. Branching trunk, 49 feet long, supposed to have belonged to L. Stern-
bergii. (Foss. Flo. 203.)

Fig. 257. Branching stem with bark and leaves of JL. Sternbergii. (Foss. Flo. 4.)
Fig. 258. Portion of same nearer the root ; natural size. (Ibid.)

Lycopodiums, or club-mosses, while others of much larger
dimensions are supposed to have been intermediate between these
and coniferous plants. The annexed figures represent a large
fossil, Lepidodendron, forty-nine feet long, lately found in Jar-
row Colliery, near Newcastle, lying in shale parallel to the
planes of stratification. Fragments of others, found in the same
shale, indicate by the size of the rhomboidal scars which cover
them a still greater magnitude. The living club-mosses, of
which there are about 200 species, are abundant in tropical cli-
mates, where one species is sometimes met with attaining a
height of three feet. They usually creep on the ground, but
some stand erect, as the L. densum, from New Zealand. (Fig.
259.)

Calamites. — These fossils have a jointed stem, longitudinally
striated, and are supposed by M. Brongniart to have been allied
to the Equisetacea, or horse-tail tribe ; aquatic plants which, in
a living state, are only two or three feet high in our climates,

PART II. CHAPTER XX.

251

Fossil Plants of the Coal Strata.

a. Lycopodium densum; banks of R. Thames, New Zealand.
I. branch, natural size. c. part of same, magnified.

and even in tropical countries only attain, as in the case of
Equisetum giganteum, discovered by Humboldt and Bonpland,
in South America, a height of about five feet, the stem being an
inch in diameter. The Calamites, however, of the Coal differed
from these, principally in being furnished with a thin bark,
which is represented in the stem of C. Suckowii, (Fig. 261.) in
which it will be seen that the striped external pattern does not

Fig. 260.

Fig. 261.

HKillllllil

Calamites canaformis, Schlot.
(Foss. Flo. 79.) Common in
English coal.

Calamites Suckowii, Brong.
natural size. Common in
coal throughout Europe.

agree with that left on the stone where the bark is stripped off,
so that if the two impressions were seen separately, they might
be mistaken for two distinct species.

Coniferce. — The structure of the wood of certain coal-plants
displays so great an analogy to that of certain pines of the genus
Araucaria, as to lead to the opinion that some species of fira
existed at this period. (See above, p. 56.)

252

LYELL'S ELEMENTS OF GEOLOGY.

Fossil Plants of the Coal Strata.

Fig. 262.

Fig. 263.

Stigmaria ficoides, Brong. One-fourth of nat. size. (Foss. Flo. 32.)

StigmaricB. — Fragments of a plant which has been called
Stigmaria Jicoides^ occur in great numbers in almost every
coal-pit. It is supposed to have been a huge succulent water-
plant of an extinct family ; thin transparent sections of the stem
exhibiting an anatomical structure quite different from the wood
of any living tree.* According to the conjectures of some bota-
anists, it approached most nearly to the
family Lycopodiacecz ; according to
others, to Euphorbiacete. Mr. Hutton
discovered one of these Stigmarise form-
ing a huge dome-shaped body, from which
. ._._ . . twelve branches spread horizontally in

Surface of another individual of „ ,. n v • i- •

same species, showing form of all directions, each, usually dividing into
tubercles. (Foss. Fio. 34.) two armSj from twenty to thirty feet
long, to which leaves of great length were attached. Dr. Buck-
land imagines these plants to have grown in swamps, or to have
floated in lakes like the modern Stratiotes.f

I shall postpone some general remarks on the climate of the
Carboniferous period, arising out of the contemplation of its flora,
until something has been said of the contemporaneous Mountain
limestone and its marine fossils.

Lindley, Foss. Flora, p. 166.

t Bridgew. Treat, p. 478.

PART II. CHAPTER XXI. 253

Mountain Limestone.

CHAPTER XXI.

CARBONIFEROUS GROUP Continued, AND OLD RED SANDSTONE.

Corals and shells of the Mountain limestone — Hot climate of the Carboniferous
period inferred from the marine fossils of the Mountain limestone and the plants
of the Coal — Origin of the Coal-strata — Contemporaneous freshwater and marine
deposits — Modern analogy of strata now in progress in and around New Zealand
— Vertical and oblique position of fossil trees in the Coal — How enveloped —
How far they prove a rapid rate of deposition — Old Red sandstone — its subdi-
visions— its fossil shells and fish.

CARBONIFEROUS or Mountain limestone. — We have already
seen that this rock lies sometimes entirely beneath the Coal-
measures, while in other districts, it alternates with the shales
and sandstone of the Coal. In both cases it is destitute of land
plants, and usually charged with corals, which are often of large
size ; and several species belong to the lamelliferous class of La-
marck, which enter largely into the structure of coral reefs now
growing. There are also a great number of Crinoidea, and a few
Echinida, associated with the zoophytes above mentioned. The
Brachiopoda constitute a large proportion of the Mollusca, many
species being referable to two extinct genera, Spirifer (or Spiri-
fera) (Fig. 264.) and Producta (Fig. 265.). There are also

Fig. 264. Fig. 265.

Spirifera glabra, Sow* Producta Martini,SowJ

Mountain limestone. Mountain limestone.

many univalve and bivalve shells of existing genera in the
Mountain limestone, such as Turritella, Buccinum, Patella, Iso-
cardia, Nucula, and Pecten.ij: But the Cephalopoda depart, in

* Phillips, Geol. of Yorksh. pi. 10. fig. 11.

t Ibid., pi. 8. fig. 19. f Ibid., vol. ii. p. 208.

254

LYELL'S ELEMENTS OF GEOLOGY.

Corals and Shells of the Mountain Limestone.

general, more widely from living forms, some being generically
distinct from all those found in strata newer than the Coal. In
this number may be mentioned Orthoceras, a siphuncled and
chambered shell, like a Nautilus uncoiled and straightened.
Some species of this genus are several feet long (Figs. 266,

. 266.

Fig. 257.

Portion of Orthoceras laterale, Phillips.
Mountain Limestone.

O. giganteum, Sow.

Section showing the siphuncJe

reduced two-thirds.

267.). The Goniatite is another genus nearly allied to the Am-
monite, from which it differs in having the lobes of the septa free
from lateral denticulations, or crenatures ; so that the outline of
these is- continuous and uninterrupted (see a, Fig. 268.). Their

Fig. 268.

Fig. 269.

Goniatites evolutus, Phillips.*
Mountain limestone.

Bellerophon costatus, Sow.f
Mountain limestone.

siphon is small, and in the form of the strise of growth they
resemble Nautili. Another extinct generic form of Cephalopod,
abounding in the Mountain limestone, and not found in strata of
later date, is the Bellerophon (Fig. 269.), of which the shell,
like the living Argonaut, was without chambers.

Climate of the Carboniferous period. — The abundance of
lamelliferous and other corals, of large chambered Cephalopods
and Crinoidea, naturally leads us to infer that the waters of the
sea, at this period, were of a far warmer and more equable tem-
perature than is now experienced in those latitudes where the
Coal strata abound, in Europe. M. Adolphe Brongniart has teen
led to a similar conclusion in regard to the temperature of the

* Phillips, Geol. of Yorksh., pi. 20. fig. 65.

tlbid.pl. 17. fig. 15.

PART II. CHAPTER XXI. 255

Climate of the Carboniferous Period.

air from considering the Carboniferous flora. The unquestioned
existence of large tree-ferns, such as Caulopteris (Fig. 251.) now
exclusively the inhabitants of hot and humid climates, and the
great variety of fossil fronds of ferns in the Coal confirm this
idea, even if we refuse to accede to the arguments adduced to
prove that SigillariaB were tree-ferns of extinct genera. The
same views receive farther countenance, if the Lepidodendra and
Calamites are rightly conjectured to have been gigantic plants of
the orders LycopodiacecB and Equisetacece, which, although
most largely developed at present in the tropical zone, are even
there of pigmy stature in comparison with the fossil tribes just
alluded to. The Araucaria, also, is a family of pines now met
with in temperate and warm latitudes ; and the fir trees proper to
the forests of arctic regions do not appear to have any fossil re-
presentatives in the Coal. M. Ad. Brongniart, when endeavouring
to establish the great heat and moisture of the climate of the era
under consideration, may perhaps have relied too much on the
numerical preponderance of ferns over other orders of coal-
plants. We may easily be deceived by such reasoning, because
it is founded on negative facts, or the absence of plants of certain
orders, families, and genera. On this subject Professor Lindley
has observed, that the small variety in the forms of each fossil
flora must, in a great degree, depend on the relative, destructi-
bility of plants when suspended in water before they are imbedded
in strata. In illustration of this point, he threw into a vessel
containing fresh water 177 plants, among which were species
of all the orders found in the Carboniferous flora, with others
representing the remaining families and natural orders in the
living creation, and found that, at the end of two years, all had
decayed and disappeared except the ferns, palms, Lycopodia-
CCCB and Conifertz. The fructification of the ferns had also van-
ished, but the form and nervures of the leaves remained.*

No inference, however, drawn from this experiment, can en-
tirety explain away the fact of the vast preponderance in the
coal-shales of fern-leaves over those of Dicotyledonous plants.
Impressions of these last, together with their wood, are plentifully
preserved in tertiary rocks in which fossil ferns are rare ; and
had they been drifted down in as large numbers as ferns into the
estuaries of the Carboniferous period, they would have left im-
pressions of their shape in shale and sandstone, as they have
done in more recent formations.

It would, moreover, be rash to assume that the coal-plants in
general floated about in water for a year or two before they were

* Lindley, FOES. Flora, part 17.

256 LYELL'S ELEMENTS OF GEOLOGY.

Origin of the Coal Strata.

enveloped in sediment. It is more probable that a large part of
them were deposited immediately with the mud and sand swept
down with them by rivers into lakes or the sea. This must have
happened in those rare cases where the ferns still retain their
fructification. Where this has disappeared, its decomposition
may often have been subsequent to the inclosure of the frond in
mud or sand.

Origin of the Coal strata. — Detached portions of the ancient
Carboniferous group extend from Central Europe to Melville
Island and the confines of the arctic region ; but do not appear
in the south of Europe ; for the lignite and coal found south of
the Alps and Pyrenees, in Spain, Italy, Greece, and other coun-
tries bordering the Mediterranean, seem referable to the Creta-
ceous and other comparatively modern groups.

It has been already shown that, in some parts of England, as
in Shropshire, certain Coal-measures consist of freshwater strata,
and may have originated in a lake, while others, not far distant,
were deposited in estuaries to which the sea obtained access
occasionally ; while a third class were formed at the bottom of
an open sea, or in bays of salt water into which land plants were
drifted.*

In many parts of France and Germany there are isolated
patches of Coal strata, entirely free from marine fossils, which
repose on granite and other hypogene rocks. They are often
confined to an extremely small area, as at St. Etienne, in the
department of the Loire ; at Brassac, in that of Puy de Dome ;
at Sarrebruck ; also in Silesia ; and a hundred other places.
All these deposits may have been formed in lakes, existing in
the islands of that sea in which the Mountain limestone was
formed, f

If the climate of New Zealand and the surrounding ocean was
warmer, so that tree-ferns could thrive more luxuriantly on the
land, and corals build reefs in the sea, we might conceive new
strata to accumulate in that part of the globe analogous to those
of the ancient Coal. The two islands of New Zealand are be-
tween 800 and 900 miles in length ; and through the middle of
them runs a lofty chain of mountains, said in some parts to be
14,000 feet high, and covered with perpetual snow. Many
rivers descend from their sides ; and, in the spring, these are
copiously charged with sediment, and with abundance of drift
wood. Opposite the mouths of these rivers, and near the shores,
wherever these may be wasting by the action of the waves, an
irregular zone of gravel, sand, and mud, must be forming in the

* Murchison, Silurian System, p. 148. t Burat's D'Aubuisson, torn. ii. p. 26a

PART II. CHAPTER XXL 257

Erect Position of Trees in the Coal Strata.

surrounding sea — a zone several thousand miles in circumference.
No less than 57 species of ferns, some few of them arborescent,
have been already discovered in this country ; and what is re-
markable, one tree-fern ranges in this country as far south as the
46th degree, south latitude. There are no indigenous mammalia
except one rat, and a species of bat ; few reptiles, and none of
large size; so that we may anticipate a total absence of the
bones of land quadrupeds, and a scarcity of those of reptiles, in
the modern estuary and lacustrine deposits of this region. That
there are lacustrine strata now in progress is certain, since one
lake called Rotorua, in the interior of the northern island, is said
to be 40 miles long, and receives the waters of many small rivers
and torrents.*

The minor repetitions of alternate fresh and saltwater strata
in the Coal, have been ascribed to such changes as may annu-
ally occur near the mouths of rivers ; but when shale and grit,
containing coal and freshwater shells, are covered by large masses
of coralline rock, and these again by other Coal-measures, we
must suppose great movements of elevation and subsidence, like
those by which I endeavoured to explain, in Chapters XVI. and
XVIIL, the superposition of the Cretaceous group to the Wealden,
or the alternations of argillaceous and calcareous rocks in the
Oolite. In adopting such views, we must suppose the lapse of
vast periods of time ; as the thickness of the Coal strata, in some
parts of England, independently of the Mountain limestone, has
been estimated at 3000 feet. Besides, we can by no means pre-
sume that all coal-fields were in progress at once, much less that,
in the same field, each mass of strata which is parallel, or occu-
pies a corresponding level, was formed simultaneously. It is
far more consistent with analogy to suppose that rivers filled up
first one part of a fiord, gulf, or bay, nearest the land, and then
another ; so that the sea was gradually excluded from certain
spaces which it previously occupied. This is doubtless the cause
why the coal-bearing strata are generally uppermost, and the
Mountain limestone the lowest part of each series ; and why, in
certain districts in the S. W. of England, the Mountain limestone
suddenly thins out, so that coal-shales and grit rest immediately
upon older and unconformable rocks.

Erect position of fossil trees in the Coal strata. — A great
number of the fossil trees of the Coal are in a position either ob-
lique or perpendicular to the planes of stratification. This singu-
lar fact is observed on the Continent as well as in England, and
merits great attention, not only as opening a curious field for

* Account of New Zealand, published for New Zealand Association.

W*

258

LYELL'S ELEMENTS OF GEOLOGY.

Erect Position of Trees in the Coal Strata.

speculation, but because it has furnished a popular argument to
some writers who desire to prove the earth's crust to be no more
than 5000 or 6000 years old. The fact did not escape the notice
of Werner, who conceived that the trees must have lived on the
spots where they are now found fossil ; and this hypothesis was
defended by M. Alexandre Brongniart, in the account given by
him, in 1821, of the coal-mine of Treuil, at St. Etienne. near
Lyons.* (Fig. 270.) In this mine, horizontal Coal strata aro

Fig. 270.

Section showing the erect position of fossil trees in coal-sandstone at
St. Etienne. (Alex. Brongniart.)

traversed by vertical trunks of Monocotyledonous vegetables re-
sembling bamboos, or large Equiseta. These beds are represent-
ed in the above figure (270.), and are from 10 to 13 feet in
height, consisting of micaceous sandstone, distinctly stratified,
and passing into the slaty structure. Since the consolidation of
the stone, there has been here and there a sliding movement,
which has broken the continuity of the stems, throwing the upper
parts of them on one side, so that they are often not continuous
with the lower.

Now, had these trees, as some geologists contend, once formed
part of a submerged forest like that of Portland, before described,

Annales des Mines, 1821.

PART II. CHAPTER XXL 259

Erect Position of Trees in the Coal Strata.

(see p. 205.) all the roots would have been in the same stratum,
or would have been confined to certain levels, and not scattered
irregularly through the mass. Besides, when the stems have
any roots attached to them, which happens but rarely, they are
imbedded in sandstone precisely similar to that in which the
trunks are inclosed, there being no soil of different composition
like the Portland dirt-bed, — no line of demarcation, however
slight, between the supposed ancient surface of dry land and the
sediment now enveloping the trees.

Some may, perhaps, think it superfluous to advance such
objections to M. Brongniart's theory, since Dr. Buckland has
informed us that, when he visited these same quarries of Treuil
in 1826, he saw so many trunks in an inclined posture, that the
occasional verticality of others might be accidental.* Neverthe-
less, the possibility of so many of them having remained in an
upright posture demands explanation ; and there are analogous
cases on record respecting similar fossils in Great Britain of a
still more extraordinary nature.

In a colliery near Newcastle, say the authors of the Fossil
Flora, a great number of Sigillarias were placed in the rock as
if they had retained the position in which they grew. Not less
than 30, some of them 4 or 5 feet in diameter, were visible,
within an area of 50 yards square, the interior being sandstone,
and the bark having been converted into coal. The roots of one
individual were found imbedded in shale ; and the trunk, after
maintaining a perpendicular course and circular form, for the
height of about 10 feet, was then bent over so as to become
horizontal. Here it was distended laterally, and flattened so as
to be only one inch thick, the flutings being comparatively dis-
tinct.f Such vertical stems are familiar to our miners, under
the name of coal-pipes. One of them, 72 feet in length, was
discovered, in 1829, near Gosforth, about five miles from New-
castle, in coal-grit, the strata of which it penetrated. The exte-
rior of the trunk was marked at intervals with knots, indicating
the points at which branches had shot off. The wood of the
interior had been converted into carbonate of lime; and its
structure was beautifully shown by cutting transverse slices, so
thin as to be transparent. (See p. 56.)

In 1830, a slanting trunk was exposed in Craigleith quarry,
near Edinburgh, the total length of which exceeded 60 feet. Its
diameter at the top was about 7 inches, and near the base it
measured 5 feet in its greater, and 2 feet in its lesser width. The
bark was converted into a thin coating of the purest and finest

* Bridgew. Treat., p. 471. t Lindley and Hutton, Foss. Flo., part 6. p. 150.

260

LYELL'S ELEMENTS OF GEOLOGY.

&ate of Deposition of the Coal Strata.

Fig. 271.

Inclined position of a fossil tree cutting through
horizontal beds of sandstone, Craigleith quarry.
Edinburgh. Angle of inclination from a to
b, 27°.

coal, forming a striking
contrast in colour with the
white quartzose sandstone
in which it lay. The an-
nexed figure represents a
portion of this tree, about
15 feet long, which I saw
exposed in 1830, when all
the strata had been re-
moved from one side. The
beds which remained were
so unaltered and undis-
turbed at the point of

junction, as clearly to show that they had been tranquilly depo-
sited round the tree, and that the tree had not subsequently
pierced through them, while they were yet in a soft state. They
were composed chiefly of siliceous sandstone, for the most part
white ; and divided into laminae so thin, that from six to four-
teen of them might be reckoned in the thickness of an inch.
Some of these thin layers were dark, and contained coaly mat-
ter; but the lowest of the intersected beds were calcareous.
The tree could not have been hollow when imbedded, for the
interior still preserved the woody texture in a perfect state, the
petrifying matter being, for the most part, calcareous.* It is
also clear, that the lapidifying matter was not introduced late-
rally from the strata through which the fossil passes, as most of
these were not calcareous. It is well known that, in the Missis-
sippi and other great American rivers, where thousands of trees
float annually down the stream, some sink with their roots down-
wards, and become fixed in the mud. Thus placed, they have
been compared to a lance in rest ; and so often do they pierce
through the bows of vessels which run against them, that they
render the navigation extremely dangerous. But the vertical
coal-plants did not always retain their roots. Perhaps they sank
with their larger end downwards, because the specific gravity of
the wood may have been greatest near the lower end. In trees
of the Endogenous class, in particular, the wood of the inferior
and older part of the trunk is more dense than the upper and
younger portions ; and if the former should become water-logged
while the upper part of the stem still remained nearly as light
as water, or even lighter, not only would the whole trunk descend
perpendicularly, but when it reached the bottom it might stand
upright, provided a very slight support was afforded to its lower

See figures of texture, Witham, Foss. Veget., pi. 3.

PART II. CHAPTER XXI.

