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[Illustration: From a Painting by James Hall, Esq. Engraved by
S. Williams.
STRATA OF RED SANDSTONE, SLIGHTLY INCLINED, RESTING ON VERTICAL SCHIST, AT
THE SICCAR POINT, BERWICKSHIRE.
TO ILLUSTRATE UNCONFORMABLE STRATIFICATION. See page 60.
_"The mind seemed to grow giddy by looking so far into the abyss of time;
and while we listened with earnestness and admiration to the philosopher
who was now unfolding to us the order and series of these wonderful events,
we became sensible how much farther reason may sometimes go than
imagination can venture to follow."_--PLAYFAIR, Biography of Hutton.]
A MANUAL OF ELEMENTARY GEOLOGY:
OR,
THE ANCIENT CHANGES OF THE EARTH AND ITS INHABITANTS
AS ILLUSTRATED BY GEOLOGICAL MONUMENTS.
BY SIR CHARLES LYELL, M.A. F.R.S.
AUTHOR OF "PRINCIPLES OF GEOLOGY," "TRAVELS IN NORTH AMERICA," "A SECOND
VISIT TO THE UNITED STATES," ETC. ETC.
"It is a philosophy which never rests--its law is progress: a point which
yesterday was invisible is its goal to-day, and will be its starting post
to-morrow."
EDINBURGH REVIEW, No. 132. p. 83. July, 1837.
_FOURTH AND ENTIRELY REVISED EDITION._
ILLUSTRATED WITH 500 WOODCUTS.
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1852.
LONDON:
SPOTTISWOODES and SHAW,
New-street-Square.
PREFACE TO THE FOURTH EDITION.
In consequence of the rapid sale of the third edition of the "Manual," of
which 2000 copies were printed in January last, a new edition has been
called for in less than a twelvemonth. Even in this short interval some new
facts of unusual importance in palaeontology have come to light, or have
been verified for the first time. Instead of introducing these new
discoveries into the body of the work, which would render them inaccessible
to the purchasers of the former edition, I have given them in a postscript
to this Preface (printed and sold separately), and have pointed out at the
same time their bearing on certain questions of the highest theoretical
interest.[v-A]
As on former occasions, I shall take this opportunity of stating that the
"Manual" is not an epitome of the "Principles of Geology," nor intended as
introductory to that work. So much confusion has arisen on this subject,
that it is desirable to explain fully the different ground occupied by the
two publications. The first five editions of the "Principles" comprised a
4th book, in which some account was given of systematic geology, and in
which the principal rocks composing the earth's crust and their organic
remains were described. In subsequent editions this book was omitted, it
having been expanded, in 1838, into a separate treatise called the
"Elements of Geology," first re-edited in 1842, and again recast and
enlarged in 1851, and entitled "A Manual of Elementary Geology."
Although the subjects of both treatises relate to geology, as their titles
imply, their scope is very different; the "Principles" containing a view of
the _modern_ changes of the earth and its inhabitants, while the "Manual"
relates to the monuments of _ancient_ changes. In separating the one from
the other, I have endeavoured to render each complete in itself, and
independent; but if asked by a student which he should read first, I would
recommend him to begin with the "Principles," as he may then proceed from
the known to the unknown, and be provided beforehand with a key for
interpreting the ancient phenomena, whether of the organic or inorganic
world, by reference to changes now in progress.
Owing to the former incorporation of the two subjects in one work, and the
supposed identity of their subject matter, it may be useful to give here a
brief abstract of the contents of the "Principles," for the sake of
comparison.
_Abstract of the "Principles of Geology," Eighth Edition._
BOOK I.
1. Historical sketch of the early progress of geology, chaps. i. to iv.
2. Circumstances which combined to make the first cultivators of the
science regard the former course of nature as different from the
present, and the former changes of the earth's surface as the effects of
agents different in kind and degree from those now acting, chap. v.
3. Whether the former variations in climate established by geology are
explicable by reference to existing causes, chaps. vi. to viii.
4. Theory of the progressive development of organic life in former ages,
and the introduction of man into the earth, chap. ix.
5. Supposed former intensity of aqueous and igneous causes considered,
chaps. x. and xi.
6. How far the older rocks differ in texture from those now
forming, chap. xii.
7. Supposed alternate periods of repose and disorder, chap. xiii.
BOOK II.
CHANGES NOW IN PROGRESS IN THE INORGANIC WORLD.
8. Aqueous causes now in action: Floods--Rivers--Carrying power of
ice--Springs and their deposits--Deltas--Waste of cliffs and strata
produced by marine currents: chaps. xiv. to xxii.
9. Permanent effects of igneous causes now in operation: Active volcanos
and earthquakes--their effects and causes: chaps. xxiii. to xxxiii.
BOOK III.
CHANGES OF THE ORGANIC WORLD NOW IN PROGRESS.
10. Doctrine of the transmutation of species controverted,
chaps. xxxiv. and xxxv.
11. Whether species have a real existence in nature,
chaps. xxxvi. and xxxvii.
12. Laws which regulate the geographical distribution of species, chaps.
xxxviii. to xl.
13. Creation and extinction of species, chaps. xli. to xliv.
14. Imbedding of organic bodies, including the remains of man and his
works, in strata now forming, chaps. xlv. to l.
15. Formation of coral reefs, chap. li.
It will be seen on comparing this analysis of the contents of the
"Principles" with the headings of the chapters of the present work (see p.
xxiii.), that the two treatises have but little in common; or, to repeat
what I have said in the Preface to the 8th edition of the "Principles,"
they have the same kind of connection which Chemistry bears to Natural
Philosophy, each being subsidiary to the other, and yet admitting of being
considered as different departments of science.[vi-A]
CHARLES LYELL.
_11 Harley Street, London, December 10. 1851._
FOOTNOTES:
[v-A] Postscript to 4th edition of the Manual, price 6_d._
[vi-A] As it is impossible to enable the reader to recognize rocks and
minerals at sight by aid of verbal descriptions or figures, he will do well
to obtain a well-arranged collection of specimens, such as may be procured
from Mr. Tennant (149. Strand), teacher of Mineralogy at King's College,
London.
POSTSCRIPT.
Tracks of a Lower Silurian reptile in Canada--Chelonian footprints in
Old Red Sandstone, Morayshire--Skeleton of a reptile in the same
formation in Scotland--Eggs of Batrachians (?) in a lower division of
the "Old Red," or Devonian--Footprints of Lower Carboniferous reptiles
in the United States--Fossil rain-marks of the Carboniferous period in
Nova Scotia--Triassic Mammifer from the Keuper of Stuttgart--Cretaceous
Gasteropoda--Dicotyledonous leaves in Lower Cretaceous strata--Bearing
of the recent discoveries above-mentioned on the theory of the
progressive development of animal life.
_Tracks of a Lower Silurian reptile in Canada._--In the year 1847, Mr.
Robert Abraham announced in the Montreal Gazette, of which he was editor,
that the track of a freshwater tortoise had been observed on the surface of
a stratum of sandstone in a quarry opened on the banks of the St. Lawrence
at Beauharnais in Upper Canada. The inhabitants of the parish being
perfectly familiar with the track of the amphibious mud-turtles or
terrapins of their country, assured Mr. Abraham that the fossil impressions
closely resembled those left by the recent species on sand or mud. Having
satisfied himself of the truth of their report, he was struck with the
novelty and geological interest of the phenomenon. Imagining the rock to be
the lowest member of the old red sandstone, he was aware that no traces had
as yet been found of a reptile in strata of such high antiquity.
He was soon informed by Mr. Logan, at that time engaged in the geological
survey of Canada, that the white sandstone above Montreal was really much
older than the "Old Red," or Devonian. It had in fact been ascertained many
years before, by the State surveyors of New York (who called it the
"Potsdam Sandstone"), to lie at the base of the whole Silurian series. As
such it had been pointed out to me in 1841, in the valley of the Mohawk, by
Mr. James Hall[vii-A], and its position was correctly described by Mr.
Emmons, on the borders of Lake Champlain, where I examined it in 1842. It
has there the character of a shallow-water deposit, ripple-marked
throughout a considerable thickness, and full of a species of Lingula. The
flat valves of this shell, of a dark colour, are so numerous, and so
arranged in horizontal layers, as to play the part of mica, causing the
rock to divide into laminae, as in some micaceous sandstones.
When I mentioned this rock in my Travels[vii-B] as occurring between
Kingston and Montreal, (the same in which the Chelonian foot-prints have
since been found,) I spoke of it as the lowest member of the Lower Silurian
series; but no traces of any organic being of a higher grade than the
Lingula had then been seen in it, and I called attention to the singular
fact, that the oldest fossil form then known in the world, was a marine
shell strictly referable to a genus now existing.
Early in the year 1851, Mr. Logan laid before the Geological Society of
London a slab of this sandstone from Beauharnais, containing no less than
twenty-eight foot-prints of the fore and hind feet of a quadruped, and six
casts in plaster of Paris, exhibiting a continuation of the same trail.
Each cast contained from twenty-six to twenty-eight impressions with a
median channel equidistant from the two parallel rows of foot-prints, the
one made by the feet of the right side, the other by those of the left. In
these specimens a greater number of successive foot-marks belonging to one
and the same series were displayed than had ever before been observed in
any rock ancient or modern. Mr. Abraham has inferred that the breadth of
the quadruped was from five to seven inches. A detailed account of the
trail was published by Professor Owen, in April 1851, from which the
following extracts are made.
"The foot-prints are in pairs, and the pairs extend in two parallel series,
with a channel exactly midway between the right and left series. The pairs
of the same side succeed each other at intervals, varying from one inch and
a half to two inches and a half, the common distance being about two
inches. The interval between the right and left pairs, measured from the
inner border of the small prints, is three inches and a half, and from the
outer border of the exterior or large prints, is seven inches. The shallow
median track is one inch and a quarter in breadth, varying in depth, but
not in its relative position to the right and left foot prints."
