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.
[Illustration: Fig. 153. Map of the principal tertiary basins of
the Eocene period.
N. B. The space left blank is occupied by secondary formations from the
Devonian or old red sandstone to the chalk inclusive.]
The tertiary strata described in the preceding chapters are all of them
characterized by fossil shells, of which a considerable proportion are
specifically identical with the living mollusca; and the greater the
number, the more nearly does the entire fauna approach in species and
genera to that now inhabiting the adjoining seas. But in the Eocene
formations next to be considered, the proportion of recent species is very
small, and sometimes scarcely appreciable, and those agreeing with the
fossil testacea often belong to remote parts of the globe, and to various
zoological provinces. This difference in conchological character implies a
considerable interval of time between the Eocene and Miocene periods,
during which the whole fauna and flora underwent other changes as great,
and often greater, than those exhibited by the mollusca. In the
accompanying map, the position of several Eocene areas is pointed out, such
as the basin of the Thames, part of Hampshire, part of the Netherlands,
and the country round Paris. The deposits, however, occupying these spaces
comprise a great succession of marine and freshwater formations, which,
although they may all be termed Eocene, as being newer than the chalk, and
older than the faluns, are nevertheless divisible into separate groups, of
high geological importance.
The newest of these, like the Faluns of the Loire, have no true
representatives, or exact chronological equivalents, in the British Isles.
Their place in the series will best be understood by referring to the order
of superposition of the successive deposits found in the neighbourhood of
Paris. The area which has been called the Paris basin is about 180 miles in
its greatest length from north-east to south-west, and about 90 miles from
east to west. This space may be described as a depression in the chalk,
which has been filled up by alternating groups of marine and freshwater
strata. MM. Cuvier and Brongniart attempted, in 1810, to distinguish five
different formations, comprising three freshwater and two marine, which
alternated with each other. It was imagined that the waters of the ocean
had been by turns admitted and excluded from the same region; but the
subsequent investigations of several geologists, especially of M. Constant
Prevost,[175-A] have led to great modifications in these theoretical views;
and now that the true order of succession is better understood, it appears
that several of the deposits, which were supposed to have originated one
after the other, were, in fact, in progress at the same time by the joint
action of the sea and rivers.
The whole series of strata may be divided into three groups, as expressed
in the following table:--
{ _a._ Upper freshwater limestone, marls, and siliceous
1. Upper Eocene { millstone.
{ _b._ Upper marine sands, or Fontainebleau sandstone
{ and sand.
{ _a._ Lower freshwater limestone and marl, or
{ gypseous series.
{ _b._ Sandstone and sands with marine shells (_Sables_
2. Middle Eocene { _moyens_, or _gres de Beauchamp_).
{ _c._ Calcaire grossier, limestone with marine shells.
{ _d._ Calcaire siliceux, hard siliceous freshwater
{ limestone, for the most part contemporaneous
{ with _c_.
{ _a._ Lower sands with marine shelly beds (_Sables_
3. Lower Eocene { _inferieurs et lits coquilliers_).
{ _b._ Lower sands, with lignite and plastic clay
{ (_Sables inferieurs et argiles plastiques_).
Postponing to the next chapter the consideration of the Middle and Lower
Eocene groups, I shall now speak of the Upper Eocene of Paris, and
its foreign equivalents.
The upper freshwater marls and limestone (1. _a_) seem to have been formed
in a great number of marshes and shallow lakes, such as frequently
overspread the newest parts of great deltas. It appears that many layers of
marl, tufaceous limestone, and travertin, with beds of flint, continuous
or in nodules, accumulated in these lakes. _Charae_, aquatic plants, already
alluded to (see p. 32.) left their stems and seed-vessels imbedded both in
the marl and flint, together with freshwater and land shells. Some of the
siliceous rocks of this formation are used extensively for millstones. The
flat summits or platforms of the hills round Paris, large areas in the
forest of Fontainebleau, and the Plateau de la Beauce, between the Seine
and the Loire, are chiefly composed of these upper freshwater strata.