Rate of Deposition of the Coal Strata.

extremity by penetrating to the depth of a foot or two into soft
mud. How long such trunks, if constantly submerged, might
resist decomposition, is a question which cannot, perhaps, be
determined ; but, judging from the duration of wooden piles
constantly covered by water, and trees naturally submerged, like
those in Louisiana,* we may conclude that they might endure
for many years, so that their envelopment in strata, like those
of the Coal, may have been effected without a very rapid rate
of deposition.

If, however, we assume that strata 30 or 40 feet thick were
often thrown down in a few years, months, or even days, this
fact affords no ground for calculating the time required for the
formation of a wide coal-field.

Suppose, for example, the structure of a coal-field always re-
sembled that exhibited in the annexed section (Fig. 272.), we
might then infer, that if the lowest set of strata, a, having a
thickness of fifty feet, required half a century for its accumula-
tion, the strata, a, &, c, constituting the entire coal-field, and being
150 feet thick, might have been completed in a century and a

Fig. 272.

half. But as the beds are wedge-shaped, and often thin out ; and
as the successive beds of a single coal-field are usually arranged
in the form of a, 6, c, d, e (Fig. 273.), we cannot calculate their

Fig. 273.

number from considering any one section. The deposits, a, &,
c, tZ, e, traced in a given direction, may have taken each fifty
years for their deposition ; but they may have been as limited in
breadth as in length. They may have constituted originally a
narrow strip of land like part of the delta formed by the Missis-
sippi, since New Orleans was built, by the incessant discharge
of mud and drift timber into the Gulf of Mexico. Although by
this means a narrow tongue of land has been made to protrude
for several leagues into the sea, yet thousands of years may

* See Principles of Geology, Index, " Bistineau."

262 LYELL'S ELEMENTS OF GEOLOGY.

Old Red Sandstone.

elapse before a square area of low land, having a diameter of as
many leagues, can be gained from the Gulf of Mexico.

OLD RED SANDSTONE.

It was stated that the Carboniferous formation was surmount-
ed by one called the " New Red Sandstone," and underlaid by
another called the Old Red, which last was formerly merged in
the Carboniferous system, but is now found to be distinguishable
by its fossils. The Old Red Sandstone is of enormous thickness
in Herefordshire, Worcestershire, Shropshire, and South Wales,
where it is seen to crop out from beneath the Coal-measures and
to repose upon the Silurian rocks. In that region its thickness
has been estimated by Mr. Murchison at no less than 10,000
feet. It consists there of

1st A quartzose conglomerate passing downwards into chocolate-red and
green sandstone and marl.

2d. Cornstone and marl (red and green argillaceous spotted marls, with irre-
gular courses of impure concretionary limestone, provincially called
Cornstone, mottled, red, and green ; remains of fishes).

3d. Tilestone (finely laminated hard reddish or green micaceous or quartzose
sandstones, which split into tiles ; remains of mollusca and fishes).

I have already observed that the fossils are rare in marls and
sandstones, in which the red oxide of iron prevails ; in the Corn-
stone, however, of the counties above-mentioned, fishes of the
genera Cephalaspis and Onchus have been discovered.* In the
Tilestones also, Ictrfyodorulites, of the genus Onchus, have been
obtained ; and a species of Dipterus, with mollusca of the genera
Avicula, Area, Cucullsea, Terebratula, Lingula, Turbo, Trochus,
Turritella, Bellerophon, Orthoceras, and others.f

By consulting geological maps, the reader will perceive that
from Wales to the north of Scotland, the Old Red sandstone
appears in patches, and often in large tracts. Many fishes have
been found in it at Caithness,^: and various organic remains in
the northern part of Fifeshire, where it crops out from beneath
the Coal formation, and spreads into the adjoining southern half
of Forfarshire ; forming, together with trap, the Sidlaw hills
and valley of Strathmore. (See section, p. 99.) A large belt
of this formation skirts the southern borders of the Grampians,
from the sea-coast at Stonehaven and the Frith of Tay to the
opposite western coast of the Frith of Clyde. In Forfarshire,
where, as in Herefordshire, it is many thousand feet thick, it

* Murchison's Silurian System, p. 180. t Ibid., p. 183,

t See Geol. Trans. 2d series, vol. in. plates 15, 16, 17.

PART II. CHAPTER XXI.

263

Old Red Sandstone.

may be divided into three principal masses : 1st, red and mottled
marls, cornstone and sandstone ; 2d, Conglomerate, often of vast
thickness ; 3d, Tilestones and paving stone, highly micaceous,
and containing a slight admixture of carbonate of lime. (See
section, p. 65.) In the uppermost of these divisions, but chiefly
in the lowest, the remains of fish have been found, of the genus
named by M. Agassiz, Cephalaspis, or tuckler-headed, from the
extraordinary shield which covers the head, and which has often
been mistaken for that of a trilobite, of the division Asaphus.
(See Fig. 276. p. 266.)

Fig. 274.

Cephalaspis Lyellii, Agass. Length 6f inches.

This figure is from a specimen now in my collection, which I procured at Glam-
mis, in Forfarshire ; see other figures, Agassiz, vol. ii. Tab. 1. a, & 1. 6.

a. one of the peculiar scales with which the head is covered when perfect. —
These scales are generally removed, as in the specimen above figured.

b, c. scales from different parts of the body and tail.

A gigantic species of fish of the genus Gyrolepis has also been
found by Dr. Fleming in the Old Red sandstone of Fifeshire.*

* See Agassiz, Poissons Fossiles, torn. ii. p. 139.

LYELL'S ELEMENTS OF GEOLOGY.

Primary Fossiliferous Strata.

CHAPTER XXII.

PRIMARY FOSSILIFEROUS STRATA.

Primary Fossiliferous or Transition Strata — Terra "Grauwacke" — Silurian
Group — Upper Silurian and Fossils — Lower Silurian and Fossils — Trilobites —
Graptolites — Orthocerata — Occasional horizontality of Silurian Strata — Cambrian
Group — Endosiphonite.

WE have now arrived in the descending order at those more
ancient sedimentary rocks, which I have called the Primary
Fossiliferous (see p. 157.), and to which Werner first gave the
name of Transition, for reasons fully explained and discussed in
the 12th chapter. Many geologists have also applied to these
older strata the general name of " grauwacke," by which the
German miners designate a variety of quartzose sandstone,
which is usually an aggregate of small fragments of quartz,
flinty-slate (or Lydian stone), and clay-slate, cemented together
by argillaceous matter. But far too much importance has been
attached to this kind of rock, as if it were peculiar to a certain
epoch in the earth's history, whereas a similar sandstone or grit
is not only found sometimes in the Old Red, and in the millstone
grit of the Coal, and in certain cretaceous ^formations of the
Alps — but even in some tertiary deposits.

In England, the Old Red sandstone has been generally regard-
ed as the base of the secondary series ; but by some writers on
the Continent, the Old Red and Coal formations have been class-
ed as the upper members of the Transition series, a method
adopted by Dr. Buckland, in his late Bridgewater Treatise. This
classification, however, requires us to draw a strong line of de-
marcation between the Coal and the lower New Red sandstone
group, which, now that the fossils of these two groups are ascer-
tained to be very analogous, becomes a more arbitrary division
than that which separates the Old Red from the uppermost of
the primary fossiliferous strata.

Professor Sedgwick and Mr. Murchison have lately proposed
to subdivide all the English sedimentary strata below the Old
Red sandstone into two leading groups, the upper of which may
be termed the Silurian, and the inferior the Cambrian system.
Mr. Murchison has applied the name of Silurian to the newer
group, because these rocks may be best studied in that part of

PART II. CHAPTER XXII.

265

Upper Silurian Rocks.

England and Wales which was included in the ancient British
kingdom of the Silures. He has also formed four subdivisions
of the Silurian system, which he has designated as the Ludlow,
Wenlock, Caradoc, and Llandeilo, indicating thereby the places
where the prevailing characters of each formation are most per-
fectly exhibited. The following Table explains the succession
of these deposits.*

UPPER SILURIAN ROCKS.

Prevailing Lithological Thick-
Characters, ness.

2000 ft. <

2. Wenlock
formation.

formation.

Upper
Ludlow.

> Micaceous grey"
$ sandstone.

1. Ludlow ,
formation. '

Aymestry
limestone.

) Argillaceous
) limestone.

Lower
Ludlow.

C Shale with con-
< cretions of lime-
( stone.

Mollusca marine, of al-
most every order, the
Brachiopoda most
abundant. Serpula,
Corals, Sauroid fish,
Fuci.

Wenlock ) Concretionary
limestone. ( limestone.

Wenlock
shale.

1 Argillaceous
I shale.

f Marine mollusca of va-
rious orders as before.

}-1800ft.^ Crustaceans of the
Trilobite family.

j LNo vertebrata or plants.

LOWER SILURIAN ROCKS.

Flags of shelly'
limestone and

bedded white
^ freestone.

) Dark coloured
$ calcareous flags.

• I Crinoidea, Corals, Mol-

2500 ft. «{ lusca, chiefly Brachio-
poda, Trilobites.

1200ft. $ Mollusca, Trilobites.

UPPER SILURIAN ROCKS.

Ludlow formation. — This member of the upper Silurian
group, as will be seen by the above table, is of great thickness,
and subdivided into three parts. Each of these may be distin-
guished near the town of Ludlow, and at other places in Shrop-
shire and Herefordshire, by peculiar organic remains. The
most remarkable fossils are scales, ichthyodorulites, jaws, teeth,
and coprolites of fish, of the upper Ludlow rock.f As they
are the oldest remains of vertebrated animals yet known to geo-
logists, it is worthy of notice that they belong to fish of a high
or very perfect organization.

See Murchison's Silurian System.
x

t Ibid. p. 198, 199.

266 LYELL'S ELEMENTS OF GEOLOGY.

Upper Silurian Bocks, and their Fossils.

Among the fossil shells are species of leptoena, orthis, terebra-
tula, avicula, trochus, orthoceras, bellerophon, and others.*

, Fig. 275.

Tcrebratula Wilsoni, Sow. Ludlow formation.

Several species also of trilobite, an extinct species of crusta-
cean, characteristic of the Silurian period in general, are found in
the lower Ludlow limestone. Those represented in the annexed
figures, Calymene Blumenbachii and Asaphus caudatus, are
common to this limestone, and to the Wenlock formation which
succeeds next in the descending order.

Fig. 276.

Calymene Blumenbachii,

Brongniart, commonly called

" Dudley trilobite."

Jlsaphus oaudatus.

Some of the Upper Ludlow .sandstones are ripple-marked,
thus affording evidence of gradual deposition; and the same
may be said of the fine argillaceous shales of the Ludlow form-
ation, which are of great thickness, and have been provincially
named " mudstones," from their tendency to dissolve into mud.
In these shales many zoophytes are found enveloped in an erect
position, having evidently become fossil on the spots where they
grew at the bottom of the sea. Among others, the graptolite is
abundant. (See p. 268.) The facility with which these upper
Silurian shales, when exposed to the weather, are resolved into
mud, proves that, notwithstanding their antiquity, they are nearly
in the state in which they were first thrown down at the bottom
of the sea. All rocks, therefore, of the transition era of Wer-
ner, were hot originally precipitated in a semi-crystalline state as
was formerly pretended. (See p. 153.)

•* Murchison, Silurian System, p. 199.

PART II. CHAPTER XXII.

267

Lower Silurian Rocks, and their Fossils.

WenlocJc formation. The well-known Fig. 278.

rock of Dudley, so rich in organic re-
mains, belongs to this member of the
upper Silurian group, which consists
in its higher division of limestone more
or less crystalline, and highly charged
with corals and encrinites of species
distinct from those of the mountain
limestone. In its lower part it is prin-
cipally composed of argillaceous shale.
In the Wenlock limestone, the chain-
coral, called Catenipora cscharoidcs,
abounds. Among the shells appear the
genera euomphalus, productus, atrypa,

and many Others. Catenipora escharoides.

LOWER SILURIAN ROCKS.

Caradoc sandstone. — This formation, which is 2500 feet
thick, consists chiefly of sandstones of various colours, with
some subordinate beds of calcareous matter. Almost all the
more abundant fossils belong to the same genera as those of the
upper Silurian rocks, but the species are distinct.

Llandeilo formation. — This division, forming the base of the
Silurian system, consists of hard dark-coloured flags, sometimes
slightly micaceous, frequently calcareous, and especially distin-
guished by containing the large trilobites
Fig. 279. Asaphus Buchii (Fig. 279.), and A. ty-

ranus.* There are also several genera
of mollusca in this deposit ; and it is an
interesting fact, that with many extinct
forms of testacea peculiar to the lower
Silurian rocks, such as orthoceras, pen-
temerus, spirifer, and productus, others
JBucMi, Brong. are associated belonging to genera still

existing, as nautilus, turbo, buccinum, turritella, terebratula,
and orbicwla.'l"

No land plants seem yet to have been discovered in strata
which can be unequivocally demonstrated to belong to the Silu-
rian period.

In Norway and Sweden, the Silurian strata extend over a
wide area, and so much resemble those of England in lithologi-
cal character and fossils, that they will probably be found to be

Murchison, Silurian System, p. 222.

t Ibid. p. 351.

268

LYELL'S ELEMENTS OF GEOLOGY.

Lower Silurian Rocks, and their Fossils.

divisible into similar groups. They are composed of large de-
posits of sandstone, which is sometimes found at the base of the
system, resting on gneiss and calcareous rocks, with orthocerata
and corals ; the chain-coral, (Fig. 278.) before mentioned, being
very conspicuous ; also fine bituminous shales containing grap-
tolites. (Fig. 280.)

These bodies are supposed by Dr. Beck, of Copenhagen, to be
fossil zoophytes, related to the family of sea-pens, of which the
living animals inhabit mud and slimy sediment.

Fig. 280.

Graptotttcs, Linn.

a. 6. Graptolites from Christiania, Norway.
c. d. Graptolites from the south of Sweden.

In the limestones of Lake Michigan, in North America, and
other regions bordering the great Canadian lakes, chain-corals
and trilobites are also found, and from their fossils generally
they seem to belong to the Silurian period. They contain cer-
tain orthocerata, which have a very peculiar structure. The
siphuncle is very large, and has a tube running through its
whole length, from the outside of which radii set off in verticila-
tions extending to the inner wall of the syphon, these verticila-
tions corresponding in number to the chambers of the shell.
Mr. Stokes, who has described this division of orthoceratites, has
formed them into a distinct 'genus, for which he has adopted the

Fig. 281.

Actinoceras Simmsii, Stokes.
County of Down, Ireland. Length of original, 2 feet.

name of Actinoceras, proposed by Professor Bronn.* The ac-
tinoceras was not known as a British fossil, until lately disco-

* See Proceedings, Geol. Soc. 1838.

PART II. CHAPTER XXII.

Horizontal Silurian Strata.

Fig. 282.

A, Simmsii, portion of the shell at a, Fig. 281., natural size,
showing the tube and its radii within the siphuncle.

vered at Castle Espie, in the county of Down, in Ireland. (See
Figs. 281, 282.)

Silurian strata occasionally horizontal. — The Silurian strata
throughout a large part of the province of Skaraborg, in the
south of Sweden, are perfectly horizontal ; the different subor-
dinate formations of sandstone, shale, and limestone, occurring
at corresponding heights in hills many leagues distant from each
other, with the same mineral characters and organic remains.
It is clear that they have never been disturbed since the time of
their deposition, except by such gradual movements as those by
which large areas in Sweden and Greenland are now slowly and
insensibly rising above or sinking below their former level. The
ancient limestone and shale; also of the Canadian lake district
before mentioned, are for the most part horizontal.

These facts are very important, as the more ancient rocks are
usually much disturbed, and horizontality is a common charac-
ter of newer strata. Similar exceptions, however, occur in re-
gard to the more modern or tertiary formations which, in some
places, as in the Alps, are not only vertical, but in a reversed
position. These appearances accord best with the theory which
teaches that, at all periods, some parts of the earth's crust have
been convulsed by violent movements, which have been some-
times continued so long, or so often repeated, that the derange-
ment has become excessive, while other spaces have escaped
again and again, and have never once been visited by the same
kind of movement. Had paroxysmal convulsions ever agitated
simultaneously the entire crust of the earth, as some have ima-
gined, the primary fossiliferous strata would nowhere have re-
mained horizontal.
..*

270 LYELL'S ELEMENTS OF GEOLOGY.

Cambrian Rocks.

• Cambrian Group. — Below the Silurian strata in the region
of the Cumberland lakes, in N. Wales, Cornwall, and other
parts of Great Britain, there is a vast thickness of stratified
rocks, for the most part slaty, and devoid of fossils. In some
few places a few organic remains are detected specifically, and
some of them generically, distinct from those of the Silurian
period. These rocks have been called Cambrian by Professor
Sedgwick, because they are largely developed in N. Wales,
where they attain a thickness of several thousand yards. They
are chiefly formed of slaty sandstone and conglomerate, in the
midst of which is a limestone containing shells and corals, as at
Bala in Merionethshire. A slaty sandstone, forming the bottom
of the Cambrian system in Snowdon, contains shells of the
family Brachiopoda, and a few zoophytes.*

In some of the slate rocks of Cornwall, referred by Professor
Sedgwick to the Cambrian group, cephalopoda of a very pecu-
liar structure, called Endosiphonites, have been detected, a form
which appears not yet to have been observed in the Silurian
formation. The siphuncle in this shell is ventral, in which cha-
racter it differs both from ammonite, in which it is dorsal, and
from nautilus, in which it is central, or nearly central.

Fig. 283.

Endosiphonites carinatus, Ansted.f Cambrian strata, Cornwall.

Although the Cambrian group can scarcely yet be said to be
established on the evidence of a distinct assemblage of fossils,
yet so great is the thickness of strata beneath the lowest of the
well-determined Silurian rocks, all of a date posterior to the
creation of organic beings, that we may reasonably expect to be
able to divide the primary fossiliferous strata into two groups.

* Phillips's Geology, vol. i. p. 129. Lafdner's Cyclop, vol. xcvii.
t Camb. Phil. Trans, vol. vi. pi. 8. fig. 2.

PART II. CHAPTER X£III. 271

Tests of Relative Age of Volcanic Rocks.

CHAPTER XXIII.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.

Tests of relative age of volcanic rocks — Test by superposition and intrusion
— By alteration of rocks in contact — Test by organic remains — Test of age by
mineral character — Test by included fragments — Volcanic rocks of the Recent
and Newer Pliocene periods — Miocene — Eocene — Cretaceous — Oolitic — New
Red sandstone period — Carboniferous — Old Red sandstone period — Silurian —
Upper and lower Cambrian periods — Relative ages of intrusive traps.

HAVING referred the sedimentary strata to a long succession
of geological periods, we have next to consider how far the vol-
canic formations can be classed in a similar chronological order.
The tests of relative age in this class of rocks are four: — 1st,
superposition and intrusion, with or without alteration of the
rocks in contact ; 2d, organic remains ; 3d, mineral character ;
4th, included fragments of older rocks.

Test by superposition, fyc. — If a volcanic rock rest upon an
aqueous deposit, the former must be the newest of the two, but
the like rule does not hold good where the aqueous formation
rests upon the volcanic, for we have already seen (p. 110.) that
melted matter, rising from below, may penetrate a sedimentary
mass without reaching the surface, or may be forced in con-
formably between two strata, as b at D in the annexed figure
(Fig. 284.) after which it may cool down and consolidate. Su-

Fig. 284.

^— ^____

D

iiziziSI^S

',',' i ,«. ' ,'• ' • '£.' i '

,','•' i1 ' i

c

c

perposition, therefore, is not of the same value as a test of age
in the unstratified volcanic rocks as in fossiliferous formations.
We can only rely implicitly on this test where the volcanic
rocks are contemporaneous, not where they are intrusive. Now
they are said to be contemporaneous if produced by volcanic
action, which was going on simultaneously with the deposition
of the strata with which they are associated. Thus in the sec-

272

LYELL'S ELEMENTS OF GEOLOGY.