"The inference to be deduced from these characters is, that the impressions
were made by a quadruped with the hind feet larger and somewhat wider apart
than the fore feet, with both hind and fore feet either very short, or
prevented by some other part of the animal's structure from making long
steps; and with the limbs of the right side wide apart from those of the
left; consequently, that the quadruped had a broad trunk in proportion to
its length, supported on limbs either short, or capable only of short
steps, and with rounded and stumpy feet, not provided with long claws.
There are faint traces of a fine reticulate pattern of the cuticle of the
sole at the bottom of some of the foot-prints on one portion of the
sandstone; and the surface of the sand is generally smoother there than
where not impressed, which, with the rising of the sand at the border of
the prints, indicates the weight of the impressing body. The median groove
may be interpreted as due either to the abdomen or the tail of the animal;
but as there is no indication of any bending or movement of a tail from
side to side, it was probably scooped out of the soft sand by a hard
breast-plate or plastron. If this were so, it may be inferred that
the species was a freshwater or estuary tortoise rather than
a land tortoise."[viii-A]
Previously to this discovery, the trias was the oldest stratum in which
any remains or signs of a Chelonian had been detected. Numerous other
trails have since been observed (1850-51) in various localities in Canada,
all in the same very ancient fossiliferous rock; and Mr. Logan, who has
visited the spots, will shortly publish a description of the phenomena.
_Chelonian foot-prints in Old Red Sandstone, Morayshire._--Captain Lambart
Brickenden has just communicated to the Geological Society of London a
drawing and description of a continuous series of no less than thirty-four
foot-prints of a quadruped observed in the course of last year (1850), on a
slab of sandstone quarried at Cummingstone, near Elgin, in Morayshire, a
rock which has always been considered as an upper member of the Devonian or
"Old Red."[ix-A] A part of the track, the course of which was from A to B,
is represented in the annexed woodcut, fig. 521. The foot-prints are in
pairs, forming two parallel rows, which are somewhat less distant from each
other than those of the Lower Silurian tortoise of Canada above mentioned.
The stride, on the other hand, is four inches, or twice that of the
Beauharnais Chelonian. The hind foot is exactly of the same size,
being one inch in diameter, and larger than the fore foot in the
proportion of four to three.
[Illustration: Fig. 521. Scale one-sixth the original size.
Part of the trail of a (Chelonian?) quadruped from the Old Red Sandstone of
Cummingstone, near Elgin, Morayshire.--Captain Brickenden.]
_Skeleton of a reptile, allied to the Batrachians, in the Old Red Sandstone
of Morayshire._--Mr. Patrick Duff, author of a "Sketch of the Geology of
Morayshire" (Elgin, 1842), obtained recently (October, 1851), from the rock
above alluded to, the first example ever seen of the skeleton of a reptile
in the Old Red Sandstone. He has kindly allowed me to give a figure of this
fossil, of which Dr. Mantell has drawn up a detailed osteological account
for publication in the "Journal of the Geological Society of London." The
bones in this specimen have decomposed, but the natural position of almost
all of them can be seen, and nearly perfect casts of their form taken from
the hollow moulds which they have left. The matrix is a fine-grained,
whitish sandstone, with a cement of carbonate of lime. The skeleton
exhibits the general characters of the Lacertians, blended with
peculiarities that are Batrachian. Hence Dr. Mantell infers that this
reptile was either a freshwater Batrachian, resembling the Triton, or a
small terrestrial Lizard. Slight indications are visible of very minute
conical teeth. Captain Brickenden, who is well acquainted with the geology
of that part of Scotland, informs me that this fossil was found in the Hill
of Spynie, north of the town of Elgin, in a rock quarried for building, and
the same in which the Chelonian foot-prints, alluded to in the last page,
occur. The skeleton is about four and a half inches in length, but part of
the tail is concealed in the rock. Dr. Mantell has proposed for it the
generic name of Telerpeton, from +tele+, afar off, and +herpeton+, a
reptile; while the specific name Elginense commemorates the principal place
near which it was obtained.
[Illustration: Fig. 522. Natural size. _Telerpeton Elginense._ (Mantell.)
Reptile of Old Red Sandstone, from near Elgin, Morayshire.]
_Eggs of Batrachians (?) in the Old Red Sandstone of Scotland._--At page
344. of this work I have given two figures (figs. 397 and 398.) of small
groups of eggs, very common in the shales and sandstones of the "Old Red"
of Kincardineshire, Forfarshire, and Fife. I threw out as a conjecture,
that they might belong to gasteropodous mollusca, like those represented in
fig. 399. p. 345.; but Dr. Mantell, some years ago, showed me a small
bundle of the dried-up eggs of the common English frog (see fig. 524 _a_.),
black and carbonaceous, and so identical in appearance with the fossils in
question, that he suggested the probability of these last being of
Batrachian origin. The plants by which they are often accompanied (fig.
398. p. 344.), I formerly supposed to be Fuci, but Mr. Bunbury tells me
that their grass-like form agrees well with the idea of their being
freshwater, and of the family Fluviales.
The absence of all shells, so far as our researches have yet gone, in the
slates and sandstones of Scotland above alluded to, raises a presumption
against their marine origin, and a still stronger one against the fossil
eggs being those of Gasteropoda. It is well known that a single female of
the Batrachian tribe ejects annually an astonishing quantity of spawn. Mr.
Newport, author of many accurate researches into the metamorphoses of the
Amphibia, having examined my fossils from Forfarshire, concurs in Dr.
Mantell's opinion that the clusters of eggs (figs. 397. 398. p. 344.) may
be those of frogs; while other larger ones, occurring singly or in pairs in
the same slates, and often attached to a leaf, may be the ova of a gigantic
Triton or Salamander. (See figs. 523, 524, 525.) I may observe that the
subdivision of the Old Red Sandstone, in which these plants and ova occur
(No. 4. of the section, fig. 62. p. 48.), is considerably lower in position
than the rock in which the Telerpeton of Elgin is imbedded.
[Illustration: Fig. 523. Fossil.--Old Red.
Fig. 523. Slab of Old Red Sandstone, Forfarshire, with eggs of Batrachians.
_a._ Ova in a carbonized state.
_b._ Egg cells; the ova shed.]
[Illustration: Fig. 524. Recent.
Fig. 524. Eggs of the common frog, _Rana temporaria_, in a carbonized
state, from a dried-up pond in Clapham Common.
_a._ The ova.
_b._ A transverse section of the mass exhibiting the form of the
egg-cells.]
[Illustration: Fig. 525. Eggs of Batrachians.--Old Red Sandstone.
Fig. 525. Shale of Old Red Sandstone, or Devonian, Forfarshire, with
impression of plants and eggs of Batrachians.
_a._ Two pair of ova resembling those large Salamanders or Tritons
on the same leaf.
_b b._ Detached ova.
_c._ Egg-cells of frogs or _Ranina_.]
_Foot-prints of Lower Carboniferous reptiles in the United States._--I have
stated, at p. 340., that in 1849, Mr. Isaac Lea observed the foot-marks of
a large reptile in the lowest beds of the coal formation at Pottsville,
about seventy miles N.E. of Philadelphia. These researches have since been
carried farther by Professor H. D. Rogers, in the same region of
anthracitic coal, lying on the eastern flank of the Alleghany Mountains.
Beneath the productive coal-measures of that country occurs a dense mass of
red shales and sandstones, which correspond nearly in position to the
millstone grit and Mountain Limestone of the south-east of England. In
these beds foot-prints, referred to three species of quadrupeds, have
lately been detected, all of them five-toed and in double rows, with an
opposite symmetry, as if made by right and left feet, while they likewise
display the alternation of fore foot and hind foot. One species, the
largest of the three, presents a diameter for each foot-print of about two
inches, and shows the fore and hind feet to be nearly equal in dimensions.
It exhibits a length of stride of about nine inches, and a breadth between
the right and left treads of nearly four inches. The impressions of the
hind feet are but little in the rear of the fore feet. The animal which
made them is supposed to have been allied to a Saurian, rather than to a
Batrachian or Chelonian; but more information is required before so
difficult a point can be decided. With these foot-marks were seen shrinkage
cracks, such as are caused by the sun's heat in mud, and rain-spots, with
the signs of the trickling of water on a wet, sandy beach; all confirming
the conclusion derived from the foot-prints, that the quadrupeds belonged
to air-breathers, and not to aquatic races.[xii-A] The Cheirotherian
foot-prints, figured by me at p. 338., in which the fore and hind feet are
very unequal in size, betoken a distinct genus, and occur in the midst of
the productive coal measures, being consequently less ancient.
_On Fossil Rain-marks of the Carboniferous Period in North
America._--Having alluded to the spots left by rain on the surface of
carboniferous strata in the Alleghanies, on which quadrupedal foot-prints
are seen, I may mention that similar rain-prints are conspicuous in the
coal measures of Cape Breton, in Nova Scotia, in which Mr. Richard Brown
has described Stigmariae and erect trunks of trees, and where there are
proofs, as stated at p. 324., of many fossil forests ranged one above the
other. In such a region, if anywhere, might we expect to detect evidence of
the fall of rain on a sea-beach, so repeatedly must the conditions of the
same area have oscillated between land and sea. The intercalation of
deposits, containing shells of marine or brackish water, indicate the
constant proximity of a body of salt water when the clays which supported
the upright trees were formed. In the course of 1851, Mr. Brown had the
kindness to send me some greenish slates from Sydney, Cape Breton, on which
are imprinted very delicate impressions of rain-drops, with several
worm-tracks (_a_, _b_, fig. 526.), such as usually accompany rain-marks on
the recent mud of the Bay of Fundy, and other modern beaches.[xii-B]
[Illustration: Fig. 526. Carboniferous rain-prints with worm-tracks (_a_,
_b_) on green shale, from Cape Breton, Nova Scotia.]