The upper marine sands (1. _b_), consist chiefly of micaceous and quartzose
sands, 80 feet thick. As they succeed throughout an extensive area deposit
of a purely freshwater origin (2 _a_.), they appear to mark a subsidence of
the subjacent soil, whether it had formed the bottom of an estuary or a
lake. The sea, which afterwards took possession of the same space, was
inhabited by testacea, almost all of them differing from those found in the
lower formations (2. _b_ and 2. _c_) and equally or still more distinct
from the Miocene Faluns of subsequent date. One of these upper Eocene
strata in the neighbourhood of Paris has been called the oyster bed,
"couche a _Ostrea cyathula_, Lamk.," which is spread over a remarkably wide
area. From the manner in which the oysters lie, it is inferred that they
did not grow on the spot, but that some current swept them away from a bed
of oysters formed in some other part of the bay. The strata of sand which
immediately repose on the oyster-bed are quite destitute of organic
remains; and nothing is more common in the Paris basin, and in other
formations, than alternations of shelly beds with others entirely devoid of
them. The temporary extinction and renewal of animal life at successive
periods have been rashly inferred from such phenomena, which may
nevertheless be explained, as M. Prevost justly remarks, without appealing
to any such extraordinary revolutions in the state of the animate creation.
A current one day scoops out a channel in a bed of shelly sand and mud, and
the next day, by a slight alteration of its course, ceases to prey upon the
same bank. It may then become charged with sand unmixed with shells,
derived from some dune, or brought down by a river. In the course of ages
an indefinite number of transitions from shelly strata to those without
shells may thus be caused.
Besides these oysters, M. Deshayes has described 29 species of shells,
in his work (Coquilles fossiles de Paris), as belonging to this
formation, all save one regarded by him as differing from fossils of the
calcaire grossier. Since that time the railway cuttings near Etampes
have enabled M. Hebert to raise the number to 90. I have myself
collected fossils in that district, where the shells are very entire,
and detachable from the yellow sandy matrix. M. Hebert first pointed out
that most of them agree specifically with those of Kleyn Spauwen, Boom,
and other localities of Limburg in Flanders, where they have been
studied by MM. Nyst and De Koninck.[176-A]
The position in Belgium of this formation above the older Eocene group is
well seen in the small hill of Pellenberg, rising abruptly from the great
plain, half a mile south-east of the city of Louvain, where I examined it
in company with M. Nyst in 1850. At the top of the hill, a thin bed of dark
greyish green tile-clay is seen 1-1/2 foot thick, with casts of _Nucula
Deshaysiana_. This clay rests on 12 feet of yellow sand, separated, by a
band of flint and quartz pebbles, from a mass of subjacent white sand 15
feet thick, in which casts of the Kleyn Spauwen fossils have been met with.
Under this is a bed of yellow sand 12 feet thick, and, at a lower level,
the railway cuttings have passed through calcareous sands like those of
Brussels, in which the _Nautilus Burtini_, and various shells common to the
older Eocene strata of the neighbourhood of London, have been obtained.
Every new fact which throws light on the true paleontological relations of
the strata now under consideration, (the Upper Marine or Fontainebleau beds
of the Paris basin, 1. _b_, p. 175.), deserves more particular attention,
because geologists of high authority differ in opinion as to whether they
should be classed as Eocene or Miocene.
Professor Beyrich has lately described a formation of the same age,
occurring within 7 miles of the gates of Berlin, near the village of
Hermsdorf, where, in the midst of the sands of which that country
chiefly consists, a mass of tile-clay, more than 40 feet thick, and of a
dark blueish grey colour, is found, full of shells, among which the
genera _Fusus_ and _Pleurotoma_ predominate, and among the bivalves,
_Nucula Deshaysiana_, Nyst, an extremely common shell in the Belgian
beds above-mentioned. M. Beyrich has identified eighteen out of
forty-five species of the Hermsdorf fossils with the Belgian species;
and I believe that a much larger proportion agree with the Upper Eocene
of Belgium, France, and the Rhine. On the other hand, eight of the
forty-five species are supposed by him to agree with English Eocene
shells. Messrs. Morris, Edwards, and S. Wood have compared a small
collection, which I obtained of these Berlin shells, with the Eocene
fossils of their museums, and confirmed the result of M. Beyrich, the
species common to the English fossils belonging not simply to the
uppermost of our marine beds, or those of Barton, but some of them to
lower parts of the series, such as Bracklesham and Highgate. On the
other hand, while these testacea, like those of Kleyn Spauwen and
Etampes, present many analogies to the Middle and Lower Eocene group,
they differ widely from the Falun shells,--a fact the more important in
reference to Etampes, as that locality approaches within 70 miles of
Pontlevoy, near Blois, and within 100 miles of Savigne, near Tours,
where Falun shells are found. It is evident that the discordance of
species cannot be attributed to distance or geographical causes, but
must be referred to time, or the different epoch at which the upper
marine beds of the Paris basin and the Faluns of the Loire originated.