Tests of Relative Age of Volcanic Rocks

tion at D (Fig. 284.), wre may perhaps ascertain that the trap b
flowed over the fossiliierous bed c, and that, after its consolida-
tion, a was deposited upon it, a and c both belonging to the same
geological period. But if the stratum a be altered by b at the
point of contact, we must then conclude the trap to have been
intrusive, or if, in pursuing b for some distance, we find at length
that it cuts through the stratum a, and then overlies it.

We may, however, be easily deceived in supposing a volcanic
rock to be intrusive, when in reality it is contemporaneous, for a
sheet of lava, as it spreads over the bottom of the sea, cannot
rest every where upon the same stratum, either because these
have been denuded, or because, if newly thrown down, they thin
out in certain places, thus allowing the lava to cross their edges.
Besides, the heavy igneous fluid will often, as it moves along,
cut a channel into beds of soft mud and sand. Suppose the sub-
marine lava F, to have come in contact in this manner with the

strata a, &, c, and that, after its
Fig. 285. consolidation, the strata d, e, are

thrown down in a nearly horizon-
tal position, yet so as to lie uncon-
formably to F, the appearance of
subsequent intrusion will here be
complete, although the trap is in
fact contemporaneous. We must,
unless we find the strata d or e to
have been altered at thoir junction, as if by heat, not therefore
hastily infer that the rock F is intrusive.

When trap dikes were described in the 8th chapter, they were
shown to be more modern than all the strata which they tra-
verse. The ninety-fathom dike in the Northumberland coal-
field (see section, Fig. 286.) passes through coal-measures which

Magnesian limestone.

Coal.

Dike.
Section in a quarry at Cullercoats, Northumberland.

are much disturbed.* The beds of overlying Magnesian lime-
stone are not cut through by the dike, but appear to be in the

* See Mr. Winch's account, Geol. Trans. 1st ser. vol. iv. p. 1.

PART II. CHAPTER XXIII. 273

Tests of Relative Age of Volcanic Rocks.

position in which they were originally deposited in a hollow,
existing in the denuded surface formed by the carboniferous
strata and intrusive dike. Now here the coal-measures were
not only deposited, but had been fissured before the fluid trap
was introduced to form the dike. It also appears by the trun-
cated edges of the Coal strata, and the abrupt termination of the
dike on which the Magnesian limestone rests, that denudation
had taken place at a period intervening between the injection of
the volcanic matter and the deposition of the Magnesian lime-
stone. Even in this case, however, although the date of the vol-
canic eruption is brought within Harrow limits, it cannot be
defined with precision ; it may have happened either at the close
of the carboniferous period, or early in that of the lower New
Red sandstone, or between these two periods, when the state of
the animate creation, and the physical geography of Europe
were gradually changing from the type of the carboniferous era
to that of the lower New Red formation.

In regard to all stratified volcanic tuffs, the test of age by
superposition is strictly applicable to them, according to the
already explained rules in the case of other sedimentary deposits
(See p. 159.)

Test of age by organic remains. — We have seen how, in the
vicinity of active volcanos, scoriae, pumice, fine sand, and frag-
ments of rock are thrown up into the air, and then showered
down upon the land, or into neighbouring lakes or seas. In the
tuffs so formed, shells, corals, or any other durable organic
bodies which may happen to be strewed over the bottom of a
lake or sea, will be imbedded in tuff, and thus continue as per-
manent memorials of the geological period when the volcanic
eruption occurred. Tufaceous strata thus formed in the neigh-
bourhood of Vesuvius, Etna, Stromboli, and other volcanos now
active in islands or near the sea, may give information of the
relative age of these tuffs at some remote..future period when the
fires of these mountains are extinguished. By such evidence we
can distinctly establish the coincidence in age of volcanic rocks,
and the different primary, secondary, and tertiary fossiliferous
strata already considered.

The tuffs now alluded to are not exclusively marine, but
include, in some places, freshwater shells, in others, the bones
of terrestrial quadrupeds. The diversity of organic remains in
formations of this nature is perfectly intelligible, if we reflect
on the wide dispersion of ejected matter during late eruptions,
such as that of the volcano of Coseguina, in the province of
Nicaragua, January 19, 1835. Hot cinders and fine scoriae
were then cast up to a vast height, and covered the ground as

274 LYELL'S ELEMENTS OF GEOLOGY.

Tests of Relative Age of Volcanic Rocks.

they fell to the depth of more than ten feet, and for a distance
of eight leagues-from the crater in a southerly direction. Birds,
cattle, and wild animals were scorched to death in great num-
bers, and buried in these ashes. Some volcanic dust fell at
Chiapa, upwards of 1200 miles to windward of the volcano, a
striking proof of a counter-current in the upper region of the
atmosphere, and some on Jamaica, about 700 miles distant to
the north-east. In the sea also, at the distance of 1100 miles
from the point of eruption, Captain Eden of the Conway sailed
40 miles through floating pumice, among which were some pieces
of considerable size.*

Test of age by mineral -composition. — As sediment of homo-
geneous composition, when discharged from the mouth of a large
river, is often deposited simultaneously over a wide space, so a
particular kind of lava flowing from a crater, during one erup-
tion, may spread over an extensive area, as in Iceland, in 1783,
when the melted matter, pouring from Skaptar Jokul, flowed in
streams in opposite directions, and caused a continuous mass,
the extreme points of which were 90 miles distant from each
other. This enormous current of lava varied in thickness from
100 feet to 600 feet, and in breadth, from that of a narrow river
gorge to 15 miles.f Now, if such a mass should afterwards be
divided into separate fragments by denudation, we might still
perhaps identify the detached portions by their similarity in
mineral composition. Nevertheless, this test will not always
avail the geologist, for, although there is usually a prevailing
character in lava emitted during the same eruption, and even in
the successive currents flowing from the same volcano, still, in
many cases, the different parts even of one lava-stream, or, as
before stated, of one continuous mass of trap, vary so much in
mineral composition and texture, as to render these characters
of minor importance when compared to their value in the chro-
nology of the fossiliferous rocks.

It will, however, be seen in the description which follows, of
the European trap rocks of different ages, that they had often a
peculiar lithological character, resembling the differences before
remarked as existing between the modern lavas of Vesuvius,
Etna, and Chili. (See p. 98.)

It has been remarked that in Auvergne, the Eifel, and other
countries where trachyte and basalt are both present, the trachytic
rocks are for the most part older than the basaltic. These rocks do,
indeed, sometimes alternate partially, as in the volcano of Mount

* Caldcleugh,.Phil. Trans. 1836, p. 27., and Official Documents of Nicaragua,
t See Principles of Geology, Index, " Skaptar Jokul."

PART II. CHAPTER XXIII. 275

Different Ages of Volcanic Rocks.

Dor, in Auvergne ; but the great mass of trachyte occupies in
general an inferior position, and is cut through arid overflowed
by basalt. It can by no means be inferred that trachyte pre-
dominated greatly at one period of the earth's history, and basalt
at another, for we know that trachytic lavas have been formed
at many successive periods, and are still emitted from many
active craters ; but it seems that in each region, where a long
series of eruptions have occurred, the more felspathic lavas have
been first emitted, and the escape of the more augitic kinds has
followed. The hypothesis suggested by Mr. Scrope may, per-
haps, afford a solution of this problem. The minerals, he ob-
serves, which abound in basalt are of greater specific gravity
than those composing the felspathic lavas ; thus, for example,
hornblende, augite, and olivine, are each more than three times
the weight of water ; whereas common felspar, albite, and La-
brador felspar, have each scarcely more than "2\ times the specific
gravity of water ; and the difference is increased in Consequence
of there being much more iron in a metallic state in basalt and
greenstone than in trachyte and other felspathic lavas and traps.
If, therefore, a large quantity of rock be melted up in the bowels
of the earth by volcanic heat, the denser ingredients of the boil-
ing fluid will sink to the bottom, and the lighter remaining above
will be first propelled- upwards to the surface by the expansive
power of gases. Those materials, therefore, which occupied
the lowest place in the subterranean reservoir will always be
emitted last, and take the uppermost, place on the exterior of the
earth's crust.

Test by included fragments. — We may sometimes discover
the relative age of two trap rocks, or of an aqueous deposit and
the trap on which it rests, by finding fragments of one included
in the other, in cases such as those before alluded to, where the
evidence of superposition alone would be insufficient. It is also
not uncommon to find conglomerates almost exclusively com-
posed of rolled pebbles of trap, associated with stratified rocks in
the neighbourhood of masses of intrusive trap. If the pebbles
agree generally in mineral character with the latter, we are then
enabled to determine the age of the intrusive rock by knowing
that of the fossiliferous strata associated with the conglomerate.
The origin of such conglomerates is explained by observing the
shingle beaches composed of trap pebbles in modern volcanic
islands, or at the base of Etna.

Recent and newer Pliocene period. — J shall now select ex-
amples of contemporaneous volcanic rocks of successive geolo-
gical periods, that the reader may be convinced that the igneous
causes have been in activity in all past ages of the world, and

276 LYELL'S ELEMENTS OF GEOLOGY.

Different Ages of Volcanic Rocks.

that they have been ever shifting the places where they have
broken out at the earth's surface. One portion of the lavas, tuffs,
and trap-dikes of Etna, Vesuvius, and the island of Ischia, have
been produced within the historical era ; another and a far more
considerable part have originated at times immediately antece-
dent, when the waters of the Mediterranean were already inha-
bited by the existing species of testacea. The submarine foun-
dations of Etna and Ischia have been upheaved to the great
height of between 500 and 1500 feet above the level of the sea ;
and the same observations may be made respecting the base of
many active volcanos which were first subaqueous vents, or, like
Stromboli, half submerged, and then became subaerial, when the
ancient bed of the sea was laid dry by elevation.

Older Pliocene period. — In Tuscany and the Campagna di
Roma, submarine volcanic tuffs are interstratified with the Older
Pliocene strata of the Subapennine hills in such a manner as to
leave no doubt that they were the products of eruptions which
occurred wnen the shelly marls and sands of the Subapennine
hills were in the course of deposition.

Miocene period. — The most ancient volcanic rocks, consisting
chiefly of trachyte, of the Upper and Lower Eifel, are interca-
lated between Miocene strata in such a manner, as to prove
them to have been coeval in origin. The eruptions, however,
of the same district were continued down to the Newer Pliocene
era, or were at least renewed at that later period, so that show-
ers of ashes from the Rhenish volcanos are interstratified with
the loess, in which, we have already stated, shells of land and
freshwater species occur identical with those now living in
Europe.*

Eocene period. — The extinct volcanos of Auvergne and Can-
tal, in central France, commenced their eruptions in the Eocene
period, but were most active during the Miocene era. In the
lacustrine deposits, near those ancient volcanos, the lowest strata
were evidently formed before any eruptions had occurred. They
consist of sandstone and conglomerate, containing rounded peb-
bles of quartz, mica-schist, granite, and other hypogene rocks,
composing the borders of the ancient lakes, but not the slightest
intermixture of volcanic products can be detected. To these
conglomerates succeeded argillaceous and calcareous marls,
containing Eocene shells, during the deposition of which some
feeble signs of volcanic action began to show themselves.
Above these, freshwater marls and limestones are seen frequently
to alternate with volcanic tuff, and in them some fossils of the

*See above, p. 172., and Principles of Geology, book iv.

PART II. CHAPTER XXIII. 277

Volcanic Rocks of the Cretaceous Period.

Miocene period are discovered. After the filling up or drainage
of the ancient lakes, huge piles of trachytic and basaltic rocks,
with volcanic breccias and conglomerates, accumulated to a
thickness of several thousand feet, and were superimposed upon
granite, or the contiguous lacustrine strata. The greater portion
of these igneous rocks appear to have originated during the Mio-
cene period, and extinct quadrupeds of that era, belonging to the
genera Mastodon, Rhinoceros, and others, were buried in ashes
and beds of alluvial sand and gravel, which owe their preserva-
tion to sheets of lava which spread over them.

Cretaceous period. — Although we have no proof of volcanic
rocks erupted in England during the deposition of the chalk and
green-sand, it must not be supposed that no theatres of igneous
action existed in the cretaceous period. M. Virlet, in his account
of the geology of the Morea,* (p. 205.) has clearly shown that
certain traps in Greece, called by him ophiolites, are of this date;
as those, for example, which alternate conformably with creta-
ceous limestone and green-sand between Kastri and Damala in
the Morea. They consist in great part of dial! age rocks and
serpentine, and of an amygdaloid with calcareous kernels, and a
base of serpentine.

In certain parts of the Morea, the age of these volcanic rocks
is established by the following proofs : first, the lithographic
limestones (see p. 199.) of the cretaceous era are cut through by
trap, and then a conglomerate occurs at Nauplia and other places,
containing in its calcareous cement many well-known fossils of
the chalk and green-sand, together with pebbles formed of rolled
pieces of the same ophiolite, which appear in the dikes above
alluded to.

It was before stated that at Tercis, near Dax, in the depart-
ment of the Landes, in the south of France, highly inclined

Fig. 287. AdourR. Luy R. Puy Arzet.

'

Chalk and volcanic tuff in the environs of Dax.
E Inclined beds of chalk, and conformable volcanic tuff
a. b. c. d. Gravel, sand, and tertiary strata.

strata of limestone and marl occur, containing the fossils of the
chalk, the inclined strata being in great part concealed by
unconformable tertiary formations. In one section in this dis-
trict I observed, alternating with thin layers of volcanic tuff,
vertical cretaceous beds, which are perfectly conformable. Such

278 LYELL'S ELEMENTS OF GEOLOGY.

Volcanic Rocks.

tuffs were probably the product of submarine eruptions in the
cretaceous sea.

The traps of this country and of the neighbouring Pyrenees
are generally ophitic, and many French geologists consider them
to be newer than the cretaceous period, and therefore tertiary ;
but I know of no sections which demonstrate this point. M.
Charpentier has argued that the ophites of the Pyrenees were
more modern than all the secondary strata of that chain, because
in the conglomerates constituting the upper part of the creta-
ceous series on the flanks of the Pyrenees, no rolled pebbles of
ophite have been found.* But this negative fact may be ex-
plained by supposing that, in the cretaceous sea, which occupied
the space where the Pyrenees now stand, the ophitic eruptions
were submarine, and never formed islands or shoals exposed to
denudation.

The age of the trap of Antrim in Ireland, before described, as
altering the chalk by its dikes (p. 107.), is uncertain. It is
newer than the chalk of that region, which it cuts through and
overflows ; and perhaps it belongs to some one of the tertiary
periods. As wood-coal and coniferous fossil trees have been
found associated with it on the eastern shore of Lough Neagh,
these plants may hereafter throw light on this chronological
question, f

Period of Oolite and Lias. — Although the green and serpen-
tinous trap rocks of the Morea belong chiefly to the cretaceous
era, as before-mentioned, yet it seems that some eruptions of
similar rocks began during the oolitic period ;f and it is proba-
ble, that a large part of the trappean masses, called ophiolites in
the Apennines, and associated with the limestone of that chain,
are of corresponding age.

Whether part of the volcanic rocks of the Hebrides, in our
own country, originated contemporaneously with the lias and
oolite which they traverse and overlie, remains to be ascer-
tained.

Trap of the New Red sandstone period. — In the southern
part of Devonshire, trappean rocks are associated with new red
sandstone, and, according to Mr. De la Beche, have not been
intruded subsequently into the sandstone, but were produced by
contemporaneous volcanic action. Some beds of grit, mingled
with ordinary red marl, resemble sands ejected from a crater;
and in the stratified conglomerates occurring near Tiverton are

* Charpentier, Essai Geog. sur ies Pyrenees, p. 524.
t Dr. Berger, Geol. Trans. 1st series, vol. iii. p. 183.
t Boblaye and Virlet, Morea, p. 23.

PART II. CHAPTER XXIII. 279

Trap coeval with Coal and Old Red Sandstone.

many angular fragments of trap porphyry, some of them one or
two tons in weight, intermingled with pebbles of other rocks.
These angular fragments were probably thrown out from vol-
canic vents, and fell upon sedimentary matter, then in the course
of^deposition.*

Carboniferous period.— -Two classes of contemporaneous trap
rocks have been ascertained by Dr. Fleming to occur in the coal-
field of the Forth in Scotland. The newest of these, connected
with the higher series of coal-measures, is well exhibited along
the shores of the Forth, in Fifeshire, where they consist of basalt
with olivine, amygdaloid, greenstone, wacke, and tuff. They
appear to have been erupted while the sedimentary strata were
in a horizontal position, and to have suffered the same disloca-
tions which those strata have subsequently undergone. In the
volcanic tuffs of this age are found not only fragments of lime-
stone, shale, flinty slate, and sandstone, but also pieces of coal.

The other or older class of carboniferous traps are traced
along the south margin of Stratheden, and constitute a ridge
parallel with the Ochils, and extending from Stirling to near St.
Andrew's. They consist almost exclusively of greenstone, be-
coming, in a few instances, earthy and amygdaloidal. They
are regularly interstratified with the sandstone, shale, and iron-
stone of the lower coal-measures, and, on the East Lomond,
with Mountain limestone, f

Trap of the Old Red sandstone period. — By referring to the
section explanatory of the structure of Forfarshire, already given
(p. 65.), the reader will perceive that the beds of conglomerate,
No. 3., occur in the middle of the old red sandstone system, 1,
2, 3, 4. The pebbles in these conglomerates are sometimes
composed of granitic and quartz rocks, sometimes exclusively
of different varieties of trap, which, although purposely omitted
in the above section, are often found, either intruding themselves
in amorphous masses and dikes into the older fossiliferous tile-
stones, No. 4., or alternating with them in conformable beds.
All the different divisions of the red sandstone, 1, 2, 3, 4, are
occasionally intersected by dikes, but they are very rare in Nos.
1. and 2., the upper members of the group consisting of red
shale and red sandstone. These phenomena, which occur at the
foot of the Grampians, are repeated in the Sidlaw Hills ; and it
appears that in this part of Scotland volcanic eruptions were
most frequent in the earlier part of the old red sandstone period.

The trap rocks alluded to consist chiefly of felspathic porphy-

*De la Beche, Geol. Proceedings, No. 41. p. 196.

t Fleming MS. Part of this tract I have myself examined with Dr. F.

280 LYELL'S ELEMENTS OF GEOLOGY.

Silurian Volcanic Rocks Cambrian Volcanic Rocks.

ry and amygdaloid, the kernels of the latter being sometimes
calcareous, often chalcedonic, and forming beautiful agates. We
meet also with claystone, clinkstone, greenstone, compact felspar,
and tuff. Some of these rocks flowed as lavas over the bottom
of the sea, and enveloped quartz pebbles which were lying there,
so as to form conglomerates with a base of greenstone, as is
seen in Lumley Den, in the Sidlaw Hills. On either side of
the axis of this chain of hills (see section, p. 65.), the beds of
massive trap, and the tuffs composed of volcanic sand and ashes,
dip regularly to the south-east or north-west, conformably with
the shales and sandstones.

Dr. Fleming has observed similar . trap rocks in the old red
sandstone of northern Fifeshire, where they are covered imme-
diately by the yellow sandstone which forms the base of the
mountain limestone and coal-measures.

Silurian period. — It appears from the investigations of Mr.
Murchison in Shropshire, that when the lower Silurian strata of
that county were accumulating, there were frequent, volcanic
eruptions beneath the sea ; and the ashes and scoriae then ejected,
gave rise to a peculiar kind of tufaceous sandstone or grit, dis-
similar to the other rocks of the Silurian series, and only ob-
servable in places where syenitic and other trap rocks protrude.*
These tuffs occur on the flanks of the Wrekin and Caer Cara-
doc, and contain Silurian fossils, such as casts of encrinites, tri-
lobites and mollusca. Although fossil iferous, the stone resembles
a sandy claystone of the trap family.f

Thin layers of trap, only a few inches thick, alternate, in
some parts of Shropshire and Montgomeryshire, with sedimen-
tary strata of the lower Silurian system. This trap consists of
slaty porphyry and granular felspar rock, the beds being tra-
versed by joints like those in the associated sandstone, limestone,
and shale, and having the same strike and dip.J

In Radnorshire, there is an example of twelve bands of stra-
tified trap alternating with Silurian schists and flagstones in a
thickness of 350 feet. The bedded traps consist of felspar-por-
phyry, clinkstone, and other varieties ; and the interposed Llan-
deilo flags are of sandstone and shale, with trilobites and grap-
tolites.§

Cambrian volcanic rocks. — In Pembrokeshire stratified,
greenstone, felspar-rock, and a breccia containing fragments of
trap, alternate conformably in thick parallel masses with regu-
larly stratified sandstone and schist of the upper Cambrian sys-

* Murchison, Silurian System, &c. p. 230.

t Ibid. J Ibid. p. 272. $ Ibid. p. 325.