[Illustration: Fig. 527. Casts of rain-prints on a portion of the
same slab, No. 526. seen on the under side of an incumbent layer
of arenaceous shale.
The arrow represents the direction of the shower.]
[Illustration: Fig. 528. Casts of carboniferous rain-prints and
shrinkage-cracks, (_a_) on the under side of a layer of sandstone, Cape
Breton, Nova Scotia.]
The casts of rain-prints, in figs. 527. and 528., project from the under
side of two layers, occurring at different levels, the one a sandy shale,
resting on the green shale (fig. 526.), the other a sandstone presenting a
similar warty or blistered surface, on which are also observable some small
ridges as at _a_, which stand out in relief, and afford evidence of cracks
formed by the shrinkage of subjacent clay, on which rain had fallen. Many
of the associated sandstones are described by Mr. Brown as ripple-marked.
The great humidity of the climate of the coal period had been previously
inferred from the nature of its vegetation and the continuity of its
forests for hundreds of miles; but it is satisfactory to have at length
obtained such positive proofs of showers of rain, the drops of which
resembled in their average size those which now fall from the clouds. From
such data we may presume that the atmosphere of the carboniferous period
corresponded in density with that now investing the globe, and that
different currents of air varied then as now, in temperature, so as to give
rise, by their mixture, to the condensation of aqueous vapour.
_Triassic Mammifer (Microlestes antiquus Plieninger.)_--In the year 1847,
Professor Plieninger, of Stuttgart, published a description of two fossil
molar teeth, referred by him to a warm-blooded quadruped[xiii-A], which he
obtained from a bone-breccia in Wuertemberg occurring between the lias and
the keuper. As the announcement of so novel a fact has never met with the
attention it deserved, we are indebted to Dr. Jaeger, of Stuttgart, for
having recently reminded us of it in his Memoir on the Fossil Mammalia of
Wuertemberg.[xiii-B]
Fig. 529. represents the tooth first found, taken from the plate published
in 1847, by Professor Plieninger; and fig. 530. is a drawing of the same
executed from the original by Mr. Hermann von Meyer, which he has been
kind enough to send me. Fig. 529. is a second and larger molar, copied from
Dr. Jaeger's plate lxxi., fig. 15.
[Illustration: Fig. 529. _Microlestes antiquus_, Plieninger. Molar tooth
magnified. Upper Trias, Diegerloch, near Stuttgart, Wuertemberg.
_a._ View of inner side?
_b._ same, outer side?
_c._ Same in profile.
_d._ Crown of same.]
[Illustration: Fig. 530. _Microlestes antiquus_, Plien.
View of same molar as No. 529. From a drawing by Herman von Meyer.
_a._ View of inner side?
_b._ Crown of same.]
[Illustration: Fig. 531. Molar of _Microlestes_? Plien. 4 times as large as
fig. 529. From the trias of Diegerloch, Stuttgart.]
Professor Plieninger inferred in 1847, from the double fangs of this tooth
and their unequal size, and from the form and number of the protuberances
or cusps on the flat crowns, that it was the molar of a Mammifer; and
considering it as predaceous, probably insectivorous, he called it
Microlestes, from +mikros+, little, and +lestes+, a beast of prey. Soon
afterwards, he found the second tooth also, at the same locality,
Diegerloch, about two miles to the south-east of Stuttgart. Some of its
cusps are broken, but there seem to have been six of them originally. From
its agreement in general characters, it is supposed by Professor Plieninger
to be referable to the same animal, but as it is four times as big, it may
perhaps have belonged to another allied species. This molar is attached to
the matrix consisting of sandstone, whereas the tooth, No. 529., is
isolated. Several fragments of bone, differing in structure from that of
the associated saurians and fish, and believed to be mammiferous, were
imbedded near them in the same rock.
Mr. Waterhouse, of the British Museum, after studying the annexed figs.
529. 531. and the descriptions of Prof. Plieninger, observes, that not only
the double roots of the teeth and their crowns presenting several cusps,
resemble those of Mammalia, but the cingulum also, or ridge surrounding the
base of that part of the body of the tooth which was exposed or above the
gum, is a character distinguishing them from fish and reptiles. "The
arrangement of the six cusps or tubercles in two rows, in fig. 529., with a
groove or depression between them and the oblong form of the tooth, lead
him, he says, to regard it as a molar of the lower jaw. Both the teeth
differ from those of the Stonesfield Mammalia[xiv-A], but do not supply
sufficient data for determining to what order they belonged. Even in regard
to the Stonesfield jaws, where we possess so much ampler materials, we
cannot safely pronounce on the order."
Professor Plieninger has sent me a cast of the smaller tooth, which
exhibits well the characteristic mammalian test, the double fang; but Mr.
Owen, to whom I have shown it, is not able to recognize its affinity with
any mammalian type, recent or extinct, known to him.
It has already been stated that the stratum in which the above-mentioned
fossils occur is intermediate between the lias and the uppermost member of
the trias. That it is really triassic may be deduced from the following
considerations. In Wuertemberg there are two "bone-beds," one of great
extent, and very rich in the remains of fish and reptiles, which intervenes
between the muschelkalk and keuper, the other, containing the Microlestes,
less extensive and fossiliferous, which rests on the keuper, or superior
member of the trias, and is covered by the sandstone of the lias. The
last-mentioned breccia therefore occupies the same place as the well-known
English "bone-bed" of Axmouth and Aust-cliff near Bristol, which is
shown[xv-A] to include characteristic species of muschelkalk fish, of the
genus Saurichthys, Hybodus, and Gyrolepis. In both the Wuertemberg bone-beds
these three genera are also found, and one of the _species_, Saurichthys
Mougeotii, is common to both the lower and upper breccias, as is also a
remarkable reptile called Nothosaurus mirabilis. The Saurian called Belodon
by H. Von Meyer of the Thecodont family, is another Triassic form,
associated at Diegerloch with Microlestes.
Previous to this discovery of Professor Plieninger, the most ancient of
known fossil Mammalia were those of the Stonesfield slate, a subdivision of
the Lower Oolite[xv-B] no representative of this class having as yet been
met with in the Fuller's earth, or inferior Oolite (see Table, p. 258.),
nor in any member of the lias.
_Thecodont Saurians._--This family of reptiles is common to the Trias and
Permian groups in Germany, and the geologists employed in the government
survey of Great Britain have come to the conclusion, that the rock
containing the two species alluded to at p. 306., and of which the teeth
are represented in figs. 348, 349., ought rather to be referred to the
Trias than to the Permian group.
CRETACEOUS GASTEROPODA.
In speaking of the chalk of Faxoe in Denmark (p. 210.) or the highest
member of the Cretaceous series, I have remarked that it is characterized
by univalve Mollusca, both spiral and patelliform, which are wanting or
rare in the white chalk of Europe. This last statement requires, I find,
some modification. It holds true in regard to certain forms, such as Cypraea
and Oliva, found at Faxoe; but M. A. d'Orbigny enumerates 24 species of
Gasteropoda from the white chalk (Terrain Senonien) of France alone. The
same author describes 134 French species of Gasteropoda from the chloritic
chalk marl and upper greensand (Turonien), 77 from the gault, and 90 from
the lower greensand (Neocomien), in all 325 species of Gasteropoda, from
the cretaceous group below the Maestricht beds. Among these he refers 1 to
the genus Mitra, 17 to Fusus, 17 to Trochus, 4 to Emarginula, and 36 to
Cerithium. Notwithstanding, therefore, the peculiarity of the chambered
univalves of various genera, so abundant in the chalk, the Mollusca of the
period approximate in character to the tertiary and recent Fauna far more
than was formerly supposed.
DICOTYLEDONOUS LEAVES IN LOWER CRETACEOUS STRATA.
M. Adolphe Brongniart when founding his classification of the fossiliferous
strata in reference to their imbedded fossil plants, has placed the
cretaceous group in the same division with the tertiary, that is to say, in
his "Age of Angiosperms."[xvi-A] This arrangement is based on the fact,
that the cretaceous plants display a transition character from the
vegetation of the secondary to that of the tertiary periods. Coniferae and
Cycadeae still flourished as in the preceding oolitic and triassic epochs;
but with these fossils, some well-marked leaves of dicotyledonous trees
referred to several species of the genus Credneria, had been found in
Germany in the Quadersandstein and Plaener-kalk. Still more recently, Dr.
Debey of Aix-la-Chapelle has met with a great variety of other leaves of
dicotyledonous plants in the cretaceous flora[xvi-B], of which he
enumerates no less than 26 species, some of the leaves being from four to
six inches in length, and in a beautiful state of preservation. In the
absence of the organs of fructification and of fossil fruits, the number of
species may be exaggerated; but we may nevertheless affirm, reasoning from
our present data, that in the lower chalk of Aix-la-Chapelle,
Dicotyledonous Angiosperms flourished nearly in equal proportions with
Gymnosperms; a fact of great significance, as some geologists had wished to
connect the rarity of dicotyledonous trees with a peculiarity in the state
of the atmosphere in the earlier ages of the planet, imagining that a
denser air and noxious gases, especially carbonic acid in excess, were
adverse to the prevalence, not only of the quick-breathing classes of
animals, (mammalia and birds,) but to a flora like that now existing, while
it favoured the predominance of reptile life, and a cryptogamic and
gymnospermous flora. The co-existence, therefore, of dicotyledonous
angiosperms in abundance with Cycads and Coniferae, and with a rich
reptilian fauna comprising the Iguanodon, Ichthyosaurus, Pliosaurus, and
Pterodactyl, in the lower cretaceous series tends, like the oolitic
mammalia of Stonesfield and Stuttgart, and the triassic birds of
Connecticut, to dispel the idea of a meteorological state of things in the
secondary periods widely distinct from that now prevailing.