PART II. CHAPTER XXIV. 281

Tests of Relative Age of Plutonic Rocks.

tern. These trappean masses, says Mr. Murchison, must have
been evolved at intervals from volcanic fissures at the bottom of
the sea, when the sand, pebbles, and mud, now forming the ac-
companying sedimentary rocks, were deposited.*

Professor Sedgwick, in his account of the geology of Cum-
berland, has described various trap-rocks which accompany the
green slates of the Cambrian system, beneath a limestone con-
taining organic remains. Different felspathic and porphyritic
rocks and greenstones occur, not only in dikes, but in conform-
able beds ; and there is occasionally a passage from these igne-
ous rocks to some of the green quartzose slates. Professor
Sedgwick supposes these porphyries to have originated contem-
poraneously with the stratified chloritic slates, the materials of
the slates having been supplied, in part at least, by submarine
eruptions oftentimes repeated. f

CHAPTER XXIV.

ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.

Difficulty in ascertaining the precise age of a plutonic rock— Test of age by
relative position — Test by intrusion and alteration — Test by mineral composition
— Test by included fragments — Recent and Pliocene plutonic rocks, why invisi-
ble— Tertiary plutonic rocks in the Andes — Granite altering Cretaceous rocks —
Granite altering Lias in the Alps and in Sky — Granite of Dartmoor altering Car-
boniferous strata — Granite of the Old Red sandstone period — Syenite altering
Silurian strata in Norway — Blending of the same with gneiss — Most ancient
plutonic rocks — Granite protruded in a solid form — On the probable age of the
granite of Arran, in Scotland.

WHEN we adopt the igneous theory of granite, as explained
in the 9th chapter, and believe that different plutonic rocks have
originated at successive periods beneath the surface of the planet,
we must be prepared to encounter greater difficulty in ascertain-
ing the precise age of such rocks, than in the case of volcanic
and fossiliferous formations. We must bear in mind, that the
evidence of the age of each contemporaneous volcanic rock was
derived, either from lavas poured out upon the ancient surface,

* Murchison, Silurian System, &c. p. 404.
t Geol. Trans. 2d series, vol. iv. p. 55.

282 LYELL'S ELEMENTS OF GEOLOGY.

Tests of Relative Age of Plutonic Rocks.

whether in the sea or in the atmosphere, or from tuffs and con-
glomerates, also deposited at the surface, and either containing
organic remains themselves, or intercalated between strata con-
taining fossils. But all these tests fail when we endeavour to fix
the chronology of a rock, which has crystallized from a state of
fusion in the bowels of the earth. In that case, we are reduced
to the following tests ; 1st, relative position ; 2dly, intrusion and
alteration of the rocks in contact; 3dly, mineral characters;
4thly, included fragments.

Test of age by relative position. — Unaltered fossiliferous
strata of every age are met with reposing immediately on plu-
tonic rocks, as at Christiariia, in Norway, where the Newer
Pliocene deposits rest on granite ; in Auvergne, where the fresh-
water Eocene strata, and at Heidelberg, on the Rhine, where the
New Red sandstone occupy a similar place. In all these, and
similar instances, inferiority in position is connected with the
superior antiquity of granite. The crystalline rock was solid
before the sedimentary beds were superimposed, and the latter
usually contain in them rounded pebbles of the subjacent
granite.

Test by intrusion and alteration. — But when plutonic rocks
send veins into strata, and alter them near the point of contact,
in the manner before described (p. 123.), it is clear that, like in-
trusive traps, they are newer than the strata which they invade
and alter. Examples of the application of this test will be given
in the sequel.

Test by mineral composition. — Notwithstanding a general
uniformity in the aspect of plutonic rocks, we have seen in the
9th chapter that there are many varieties, such as Syenite, Tal-
cose granite, and others. One of these varieties is sometimes
found exclusively prevailing throughout an extensive region,
where it preserves a homogeneous character; so that having
ascertained its relative age in one place, we can easily recognize
its identity in others, and thus determine from a single section
the chronological relations of large mountain masses. Having
observed, for example, that the syenitic granite of Norway, in
which the mineral called zircon abounds, has altered the Silurian
strata wherever it is in contact, we do not hesitate to refer all
masses of the same zircon-syenite in the south of Norway to the
same era. (See p. 143.)

Some have imagined that the age of different granites might,
to a great extent, be determined by their mineral characters
alone ; syenite, for instance, or granite with hornblende, being
more modern than common or micaceous granite. But modern
investigations have proved these generalizations to have been

PART II. CHAPTER XXIV. 283

Recent and Pliocene Plutonic Rocks.

premature. The syenitic granite of Norway already alluded to
may be of the same age as the Silurian strata, which it traverses
and alters, or may belong to the Old Red sandstone period;
whereas the granite of Dartmoor, although consisting of mica,
quartz, and felspar, is newer than the Coal. (See p. 499.)

Test by included fragments — This criterion can rarely be
of much importance, because the fragments involved in granite
are usually so much altered, that they cannot be referred with
certainty to the rocks whence they were derived. In the White
Mountains in North America, according to Professor Hubbard, a
granite vein traversing granite, contains fragments of slate and
trap, which must have fallen into the fissure when the fused ma-
terials of the vein were injected from below,* and thus the granite
is shown to be newer than certain superficial and slaty trappean
formations.

Recent and Pliocene plutonic rocks, why invisible. — The
explanation already given in the 8th and 9th chapters of the
probable relation of the plutonic to the volcanic formations, will
naturally lead the reader to infer that rocks of the one class can
never be produced at or near the surface without some members
of the other being formed below simultaneously, or soon after-
wards. It is not uncommon for lava streams to require more
than ten years to cool in the open air ; and where they are of
great depth, a much longer period. The melted matter poured
from Jorullo, in Mexico, in the year 1759, which accumulated
in some places to the height of 550 feet, was found to retain a
high temperature half a century after the eruption.f We may
conceive, therefore, that great masses of subterranean lava may
remain in a red-hot or incandescent state in the volcanic foci for
immense periods, and the process of refrigeration may be ex-
tremely gradual. Sometimes, indeed, this process may be
retarded for an indefinite period, by the accession of fresh sup-
plies of heat ; for we find that the lava in the crater of Strom-
boli, one of the Lipari islands, has been in a state of constant
ebullition for the last two thousand years ; and we must suppose
this fluid mass to communicate with some caldron or reservoir
of fused matter below. In the Isle of Bourbon, also, where there
has been an emission of lava once in every two years for a long
period, the lava below can scarcely fail to have been perma-
nently in a state of liquefaction. If then it be a reasonable con-
jecture, that about 2000 volcanic eruptions occur in the course
of every century, either above the waters of the sea or beneath

* Silliraan's Journ. No. 69. p. 123.

t See Principles of Geology, Index, "Jorullo."

284 LYELL'S ELEMENTS OF GEOLOGY.

Relative Age and Position of Volcanic Rocks.

them,* it will follow, that the quantity of plutonic rock generated,
or in progress during the recent epoch, must already have been
considerable.

But as the plutonic rocks originate at some depth in the
earth's crust, they can only be rendered accessible to human
observation by subsequent upheaval and denudation. Between
the period when a plutonic rock crystallizes in the subterranean
regions, and the era of its protrusion at any single point of the
surface, one or two geological periods must usually intervene.
Hence, we must not expect to find the Recent or Newer Pliocene
granites laid open to view, unless we are prepared to assume that
sufficient time has elapsed since the commencement of the Newer
Pliocene period for great upheaval and denudation. A plutonic
rock, therefore, must, in general, be of considerable antiquity rela-
tively to the fossiliferous and volcanic formations, before it be-
comes extensively visible. As we know that the upheaval of land
has been sometimes accompanied in South America by volcanic
eruptions and the emission of lava, we may conceive the more
ancient plutonic rocks to be forced upwards to the surface by
the newer rocks of the same class formed successively below,
— subterposition in the plutonic, like superposition in the sedi-
mentary rocks, being usually characteristic of a newer origin.

In the accompanying diagram, Fig. 288., an attempt is made
to show the inverted order in which sedimentary and plutonic
formations may occur in the earth's crust.

. The oldest plutonic rock, No. I., has been upheaved at suc-
cessive periods until it has become exposed to view in a moun-
tain chain. This protrusion of No. I. has been caused by the
igneous agency which produced the new plutonic rocks Nos. II.
III. and IV. Part of the primary fossiliferous strata, No. 1.,
has also been raised to the surface by the same gradual pro-
cess. It will be observed that the Recent strata No. 4., and the
Recent granite or plutonic rock No. IV., are the most remote
from each other in position, although of contemporaneous date.
According to this hypothesis, the convulsions of many periods
will be required before Recent granite will be upraised so as to
form the highest ridges and central axes of mountain-chains.
During that time the Recent strata No. 4. might be covered by a
great many newer sedimentary formations.

Tertiary plutonic rocks. — We have seen that great upheaving
movements have been experienced in the region of the Andes,
during the Recent and Newer Pliocene periods. In some part,
therefore, of this chain, if any where, we may hope to discover

* See Principles of Geology, Index, " Volcanic Eruptions.''

PART II. CHAPTER XXIV.

285

Tertiary Plutonic Rocks in the Andes.

- s

2<*

:•§

II

tertiary plutonic rocks laid open to view. What we already
know of the structure of the Chilian Andes seems to realize this
expectation. In a transverse section, examined by Mr. Darwin,
between Valparaiso and Mendoza, the Cordillera was found to
consist of two separate and parallel chains, formed of sedimen-

286 LYELL'S ELEMENTS OF GEOLOGY.

Plutonic Rocks of the Chalk, Oolite, and Lias.

tary rocks of different ages, the strata in both resting on plutonic
rocks, by which they have been altered. In the western or old-
est range, called the Peuquenes, are black calcareous clay-slates,
rising to the height of nearly 14,000 feet above the sea, in which
are shells of the genera Gryphoea, Turritella, Terebratula, and
Ammonite. These rocks are supposed to be of the age of the
central parts of the. secondary series of Europe. They are
penetrated and altered by dikes and mountain masses of a
plutonic rock, which has the texture of ordinary granite, but
rarely contains quartz, being a compound of albite and horn-
blende.

The second or eastern chain consists chiefly of sandstones
and conglomerates, of vast thickness, the materials of which are
derived from the ruins of the western chain. The pebbles of
the conglomerates are, for the most part, rounded fragments of
the fossiliferous slates before mentioned. The resemblance of the
whole series to certain tertiary deposits on the shores of the
Pacific, not only in mineral character, but in the imbedded lig-
nite and silicified wood, leads to the conjecture that they also are
tertiary. Yet these strata are not only associated with trap
rocks and volcanic tuffs, but are also altered by a granite newer
than that of the western chain, and consisting of quartz, felspar,
and talc. They are traversed, moreover, by dikes of the same
granite, and by numerous veins of iron, copper, arsenic, silver,
and gold ; all of which can be traced to the underlying granite.*
We have, therefore, strong ground to presume that the plutonic
rock, here exposed on a large scale in the Chilian Andes, is of
later date than certain tertiary formations.

Cretaceous period. — It was stated (p. 145.) that chalk as
well as lias have been altered by granite in the eastern Pyre-
nees. Whether such granite be cretaceous or tertiary cannot
easily be decided.

Suppose b, r, d, to be three

Fig. 289. members of the Cretaceous series,

the lowest. of which, ft, has been
altered by the granite A, the modi-
fying influence not having extended
"so far as c, or having but slightly
affected its lowest beds. Now it
can rarely be possible for the ge-.
ologist to decide whether $e beds
d existed at the time of the intrusion of A, and alteration of b
and c, or whether they were subsequently thrown down upon c.

* Darwin, pp. 390. 406.

PART II. CHAPTER XXIV.

287

Trap Rocks coeval with Oolite and Coal.

As some Cretaceous rocks, however, have been raised to the
height of more than 9000 feet in the Pyrenees, we must not
assume that plutonic formations of the same age may not have
been brought up and exposed by denudation, at the heigut of
2000 or 3000 feet on the flanks of that chain.

Period of Oolite and Lias. — In the department of the Hautes
Alpes, in France, near Vizille, M. Elie de Beaumont traced a
black argillaceous limestone, charged with belemnites, to within

a few yards of a mass
Fig. 290. of granite. Here the

limestone begins to put
on a granular texture,
but is extremely fine-
grained. When nearer
the junction, it becomes
grey, and has a saccha-
roid structure. In ano-
ther locality, near Cham-
poleon, a granite com-
posed of quartz, black
mica, and rose-coloured
felspar, is observed partly
to overlie the secondary
rocks, producing an al-
teration which extends
for about thirty feet down-
wards, diminishing in the
beds which lie farthest
from the granite. (See Fig. 290.) In the altered mass the argil-
laceous beds are hardened, the limestone is saccharoid, the grits
quartzose, and in the midst of them is a thin layer of an imper-
fect granite. It is also an important circumstance, that near the
point of contact, both the granite and the secondary rocks
become metalliferous, and contain nests and small veins of
blende, galena, iron, and copper pyrites. The stratified rocks
become harder and more crystalline, but the granite, on the con-
trary, softer and less perfectly crystallized-near the junction.*

Although the granite is incumbent in the above section, (Fig.
290.) we cannot assume that it overflowed the strata, for the dis-
turbances of the rocks are so great in this part of the Alps that
they seldom retain the position which they must originally have
occupied.

Junction of granite with Jurassic or Oolite strata
in the Jllps, near Champoleon.

* Elie de Beaumont, sur les Montagnes de i'Oisans, &c., Mem. de la Soc.
d'Hist. Nat. de Paris, tome v.

288 LYELL'S ELEMENTS OF GEOLOGY.

Granite of Carboniferous and Old Red Sandstone Period.

A considerable mass of syenite, in the Isle of Sky, is de-
scribed by Dr. MacCulloch as intersecting limestone and shale,
which are of the age of the lias.* The limestone, which, at a
greater distance from the granite, contains shells, exhibits no
traces of them near its junction, where it has been converted
into a pure crystalline marble. f

At Predazzo, in the Tyrol, secondary strata, some of which
are limestones of the Oolitic period, have been traversed and
altered by plutonic rocks, one portion of which is an augitic por-
phyry, which passes insensibly into granite. The limestone is
changed into granular marble, with a band of serpentine at the
junction.^:

Carboniferous period. — The granite of Dartmoor, in Devon-
shire, was formerly supposed to be one of the most ancient of
the plutonic rocks, but is now ascertained to be posterior in date
to the culm-measures of that county, which, from their position,,
and as containing true coal-plants, are regarded by Professor
Sedgwick and Mr. Murchison as members of the true carbonife-
rous series. This granite, like the syenitic granite of Christi-
ania, has broken through the stratified formations without much
changing their strike. Hence, on the north-west side of Dart-
moor, the successive members of the culm-measures abut against
the granite, and become metamorphic as they approach. These
strata are also penetrated by granite veins and plutonic dikes,
called " elvans."§ The granite of Cornwall is probably of the
same date, and, therefore, as modern as the Carboniferous strata,
if not much newer.

Old Red sandstone period. — The plutonic rocks of the Mal-
vern hills, in Worcestershire, consist of a granitic compound of
quartz, felspar, and hornblende, or occasionally of quartz, mica,
and felspar, which passes into syenite and greenstone.|| This
rock has altered the adjacent Silurian strata into well character-
ized metamorphic schists, principally chloride and micaceous-
schist, with some gneiss, and has dislocated and reversed the
position of the beds of the Silurian and Old Red sandstone.
There are indications, says Mr. Murchison, of several periods
of movement, by which the strata were forced up and folded
back, but the chief outburst was after the accumulation of the
Silurian and part of the Old Red system, and anterior to the for-
mation of the coal-beds, which are undisturbed.lF

* See Murchison, Geol. Trans., 2d series, vol. ii. part ii. pp. 311 — 321.

t Western Islands, vol. I p. 330. plate 18. figs. 3, 4.

t Von Buch, Annales de Chimie, &c.

§ Proceedings of Geol. Soc., vol. ii. p. 562.

II Mr. L. Horner, Geol. Trans., 1st ser., vol. i. p. 281.

tf Silurian System, p 425.

PART II. CHAPTER XXIV. 289

Granite of Silurian Period.

Silurian period. — I have already alluded to the granite near
Christiania, in Norway, as being newer than the Silurian strata
of that region. Its posteriority in date to limestones containing
orthocerata and trilobites, has long been celebrated, it being
twenty-five years since Von Buch first announced the discovery.
The proofs consist in the penetration of granite veins into the
shale and limestone, and the alteration of the strata, for a con-
siderable distance from the point of contact, both of these veins
and the central mass from which they emanate. (See p. 129.)
Von Buch supposed that the plutonic rock alternated with the
fossiliferous strata, and that large masses of granite were some-
times incumbent upon the strata ; but this idea was erroneous,
and arose from the fact that the beds of shale and limestone
often dip towards the granite up to the point of contact, appear-
ing as if they would pass under it in mass, as at a, Fig. 291.,

Fig. 291.

Silurian. Granite. Silurian strata.

and then again on the opposite side of the same mountain, as at
&, dip away from the same granite. When the junctions, how-
ever, are carefully examined, it is found that the plutonic rock
intrudes itself in veins, and nowhere covers the fossiliferous
strata in large overlying masses, as is so commonly the case
with trappean formations.*

Now this granite, which is more modern than the Silurian
strata of Norway, also sends veins in the same country into an
ancient formation of gneiss ; and the relations of the plutonic
rock and the gneiss, at their junction, are full of interest when
we duly consider the wide difference of epoch which must have
separated their origin.

The length of this interval of time is attested by the following
facts : — The fossiliferous, or transition beds, rest unconformably
upon the truncated edges of the gneiss, the inclined strata of
which had been disturbed and denuded before the sedimentary
beds were superimposed. (See Fig. 292.)

The signs of denudation are twofold ; first, the surface of the
gneiss is seen occasionally on the removal of the newer beds,
containing organic remains, to be scored and polished ; secondly,

* See the Gsea Norvegica and other works of Keilhau, with whom I examined
this country.

290

LYELL'S ELEMENTS OF GEOLOGY.

Granite altering Silurian Strata Most ancient Granites.

Fig. 292.

Silurian strata.

Gneiss. Granite. Gneiss.

Granite sending veins into Silurian strata and Gneiss,— Christiania, Norway.

pebbles of gneiss have been found in some of the transition
strata. Between the origin, therefore, of the gneiss and the gra-
nite there intervened, first, the period when the strata of gneiss
were inclined ; secondly, the period when they were denuded ;
thirdly, the period of the deposition of .the transition deposits.
Yet the granite produced, after this long interval, is often so inti-
mately blended with the ancient gneiss, at the point of junction,
that it is impossible to draw any other than an arbitrary line of
separation between them ; and where this is not the case, tortu-
ous veins of granite pass freely through gneiss, ending some-
times in threads, as if the older rock had offered no resistance
to their passage. It seems necessary, therefore, to conceive that
the gneiss was softened and more or less melted when penetrated
by the granite. But had such junctions alone been visible, and
had we not learnt, from other sections, how long a period elapsed
between the consolidation of the gneiss and the injection of this
granite, we might have suspected that the gneiss was scarcely
solidified, or had not yet assumed its complete metamorphic cha-
racter, when invaded by the plutonic rock. From this example
we may learn how impossible it is to conjecture whether certain
granites in Scotland, and other countries, which send veins into
gneiss and other metamorphic rocks, are primary, or whether
they may not belong to some secondary or tertiary period.