_General remarks._--In the preliminary chapters of "The Principles of
Geology," in the first and subsequent editions, I have considered the
question, how far the changes of the earth's crust in past times confirm or
invalidate the popular hypothesis of a gradual improvement in the
habitable condition of the planet, accompanied by a contemporaneous
development and progression in organic life. It had long been a favourite
theory, that in the earlier ages to which we can carry back our geological
researches, the earth was shaken by more frequent and terrible earthquakes
than now, and that there was no certainty nor stability in the order of the
natural world. A few sea-weeds and zoophytes, or plants and animals of the
simplest organization, were alone capable of existing in a state of things
so unfixed and unstable. But in proportion as the conditions of existence
improved, and great convulsions and catastrophes became rarer and more
partial, flowering plants were added to the cryptogamic class, and by the
introduction of more and more perfect species, a varied and complex flora
was at last established. In like manner, in the animal kingdom, the
zoophyte, the brachiopod, the cephalopod, the fish, the reptile, the bird,
and the warm-blooded quadruped made their entrance into the earth, one
after the other, until finally, after the close of the tertiary period,
came the quadrumanous mammalia, most nearly resembling man in outward form
and internal structure, and followed soon afterwards, if not accompanied at
first, by the human race itself.
The objections which, in 1830, I urged against this doctrine[xvii-A], in so
far as relates to the passage of the earth from a chaotic to a more settled
condition, have since been embraced by a large and steadily increasing
school of geologists; and in reference to the animate world, it will be
seen, on comparing the present state of our knowledge with that which we
possessed twenty years ago, how fully I was justified in declaring the
insufficiency of the data on which such bold generalizations, respecting
progressive development, were based. Speaking of the absence, from the
tertiary formations, of fossil Quadrumana, I observed, in 1830, that "we
had no right to expect to have detected any remains of tribes which live in
trees, until we knew more of those quadrupeds which frequent marshes,
rivers, and the borders of lakes, such being usually first met with in a
fossil state."[xvii-B] I also added, "if we are led to infer, from the
presence of crocodiles and turtles in the London clay, and from the
cocoa-nuts and spices found in the isle of Sheppey, that at the period when
our older tertiary strata were formed, the climate was hot enough for the
Quadrumana, we nevertheless could not hope to discover any of their
skeletons, until we had made considerable progress in ascertaining what
were the contemporary Pachydermata; and not one of these has been
discovered as yet in any strata of this epoch in England."
Nine years afterwards, when these fossil Pachyderms had been found in the
London clay, and in the sandy strata at its base, the remains of a monkey,
of the genus Macacus, were detected near Woodbridge, in Suffolk; and other
Quadrumana had been met with, a short time previously, in different stages
of the tertiary series, in India, France, and Brazil.
When we consider the small area of the earth's surface hitherto examined
geologically, and our scanty acquaintance with the fossil Vertebrata, even
of the environs of great European capitals, it is truly surprising that any
naturalist should be rash enough to assume that the Lower Eocene deposits
mark the era of the first creation of Quadrumana. It is, however, still
more unphilosophical to infer from a single extinct species of this order,
obtained in a latitude far from the tropics, that the Eocene Quadrumana had
not attained as high a grade of organization as those of our own times,
when the naturalist is acquainted with all, or nearly all, the species of
monkeys, apes and orangs which are contemporary with man.
To return to the year 1830, Mammalia had not then been traced to rocks of
higher antiquity than the Stonesfield Oolite, whereas we have just seen
that memorials of this class have at length made their appearance in the
Trias of Germany. In 1830 birds had been discovered no lower in the series
than the Paris gypsum, or Middle Eocene. Their bones have now been found
both in England and the Swiss Alps in the Lower Eocene, and their existence
has been established by foot-prints in the triassic epoch in North America
(p. 297.). Reptiles in 1830 had not been detected in rocks older than the
Magnesian limestone, or Permian formation; whereas the skeletons of four
species have since been brought to light (see p. 336.) in the
coal-measures, and one in the Old Red sandstone, of Europe, while the
footprints of three or four more have been observed in carboniferous rocks
of North America, not to mention the chelonian trail above described, from
the most ancient of the fossiliferous rocks of Canada, the "Potsdam
Sandstone," which lies at the base of the Lower Silurian system.
(See above, p. vii.)
Lastly, the remains of fish, which in 1830 were scarcely recognized in
deposits older than the coal, have now been found plentifully in the
Devonian, and sparingly in the Silurian, strata; though not in any
formation of such high antiquity as the Chelonian of Montreal.
Previously to the discovery last mentioned, it was by no means uncommon for
paleontologists to speak with confidence of fish as having been created
before reptiles. It was deemed reasonable to suppose that the introduction
of a particular class or order of beings into the planet coincided, in
date, with the age of the oldest rock to which the remains of that class or
order happened then to have been traced back. To be consistent with
themselves, the same naturalists ought now to take for granted that
reptiles were called into existence before fish. This they will not do,
because such a conclusion would militate against their favourite hypothesis
of an ascending scale, according to which Nature "evolved the organic
world," rendering it more and more perfect in the lapse of ages.
In our efforts to arrive at sound theoretical views on such a question, it
would seem most natural to turn to the marine invertebrate animals as to a
class affording the most complete series of monuments that have come down
to us, and where we can find corresponding terms of comparison, in strata
of every age. If, in this more complete series of her archives, Nature had
really exhibited a more simple grade of organization in fossils of the
remotest antiquity, we might have suspected that there was some foundation
of facts in the theory of successive development. But what do we find? In
the Lower Silurian there is a full representation of the Radiata, Mollusca,
and Articulata proper to the sea. The marine Fauna, indeed, in those three
classes, is so rich as almost to imply a more perfect development than that
which now peoples the ocean. Thus, in the great division of the Radiata, we
find asteroid and helianthoid zoophytes, besides crinoid and cystidean
echinoderms. In the Mollusca of the same most ancient epoch M. Barrande
enumerates, in Bohemia alone, the astonishing number of 253 species of
Cephalopoda. In the Articulata we have the crustaceans, represented by more
than 200 species of Trilobites, not to mention other genera.
It is only then, in reference to the Vertebrata, that the argument of
degeneracy in proportion as we trace fossils back to older formations can
be maintained; and the dogma rests mainly for its support on negative
evidence, whether deduced from the entire absence of the fossil
representatives of certain classes in particular rocks, or the low grade of
the first few species of a class which chance has thrown in our way.
The scarcity of all memorials of birds in strata below the Eocene, has been
a subject of surprise to some geologists. The bones formerly referred to
birds in the Wealden and Chalk, are now admitted to have belonged to flying
reptiles, of various sizes, one of them from the Kentish chalk so large as
to have measured 16 feet 6 inches from tip to tip of its outstretched
wings. Whether some elongated bones of the Stonesfield Oolite should be
referred to birds, which they seem greatly to resemble in microscopic
structure or to Pterodactyles, is a point now under investigation. If it
should be proved that no osseous remains of the class Aves have hitherto
been derived from any secondary or primary formation, we must not too
hastily conclude that birds were even scarce in these periods. The rarity
of such fossils in the Eocene marine strata is very striking. In 1846,
Professor Owen, in his "History of the Fossil Mammalia and Birds of Great
Britain," was unable to obtain more than four or five fragments of bones
and skulls of birds from the London Clay, by the aid of which four species
were recognized. Even so recently, therefore, as 1846, as much was known of
the Mammalia of the Stonesfield Oolite, as of the ornithic Fauna of our
English Eocene deposits.
To reason correctly on the value of negative facts in this branch of
Paleontology, we must first have ascertained how far the relics of birds
are now becoming preserved in new strata, whether marine, fluviatile, or
lacustrine. I have explained, in the "Principles of Geology," that the
imbedding of the bones of living birds in deposits now in progress in
inland lakes appears to be extremely rare. In the shell-marl of Scotland,
which is made up bodily of the shells of the genera Limneus, Planorbis,
Succinea, and Valvata, and in which the skeletons of deer and oxen abound,
we find no bones of birds. Yet we know that, before the lakes were drained
which yield this marl used in agriculture, the surface of the water and the
bordering swamps were covered with wild ducks, herons, and other fowl. They
left no memorials behind them, because, if they perished on the land, their
bodies decomposed or became the prey of carnivorous animals; if on the
water, they were buoyant and floated till they were devoured by predaceous
fish or birds. The same causes of obliteration have no power to efface the
foot-prints which the same creatures may leave, under favourable
circumstances, imprinted on an ancient mud-bank or shore, on which new
strata may be from time to time thrown down. In the red mud of recent
origin spread over wide areas by the high tides of the bay of Fundy,
innumerable foot-tracks of recent birds (Tringa minuta) are preserved in
successive layers, and hardened by the sun. Yet none of the bones of these
birds, though diligently searched for, have yet been discovered in digging
trenches through the red mud. It is true that, in a few spots, the bones of
birds have been met with plentifully in the older tertiary strata, but
always in rocks of freshwater origin, such as the Paris gypsum or the
lacustrine limestone of the Limagne d'Auvergne. In strata of the same
age, in Belgium and other European countries, or in the United States,
where no less careful search has been made, few, if any, fossil birds
have come to light.
We ought, therefore, most clearly to perceive that it is no part of the
plan of Nature to hand down to after times a complete or systematic record
of the former history of the animate world. The preservation of the relics,
even of aquatic tribes of animals, is an exception to the general rule,
although time may so multiply exceptional cases that they may seem to
constitute the rule; and may thus impose upon the imagination, leading us
to infer the non-existence of creatures of which no monuments are extant.
Hitherto our acquaintance with the birds, and even the Mammalia, of the
Eocene period has depended, almost everywhere, on single specimens, or on a
few individuals found in one spot. It has therefore depended on what we
commonly call chance; and we must not wonder if the casual discovery of a
tertiary, secondary, or primary rock, rich in fossil impressions of the
foot-prints of birds or quadrupeds, should modify or suddenly overthrow all
theories based on negative facts.
The chief reason why we meet more readily with the remains of every class
in tertiary than in secondary strata, is simply that the older rocks are
more and more exclusively marine in proportion as we depart farther and
farther from periods during which the existing continents were built up.
The secondary and primary formations are, for the most part, marine,--not
because the ocean was more universal in past times, but because the epochs
which preceded the Eocene were so distant from our own, that entire
continents have been since submerged.
I have alluded at p. 299. to Mr. Darwin's account of the South American
Ostriches, seen on the coast of Buenos Ayres, walking at low water over
extensive mud-banks, which are then dry, for the sake of feeding on small
fish. Perhaps no bird of such perfect organization as the eagle or vulture
may ever accompany these ostriches. Certainly, we cannot expect the condor
of the Andes to leave its trail on such a shore; and no traveller, after
searching for footprints along the whole eastern coast of South America,
would venture to speculate, from the results of such an inquiry, on the
extent, variety, or development of the feathered Fauna of the interior
of that continent.
The absence of Cetacea from rocks older than the Eocene has been frequently
adduced as lending countenance to the theory of the late appearance of the
highest class of Vertebrata on the earth. That we have hitherto failed to
detect them in the Oolite or Trias, does not imply, as we have now seen,
that Mammalia were not then created. Even in the Eocene strata of Europe,
the discovery of Cetaceans has never kept pace with that of land
quadrupeds. The only instance cited in Great Britain is a species of
Monodon, from the London clay, of doubtful authenticity as to its
geological position. On the other hand, the gigantic Zeuglodon of North
America (see p. 207.), occurs abundantly in the Middle Eocene strata
of Georgia and Alabama, from which as yet no bones of land-quadrupeds
have been obtained.
Professor Sedgwick states in a recent work[xxi-A], that he possesses in the
Woodwardian Museum, a mass of anchylosed cervical vertebrae of a whale which
he found near Ely, and which he believes to have been washed out of the
Kimmeridge clay, a member of the Upper Oolite; but its true geological site
is not well determined. It differs, says Professor Owen, from any other
known fossil or recent whale.
In the present imperfect state then of our information, we can scarcely say
more than that the Cetacea may have been scarce, in the secondary and
primary periods. It is quite conceivable that when aquatic saurians, some
of them carnivorous, like the Ichthyosaurus, were swarming in the sea, and
when there were large herbivorous reptiles, like the Iguanodon, on the
land, such reptiles may, to a certain extent, have superseded the Cetacea,
and discharged their functions in the animal economy.
The views which I proposed originally in the Principles of Geology in
opposition to the theory of progressive development may be thus briefly
explained. From the earliest period at which plants and animals can be
proved to have existed, there has been a continual change going on in the
position of land and sea, accompanied by great fluctuations of climate. To
these ever-varying geographical and climatal conditions the state of the
animate world has been unceasingly adapted. No satisfactory proof has yet
been discovered of the gradual passage of the earth from a chaotic to a
more habitable state, nor of a law of progressive development governing the
extinction and renovation of species, and causing the Fauna and Flora to
pass from an embryonic to a more perfect condition, from a simple to a
more complex organization.
The principle of adaptation above alluded to, appears to have been
analogous to that which now peoples the arctic, temperate, and tropical
regions contemporaneously with distinct assemblages of species and
genera, or which independently of mere temperature gives rise to a
predominance of the marsupial tribe of quadrupeds in Australia, and
of the placental tribe in Asia and Europe, or to a profusion of reptiles
without mammalia in the Galapagos Archipelago, and of mammalia without
reptiles in Greenland.[xxii-A]
This theory implies, almost necessarily, a very unequal representation at
successive periods of the principal classes and orders of plants and
animals, if not in the whole globe, at least throughout very wide areas.
Thus, for example, the proportional number of genera, species, and
individuals in the vertebrate class may differ, in two different and
distinct epochs, to an extent unparalleled by any two contemporaneous
Faunas, because in the course of millions of ages, the contrast of climate
and geographical conditions may exceed the difference now observable in
polar and equatorial latitudes.
I shall conclude by observing, that if the doctrine of successive
development had been paleontologically true, as the new discoveries above
enumerated show that it is not; if the sponge, the cephalopod, the fish,
the reptile, the bird, and the mammifer had followed each other in regular
chronological order--the creation of each class being separated from the
other by vast intervals of time; and if it were admitted that Man was
created last of all, still we should by no means be able to recognize, in
his entrance upon the earth, the last term of one and the same series of
progressive developments. For the superiority of Man, as compared to the
irrational mammalia, is one of kind, rather than of degree, consisting in a
rational and moral nature, with an intellect capable of indefinite
progression, and not in the perfection of his physical organization, or
those instincts in which he resembles the brutes. He may be considered as a
link in the same unbroken chain of being, if we regard him simply as a new
species--a member of the animal kingdom--subject, like other species, to
certain fixed and invariable laws, and adapted like them to the state of
the animate and inanimate world prevailing at the time of his creation.
Physically considered, he may form part of an indefinite series of
terrestrial changes past, present, and to come; but morally and
intellectually he may belong to another system of things--of things
immaterial--a system which is not permitted to interrupt or disturb the
course of the material world, or the laws which govern its changes.[xxii-B]
FOOTNOTES:
[vii-A] Travels in North America by the Author, vol. ii. chap. 22.
[vii-B] Ibid. 1842.
[viii-A] Quart. Journ. Geol. Soc. 1851, vol. vii. p. 250.
[ix-A] The generally received determination of the age of this rock is
probably correct; but as there are no overlying coal-measures and no
well-known Devonian fossils in the whitish stone of Elgin, and as I have
not personally explored the geology of that district, I cannot speak as
confidently as in regard to the age of the Montreal Chelonian.
[xii-A] H. D. Rogers, Proceedings of Amer. Assoc. of Science, Albany, 1851.
[xii-B] See Memoir by the Author, Quart. Journ. Geol. Soc., vol.
vii. p. 240.
[xiii-A] Wuertembergisch. Naturwissen. Jahreshefte, 3 Jahr. Stuttgart, 1847.
[xiii-B] Nov. Act. Acad. Caesar. Leopold. Nat. Cur. 1850, p. 902. For
figures, see ibid. plate xxi. figs. 14, 15, 16, 17.
[xiv-A] See Manual, p. 268.
[xv-A] Manual, p. 289.
[xv-B] Ibid. p. 268.
[xvi-A] For Terminology, see Note, p. 223.
[xvi-B] Quart. Journ. vol. vii. Memoirs, p. 111.
[xvii-A] Principles, 1st ed. chaps. v. and ix.
[xvii-B] Ibid. p. 153.
[xxi-A] Preface to 5th ed. of Studies of University of Cambridge.
[xxii-A] Principles, 4th ed. 1835, vol. i. p. 231, and vol. i. chap. 9.
subsequent ed.
[xxii-B] In my Anniversary Address, for 1851, to the Geological Society,
the reader will find a full discussion of the facts and arguments which
bear on the theory of progressive development.--Quart. Journ. Geol.
Soc., vol. vii.
CONTENTS.
CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.
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--Leading division of the work Page 1
CHAPTER II.
AQUEOUS ROCKS--THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral composition of strata--Arenaceous
rocks--Argillaceous--Calcareous--Gypsum--Forms of
stratification--Original horizontality--Thinning out--Diagonal
arrangement--Ripple mark 10
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--Wood bored by Teredina--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 21
CHAPTER IV.
CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and mechanical deposits--Cementing together of
particles--Hardening by exposure to air--Concretionary
nodules--Consolidating effects of pressure--Mineralization of organic
remains--Impressions and casts how formed--Fossil wood--Goeppert'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 33
CHAPTER V.
ELEVATION OF STRATA ABOVE THE SEA--HORIZONTAL AND INCLINED
STRATIFICATION.
Why the position of marine strata, above the level of the sea, 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--Bent strata in
east of Scotland--Theory of folding by lateral movement--Creeps--Dip and
strike--Structure of the Jura--Various forms of outcrop--Rocks broken by
flexure--Inverted position of disturbed strata--Unconformable
stratification--Hutton and Playfair on the same--Fractures of
strata--Polished surfaces--Faults--Appearance of repeated alternations
produced by them--Origin of great faults Page 44
CHAPTER VI.
DENUDATION.
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--Coalbrook Dale--Denuding power of the ocean during the emergence
of land--Origin of Valleys--Obliteration of sea-cliffs--Inland
sea-cliffs and terraces in the Morea and Sicily--Limestone pillars at
St. Mihiel, in France--in Canada--in the Bermudas 66
CHAPTER VII.
ALLUVIUM.
Alluvium described--Due to complicated causes--Of various ages, as shown
in Auvergne--How distinguished from rocks _in
situ_--River-terraces--Parallel roads of Glen Roy--Various theories
respecting their origin 79
CHAPTER VIII.
CHRONOLOGICAL CLASSIFICATION 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--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--Explanation of the
chronological nomenclature adopted in this work, so far as regards
primary, secondary, and tertiary periods 89
CHAPTER IX.
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--Distinct provinces of
indigenous species--Great extent of single provinces--Similar laws
prevailed at successive geological periods--Relative importance of
mineral and palaeontological characters--Test of age by included
fragments--Frequent absence of strata of intervening periods--Principal
groups of strata in western Europe 96
CHAPTER X.