Most ancient granites. — It is riot half a century since the
doctrine was very general that all granitic rocks were primitive,
that is to say, that they originated before the deposition of the
first sedimentary strata, and before the creation of organic
beings. (See p. 23.) But so greatly are our views now changed,
that we find it no easy task to point out a single mass of granite
demonstrably more ancient than all the known fossiliferous depo-
sits. Could we discover some Lower Cambrian strata resting
immediately on granite, there being no alterations at the point
of contact, nor any intersecting granitic veins, we might then
affirm the plutonic rock to have originated before the oldest
known fossiliferous strata. Still it would be presumptuous to

PART II. CHAPTER XXIV. 291

Protrusion of Solid Granite.

suppose that when a small part only of the globe has been inves-
tigated, we are acquainted with the oldest fbssiliferous strata in
the crust of our planet. Even when these are found, we cannot
assume that there never were any antecedent strata containing
organic remains, which may have become metamorphic. If we
find pebbles of granite in a conglomerate of the Lower Cam-
brian system, we may then feel assured that the parent granite
was formed before the Lower Cambrian formation. But if the
incumbent strata be merely Silurian or Upper Cambrian, the
fundamental granite, although of high antiquity, may be poste-
rior in date to known fossiliferous formations.

Protrusion of solid granite. — In part of Sutherlandshire,
near Brora, common granite, composed of felspar, quartz, and
mica, is in immediate contact with Oolitic strata, and has clearly
been elevated to the surface at a period subsequent to the deposi-
tion of those strata.* Professor Sedgwick and Mr. Murchison
conceive that this granite has been upheaved in a solid form ;
and that in breaking through the submarine deposits, with which
it was not perhaps originally in contact, it has fractured them so
as to form a breccia along the line of junction. This breccia
consists of fragments of shale, sandstone, and limestone, with
fossils of the oolite, all united together by .a calcareous cement.
The secondary strata, at some distance from the granite, are but
slightly disturbed, but in proportion to their proximity the
amount of dislocation becomes greater.

If we admit that solid hypogene rocks, whether stratified or
unstratified, have in such cases been driven upwards, so as to
pierce through yielding sedimentary deposits, we shall be enabled
to account for many geological appearances otherwise inexplica-
ble. Thus, for example, at Weinbohla and Hohnstein, near
Meissen, in Saxony, a mass of granite has been observed cover-
ing strata of the cretaceous and oolitic periods for the space of
between 300 and 400 yards square. It appears clearly from a
recent memoir of Dr. B. Cotta on this subject,! that the granite
was thrust into its actual position when solid. There are no
intersecting veins at the junction — no alteration as if by heat,
but evident signs of rubbing, and a breccia in some places, in
which pieces of granite are mingled with broken fragments of
the secondary rocks. As the granite overhangs both the lias
and chalk, so the lias is in some places bent over strata of the
cretaceous era.

Age of the granite of Arran — In this island, the largest in

* Murchison, Geol. Trans. 3d series, vol. ii. p. 307.
t Geognostische Wanderungen, Leipzic, 1838.

292 LYELL'S ELEMENTS OF GEOLOGY.

Age of the Granite of the Isle of Arran.

the Firth of Clyde, on the west coast of Scotland, the four great
classes of rocks, the fossiliferous, volcanic, plutonic and meta-
morphic, are all conspicuously displayed within a very small
area, arid with their peculiar characters strongly contrasted. In
the north of the island the granite rises to the height of nearly
3000 feet above the sea, terminating in mountainous peaks. On
the flanks of the same mountains are chloritic-schists, blue roof-
ing-slate, and other rocks of the metamorphic order (A), into

Fig. 293. Section of Arran.

B

A, Crystalline, or metamorphic schist. B, Granite,

c, Conglomerate, sandstone, limestone, and shale. D, Trap.

which the granite (B) sends veins. These schists are highly
inclined. On their truncated edges rest unconformable beds of
conglomerate and sandstone, to which succeed various shales
and limestones, containing fossils of the carboniferous period.
All these secondary strata (c) are much tilted and inclined near
the hypogene rocks ; but are horizontal at a distance from them,
as in the south of Arran. Lastly, the volcanic rocks (D), con-
sisting of greenstone, pitchstone, claystone, porphyry, and other
varieties, traverse all the preceding formations, cutting through
the granite in dikes (d), as well as through the sandstone ;
which last they also overlie in dense masses, from 50 to 700
feet in thickness.

Now as the different kinds of trap intersect all the other form-
ations, they are certainly the newest rocks in Arran. The red
sandstone and other secondary strata are older than the trap,
but newer than the metamorphic schists, for the red sandstone
conglomerates not only rest unconformably upon the schists, but
contain rounded pebbles of those crystalline strata. It is equally
certain that the schists are the oldest rocks in the island : they
are more ancient than the trap and red sandstone, for reasons
already stated ; and the granite must be of newer origin, because
it penetrates them in veins. The only chronological point,
therefore, in which there can be any ambiguity, relates to the
plutonic formations. They are more modern, as before remarked,

PART II. CHAPTER XXIV. 293

Age of the Granite of the Isle of Arran.

than the crystalline schists ; but can we decide them to be like-
wise younger than the secondary sandstones ?

Now it is a curious and most striking fact, that no pebbles of
granite have ever been found in the conglomerates of the red
sandstone in Arran, although careful search has been made for
them by many geologists ; and although puddingstones in gene-
ral are chiefly made up of fragments of older rocks of the imme-
diate vicinity. The total absence of such pebbles has justly been
a theme of wonder to those who have visited Arran, and have
seen that the conglomerates are several hundred feet in thick-
ness, and that they occur at the base of the granite mountains,
which tower above them in far bolder and more picturesque
forms than those of similar composition in other parts of Scot-
land. We may at once infer, with confidence, that when the
sandstone and conglomerate were formed, no granite had reached
the surface, or had been exposed to denudation in this region :
the crystalline schists were ground into sand and shingle when
these puddingstones were accumulated, but the waves had never
acted upon the granite which sends its veins into the schist.
Are we then to conclude, that the schists suffered denudation
before they had been invaded by granite? This opinion, al-
though it cannot be disproved, is by no means fully borne out.
by the evidence. At the time when the red sandstone was
formed, the metamorphic strata may have formed islands in the
sea, as in Fig. 294., over which the breakers rolled, or from

Fig. 294.

which torrents and rivers descended, carrying down gravel and
sand. The plutonic rock (B) may have been previously injected
at a certain depth below, and yet may never have been exposed
to denudation.

As to the time and manner of the subsequent protrusion of
the hypogene rocks in Arran, these are questions into which I
have not space to enter at present : I shall merely observe, that
those crystalline rocks may have been thrust up bodily, in a
solid form ; and it is clear that, during or since the period of
their emergence, they have undergone great aqueous denuda-
tion. This action is confirmed by three distinct kinds of proofs :
1st, The occurrence of scattered pebbles and huge erratic blocks
of granite arid schist over the surface of Arran and the adjacent^
z*

294 LYELL'S ELEMENTS OF GEOLOGY.

Different Ages of Metamorphic Rocks.

mainland ; 2dly, The abrupt truncation of dikes, such as those
at d (Fig. 293.), cut off on the surface of the granite ; 3dly, The
fact -that not only the secondary strata but the enormous masses
of trap which accompany and overlie them, terminate suddenly
on reaching the borders of the granite and schist, towards which
they often present a steep escarpment, and over which, for
some distance at least, they must originally have extended.*

CHAPTER XXV.

ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.

Age of each set of metamorphic strata twofold — Test of age by fossils and
mineral character not available — Test by superposition ambiguous — Conversion
of dense masses of fossiliferous strata into metamorphic rocks — Limestone and
shale of Carrara — Metamorphic strata of modern periods in the Alps of Switzer-
land and Savoy — Why the visible crystalline strata are none of them very
modern — Order of succession in metamorphic rocks — Uniformity of mineral cha-
racter— Why the metamorphic strata are less calcareous than the fossiliferous.

ACCORDING to the theory adopted in the llth chapter, the
age of each set of metamorphic strata is twofold, they have been
deposited at one period, they have become crystalline at another.
We can rarely hope to define with exactness the date of both
these periods, the fossils having been destroyed by plutonic
action, and the mineral characters being the same, whatever the
age. Superposition itself is an ambiguous test, especially when
we desire to determine the period of crystallization. Suppose,
for example, we are convinced that certain metamorphic strata
in the Alps, which are covered by cretaceous beds, are altered
lias ; this lias may have assumed its crystalline texture in the
cretaceous or in some tertiary period, the Eocene for example.
If in the latter, it should be called Eocene, when regardecj as a
metamorphic rock, although it be liassic, when considered in
reference to the era of its deposition. According to this view,
the superposition of chalk does not prevent the subjacent meta-
morphic rock from being Eocene. If, however, in the progress

* In the works of Drs. Hutton and MacCulloch, and in the Memoirs of Messrs.
Von Dechen and Oeynhausen, and that of Professor Sedgwick and Mr. Murchi-
eon, (Geol. Trans. 2d series) and others, whose observations I have verified on
the spot, the reader will find a full description of the geology of Arran.

PART II. CHAPTER XXV. 295

Age of the Metamorphic Rocks of the Northern Apennines.

of science, we should succeed in ascertaining the twofold chro-
nological relations of the metamorphic formations, it might be
useful to adopt a twofold terminology. We might call the strata
above alluded to Liassic-Eocene, or Liassic-Cretaceous ; the first
term referring to the era. of deposition, the second to that- of
crystallization. According to this method, the chlorite-schist,
mica-schist, and gneiss of the Malvern Hills, would belong to
the Silurian Old Red sandstone period, because they are Silurian
strata altered into metamorphic rocks during the deposition of
the Old Red sandstone. (See p. 288.)

We have seen, when discussing the ages of the plutonic rocks,
that examples occur of various primary, secondary, and tertiary
deposits converted into metamorphic strata, near their contact
with granite. There can be no doubt in these cases that strata,
once composed of mud, sand, and gravel, or of clay, marl, and
shelly limestone, have for the distance of several yards, and in
some instances several hundred feet, been turned into gneiss,
mica-schist, hornblende-schist, chlorite-schist, quartz rock, sta-
tuary marble, and the rest. (See Chapters 10. and 11.)

But when the metamorphic action has operated on a grander
scale, it tends entirely to destroy all monuments of the date of
its development. It may be easy to prove the identity of two
different parts of the same stratum ; one, where the rock has
been in contact with a volcanic or plutonic mass, and has been
changed into marble or hornblende-schist, and another not far
distant, where the same bed remains unaltered and fossiliferous ;
but when we have to compare two portions of a mountain chain —
the one metamorphic, and the other unaltered — all the labour and
skill of the most practised observers are required. I shall men-
tion one or two examples of alteration on a grand scale, in order
to explain to the student the kind of reasoning by which we are
led to infer that dense masses of fossiliferous strata have been
converted into crystalline rocks.

Northern Appenines. — Carrara. — The celebrated marble of
Carrara, used in sculpture, was once regarded as a type of primi-
tive limestone. It abounds in the mountains of Massa Carrara,
or the "Apuan Alps," as they have been called, the highest
peaks of which are nearly 6000 feet high. Its great antiquity
was inferred from its mineral texture, from the absence of fossils,
and its passage downwards into talc-schist and garnetiferous
mica-schist ; "these rocks again graduating downwards into
gneiss, which is penetrated, at Forno, by granite veins. Now
the researches, of MM. Savi, Boue, Pareto, Guidoni, De la
Beche, and especially Hoffman, have demonstrated that this
marble, once supposed to be formed before the existence of

296 LYELL'S ELEMENTS OF GEOLOGY.

Age of Met^nrorphic Rocks of the Swiss Alps.

organic beings,4*^, in fact, an altered limestone of the oolitic
period, and the underlying crystalline schists are secondary
sandstones and shales, modified by plutonic action. In order to
establish these conclusions it was first pointed out, that the cal-
careous rocks bordering the Gulf of Spezia, and abounding in
oolitic fossils, assume a texture like that of Carrara marble, in
proportion as they are more and more invaded by certain trap-
pean and plutonic rocks, such as diorite, euphodite, serpentine,
and granite, occurring in the same country.

It was then observed that, in places where the secondary form-
ations are unaltered, the uppermost consist of common Apen-
nine limestone with nodules of flint, below which are shales, and
at the base of all, argillaceous and siliceous sandstones. In the
limestone, fossils are frequent, but very rare in the underlying
shale and sandstone. Now a gradation has been traced laterally
from these rocks into another and corresponding series, which is
completely metamorphic ; for at the top of this we find a white
granular marble, wholly .devoid of fossils, and almost without
stratification, in which there are no nodules of flint, but in its
place siliceous matter disseminated through the mass in the form
of prisms of quartz. Below this, and in place of the shales, are
talc-schists, jasper, and hornstone ; and at the bottom, instead
of the siliceous and argillaceous sandstones, are quartzite and
gneiss.* Had these secondary strata of the Apennines under-
gone universally as great an amount of transmutation, it would
have been impossible to form a conjecture respecting their true
age ; and then, according to the common method of geological
classification, they would have ranked as primary rocks. In that
case the date of their origin would have been thrown back to an
era antecedent to the deposition of the Lower Cambrian strata,
although in reality they were formed in the oolitic period, and
altered at some subsequent and unknown epoch.

Alps of Switzerland. — In the Alps, analogous conclusions
have been drawn respecting the alteration of strata on a still
more extended scale. In the eastern part of that chain, some
of the primary fossiliferous strata, as well as the older secondary
formations, together with the oolitic and cretaceous rocks, are
distinctly recognizable. Tertiary deposits also appear in a less
elevated position on the flanks of the Eastern Alps ; but in the
Central or Swiss Alps, the primary fossiliferous, and older second-
ary formations disappear, and the cretaceous, oolitic, and liassic
strata graduate insensibly into metamorphic rocks, consisting of

* See notices of Savi, Hoffman, and others, referred to by Boue, Bull, de la Soe.
Geol. de France, torn. v. p. 317. and torn. iii. p. 44.

PART II. C1IA

Age of Metamorphic Rocks

granular limestone, talc-schist, talcose-gneiss,rmca^^usschist,"
and other varieties. In regard to the age of this vast assemblage
of crystalline strata, we can merely affirm that some of the upper
portions are altered newer secondary deposits : but we cannot
avoid suspecting that the disappearance both of the older second-
ary and primary fossiliferous rocks may be owing to their hav-
ing been all converted in this region into crystalline schist.

It is difficult to convey to those who have never visited the
Alps a just idea of the various proofs which concur to produce
this conviction. In the first place, there are certain points where
strata of the Oolite, Lias, and Chalk have been turned into gra-
nular marble, gneiss, and other metamorphic schists, near their
contact with granite. This fact shows undeniably that plutonic
causes continued to be in operation in the Alps down to a late
period, even after the deposition of some of the newer secondary
formations. Having established this point, we are the more
willing to believe that many inferior fossiliferous rocks, probably
exposed for longer periods to a similar action, may have become
metamorphic to a still greater extent.

We also discover in parts of the Swiss Alps dense masses of
strata of the age of the Green-sand and Chalk, which have as-
sumed that semi-crystalline texture which Werner called transi-
tion, and which naturally led his followers, who attached great
importance to mineral characters taken alone, to class them as
transition formations, or as groups older than the lowest second-
ary rocks. (See p. 154.) Now, it is probable that these strata
have been affected, although in a less intense degree, by that
same plutonic action which has entirely altered and rendered
metamorphic so many of the subjacent formations ; for in the
Alps, this action has by no means been confined to the immediate
vicinity of granite. Granite, indeed, and other plutonic rocks
rarely make their appearance at the surface, notwithstanding the
deep ravines which lay open to view the internal structure of
these mountains. That they exist below at no great depth we
cannot doubt, and we have already seen (p. 127.) that at some
points, as in the Valorsine, near Mont Blanc, granite and grani-
tic veins are observable, piercing through talcose gneiss, which
passes insensibly upwards into secondary strata.

It is certainly in the Alps of Switzerland and Savoy, more
than in any other district in Europe, that the geologist is prepared
to meet with the signs of an intense development of plutonic
action ; for here we find the most stupendous monuments of
mechanical violence, by which strata thousands of feet thick have
been bent, folded, and overturned. (See p. 74.) It is here
that marine secondary formations of a comparatively modern

298 LYELL'S ELEMENTS OF GEOLOGY.

Age of Mejajfcorphic Rocks of the Swiss Alps.

date, such as tff<f oolitic and cretaceous, have been upheaved to
the height of 10,000, or even 12,000 feet above the level of the
sea ; and even tertiary strata, apparently of the Miocene era,
have been raised to an elevation of 4000 or 5000 feet, so as to
rival in height the loftiest mountains in Great Britain.

If the reader will consult the works of many eminent geolo-
gists who have explored the Alps, especially those of MM. De
Beaumont, Studer, Necker, and Boue, he will learn that they all
share, more or less fully, in the opinions above expressed. It
has, indeed, been stated by MM. Studer and Hugi, that there are
Complete alternations on a large scale of secondary strata, con-
taining fossils, with gneiss and other rocks, of a perfectly meta-
morphic structure. I have visited some of the most remarkable
localities referred to by these authors, but although agreeing with
them that there are passages from the fossiliferous to the meta-
morphic series far from the contact of granite or other plutonic
rocks, I was unable to convince myself that the distinct al-
ternations of highly crystalline, with unaltered strata above
alluded to, might not admit of a different explanation. In one
of the sections described by M. Studer in the highest of the Ber-
nese Alps, namely in the Roththal, a valley bordering the line
of perpetual snow on the northern side of the Jungfrau, I observ-
ed a mass of gneiss 1000 feet thick, and 15,000 feet long, not
only resting upon, but also again covered by strata containing
oolitic fossils. These anomalous appearances may partly be
explained by supposing great solid wedges of intrusive gneiss to
have been forced in laterally between strata to which I found
them to be in many sections unconformable. The superposition,
also, of the gneiss to the oolite may, in some cases, be due to a
reversal of the original position of the beds in a region where
the convulsions have been on so stupendous a scale.

On the Sattel also, at the base of the Gestellihorn, above
Enzen, in the valley of Urbach, near Meyringen, some of the
intercalations of gneiss between fossiliferous strata may, I con-
ceive, be ascribed to mechanical derangement. Almost any
hypothesis of repeated changes of position may be resorted to
in a region of such extraordinary confusion. The secondary
strata may first have been vertical, and then certain portions
may have become metamorphic (the plutonic influence ascending
from below) while intervening strata remained unchanged. The
whole series of beds may then agani have been thrown into a
nearly horizontal position, giving rise to the superposition of
crystalline upon fossiliferous formations.

It was remarked, in the last chapter, that as the hypogene
rocks, both stratified and unstratified, crystallize originally at a

PART II. CHAPTER XXV. 299

Age of Metamorphic Rocks, and Order of Superposition.

certain depth beneath the surface, they must always, before they
are upraised and exposed at the surface, be of considerable anti-
quity, relatively to a large portion of the fossiliferous and vol-
canic rocks. They may be forming at all periods ; but before
any of them can become visible, they must be raised above the
level of the sea, and some of the rocks which previously con-
cealed them must have been removed by denudation. If the
student will refer to the frontispiece, he will see that the strata
A, which were the last deposited, are every where hidden from
human observation by the sea, while the contemporaneous meta-
morphic rocks C are concealed at a still greater depth, as are
also the plutonic rocks D of the same age. He will also observe
that the strata C, which have recently become metamorphic, are
not parts of A, nor even of the groups immediately antecedent
in date a, 6, c, but they are portions of much older formations,
d, e, f,g,h,i. Now, suppose that part of the earth's crust,
which is represented in the frontispiece to be subjected, in vari-
ous places, to a long series of upheaving and depressing move-
ments ; the beds A will, here and there, be partially upraised
and converted into dry land, but the hypogene rocks C, D, al-
though brought up nearer to the surface, will still, very probably,
remain hidden from sight. Let a second period elapse, and the
rocks A may be raised in some countries to a height of several
thousand feet ; and still the rocks C and D may be almost every
where hidden. During a third period, when the stratified form-
ations A have been laid dry over large continental areas, and
have reached the summits of some Alpine chains, the hypogene
formations C D may also be forced up and exposed to view above
the level of the ocean by similar causes ; but they will rank no
longer as modern rocks, the geologist being already acquainted
with newer groups, both fossiliferous and volcanic. The student
will also perceive how impossible it may then be to prove that
the strata C became metamorphic at the period of the deposition
of A, and how difficult not to exaggerate the antiquity of C as a
series of metamorphic rocks, when the remote period of their
deposition has been ascertained, and the comparatively modern
era of their crystallization remains uncertain.