CLASSIFICATION OF TERTIARY FORMATIONS.--POST-PLIOCENE GROUP.
General principles of classification of tertiary strata--Detached
formations scattered over Europe--Strata of Paris and London--More
modern groups--Peculiar difficulties in determining the chronology of
tertiary formations--Increasing proportion of living species of shells
in strata of newer origin--Terms Eocene, Miocene, and
Pliocene--Post-Pliocene strata--Recent or human period--Older
Post-Pliocene formations of Naples, Uddevalla, and Norway--Ancient
upraised delta of the Mississippi--Loess of the Rhine Page 104
CHAPTER XI.
NEWER PLIOCENE PERIOD.--BOULDER FORMATION.
Drift of Scandinavia, northern Germany, and Russia--Its northern
origin--Not all of the same age--Fundamental rocks polished, grooved,
and scratched--Action of glaciers and icebergs--Fossil shells of glacial
period--Drift of eastern Norfolk--Associated freshwater deposit--Bent
and folded strata lying on undisturbed beds--Shells on Moel
Tryfane--Ancient glaciers of North Wales--Irish drift 121
CHAPTER XII.
BOULDER FORMATION--_continued_.
Difficulty of interpreting the phenomena of drift before the glacial
hypothesis was adopted--Effects of intense cold in augmenting the
quantity of alluvium--Analogy of erratics and scored rocks in North
America and Europe--Bayfield on shells in drift of Canada--Great
subsidence and re-elevation of land from the sea, required to account
for glacial appearances--Why organic remains so rare in northern
drift--Mastodon giganteus in United States--Many shells and some
quadrupeds survived the glacial cold--Alps an independent centre of
dispersion of erratics--Alpine blocks on the Jura--Recent transportation
of erratics from the Andes to Chiloe--Meteorite in Asiatic drift 131
CHAPTER XIII.
NEWER PLIOCENE STRATA AND CAVERN DEPOSITS.
Chronological classification of Pleistocene formations, why
difficult--Freshwater deposits in valley of Thames--In Norfolk
cliffs--In Patagonia--Comparative longevity of species in the mammalia
and testacea--Fluvio-marine crag of Norwich--Newer Pliocene strata of
Sicily--Limestone of great thickness and elevation--Alternation of
marine and volcanic formations--Proofs of slow accumulation--Great
geographical changes in Sicily since the living fauna and flora began to
exist--Osseous breccias and cavern deposits--Sicily--Kirkdale--Origin of
stalactite--Australian cave-breccias--Geographical relationship of the
provinces of living vertebrata and those of the fossil species of the
Pliocene periods--Extinct struthious birds of New Zealand--Teeth of
fossil quadrupeds 146
CHAPTER XIV.
OLDER PLIOCENE AND MIOCENE FORMATIONS.
Strata of Suffolk termed Red and Coralline crag--Fossils, and proportion
of recent species--Depth of sea and climate--Reference of Suffolk crag
to the older Pliocene period--Migration of many species of shells
southwards during the glacial period--Fossil whales--Subapennine
beds--Asti, Sienna, Rome--Miocene formations--Faluns of Touraine--Depth
of sea and littoral character of fauna--Tropical climate implied by the
testacea--Proportion of recent species of shells--Faluns more ancient
than the Suffolk crag--Miocene strata of Bordeaux and Piedmont--Molasse
of Switzerland--Tertiary strata of Lisbon--Older Pliocene and Miocene
formations in the United States--Sewalik Hills in India 161
CHAPTER XV.
UPPER EOCENE FORMATIONS.
Eocene areas in England and France--Tabular view of French Eocene
strata--Upper Eocene group of the Paris basin--Same beds in Belgium and
at Berlin--Mayence tertiary strata--Freshwater upper Eocene of Central
France--Series of geographical changes since the land emerged in
Auvergne--Mineral character an uncertain test of age--Marls containing
Cypris--Oolite of Eocene period--Indusial limestone and its
origin--Fossil mammalia of the upper Eocene strata in
Auvergne--Freshwater strata of the Cantal, calcareous and siliceous--Its
resemblance to chalk--Proofs of gradual deposition of strata 174
CHAPTER XVI.
EOCENE FORMATIONS--_continued_.
Subdivisions of the Eocene group in the Paris basin--Gypseous
series--Extinct quadrupeds--Impulse given to geology by Cuvier's
osteological discoveries--Shelly sands called sables moyens--Calcaire
grossier--Miliolites--Calcaire siliceux--Lower Eocene in France--Lits
coquilliers--Sands and plastic clay--English Eocene strata--Freshwater
and fluvio-marine beds--Barton beds--Bagshot and Bracklesham
division--Large ophidians and saurians--Lower Eocene and London Clay
proper--Fossil plants and shells--Strata of Kyson in Suffolk--Fossil
monkey and opossum--Mottled clays and sand below London Clay--Nummulitic
formation of Alps and Pyrenees--Its wide geographical extent--Eocene
strata in the United States--Section at Claiborne, Alabama--Colossal
cetacean--Orbitoid limestone--Burr stone 190
CHAPTER XVII.
CRETACEOUS GROUP.
Divisions of the cretaceous series in North-Western Europe--Upper
cretaceous strata--Maestricht beds--Chalk of Faxoe--White
chalk--Characteristic fossils--Extinct cephalopoda--Sponges and corals
of the chalk--Signs of open and deep sea--White area of white chalk--Its
origin from corals and shells--Single pebbles in chalk--Siliceous
sandstone in Germany contemporaneous with white chalk--Upper greensand
and gault--Lower cretaceous strata--Atherfield section, Isle of
Wight--Chalk of South of Europe--Hippurite limestone--Cretaceous
Flora--Chalk of United States 209
CHAPTER XVIII.
WEALDEN GROUP.
The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck
Beds--Intercalated between two marine formations--Weald clay and
Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata
formed in shallow water--Brackish water-beds--Upper, middle, and lower
Purbeck--Alternations of brackish water, freshwater, and land--Dirt-bed,
or ancient soil--Distinct species of fossils in each subdivision of the
Wealden--Lapse of time implied--Plants and insects of
Wealden--Geographical extent of Wealden--Its relation to the cretaceous
and oolitic periods--Movements in the earth's crust to which it owed its
origin and submergence 225
CHAPTER XIX.
DENUDATION OF THE CHALK AND WEALDEN.
Physical geography of certain districts composed of Cretaceous and
Wealden strata--Lines of inland chalk-cliffs on the Seine in
Normandy--Outstanding pillars and needles of chalk--Denudation of the
chalk and Wealden in Surrey, Kent, and Sussex--Chalk once continuous
from the North to the South Downs--Anticlinal axis and parallel
ridges--Longitudinal and transverse valleys--Chalk escarpments--Rise and
denudation of the strata gradual--Ridges formed by harder, valleys by
softer beds--Why no alluvium, or wreck of the chalk, in the central
district of the Weald--At what periods the Weald valley was
denuded--Land has most prevailed where denudation has been
greatest--Elephant bed, Brighton 238
CHAPTER XX.
OOLITE AND LIAS.
Subdivisions of the Oolitic or Jurassic group--Physical geography of the
Oolite in England and France--Upper Oolite--Portland stone and
fossils--Lithographic stone of Solenhofen--Middle Oolite, coral
rag--Zoophytes--Nerinaean limestone--Diceras limestone--Oxford clay,
Ammonites and Belemnites--Lower Oolite, Crinoideans--Great Oolite and
Bradford clay--Stonesfield slate--Fossil mammalia, placental and
marsupial--Resemblance to an Australian fauna--Doctrine of progressive
development--Collyweston slates--Yorkshire Oolitic coal-field--Brora
coal--Inferior Oolite and fossils 257
CHAPTER XXI.
OOLITE AND LIAS--_continued_.
Mineral character of Lias--Name of Gryphite limestone--Fossil shells and
fish--Ichthyodorulites--Reptiles of the Lias--Ichthyosaur and
Plesiosaur--Marine Reptile of the Galapagos Islands--Sudden destruction
and burial of fossil animals in Lias--Fluvio-marine beds in
Gloucestershire and insect limestone--Origin of the Oolite and Lias, and
of alternating calcareous and argillaceous formations--Oolitic
coal-field of Virginia, in the United States 273
CHAPTER XXII.
TRIAS OR NEW RED SANDSTONE GROUP.
Distinction between New and Old Red Sandstone--Between Upper and Lower
New Red--The Trias and its three divisions--Most largely developed in
Germany--Keuper and its fossils--Muschelkalk--Fossil plants of
Bunter--Triassic group in England--Bone-bed of Axmouth and Aust--Red
Sandstone of Warwickshire and Cheshire--Footsteps of _Chirotherium_ in
England and Germany--Osteology of the _Labyrinthodon_--Identification of
this Batrachian with the Chirotherium--Origin of Red Sandstone and
rock-salt--Hypothesis of saline volcanic exhalations--Theory of the
precipitation of salt from inland lakes or lagoons--Saltness of the Red
Sea--New Red Sandstone in the United States--Fossil footprints of birds
and reptiles in the Valley of the Connecticut--Antiquity of the Red
Sandstone containing them 286
CHAPTER XXIII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Fossils of Magnesian Limestone and Lower New Red distinct from the
Triassic--Term Permian--English and German equivalents--Marine shells
and corals of English Magnesian limestone--Palaeoniscus and other fish
of the marl slate--Thecodont Saurians of dolomitic conglomerate of
Bristol--Zechstein and Rothliegendes of Thuringia--Permian Flora--Its
generic affinity to the carboniferous--Psaronites or tree-ferns 301
CHAPTER XXIV.