Order of succession in Metamorphic rocks. — There is no
universal and invariable order of superposition in metamorphic
rocks, although a particular arrangement may prevail through-
out countries of great extent, for the same reason that it is trace-
able in those sedimentary formations from which crystalline
strata are derived. Thus, for example, we have seen that in the
Apennines, near Carrara, the descending series, where it is

300 LYELL'S ELEMENTS OF GEOLOGY.

Chronological Relations of Metamorphic Rocks.

metamorphic, consists of, 1st, saccharine marble; 2dly, talcose-
schist ; and 3dly, of quartz rock and gneiss ; where unaltered,
of, 1st, fossiliferous limestone; 2dly, shale; and 3dly, sand-
stone.

But if we investigate different mountain chains, we find gneiss,
mica-schist, hornblende-schist, chlorite-schist, hypogene lime-
stone, and other rocks, succeeding each other, and alternating
with each other, in every possible order. It is, indeed, more
common to meet with some variety of clay-slate forming the
uppermost member of a metamorphic series than any other rock ;
but this fact by no means implies, as some have imagined, that
all clay-slates were formed at the close of an imaginary period,
when the deposition of the crystalline strata gave way to that of
ordinary sedimentary deposits. Such clay-slates, in fact, are
variable in composition, and sometimes alternate with fossilife-
rous strata, so that they may be said to belong almost equally to
the sedimentary and metamorphic order of rocks. It is proba-
ble that had they been subjected to more intense plutonic action,
they would have been transformed into hornblende schist, foliated
chlorite-schist, scaly talcose-schist, mica-schist, or other more
perfectly crystalline rocks, such as are usually associated with
gneiss.

Uniformity of mineral character in Hypogene rocks. — Hum-
boldt has emphatically remarked that when we pass to another
hemisphere, we see new forms of animals and plants, and even
new constellations in the heavens ; but in the rocks we still re-
cognize our old acquaintances, — the same granite, the same
gneiss, the same micaceous schist, quartz rock, and the rest. It
is certainly true that there is a great and striking general resem-
blance in the principal kinds of hypogene rocks, although of very
different ages and countries ; but it has been shown that each of
these are, in fact, geological families of rocks, and not definite
mineral compounds. They are much more uniform in aspect
than sedimentary strata, because these last are often composed
of fragments varying greatly in form, size, and colour, and con-
tain fossils of different shapes and mineral composition, and ac-
quire a variety of tints from the mixture of various kinds of sedi-
ment. The materials of such strata, if melted, and made to
crystallize, would be subject to chemical laws, simple and
uniform in their action, the same in every climate, and wholly
undisturbed by mechanical and organic causes.

Nevertheless, it would be a great error to assume that the
hypogene rocks, considered as aggregates of simple minerals,
are really more homogeneous in their composition than the

PART II. CHAPTER XXV. 301

Hypogene Rocks, why less calcareous.

several members of the sedimentary series. In the first place,
different assemblages of hypogene rocks occur in different coun-
tries ; and secondly, in any one district, the rocks which pass
under the same name are often extremely variable in their com-
ponent ingredients, or at least in the proportions in which each
of these are present. Thus, for example, gneiss and mica-
schist, so abundant in the Grampians, are wanting in Cumber-
land, Wales, and Cornwall ; in parts of the Swiss and Italian
Alps, the gneiss and granite are talcose, and not micaceous, as
in Scotland ; hornblende prevails in the granite of Scotland —
schorl in that of Cornwall — albite in the plutonic rocks of the
Andes — common felspar in those1 of Europe. In one part of
Scotland, the mica-schist is full of garnets; in another, it is
wholly devoid of them ; while in South America, according to
Mr. Darwin, it is the gneiss, and not the mica-schist, which is
most commonly garnetiferous. And not only do the propor-
tional quantities of felspar, quartz, mica, hornblende, and other
minerals, vary in hypogene rocks bearing the same name ; but,
what is still more important, the ingredients, as we have seen,
of the same simple mineral are not always constant, (p. 92.,
and table, p. 102.)

The Metamorphic strata, why less calcareous than the fos-
siliferous. — It has been remarked, that the quantity of calcare-
ous matter in metamorphic strata, or, indeed, in the hypogene
formations generally, is far less than in fossiliferous deposits.
Thus the crystalline schists of the Grampians in Scotland, con-
sisting of gneiss, mica-schist, hornblende-schist, and other rocks,
many thousands of yards in thickness, contain an exceedingly
small proportion of interstratified calcareous beds, although
these have been the objects of careful search for economical pur-
poses. Yet limestone is not wanting in the Grampians, and it is
associated sometimes with gneiss, sometimes with mica-schist,
and in other places with other members of the metamorphic
series. But where limestone occurs abundantly, as at Carrara,
and in parts of the Alps, in connexion with hypogene rocks, it
usually forms one of the superior members of the crystalline
group.

The scarcity, then, of carbonate of lime in the plutonic and
metamorphic rocks generally, seems to be the result of some
general cause. So long as the hypogene rocks were believed to
have originated antecedently to the creation of organic beings, it
was easy to impute the absence of lime to the non-existence of
those mollusca and zoophytes by which shells and corals are
secreted ; but when we ascribe the crystalline formations to plu-
Aa

302 LYELL'S ELEMENTS OF GEOLOGY.

Scarcity of Lime in Metamorphic Rocks.

tonic action, it is natural to inquire whether this action itself may
not tend to expel carbonic acid and lime from the materials
which it reduces to fusion or semi-fusion. Although we cannot
descend into the subterranean regions where volcanic heat is
developed, we can observe in regions of spent volcanos, such as
Auvergne and Tuscany, hundreds of springs both cold and ther-
mal, flowing out from granite and other rocks, and having their
waters plentifully charged with carbonate of lime. The quan-
tity of calcareous matter which thqse springs transfer, in the
course of ages, from the lower parts of the earth's crust to the
superior or newly formed parts of the same, must be con-
siderable.*

If the quantity of siliceous and aluminous ingredients brought
up by such springs were great, instead of being utterly insigni-
ficant, it might be contended that the mineral matter thus expelled
implies simply the decomposition of ordinary subterranean rocks ;
but the prodigious excess of carbonate of lime over every other
element must, in the course of time, cause the crust of the earth
below to be almost entirely deprived of its calcareous constitu-
ents, while we know that the same action imparts to newer depo-
sits, ever forming in seas and lakes, an excess of carbonate of
lime. Calcareous matter is poured into these lakes and the
ocean by a thousand springs and rivers ; so that part of almost
every new calcareous rock chemically precipitated, and of many
reefs of shelly and coralline stone, must be derived from mineral
matter subtracted by plutonic agency, and driven up by gas and
steam from fused dnd heated rocks in the bowels of the earth.

Not only carbonate of lime, but also free carbonic acid gas is
given off plentifully from the soil and crevices of rocks in
regions of active and spent volcanos, as near Naples, and in
Auvergne. By this process, fossil shells or corals may often
lose their carbonic acid, and the residual lime may enter into the
composition of augite, hornblende, garnet, and other hypogene
minerals. That the removal of the calcareous matter of fossil
shells is of frequent occurrence, is proved by the fact of such
organic remains being often replaced by silex or other minerals,
and sometimes by the space once occupied by the fossil being
left empty, or only marked by a faint impression. We ought
not indeed to marvel at the general absence of organic remains
from the crystalline strata, when we bear in mind how often fos-
sils are obliterated, wholly or in part, even in tertiary forma-
tions— how often vast masses of sandstone and shale, of different

* See Principles of Geology, Index, " Calcareous Springs."

PART II. CHAPTER XXV. 303

Scarcity of Lime in Metamorphic Rocka.

ages, and thousands of feet thick, are devoid of fossils — how
certain strata may first have been deprived of a portion of their
fossils when they became semi-crystalline, or assumed the tran-
sition state of Werner — and how the remaining organic remains
may have been effaced when they were rendered metamorphic.
Some rocks of the last-mentioned class, moreover, must have
been exposed again and again to renewed plutonic action.

INDEX.

ABERDEENSHIRE, granite of, 123.
Acephalous mollusca, 44.

Acrodus nobilis, 226.

Actinoceras Simmsii, 268.

Actinolite, 102. 134.

Agassiz, on fossil fish, 180. 225, 226
237. 241. 246. 263.

Age of aqueous strata, how deter-
mined, 159.

, of volcanic rocks, 271.

, of the plutonic rocks, 281.

, of the metamorphic rocks, 293.

Airdnamurchan, trap veins in. 104.

Albite, 102.

Alluvium described, 83.

, passes into regular strata, 84.

, marine, 85.

Alps, reversed position of strata in, 73.
297.

, curved strata of, 73, 74.

, metamorphic rocks of the, 287.

296.

Altered rocks, 21. 107. 123. 140. 143.
286. 293.

Alternations of coarse and fine strata,
how formed, 16. 29.

, of marine and freshwater forma-
tions, 49.

Alumine in rocks, how to detect, 26.

Amblyrhynchus cristalus, 229.

America, Recent and Tertiary strata
of, 171.

Silurian strata in, 268.

Amici, Professor, on recent Charae, 48.

Ammonites, figures of, 189. 220. 238. 246.

Ampelite, 134.

Amphibolite, 93. 134.

Ampullaria glauca, 46.

Amygdaloid described, 96.

Ananchytes ovatus, 185.

Ancylus elegans, 45.

Andes, geological structure of, 284, 285.

, tertiary plutonic rocks of, 286.

Anglesea, rocks altered by a dike in,

Anodonta, figures of, 44.
Anoplotherium, 180.
Ansted, Mr., on Cambrian fossils, 270.
Anticlinal line explained, 66. 72.
Antrim, rocks altered by dikes in, 107.

, on age of trap rocks of, 278.

Apennines, age of metamorphic rocks

of, 295.

Apes, fossil, 180.
Apiocrinites rotundus, 216.
Aa*

Aqueous rocks described, 15. 159.

Arbroath, section from, to the Gram-
pians", 65.

Arenaceous rocks described, 25.

Argillaceous rocks described, 26. 133.

Arran, dikes in, 104. ,

, geology of, 291, 292.

, section of, 292.

Arthur's seat, strata altered in, 109.

Asapkus, figures of, 266, 267.

Ashby, faults in coal-field of, 81.

Ashes, volcanic, hollows filled up by,
30, 31.

, wide dispersion of, 273, 274.

Astarte, 176.

Atlantis, 210.

Auch, ape fossil near, 180.

Augite and hornblende, analogy of, 92,
9o.

, analysis of, 102.

Augite rock, 99.

Augitic porphyry, 99.

Auricula, 45.

Autreppe, unconformable strata, 74, 75.

Auvergne, volcanos of, 18. 91. 276.

, freshwater strata of, 43.

Auvergne, rocks decomposed by car-
bonic acid in, 147.

, tertiary red sandstone of, 243.

, calcareous springs of, 302.

Amcula, figures of, 224. 238.

JBacillaria in tripoli, 39.

Baculites, figures of, 184.

Bakewell, Mr.,on structure ofrocks, 139.

Bala limestone, 270.

Baltic, rocks drifted by ice in, 87.

Basalt described, 95. 100.

— , columnar, 110.

— , sandstone rendered columnar by,

140.
and trachyte, relative position of,

274, 275.

Basin, or trough, described, 66.
Basset, term explained, 72.
Bayfield, Captain, on transportation of

rocks by ice, 72.

— , on worn limestone pillars, 194.
Beaumont, M. E. de, on lias of the Vos-

ges, 225.
, on metamorphic rocks in the Alps,

287. 298.
Beck, Dr., on recent and tertiary fossil

shells, 167.

— , on classification of tertiary strata,

168.

(305)

306

INDEX.

Beck, Dr., on proportion of species to

genera in different latitudes, 168.

, on stones carried by sea- weed, 188.

, on Graptolites, 268.

Belemnites, figures of, 184. 220.
Bellerophon costatus, 254.
Berenicea diluviana, 217.
Berger, Dr., on dikes in chalk of An-
trim, 107. 278.

Bergmann on trap rocks, 89.
Bernese Alps, sections in, 298.
Berthier, on augite, 93.
Bertrich-Baden, globular structure in

basaltic columns at, 113.
Berwickshire, curved strata on coast of,

66.

Biggleswade, section near, 32.
Bilin, tripoli of, composed of infusoria,

39.

Binstead, fossils of, 180.
Birds, fossil, in Wealden, 203.
Bischoff, Professor, cited, 146.
Blainville, on number of genera of

mollusca, 43,44.
Boase, Dr., cited, 122. 149.
Boblaye, M., on geology of the Morea,

199:278.

Bog iron-ore, infusoria fossil in, 40, 41.
Bonpland, cited, 251.
Bothnia, Gulf of, proofs of rise of land

in, 63.

Boue, Mr., bis classification of rocks, 157.
, on metamorphic rocks, 295, 296.

298.

Bradford clay, fossils of, 216.
Brash described, 83.
Bray, valley of, 182. 210.
Breccia, volcanic, 98.
Bridgnorth, tertiary strata of, 173.
Brongniart, M. Alex., on vertical trees

in coal strata, 258.
Brongniart, M. Ad., on fossil coal plants,

247, 248, 249, 250.

, on climate of carboniferous pe-
riod, 254, 255.
Bronn, Professor, on fossils of upper

New Red sandstone, 237, 238.

, on Orthocerata, 268.

Brora coal-field, 223.

, granite of, 291.

Buckland, Dr., on changes caused by

dikes, 108.

, on coprolites, 187.

, on origin of flint in chalk, 190.

, on fossils of Oolite, 217. 222.

, on dirt-bed in Portland, 206, 207.

, on Ichthyodorulites, 226, 227.

, on saurians of the Lias, 228, 229,

230.

, on the New Red sandstone, 236.

, on fossil footsteps, 238, 239.

, on fossil coal plants, 252. 259.

, his classification of secondary

rocks, 265.

Budenheim limestone,* 43.
Bulimus lubricus, 46.

Bunter sandstein and fossils, 237.
Burdiehouse fossils, 245.

Caer Caradoc, trap-tuffs of, 280.

Cairo, strata formed by the Nile at, 16.

Caithness, fossil fish of, 260.

Calamites, figures of, 251.

Calcareous rocks described, 26, 27.

Calcareous springs, action of, 302.

Caldcleugh, Mr., on eruption of Cose-
guina, 274.

Calimene Blumenbachii, 266.

Cambrian rocka and fossils, 264, 265.
270.

, volcanic rocks, 280.

Campagna di Roma, tuffs of, 276.

Cantal, volcanic rocks of, 277.

Cape Wrath, granite veins of, 126.

Caradoc sandstone, 267.

Carbonate of lime, 102.

, why least in oldest rocks, 301.

Carbonic acid, in water of sediment in
delta of Ganges, 58.

, rocks decomposed by, 146.

Carboniferous limestone and fossils, 253.

Carboniferous period, rocks of, 243.

, climate of, 254.

, trap rocks of, 279.

, plutonic rocks of, 288.

, subsidence in, 257.

. See Coal.

Carpathians, cretaceous rocks of, 193.

Carrara marble, altered oolite, 295.

Caryophyllia annularis, 215.

Castrogiovanni, bent strata near, 73.

Casts of shells, how formed, 55.

Catenipora esckaroides, 267.

Catillus cuvieri, 182, 183.

Caulopteris prirruzva, 248.

Cautley, Capt, on fossil monkey, 180.

Celsius on rise of land in Sweden, 63.

Cementing together of particles in stra-
ta, 51.

Cephalaspis Lyellii, 263.

Ceratites, 23^.

Cerithium cinctum, 46.

Chceropotamus, 180.

Chain-corals, 267, 268.

Chalk, white, composition, &c., 27. 182.

, fossils of, 41. 182, 183. 186, 187. 196.

, origin of the, 186.

, pebbles in, 187.

, its extent, 191.

, external configuration of, 193.

, needles and escarpments of, 194.

, greatest height of, in England, 195.

Chalk-flints, infusoria in, 42.

, origin of, 190.

Chalk formation, its-marine origin, 181.

, subdivisions of, 181.

, fossils of, 182. 186, 187. 196, 197.

, geographical extent, 191.

, difference of, in north and south

of Europe, 195.

, map of, in S. of France, 196.

, altered by granite, 145.

INDEX.

307

Chalk formation, covered by granite,

near Meissen, 291.
, alternating with volcanic tuff

277, 278. See Cretaceous.
Champoleon, junction of granite and

secondary rocks near, 287.
Chares, fossil in freshwater strata (see

figures), 47, 48.

Charlesworth, Mr., on the crag, 1#3.
Charpentier, M., on trap rocks of

Pyrenees, 278.

Cheese-grotto, at Bertrich-Baden, 113.
Chemical deposits, 50.
Cheshire, ripple-marked sandstone from

(see fig. 6)., 33.
Chevalier, M., on bent coal strata near

Mons, 69.
Chiastolite, 102.
Chiastolite-slate, 134.
Chimcera monslrosa, 226.
Chimney, the, basaltic dike m St.

Helena, 111.
Chirotherium, 238.
Chlorite, composition of, 102.
Chlorite-schist described, 134.
Christiania, dikes near, 105, 106.

, granitic rocks of, 128, 129.

, passage of trap into granitic rocks,

near, 123.
, porphyry conformable to strata

near, 129, 130.

, rocks altered by granite, near, 143.

, tertiary strata, near, 171.

Cidaris coronata, 219.

Ciply, cretaceous rocks at, 189.

Classification of rocks, principles on

which it is founded, 15. 25.
, of the fbssiliferous rocks, 157.

163. 264.

Clay described, 25.
Clay-slate described, 133.
, lamination of, in the Pyrenees,

137.
, position of, .m the metamorphic

series, 300.

Claystone, and Claystone porphyry, 100.
Cleavage of rocks, 137.
Climate of carboniferous period, 254.
Clinkstone, 100. 137.
Club-mosses, 250.
Coal, vegetable origin of, 42. 244.

, fossils of the, 244.

, strata, origin of, 244. 256.

, on vertical trees in, 42. 257.

, isolated patches of, 256.

, rate of deposition of, 258. 261.

, zigzag flexures of, near Mons, 68.

. See Carboniferous.

Coal-pipes, 259.
Coalbrook-dale coal-field, 245.
CockfieJd Fell, coal turned into soot by

dike at, 108.
Columnar structure in rocks, 110. 119.

140.

, in ice, 140.

CoLumnarici oblonga, 215.

Compact felspar, 100.

Concretionary structure in rocks, 53,

o4.

Cones and craters, how formed, 91.
Conglomerates described, 26.
— — , vertical, in Scotland, &c., 64.

, of New Red sandstone. 240.

Coniferse, fossil, 56. 251. 255.
Consolidation of strata, 50.
Conybeare, Rev. W. D., on faults, 78. 82.
, on changes caused by dikes, 108.

on the Chalk formation, 181.

on the Plesiosaurus, 227.

on the Oolite and Lias, 224. 232.

on the New Red sandstone, 236.

on the Coal strata, 244.
Coprolites of the Chalk, 187.
Coral islands, changes of level in, 64.
Coral rag, fossils of the, 215. 218, 219.
Coral reefs, great extent of, 191.
Corals, fossil, 175. 214, 215.
Corbula alata, 201.
Cornean, 100.

Cornwall, structure of granite of, 119,
120.

, granite veins in, 127. 145.

Coseguina, volcanic eruption of, 273.
Cotta, Dr., on granite of Weinbohla,

291.
Crag formation, and its fossils, 174. 177.

, division of, into red and coralline,

174.

, may all belong to one period,

176, 177.