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--Section in South Wales--Underclays with
Stigmaria--Carboniferous Flora--Ferns, Lepidodendra, Calamites,
Asterophyllites, Sigillariae, Stigmariae,--Coniferae--Endogens--Absence of
Exogens--Coal, how formed--Erect fossil trees--Parkfield Colliery--St.
Etienne, Coal-field--Oblique trees or snags--Fossil forests in Nova
Scotia--Brackish water and marine strata--Origin of Clay-iron-stone 308
CHAPTER XXV.
CARBONIFEROUS GROUP--_continued_.
Coal-fields of the United States--Section of the country between the
Atlantic and Mississippi--Position of land in the carboniferous period
eastward of the Alleghanies--Mechanically formed rocks thinning out
westward, and limestones thickening--Uniting of many coal-seams into one
thick one--Horizontal coal at Brownsville, Pennsylvania--Vast extent and
continuity of single seams of coal--Ancient river-channel in Forest of
Dean coal-field--Absence of earthy matter in coal--Climate of
carboniferous period--Insects in coal--Rarity of air-breathing
animals--Great number of fossil fish--First discovery of the skeletons
of fossil reptiles--Footprints of reptilians--Mountain limestone--Its
corals and marine shells 326
CHAPTER XXVI.
OLD RED SANDSTONE, OR DEVONIAN GROUP.
Old Red Sandstone of Scotland, and borders of Wales--Fossils usually
rare--"Old Red" in Forfarshire--Ichthyolites of Caithness--Distinct
lithological type of Old Red in Devon and Cornwall--Term
"Devonian"--Organic remains of intermediate character between those of
the Carboniferous and Silurian systems--Corals and shells--Devonian
strata of Westphalia, the Eifel, Russia, and the United States--Coral
reef at Falls of the Ohio--Devonian Flora 342
CHAPTER XXVII.
SILURIAN GROUP.
Silurian strata formerly called transition--Term grauwacke--Subdivisions
of Upper and Lower Silurian--Ludlow formation and fossils--Wenlock
formation, corals and shells--Caradoc and Llandeilo
beds--Graptolites--Lingula--Trilobites--Cystideae--Vast thickness of
Silurian strata in North Wales--Unconformability of Caradoc
sandstone--Silurian strata of the United States--Amount of specific
agreement of fossils with those of Europe--Great number of
brachiopods--Deep-sea origin of Silurian strata--Absence of fluviatile
formations--Mineral character of the most ancient fossiliferous rocks
350
CHAPTER XXVIII.
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 analyses of minerals most
abundant in the volcanic and hypogene rocks 366
CHAPTER XXIX.
VOLCANIC ROCKS--_continued_.
Trap dike--sometimes project--sometimes leave fissures vacant by
decomposition--Branches and veins of trap--Dikes more crystalline in the
centre--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--Inequality in the modifying influence of
dikes--Trap interposed between strata--Columnar and globular
structure--Relation of trappean rocks to the products of active
volcanos--Submarine lava and ejected matter correspond generally to
ancient trap--Structure and physical features of Palma and some other
extinct volcanos 378
CHAPTER XXX.
ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.
Tests of relative age of volcanic rocks--Test by superposition and
intrusion--Dike of Quarrington Hill, Durham--Test by alteration of rocks
in contact--Test by organic remains--Test of age by mineral
character--Test by included fragments--Volcanic rocks of the
Post-Pliocene period--Basalt of Bay of Trezza in Sicily--Post-Pliocene
volcanic rocks near Naples--Dikes of Somma--Igneous formations of the
Newer Pliocene period--Val di Noto in Sicily 397
CHAPTER XXXI.
ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.
Volcanic rocks of the Older Pliocene period--Tuscany--Rome--Volcanic
region of Olot in Catalonia--Cones and lava-currents--Ravines and
ancient gravel-beds--Jets of air called Bufadors--Age of the Catalonian
volcanos--Miocene period--Brown-coal of the Eifel and contemporaneous
trachytic breccias--Age of the brown-coal--Peculiar characters of the
volcanos of the upper and lower Eifel--Lake craters--Trass--Hungarian
volcanos 408
CHAPTER XXXII.
ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.
Volcanic rocks of the Pliocene and Miocene periods
continued--Auvergne--Mont Dor--Breccias and alluviums of Mont Perrier,
with bones of quadrupeds--River dammed up by lava-current--Range of
minor cones from Auvergne to the Vivarais--Monts Dome--Puy de Come--Puy
de Pariou--Cones not denuded by general flood--Velay--Bones of
quadrupeds buried in scoriae--Cantal--Eocene volcanic rocks--Tuffs near
Clermont--Hill of Gergovia--Trap of Cretaceous period--Oolitic
period--New Red Sandstone period--Carboniferous period--Old Red
Sandstone period--"Rock and Spindle" near St. Andrews--Silurian
period--Cambrian volcanic rocks 422
CHAPTER XXXIII.
PLUTONIC ROCKS--GRANITE.
General aspect of granite--Decomposing into spherical masses--Rude
columnar structure--Analogy and difference of volcanic and plutonic
formations--Minerals in granite, and their arrangement--Graphic and
porphyritic granite--Mutual penetration of crystals of quartz and
felspar--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
plutonic rocks are ever overlying--Their exposure at the surface due
to denudation 436
CHAPTER XXXIV.
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 invisible--Tertiary plutonic rocks in the
Andes--Granite altering Cretaceous rocks--Granite altering Lias in the
Alps and in Skye--Granite of Dartmoor altering Carboniferous
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 granites of Arran, in Scotland 449
CHAPTER XXXV.
METAMORPHIC ROCKS.
General character of metamorphic rocks--Gneiss--Hornblende-schist
--Mica-schist--Clay-slate--Quartzite--Chlorite-schist--Metamorphic
limestone--Alphabetical list and explanation of other rocks of this
family--Origin of the metamorphic strata--Their stratification is real
and distinct from cleavage--Joints and slaty cleavage--Supposed causes
of these structures--how far connected with crystalline action 463
CHAPTER XXXVI.
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--Lamination of trachyte and
obsidian due to motion--Whether some kinds of gneiss have become
schistose by a similar action 473
CHAPTER XXXVII.
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 Switzerland and Savoy--Why the visible
crystalline strata are none of them very modern--Order of succession in
metamorphic rocks--Uniformity of mineral character--Why the metamorphic
strata are less calcareous than the fossiliferous 481
CHAPTER XXXVIII.
MINERAL VEINS.
Werner's doctrine that mineral veins were fissures filled from
above--Veins of segregation--Ordinary metalliferous veins or
lodes--Their frequent coincidence with faults--Proofs that they
originated in fissures in solid rock--Veins shifting other
veins--Polishing of their walls--Shells and pebbles in lodes--Evidence
of the successive enlargement and re-opening of veins--Fournet's
observations in Auvergne--Dimensions of veins--Why some alternately
swell out and contract--Filling of lodes by sublimation from
below--Chemical and electrical action--Relative age of the precious
metals--Copper and lead veins in Ireland older than Cornish tin--Lead
vein in lias, Glamorganshire--Gold in Russia--Connection of hot springs
and mineral veins--Concluding remarks 488
* * * * *
_Dates of the successive Editions of the "Principles" and "Elements" (or
Manual) of Geology, by the Author._
Principles, 1st vol. in octavo, published in - - - Jan. 1830.
----, 2d vol. do. - - - - - - - - - - - - - - - - Jan. 1832.
----, 1st vol. 2d edition in octavo - - - - - - - 1832.
----, 2d vol. 2d edition do. - - - - - - - - - - Jan. 1833.
----, 3d vol. 1st edition do. - - - - - - - - - - May 1833.
----, New edition (called the 3d) of the whole work in 4 vols.
12mo. - - - - - - - - - - - - - - - - - - - May 1834.
----, 4th edition, 4 vols. 12mo. - - - - - - - - - June 1835.
----, 5th edition, do. do. - - - - - - - - - - - - Mar. 1837.
Elements, 1st edition in one vol. - - - - - - - - July 1838.
Principles, 6th edition, 3 vols. 12mo. - - - - - - June 1840.
Elements, 2d edition in 2 vols. 12mo. - - - - - - July 1841.
Principles, 7th edition in one vol. 8vo. - - - - - Feb. 1847.
----, 8th edition, now published in one vol. 8vo. - May 1850.
Manual of Elementary Geology (or "Elements," 3d edition), now
published in one vol. 8vo. - - - - - - - - - - - - Jan. 1851.
_Works by Sir Charles Lyell._
I.
TRAVELS IN NORTH AMERICA,--1841-2. With Geological Observations on the
United States, Canada, and Nova Scotia. With large coloured geological
Map and Plates. 2 vols. post 8vo. 21_s._
II.
A SECOND VISIT TO THE UNITED STATES,--1845-6. _Second Edition._ 2 vols.
post 8vo. 18_s._
III.
PRINCIPLES OF GEOLOGY; or the Modern Changes of the Earth and its
Inhabitants considered, as illustrative of Geology. _Eighth Edition,
thoroughly revised._ With Maps, Plates, and Woodcuts. 8vo. 18_s._
IV.
A MANUAL OF ELEMENTARY GEOLOGY; or the ANCIENT CHANGES of the Earth and
its Inhabitants, as illustrated by Geological Monuments. Fourth Edition.
_Thoroughly revised._ With 531 Woodcuts and Plates. 8vo. 12_s._
MANUAL OF ELEMENTARY GEOLOGY.
CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.
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--Leading division of the work.