, its relative position, 178.

Craigleith fossil trees, 56. 259.
Crania, figures of, 37. 183.
Crassatella sidcata, 179.
Craters, volcanic, how formed, 91.
Craven fault, 78.
Cretaceous period, 181.

, volcanic rocks of, 277.

, plutonic rocks of, 286.

. See Chalk.

Crop out, term explained, 72.
Cropthorne, tertiary deposits at, 173.
Cuba, tertiary strata in, 172.
Curved strata, 65, 66.

, experiments to illustrate, 67.

, origin of, 68.

Cutch, changes caused by earthquakes

in, 208.

Cyalhea glauca, 249.
Cyatliocrinites planus, 242.
Cyclas obovata, 44.
Cyprcea coccinelloides, 175.
Cypris, fossil in freshwater strata, 47.

, figures of, 202. 245.

Cyrena trigoluna, 44.
Cylherince of the Chalk, 41.

Dartmoor granite, 145. 288.
Daubeny, Dr., on the Solfatara, 147.
Dax, chalk near, 197.
, chalk and volcanic tuff alternat-
ing, near, 277.

308

INDEX.

Darwin, Mr. C., on gradual rise of parts
of S. America, 63.

, on coral islands, 64.

, on formation of mould, 83.

, on shivering of rocks in Chili by

earthquakes, 83.

, on transportation of rocks by ice,

87,88.

, on slaty structure in refuse of

gold mine, 142.

, on structure of Andes, 284.

, on recent strata near Lima, 171.

, on origin of chalky mud in Pa-
cific, 186.

, on drifting of stones in roots of

trees, 187.

, on stones attached to sea- weed,

188.

, on living saurian of the Galapa-
gos, 229.

, on subsidence in Pacific, 233.

De la Beche, Mr., on calcareous no-
dules in Lias, 53.

— -, on rocks altered by granite, 145.

, on dirt-bed in Portland, 206, 207.

, on saurians of the Lias, 228. 231.

, on trap rocks of New Red sand-
stone, 278.

, on metamorphic rocks, 296.

Delta of Indus, recent changes in, 208.

, the Niger, its size, 209, 210.

Deluge, fossils attributed by some to, 17.

Denmark, cretaceous coral reef in, 188.

Denudation defined, 79.

, its great amount, 80.

, valleys of, 80.

, on a great scale in Ross-shire, 81.

, proofs of, from levelled surface of

districts where great faults occur, 81.

, connexion of alluvial formations,

and, 82.

, proofs of from trap-dikes, 115.

Deshayes, M., his identifications of re-
cent and fossil shells, 165, 166, 167.
177.

Deoxydation of mineral waters by or-
ganic matter, 58.

Devonshire, trap rocks of, 278.

Diagonal stratification explained, 31.

Diallage, 102.

Diallage rocks, 100.

Diceras arietina, 219.

Diceras limestone, 219.

Didelphys, fossil, 222.

Dikes, volcanic, described, 19. 103.

• , more crystalline in the centre, 105.

, fragments of subjacent rocks in,

106.

, changes caused by, 19.21. 104. 107.

, granitic, 125.

Diluvium, ancient alluviums called, 85.

Dimyary mollusca, 44.

Dionte and dioritic porphyry, 100.

Dip, term explained, 69.

, how to measure, 70.

, reversed, how caused, 73.

Dirt-bed in isle of Portland, 205.
Dolerite described, 96. 100.
Dolomite described, 28.
Dolomitic conglomerate, fossils of, 240,
Domite, 100.

Drift-wood of American rivers, 260.
Dudley, altered coal shales of, 145.
Dufrenoy, M., on rocks altered by gra-
nite, 145.

Earth's crust, term explained, 14.
, composed of distinct substances,

13.

, its successive formation, 13.

, arrangement of its materials, 14.

, not increasing in thickness, 157.

Echini from the Chalk, parasitic fossils

on, 37.

Edinburgh coal-field, fossils of, 245, 246.
Ehrenberg, Professor, on infusoria, 39.

41, 42.

Eifel, volcanic rocks of the, 276.
Elbeuf, needles and grooved pillars of

chalk at, 194.

Elevation of land, gradual, proofs of, 62.
Encrinites fossil in Oolite, 216, 217.
Endosiphonites carinatus, 270.
England, tertiary strata of, 172. 180.
Enstone, fossil bone from, 222.
Eocene, term whence derived, 166.

strata, in England, 178.

volcanic rocks, 276.

Epidote, 102.

Equisetacece, 237. 247. 250. 255.

Erratic blocks, distribution of, 86.

, transported by ice, 86.

Escarpments of oolites, &c., 234.

Esckara dislicJia, 185.

Estuary deposits, 16.

Etna, lavas, tuffs, and dikes of, 273. 276.

Eunomia radiata, 215.

Euphorbiacece, 252.

Euphotide, 100.

Eurite and euritic porphyry described,

122.134.
Exogyra butta, 201.

Falconer, Dr., on fossil monkey, 180.
False stratification explained, 32.
Fascicularia aurantium, 176.
Faults described, 75.
cause apparent alternations of

strata, 77.

, great amount of some, 78. 82.

, origin of, 77, 78.

, grooved surfaces of, 78.

, denudation proved by, 81, 82.

Faxoe limestone and its fossils, 188.

Felspar, varieties of, 92. 102.

, its decomposition affords silex in

solution, 59.
Felspar-porphyry, 100.
Findheim.land shells in limestone of, 43.
Fish killed by submarine eruptions,

floods, &c., 231.
Fissures, polished surfaces of, 75. 78.

INDEX.

Fit ton, Dr., on the Green-sand, 181.

191, 192.

• , on the Maestricht beds, 189.

, on the Wealden strata, 200, 201.

205. 209.

, on the Portland dirt-bed, 206, 207.

Fleming, Dr., on fossil fish of Old Red

sandstone, 263.

, on trap rocks, 279, 280.

Flint, sponge fossil in, 185.

, in chalk, its origin, 190.

Flotz rocks of Werner, 153.
Footsteps, fossil, 238, 239.
Foraminifera of the Chalk, 41.
Forfarshire, geology of, 65. 262.

, decomposition of rocks in, 242.

Formation, term explained, 16.
Formations, fossiliferous, arrangement

of, 157. 163.

Forth coal-field, trap rocks of, 279.
Fortis, on columnar basalt, 112.
Fossil, term defined, 16.
Fossils in stratified rocks, 16.

, height at which they are found, 17.

, their arrangement in strata, 35.

, parasitic, prove gradual deposi-
tion, 36.

, freshwater and marine, 42.

, their absence in some rocks, how

explained, 52. 302.

, mineralization of, 55.

, casts and impressions of) how

formed, 55.
Fossiliferous strata, conversion of into

metamorphic rocks, 295.
— — , why most calcareous, 301.
Fournet, M., on disintegration of rocks,

Fox, Mr. R. W., his experiments on
lamination, 142.

Fox, Rev.W.D., on fossil mammalia, 180.

Freshwater formations, how distin-
guished from marine, 42.

, land shells numerous in, 43.

, fossils numerous, but species few

in, 44.

, figures of shells most common in,

44, 45, 46.

, Cypris fossil in, 47.

, Charae fossil in, 48.

, vertebrated animals in, 48.

, alternating with marine, causes

of, 49.

Fresh water strata of the Coal, 244.256.

Frontispiece described, 22.

Fusus contrarius, 175.

Gabbro, 100.

Gaillonella, fossil in tripoli, 39. 41.

Galapagos islands, living marine reptile

of, 229.

Ganges river, deposits in estuary of, 16.
Garnet, 102.

. , in altered rocks, 107. 301.

Gases, subterranean, rocks altered by,

146, 147.

Gault, 191.

Gavarnie, curved strata near, 74.

Geology defined, 13.

Gestellihorn, section at base of the, 298.

Giant's Cause way, volcanic rocks of, 19.

Glen Tilt, junction of granite, schist,

and limestone in, 123. 125.
Globular structure, 110.
Gneiss described, 132.
Gold mine, slaty structure in refuse of,

142.

Goniatites evolutus, 254.
Goppert, Prof, his experiments on fbs-

silization of plants, 57.
Gosforth, tree in coal strata at, 56. 259.
Graham island, 116.
Grampians, vertical conglomerates in

the, 64.

, section from to the sea, 65.

, dikes of granite in, 125.

, decomposed rocks of, 242.

, rarity of limestones in the, 301.

Granite, of igneous origin, 20. 148.

, of different ages, 23. 126. 281.

, general aspect, structure, and

composition of, 119.

, varieties of, 120, 121.

, passage from trap to, 123.

, analogy in composition of trachyte

and, 123.

, veins, 123.

, finer grained in veins, 126.

, isolated masses of, 128.

, whether it ever overlies fossilife-
rous rocks, 129. 291.
, rocks altered by, 20. 143. 145.

284. 289.

, on the most ancient, 290.

, protrusion of solid, 291.

, of Arran, age of, 291.

. See also Plutonic rocks, and

Hypogene rocks.
Graphic granite described, 120.
Graptolites, 268.
Grateloup, Dr., on chalk of S. of

France, 197.
Grauwacke, term, 264.

, different ages of, 264.

Graves, M., on valley of Bray, 210.
Gray, Mr., cited, 45.
Greenland, subsidence of part of, 63.
Green-sand formation, 191.

— , fossils of, 192.

— , its origin, 192.
Greenstone described, 96. 100.
Greystone, 100.
Grit defined, 26.

Grypheea, figures of, 36. 218. 225.
Gryphite limestone, 225.
Guadaloupe, human skeletons of, 171.
Guidoni, M., on altered oolite, 295.
Gusigny, section at, 75.
Gypsum, composition of, 28.
Gyrogonites, 48.

Hall, Sir J., on curved strata, 67.

310

INDEX.

Hail, Capt. B., on dikes in Madeira, 103.

, on granite veins, 125.

Hamiles spinig-er, 192.

Harwich, section in cliffs at, 178.

Hastings Sand, 200. 208.

Heat, consolidating effects of, 55.

Hebrides, trap rocks of, 278.

Heidelberg, granites of different apes
at, 126.

Helix plebeium, 46.

Henry, experiments of, 146.

Henslow, Prof, on changes caused by
a dike in Anglesea, 107.

, on the Portland dirt-bed, 206.

Herschel, Sir J., on slaty cleavage, 142.

Hertfordshire puddingstone, 52.

Hewett, Capt, on new channel in Yar-
mouth sands, 178.

Hibbert, Dr., on fossils of the Coal, 245.

High Teesdale, garnets in altered rock
at, 107.

— —*, intrusion of trap between strata
at, 110.

Hildburghausen, fossil footsteps at, 238.

Hippurites, figures of, 196, 197.

Hoffman, on agency of subterranean
gases, 147.

, on metamorphic rocks, 295.

Hoogly, R., analysis of water contained
in mud of, 58.

Hornblende, 92. 102.

Hornblende-rock, 100. 135.

Hornblende-schist described, 133.

Hornstone, hornslone-porphyry, 100.

Horner, Mr., on fossil fish in Coal stra-
ta, 245, 246.

, on the Malvern hills, 288.

Hubbard, Prof., on granite veins, 283.

Hugi, M., on alternation of gneiss and
fossiliferous rocks in Alps, 298.

Humboldt cited, 251.

Hungary, trachyte of, 123.

Hutton, opinions of, 153.

Hutton, Mr., on fossil coal plants, 247.
252. 259.

Hybodus relicutatus, 226.

Hypersthene, analysis of, 102.

Hypersthene rock, 101.

Hyppgene, name proposed instead of
primary for the crystalline rocks, 24.

, rocks described, 118. 134.

, must be old before they reach the

surface, 298, 299.

, age of, how determined, 282. 294.

, uniformity of mineral character

in, 300.

, why less calcareous than the fos-
siliferous, 301.

. See also Granite, Plutonic rocks,

and Metamorphic rocks.

Ice, transportation of erratic blocks by,
86, 87.

, columnar structure of, 140.

Ichthyodorulites, 226. 262.
Ichthyosaurus, 228.

Iguanodon, 203.211.
Inclined and vertical stratification, 64
Indus, recent changes in delta of, 208.
Infusoria in tripoli, &c., 39.

, figures of, 40.

Inkpen Beacon, 195.

Insects, fossil, 221, 222.

Inverted position of strata, how caused,

73.

Ipswich, section near, 175.
Ireland, tertiary strata in, 172.
Ischia, tertiary strata in, 276.
Iselten Alp, curved strata of the, 74, 75.
Isle of Bourbon, eruptions in, 283.
Isle of Wight, tertiary strata of, 43. 180.

, chalk needles of; 195.

Isomorphism, theory of, 94.

Jackson, Col., on columnar structure in

ice, 140.

Jointed structure of rocks, 138.
Jorullo, volcanic eruption of, 283.
Jungfrau, section on the, 298.
Jura, section of structure of the, 71.
, Oolite of the, 214. 218. 233.

Kander, R., land shells in delta of, 43.

Kaolin, mineral composition of, 26.

Kaup, Prof, on fossil footsteps, 239.

Keilhau, Prof, on Greenstone dike, 106.

, on granitic rocks of Norway, 128.

130. 144. 289.

Kelloway rock, cementing of the parti-
cles of; by lime derived from shells, 51.

Keuper sandstone, fossils of, 237.

Kildonan castle, dike near, 104.

Killas altered by granite, 145.

Kimmeridge clay, 221. 234.

Labradorite, 92. 102.

La Coupe d'Ayzac, columnar lava of;

Lakes, arrangement of deposits in, 15.

Lamarck, his division of bivalve mol-
lusca, 44.

Land, proofs of the elevation and sub-
sidence of, 61. 63.

Lander, Mr., on delta of Niger, 210.

Land's End, granite of, 119. 121.

Land shells, numerous in freshwater
formations, 43.

, drifted by rivers, 43.

, figures of genera most common

in strata, 46.

Lartet, M., on fossil ape, 180.

Lateral movements, folding of strata
by, 68.

Lava described, 91. 97.

Lehman's division of rocks, 152.

Leibnitz, theory of, 156.

Lepidodendra, figures of; 250.

Lepidotus figures of, 202. 225.

Leucite, 102.

Lias, mineral character of, 224, 225.
— , fossils of,— shells, 224. 231,— fish,
225 ;- reptiles, 227;— plants, 232.

INDEX.

311

Lias and Oolite, origin of, 232.

, valleys and escarpments formed

by, 234.

, calcareous nodules in, 53.

, volcanic rocks of the, 278.

, plutonic rocks of the, 287.

Lima, Recent strata near, 171.
Lime in rocks, how to detect, 27.

, whence derived, 59.

, why less in crystalline rocks, 301.

Limestones, composition of, 27. 36.

, deposited by springs, 50.

, in coral reefs formed by zoophytes,

50.

Limnea longiscata, 45.
Lindley, Mr., on fossil coal plants, 247,

248, 249. 252. 255. 259.
, on destructibility of plants in

water, 255.
Lipari islands, rocks altered by gases

in, 147.

Llandeilo formation, 267.
Loam described, 28.
Loess of the Rhine, 172.

, tuffs interstratified with, 276.

Loire, R., stratification of recent mud

of, 29.

London clay and its fossils, 38. 178.
Lonsdale, Mr., on microscopic chalk

fossils, 41.

, on Stonesfield slate, 222.

Lons-le-Saulnier, Lias and Oolite of, 224.
Louisiana, submerged trees in, 261.
Lower New Red sandstone, 242.
Ludlow formation, 265.
Lulworth cove, section in, 206.
Lumley Den, trap rocks in, 280.
Lutschine, valley of, curved strata in, 74.
Lycopodiacete, 247. 250. 255.
Lym-Fiord, invaded by the sea, 49.
, stones carried by sea-weed in, 188.

MacCulloch, Dr., termed volcanic rocks
overlying, 21.

, on consolidation of strata, 52.

, on denudation, 81.

, on compact felspar, 92.

, on trap rocks and dikes, 104. 117.

, on columnar basalt, 111.

, on passage of granite into trap, 123.

, on granite veins, 124. 126.

, on altered rocks, 109. 149. 288.

,-on isle of Arran, 294,

Madeira, dikes in, 103.

Maestricht beds and their fossils, 189.

Magnesian limestone and fossils, 240.
242.

, composition of, 28.

, concretionary structure in, 53.

Malvern hills, rocks altered by granite
in, 288.

Mammalia, extinct, found with living
shells, 172, 173.

Mammat, Mr., on faults and denuda-
tion in Ashby coal-field, 82.

Mammoth, fossil, 172, 173.

Mantell, Mr., on fossils of the Chalk,

188. 190.

, on the Iguanodon, 203.

, on Portland dirt- bed, 206.

, on plants of the Wealden, 209.

Map of chalk in south of France, 196.
Marble described, 27. 134.
Margarate, term explained, 142.
Marine formations, how distinguished

from freshwater, 42.
Markerud, strike of beds not altered

by intrusion of granite at, 128.
Marl and marl-slate described, 28.
Mechanical deposits, 50.
Megalichthys Hibberti, 246.
Meissen, granite covering chalk, near,

291.

Melanopsis buccinoidea, 45.
Melaphyre, 101.

Menai Straits, tertiary strata near, 173.
Mesotype, 102.

Messenia, puddingstone of, 199.
Metalliferous veins, 128.
Metamorphic, term whence derived, 22.

rocks, general character of, 131.

, principal members of this class

described, 132.

, their origin, 21. 135. 143. 147, 148.

, stratificalion of, distinct from

cleavage, 137.
, kind of strata from which some

may have been derived, 148.
, on the different ages of, and how

determined, 293.

, of the Apennines, Alps, &c., 295.

, must be old before they reach the

surface, 299.

, order of succession in, 299.

, why less calcareous than the fos-

siliferous, 301.

Meyringen, section near, 298.
Mica, decomposition of, affords silex in

solution, 60.

, composition of, 102.

Micaceous sandstone described, 26.

Mica-schist described, 133. 135.

Microconchus, 245.

Miller, Mr., cited, 242.

Millstone grit, 244.

Mineral character as a test of age of

rocks, 160. 274. 282. 294.
Minerals in volcanic rocks, analysis of,

102.

Mineralization of organic remains, 55.
Mingan islands, worn limestone pillars

in? 194.
Miocene, term whence derived, 166.

deposits not found in England, 173.

volcanic rocks. 276.

Mississippi R., deposits in estuary of, 16.
, lagoons alternately fresh and salt

at mouth of, 49.

, drift wood of, 260.

, delta of, 261.

Mitra scabra, 179.

Mitscherlich, Professor, on augite, 93.

312

INDEX.

Mitscherlich, his theory of Isomorphism,
94.

Moel Try fane, tertiary strata 011, 173.

Monkey, fossil, 180.

Monomyary mollusca, 44.

Mons, zigzag flexures of coal near, 68.

, unconibrmable strata near, 74.

Morea, cretaceous rocks of the, 199.

, volcanic rocks of, 277, 278.

Mosasaurus, 189.

Mould, formation of, 82.

Mountain limestone and fossils, 243. 253.

Mount Battock, granite dikes in, 125.

Munster, Count, on Solenhofen fossils,
221.

, on fossils of the Keuper, &c., 237.

Murchison, Mr., on joints and slaty
cleavage, 138. 141.

, on tertiary strata, 173.

, on Brora coal-field, 223.

, on New Red sandstone, 236. 240.

, on fossils of the coal, 245.

, on Old Red sandstone, 262.

, on the Silurian strata, 265, 266, 267.

, on Silurian and Cambrian trap-
rocks, 280.

, on granite of Dartmoor and Mal-

vern hills, 288.

, on granite of Brora, 291.

, on geology of Arran, 294.

Murex alveolatus, 175.

Muschelkalk, fossils of, 236.

Naesodden, greenstone dike of, 106.

Naples, tertiary strata of, 170.

Nassa granulata, 175.

Nautilus, figures of, 179. 225.

Keeker, Mr. L. A., terms granites un-
derlying igneous rocks, 21. 148.

, on metalliferous veins, 128.

, on the Valorsine, 128, 129.

, on metamorphic rocks of the

Alps, 298.

Needles of chalk, 194, 195.

Nelson, Lieut., on chalk formed by de-
f composition of corallines, 186.
I Neptunian theory of the origin of
V-jocks, 153.

Nefincea, figures of, 218.

Nerinaean limestone, 219.

Nerita granulosa, 45.

Neritina globulus, 45.

Newcastle coal-field, great faults in,
78. 82.

New Red sandstone group of rocks, 235.

, position and subdivisions of, 236.

, origin of the, 242.

New Zealand, 212. 256.

Niesen, slates of the, 141.

Niger, R., delta of, 209, 210.

Nile, R, stratified deposits formed by, 16.

, lagoons at mouth of, 49.

Ninety-fathom dike, 78.

North cliffi tertiary strata at, 173.

Norway, tertiary strata of, 171.

, granitic rocks of, 128, 129. 143.

Norway, Silurian strata in, 267.
Nummulite limestone, 198.
Nummuliles, figures of, 198.
Nyo'e, new island destroyed by sea,
116.

Obsidian, 101.

Oeynhausen, M., on granite veins, 127.

, on isle of Arran, 294.

Old Red sandstone, its subdivisions and

fossils, 262.

, trap rocks o'f, 279.

, pi u tonic rocks of the, 288.

Olivine, 102.

Oolite formation, name whence derived,

27. 213.

, extent and subdivisions of, 213.

, fossils, 214 to 222.

, changes in organic life during its

accumulation, 217.

, signs of land during, 221.

, volcanic rocks of the, 278.

, plutonic rocks of the, 287.

, metamorphic, in Apennines and

Alps, 296.

, and Lias, origin of the, 232.

, valleys and escarpments formed

by, 234.

Ophiolites, 101. 277. 279.
Ophites, 101. 278.
Oppenheim limestone, 43.
Orbicula reflexa, 220.
Orford, crag strata near, 174.
Organic remains, age of strata proved

by, 160. 273.

. See Fossils.

Orthoceras, figures of, 254.
Orthocerata, on structure of, 268.
Ostrea, 44. 183, 184. 218. 220.
Outcrop of strata explained, 72.
Overlying, term applied to volcanic

rocks, 21. 129.
Owen, Mr., on fossil bone from Enstone,

222.
Oxford clay, 232, 233, 234.

Palceoniscus, 241.

Palaeontology, term explained, 164.

Palaeotherium, 180.

Paludina, 45.

Pampas have been raised slowly, 63.

Parasitic fossils, 36.

Pareto, M., on altered Oolite, 295.

Passy, M.,on chalk cliffs of Normandy,
195.

Patagonia, plains of, upheaved gradu-
ally, 63.

Pearlstone, 101.

Pebbles in chalk, 187.

Pecopleris Lonchitica, 247.

Pecten, figures of, 192. 246.

Pegmatite described, 122.

Peperino described, 98.

Pepys, Mr., cited, 58.

Petrifaction of fossils, 55.

Petrosilex, 101.

INDEX.

313

Peyrehorade, numraulite limestone of,
198.

Pkasianella and cast of same, 55.

Phillips, Prof., on grooved surfaces of
faults, 78.

, on joints in rocks, 141.

, on the Coal strata, 243, 244. 246.

, on the Mountain limestone, 253.

, on Cambrian fossils, 270.

Phillips, W., on composition of clays, 26.

, on faults, 78.

Pholadomya fidicula, 219.

Phonolite, 101.

Phyllade, 135.

Physa, figures of, 45.

Pid'dington, Mr., his analysis of the
water in the mud of the Hoogly
river, 58.

Pmgel, Dr., on subsidence in Green-
land, 63.

Pitchstone, 101.

Plagiosloma, figures of, 183.

Planorbis euomphalus, 45.

Plants, fossil, of the Coal, 247.

Plas-Newydd, changes caused by a
dike near, 107.

Playfair, on rise of land in Sweden, 63.

, his description of faults, 76.

, on Huttonian theory, 154.

Plesiosaurus, 228.

Pliocene, term, whence derived, 166.

, period, newer, strata of, near Na-
ples, 170.

, in Norway, 171.

• , in S. America, 171.

, in W. Indies, 171.

, in valley of Rhine, 172.

, in Great Britain and Ireland, 172.

, (older) in England, 173.

, volcanic rocks of, 275.

, plutonic rocks of, why invisible,

283

Plutonic action, 145. 148.

Plutonic rocks, described, 20. 118.

, their relation to the volcanic, 20.

, name whence derived, 20.

, age of, how determined, 281.

, Recent and Pliocene, why invisi-
ble, 283.

, of different periods, 283.

, relative age and position of. 284.

. See also Hypogene rocks.

Poikilitic group, 236.

Polished surfaces of fissures and faults,
75. 78.

Ponza islands, globiform pitchstone in,
113.

Porcelain clay, 26.

Porphyritic granite, 121.

Porphyry described, 96. 101.

Portland dirt-bed, 205, 206.

Posidonomya minuta, 238.

Pozzolana, composition of, 53.

Predazzo, Oolite altered at, 288.

Pressure, consolidating effects of, 54.

B b

Preston, tertiary strata at, 173.

Prestwich, Mr., on the Coal strata, 245

Primary fossiliferous strata, 264.

— — , horizontal in Sweden, 269.

Primary limestone, 134.

Primitive or Primary, term, why erro-
neous, 23, 24. 154. 157.

Producta, figures of, 241. 253.

Protogine described, 122. 133. 135.

Puddingstone described, 26.

, of Hertfordshire, 52.

, of Messenia, 199.

Pumice described, 97. 101.

Pupa muscorum, 46.

Purbeck beds, 200.

Puzzuoli, elevated marine strata at, 170.

Pyrenees, bent strata in, 72. 74.

, lamination of clay-slate in, 137.

, rocks altered by granite in, 145.

286.

, chalk of, 195. 197.

, trap rocks of, 278.

Pyroxenic-porphyry, 101.

Quadersandstein, 225.
Q'uadrumana, fossil, 180.
Quaquu-versal dip explained, 72.
Quartz rock or Quartzite described, 133.
Quartz veins, 129.
Quiriquina, island of, rocks shivered by

earthquakes in, 83.
Quorra, R., delta of, 209.

Radnorshire, trap rocks of, 280.

Ramsholt, section at, 175.

Rancie, altered Lias at, 145.

Rathlin, dikes at, 108.

Recent period, how separable from ter-
tiary, 169.

, formations of, in different coun-
tries, 170. 172.

, volcanic rocks of, 275.

, plutonic rocks of, why invisible,

283

Red sandstone and marl, origin of, 242.

, of different ages, 242.

. See also Old, and New Red

sandstone.

Reptile, living marine, of the Galapa-
gos, 229.

Reptiles, fossil, of the Wealden, 202.
210.

, of the Lias, 227.

, of the Muschelkalk, 238.

, of the Magnesian Limestone, 242.

Rhine, R., land shells drifted by, 43.

, valley of, tertiary strata in, 172.

Ribboned jasper, 144.

Riley, Dr., on fossil reptiles, 242.

Ripple mark, how formed (see Fig.), 33.

Rock, term defined, 14.

Rocks, all divisible into four contempo-
raneous classes, 15. 22.

, aqueous, described, 15. 25.

, volcanic, described, 18. 89.

314

INDEX.

Rocks, platonic, described, 118.

, metamorphic, described, 131.

, how to detect alumine or lime in,

26, 27.

, hardened by exposure to air, 52.

, their particles re-arranged by

chemical action, 53.

, transported by ice, 86.

, how to be studied, 94.

, altered by dikes and granite, 104.

107. 123. 143. 145. 286. 294.
, different ages of the four great

classes of, 151. 159. 271. 281. 294.

, classification of, 15. 24. 157. 163.

Rose, G., on hornblende and augite, 93.
, on composition of volcanic rocks,

99, 100, 101.

Ross-shire, denudation in, 81.
Rostellaria macroptera, 179.
Rother, R., buried ship in old channel

of, 172.

Rothliegendes, 240.
Roththal, section in the, 298.
Rouen, chalk needles near, 194.
Rubble explained, 83.

St. Abb's Head, curved strata near, 66.

St. Etienne, vertical trees in Coal stra-
ta at, 258.

St. Helena, basaltic dike in, 111.

Salbands, 105.

Salisbury Craig, altered strata in, 109.

San Caterina, bent strata near, 73.

Sandstone described, 25.

San Lorenzo, isle of, Recent strata in,
171.

Sattel, section on the, 298.

Sauroid fish of the Coal, 246.

Saussure on vertical conglomerates, 64.

Savi, M., on metamorphic rocks, 295.

Saxony, aqueous strata rendered colum-
nar by basalt in, 140.

Scania, sinking of land in, 63.

Schorl, analysis of, 102.

Schorl rock described, 122.

Scoresby, on rents in icebergs, 140.

Scoriae described, 97. 101.

Scrope, Mr., on Auvergne volcanos, 91.

, on volcanic rocks, 98. 113. 275.

Sea, proofs that it has not sunk, but
that the land has been raised, 61.

Sea-urchins, figures of, 37.

Seale, Mr., on dikes in St. Helena, 111.

Sedgwick, Professor, on garnets in al-
tered rocks, 107.

, on changes caused by trap dike,

108.

, on intrusion of trap, 110.

, on granite veins, 126.

, on stratification, joints, and cleav-
age, 137, 138. 142.

, on lower New Red sandstone, 239.

, on the Magnesian limestone, 53.

241.

, on the Coal, 244.

, on Cambrian system, 264. 270. 281.

Sedgwick, Professor, on granite of
Dartmoor, 288.

, on granite of Brora; 291.

, on geology of Arran, 294.

Segregation, veins formed by, 129.

Seine, chalk needles in valley of, 194.

Semi-opal, infusoria fossil in, 40.

Serpentine, 101, 102. 135.

SerpulcB on fossils, prove slow deposi-
tion of strata, 36.

Sewalik hills, fossils of the, 180.

Shale defined, 26.

Shells, marine, rules for recognizing, 44.

, freshwater (see Jigures), 44, 45, 46.

, common to rivers and the sea, 46.

, amphibious, 46.

, terrestrial (see Jigures), 46.

, inferences drawn from the shape

of the mouths of, 46, 47.

Sheppey, isle of, fossils, 180.

Shetland, granites of, 126.

Ships, fossil, 172.

Sicily, tertiary strata of, 62.

Sidlaw hills, section of the, 65.

, trap rocks of, 279.

SigillaricB, figures of, 248.

, erect position of in Coal strata, 259.

Silex, in solution, source of, 59.

Siliceous limestone described, 27.

Silurian strata, origin of name, 264.

, table of succession of, 265.

, Upper, fossils of, 265.

, Lower, fossils of; 267.

, Lower, trap rocks of, 280.

, in Norway and Sweden, 62. 267

269.

, horizontal, 269.

, in N. America, 268.

, granite altering, 288.

Skaptar Jokul, eruption of, 274.

Sky, trap dikes in, 104, 105.

, columnar basalt of, 111.

, rocks altered by trap in, 109.

Slaty cleavage, 137. 141.

Slickensides, 75. 78.

Snowdon, fossils of, 270.

Sodertelje, recent strata at, 170.

Solenhofen fossils, 221.

Solfatara, rocks of, decompose, 147.

Sorgenfri, dike at, 106.

Sorting power of water, 30.

Spatangus, figures of, 37.

Sphceruliles agariciformis, 197.

Sphenopteris, figures of, 204. 248.

Spirt/era, figures of, 241. 253.

Sponge in flint, 185.

Spongilla in tripoli, 40.

Springs, calcareous, 50. 302.

Staffa, rock of, volcanic, 19.

Stammerham, cracks in clay at, 208.

Stations of species, 217.

Steatite, 102.

Steeple Ashton, fossil coral from, 215.

Sternberg on fossil coal plants, 247.

StigmarifE, figures of, 252.

Stirling Castle, rock of altered, 109.

INDEX.

315

Stokes, Mr., on lapidification of fossil

wood, 60.

, on structure of Orthocerata, 268. i

Stone-lilies, fossil, 215, 21G.
Stonesfield slate, fossils of, 222.
Strata, term defined, 15.

, original horizontally of, 30. 51. j

, thinning out of, explained, 31.

, parallelism of, 30.

, ripple-marked, 33.

, gradual deposition of, indicated

by fossils, 35. 42. 216.

, their mineral composition, 25.

, consolidation of, 50.

, horizontal at great heignts, 62.

, sometimes reversed, 73. 159.

age of, how determined, 159. 271. I

281. 294.
, fbssihferous, chronological ar- !

rangement of, 157. 163.

, oldest sometimes horizontal, 2G9.

, conversion of fossihferous into

metamorphic, 295.

Strathaird, fissures caused by decom-
posed trap dikes, 104.
Stratheden, trap rocks of, 279.
Slrathmore, valley of, 65. 2G2.
Stratification, forms and causes of, 15.

28. 32, 33. 35. 64. 74.

, proof of aqueous origin, 16.

, of deposits in lakes and estuaries,

15, 16.

, planes of, how far parallel, 30.

, distinct from cleavage, 137.

Stratum defined, 15.

Strickland, Mr., on tertiary strata, 173.

Strike and dip explained, 69.

, sometimes not altered by intruded

granite, 128.
Stromboli, 273. 276. 283.
Studer, M., on alternation of gneiss and

fbssiliferous rocks in the Alps, 298.
Stutchbury, Mr., on fossil reptiles, 242.
Subapennmes, interstratified tuffof, 276.
Subsidence of land, 63, 64.

, in Cretaceous period, 193.

, in Wealden period, 204. 212.

, in Carboniferous period, 257.

Succinea elongata, 45.

Suffolk, freshwater strata in, 173.

, Crag of, described, 173.

Suishnish, trap rocks of, 105.
Superior, Lake, recent deposits in, 52.
Superposition, relative age of strata

shown by, 159.
Sutton, section at, 175.
Sweden, gradual rising of land in, 63.

, Recent and Tertiary strata of, 171.

, Silurian strata, horizontal in, 62.

267. 269.

Swiss Alps, altered rocks of, 296.
Syenite described, 121.
Syenitic greenstone described, 96. 101.
Synclinal line described, 66. 72.

Table Mountain, stratification of, 62.

Table Mountain, granite veins in, 125.

Talc, 102.

Talcose gneiss, 135.

Talcose granite, 122.

Talcose schist, 135.

Tattingstone, crag strata at, 174.

Tephrine, 101.

Tercis, chalk of, 197.

, chalk and volcanic tuff, alternat-
ing at, 277.

Terebellum fusiforme, 179.

Terebrahdce, fossil figures of, 183. 192.
220. 266.

Teredina, fossil wood bored by, 38.

Teredo navalis, wood bored by, 38.

Terminology, 199.

Tertiary formations, their relative posi-
tion, fossils, &c., 164.

, divisible into four groups, 165.

, how classified, 165.

, of different countries described,

169.

, how distinguished from Recent,

170.

, volcanic rocks, 275.

, plutonic rocks, 284.

Testacea. See Shells.

Thermal ocean, theory of, 155.

Thinning out of formations, 162.

Thirria, M., on the Oolite, 233.

Thun.lakeof, land shells drifted into, 43.

Thurmann, M., on the Swiss Jura, 71.
218.

Tisbury, fossil coral from, 214.

Toadstone, 101.

Tourmaline, 102.

Tournedos, chalk needles at, 194.

Trachyte described, 96.101.

, analogy in composition of granite

and, 123.

and basalt, relative position of, 274.

Transition strata and fossils, 152. 154.264.

Trap conglomerates, 116.

Trap dikes described, 103.

, rocks altered by, 104, 107.

, their abrupt termination caused

by denudation, 115. 273.

Trap rocks described, 89.

, name whence derived, 89.

, step-like appearance of, 90.

, changes caused by, 104. 107.

, intrusion of, between strata, 110.

, their relation to modern lavas, 114.

117.

, pass into granite, 123.

, regarded by Werner as aqueous

deposits, 153.

, on the different ages of, 271.

. See also Volcanic rocks.

Trap tuff described, 98.

Trass, 101.

Travertin deposited by springs, 50.

Tree-ferns, figures of, 249.

Trees in Coal strata, erect position of, 257.

Treuil coal-mine, vertical trees in, 258.
Trigonia gibbosa, 218.

318

INDEX.

Tripoli composed of infusoria, 39.
Trochus, and cast of the same, 55.
Tronstad Strand, section on beach at, 129.
Trough, or basin, described, 66.
Tuff, volcanic, 19. 98.

, imbedding of fossils in, 273.

Tufaceous conglomerates, 98.
Turner, Dr., on combinations formed by

mineral matter when in a nascent

state, 58.

, on source of silex in solution, 59.

Turrilites costatus, 184.
Tuscany, volcanic rocks of, 276.

, calcareous springs of, 302.

Tynedale fault, 78.

Unconformable stratification, 74.

Underlying rocks, term proposed for
granites, 21. 129.

Unio litloralis, 44.

Upheaval of extensive masses of hori-
zontal strata, 62.

Upper New Red sandstone and fossils,

Val di Noto, volcanic rocks of the, 115.

Valley of Bray, 182. 211.

Valleys of denudation, 80.

Valorsine granite, veins of the, 127. 298.

Valvata, 45.

Veins, granitic, 193.

, metalliferous, 128.

, of segregation, 129.

. See Dikes.

Velay, volcanic rocks of, 18.

Vertical strata parts of great curves, 64.

Vesuvius, 273. 276.

Vicentin, columnar basalt in the, 112.

Virlet, M., on corrosion of rocks by

gases, 147.
, on cretaceous rocks of the Morea,

199.
, on volcanic rocks of the Morea,

277, 278.

Vivarais, volcanic rocks of, 18.
Volcanic eruptions, number of in a

century, 283.
Volcanic dikes described, 103.

, rocks altered by, 107.

Volcanic grits, 98.

Volcanic rocks described, 18. 89. 92. 99.

produce a fertile soil, 90.

, analysis of minerals found in, 102.

, their fusibility, 99.

, their relation to trap, 114. 117.

, denudation of, shown by dikes, 1 15.

, submarine, 19. 115.

, on the different ages of, 19. 271.

, age of, how determined, 271.

, See also Trap rocks.

Volcanic tuff described, 19. 99.

, imbedding of fossils in, 273, 274.

Volcanos, extinct, of different coun-
tries, 18, 19. 91.

, cones and craters of, how formed,

91.

, all near the sea, 116.

Vollzia brevifolia, 237.

Volutes, figures of; 177. 179.

Von Buch, on rise of land in Sweden,
63.

, on granite of Norway, 129. 289.

, on altered Oolite, 288.

Von Dechen, M., on granite veins, 127.

, on isle of Arran, 294.

Wacke described, 101.

Waller cited, 155.

Water, sorting power of, 30.

, levelling power of, 30, 31.

Watt, G., experiments of, 146.

Wealden strata, position and subdivi-
sions of, 200.

, fossils of, 201 to 204. 209, 210,

211.

, passage of beneath chalk, 203.

, how formed, 204. 208.

.extent of, 209.

, age of, 211.

Webster, Mr., on dirt-bed in Portland,
206.

Weinbuhla, granite of, covering chalk,
291.

Wenlock formation, 267.

Werner, his classification of rocks, 152.

West Indies, recent and tertiary strata
in, 171.

Westphalia, cretaceous rocks of, 193.

Whin-Sill, intruded trap, 110.

Whinstone, 101.

WThite Mountains, granite veins in, 283.

Whitestone, 135.

Witham, Mr., on fossil coal plants, 56.
260.

Wood, recent and fossil, drilled by per-
forating mollusca, 38.

, fossil, magnified portion of, 56.

, experiments to illustrate the petri-
faction of, 57.

, rate of its lapidification, 60.

Wrekin, trap tuffs of the, 280.

Yarmouth sands, new channel in, 178.
Yorkshire, Oolite of, 223.

Zamia, fossil, in Portland, 205.

, recent, figure of, 205.

Zechstein and fossils of, 240, 241.
Zoophytes, limestone formed by, 50.
, fossil, in Oolite, 214. 216.

THE END.

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