Of what materials is the earth composed, and in what manner are these
materials arranged? These are the first inquiries with which Geology is
occupied, a science which derives its name from the Greek +ge+, _ge_, the
earth, and +logos+, _logos_, a discourse. Previously to experience we might
have imagined that investigations of this kind would 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 such researches, we soon find ourselves led on to consider the
successive changes which have taken place in the former state of the
earth's surface and interior, 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 different periods of
the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct
substances, such as clay, chalk, sand, limestone, coal, slate, granite, and
the like; but previously to observation it is commonly 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. The geologist soon comes to a different conclusion, discovering
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, he can show that 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, or on which we are enabled
to reason by observations made at or near the surface. These reasonings may
extend to a depth of several miles, perhaps ten miles; and even then it may
be said, that such a thickness is no more than 1/400 part of the distance
from the surface to the centre. The remark is just; but although the
dimensions of such a crust are, in truth, insignificant when compared to
the entire globe, yet they are vast, and of magnificent extent in relation
to man, and to the organic beings which people 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.
The materials of this crust are not thrown together confusedly; but
distinct mineral masses, called rocks, are found to occupy definite spaces,
and to exhibit a certain order of arrangement. 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 incoherent 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 Italian, 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.
The most natural and convenient mode of classifying the various rocks which
compose the earth's crust, is to refer, in the first place, to their
origin, and in the second to their relative age. I shall therefore begin by
endeavouring briefly to explain to the student how all rocks may be divided
into four great classes by reference 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 at or near the surface.
_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 inundations. 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 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 distance from the first,
nearly the same series of beds is commonly met with, 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 sometimes 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, freshwater and marine, aqueous and volcanic, ancient and
modern, metalliferous and non-metalliferous formations.
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
deposits 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.[3-A]
When beds of sand, clay, and marl, containing shells and vegetable matter,
are found arranged in a similar 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 various heights and depths in the earth, and often far from
seas, lakes, and rivers, we meet with layers of rounded pebbles composed
of different rocks mingled together. They are like the shingle of a
sea-beach, or pebbles formed in the beds of torrents and rivers, which are
carried down into the ocean wherever these descend from high grounds
bordering a coast. There the gravel is spread out by the waves and currents
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 interior
of every continent.[4-A]
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 vegetable, 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, and sometimes, in the
case of limestone, they are in such abundance as to constitute the entire
mass of the rock itself. Shells and corals are the most frequent, and with
them are often associated the bones and teeth of fishes, fragments of wood,
impressions 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 great depths below it. They occur at all heights above the
level of the ocean, having been observed at elevations of 8000 feet in the
Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above 16,000 feet
in the Himalayas.[4-B]
These shells belong mostly to marine testacea, but in some places
exclusively to forms characteristic of lakes and rivers. Hence it is
concluded that some ancient strata were deposited at the bottom of the sea,
and others 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 deluge
of Noah; 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 simply cover the earth,
but constitute the entire mass of mountains. Nor are the fossils mingled
without reference to the original habits and natures of the creatures of
which they are the memorials; those, for example, being found associated
together which lived in deep or in shallow water, near the shore or far
from it, in brackish or in salt water.
It has, moreover, been a favourite notion of some modern writers, who were
aware that fossil bodies could not all be referred to the deluge, that
they, and the strata in which they are entombed, might have been deposited
in the bed of the ocean during the period which intervened between the
creation of man and the deluge. They have imagined 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 became the bed of the present sea. This hypothesis, although
preferable to the diluvial theory before alluded to, since it admits that
all fossiliferous strata were 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 continents exhibit, in most
regions, of having emerged from the ocean at an era far more remote than
four thousand years from the present time. Ample proofs of these reiterated
revolutions will be given in the sequel, and it will be seen 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 crust, each
containing 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, lying
immediately above or below, they as clearly prove that the deposit was
formed in a brackish estuary or lake. When the student has more fully
examined into these appearances, he will become convinced that the time
required for the origin of the rocks composing the actual continents must
have been far greater than that which is conceded by the theory above
alluded to; and likewise 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
modern accumulations of sand, mud, shingle, banks of shells, reefs of
coral, and the like, and are all 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 at or near the surface 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 are 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 from the cones into the adjoining valleys,
where they have choked 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, leaving only the harder and more solid
materials. By this erosion, and by earthquakes, their internal structure
has occasionally 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, which have burst 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 the 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,
still preserving in many cases a conical form, and having craters and often
lava-streams connected with them.
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 and other igneous 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 superficial
lava, in England and many other countries, is principally to be attributed
to the eruptions having been 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, containing
each their characteristic fossils, have been deposited at successive
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.
_Plutonic rocks_ (Granite, &c.).--We have now pointed out the existence 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 assimilate to deposits such as are now accumulated in lakes or
seas, nor to those generated by ordinary volcanic action. The members of
both these divisions of rocks agree in being highly crystalline and
destitute of organic remains. The rocks of one division have been called
plutonic, comprehending all 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 which group 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 crystalline
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. I
cannot, in reply to this question, pretend to give the student, in a few
words, an intelligible account of the long chain of facts and reasonings by
which geologists have been led to infer the analogy of the rocks in
question to others now in progress at the surface. The result, however, may
be briefly stated. All the various kinds of granite, which constitute the
plutonic family, are supposed to be of igneous origin, but to have been
formed under great pressure, at considerable depths in the earth, or
sometimes, perhaps, under a certain weight of incumbent water. Like the
lava of volcanos, they have been melted, and have afterwards cooled and
crystallized, but with extreme slowness, and under conditions very
different from those of bodies cooling in the open air. Hence they differ
from the volcanic rocks, not only by their more crystalline texture, but
also by the absence of tuffs and breccias, which are the products of
eruptions at the earth's surface, or beneath seas of inconsiderable depth.
They differ also by the absence of pores or cellular cavities, to which the
expansion of the entangled gases gives rise in ordinary lava.
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 volcanic rocks, they have been styled,
from this peculiarity, "overlying" 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, or stratified crystalline rocks._--The fourth and last great
division of rocks are the crystalline strata and slates, 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, corresponding in form and
arrangement 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 fossiliferous deposits. According to the Huttonian 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 so altered
by subterranean heat, as to assume a new texture. It is demonstrable, in
some cases at least, that such a complete conversion has actually taken
place, fossiliferous strata having exchanged an earthy for a highly
crystalline texture for a distance of a quarter of a mile from their
contact with granite. In some cases, dark limestones, replete with shells
and corals, have been turned into white statuary marble, and hard clays
into slates called mica-schist and hornblende-schist, all signs of organic
bodies having been obliterated.
Although we are in a great degree ignorant of the precise nature of the
influence exerted in these cases, yet it evidently bears some analogy to
that which volcanic heat and gases are known to produce; 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 hot water or
steam permeating stratified masses, or electricity, or any other causes
have co-operated to produce the crystalline texture, may be matter of
speculation, but it is clear that the plutonic influence has sometimes
pervaded entire mountain masses of strata.
In accordance with the hypothesis above alluded to, I proposed in the first
edition of the Principles of Geology (1833), the term "Metamorphic" for the
altered strata, a term derived from +meta+, meta, _trans_, and +morphe+,
morphe, _forma_.
Hence there are four great classes of rocks considered in reference to
their origin,--the aqueous, the volcanic, the plutonic, and the
metamorphic. In the course of this work it will be shown, that portions of
each of these four distinct classes have originated at many successive
periods. They have all been produced contemporaneously, and may even now be
in the progress of formation. It is not true, as was formerly supposed,
that all granites, together with the crystalline or metamorphic 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 secondary in the order of time. This idea was
adopted in the infancy of the science, when all formations, whether
stratified or unstratified, 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; but it
was afterwards discovered, that this opinion was by no means in every
instance a legitimate deduction from facts; for the inferior parts of the
earth's crust have often been modified, and even entirely changed, by the
influence of volcanic and other subterranean causes, while superimposed
formations have not been in the slightest degree altered. In other words,
the destroying and renovating processes have given birth to new rocks
below, while those above, whether crystalline or fossiliferous, have
remained in their ancient condition. Even in cities, such as Venice and
Amsterdam, it cannot be laid down as universally true, that the upper parts
of each edifice, whether of brick or marble, are more modern than the
foundations on which they rest, for these often consist of wooden piles,
which may have rotted and been replaced one after the other, without the
least injury to the buildings above; meanwhile, these may have required
scarcely any repair, and may have been constantly inhabited. So it is with
the habitable surface of our globe, in its relation to large masses of rock
immediately below: it may continue the same for ages, while subjacent
materials, at a great depth, are passing from a solid to a fluid state, and
then reconsolidating, so as to acquire a new texture.
As all the crystalline rocks may, in some respects, be viewed as belonging
to one great family, whether they be stratified or unstratified, plutonic
or metamorphic, it will often be convenient to speak of them by one common
name. It being now ascertained, as above stated, that they are of very
different ages, sometimes newer than the strata called secondary, the term
primary, which was formerly used for the whole, must be abandoned, as it
would imply a manifest contradiction. It 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_ sedimentary strata. I proposed
in the Principles of Geology (first edition, vol. iii.), the term
"hypogene" for this purpose, derived from +hypo+, _under_, and +ginomai+,
_to be_, or _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. This occurs in the lowest place in the order of superposition.
Even in regions such as the Alps, where some masses of granite and gneiss
can be shown to be of comparatively modern date, belonging, for example, to
the period hereafter to be described as tertiary, they are still
_underlying_ rocks. They never repose on the volcanic or trappean
formations, nor on strata containing organic remains. They are _hypogene_,
as "being under" all the rest.
From what has now been said, the reader will understand that each of the
four great classes of rocks may 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 former history
of the globe and its living inhabitants.
I shall accordingly proceed to treat of each family of rocks; first, in
reference to those characters which are not chronological, and then in
particular relation to the several periods when they were formed.
FOOTNOTES:
[3-A] See Principles of Geology, by the Author, Index, "Nile,"
"Rivers," &c.
[4-A] See p. 18.
[4-B] See Geograph. Journ. vol. iv. p. 64.