_Mayence._--The true chronological relation of many tertiary strata on
the banks of the Rhine has always presented a problem of considerable
difficulty. They occupy a tract from 5 to 12 miles in breadth, extending
along the left bank of the Rhine from Mayence to the neighbourhood of
Manheim, and are again found to the east, north, and south-west of
Frankfort. In some places they have the appearance of a freshwater
formation; but in others, as at Alzey, the shells are for the most part
marine. _Cerithia_ are in great profusion, which indicates that the sea
where the deposit was formed was fed by rivers; and the great quantity
of fossil land shells, chiefly of the genus _Helix_, confirm the same
opinion. The variety in the species of shells is small, while the
individuals are exceedingly numerous; a fact which accords perfectly
with the idea that the formation may have originated in a gulf or sea
which, like the Baltic, was brackish in some parts, and almost fresh in
others. A species of _Paludina_ (fig. 154.), very nearly resembling the
recent _Littorina ulva_, is found throughout this basin. These shells
are like grains of rice in size, and are often in such quantity as to
form entire beds of marl and limestone. They are as thick as grains of
sand, in stratified masses from 15 to 30 feet in thickness.
[Illustration: Fig. 154. _Paludina._ Mayence.]
That these Rhenish tertiary formations agree more nearly with the Upper
Eocene deposits above enumerated, than with any others, I have no doubt,
since I had the advantage of comparing (August, 1850), with the
assistance of M. De Koninck of Liege, the fossils from Kleyn Spauwen,
Boom, and other Limburg localities, with those from Mayence, Alzey,
Weinheim, and other Rhenish strata. Among the common Belgian and Rhenish
shells which are identical, I may mention _Cassidaria depressa_,
_Tritonium flandricum_ De Koninck, _Cerithium tricinctum_ Nyst,
_Tornatella simulata_, _Rostellaria Sowerbyi_, _Nucula Deshaysiana_,
_Corbula pisum_, and _Pectunculus terebratularis_.
From these Upper Eocene deposits of the Rhine M. H. von Meyer has
obtained a great number of characteristic fossil mammalia, such as
_Palaeomaeryx medius_, _Hyotherium Meissneri_, _Tapirus Helveticus_,
_Anthracotherium Alsaticum_, and others. The three first of these are
species common to some of the lignite, or brown coal beds in
Switzerland, commonly classed with the molasse, but of which the true
age has not yet been distinctly made out.
The fossils of the sandy beds of Eppelsheim, comprising bones of the
Deinotherium, Mastodon, and other quadrupeds, are regarded by H. von Meyer
as belonging to a mammiferous fauna quite distinct from that of the Mayence
basin, and they are probably referable to the Miocene period.
The upper freshwater strata (1. _a_, p. 175.), of the neighbourhood of
Paris, stretch southwards from the valley of the Seine to that of the
Loire, and in the last-mentioned region are seen to be older than the
marine faluns, so that the perforating shells of the Miocene sea have
sometimes bored the hard compact freshwater limestones; and fragments of
the Upper Eocene rocks are found at Pontlevoy and elsewhere, which have
been rolled in the bed of the Miocene sea.
[Illustration: Fig. 155. Simplified geological map south of Paris.]
_Central France._--Lacustrine strata belonging, for the most part, to
the same Upper Eocene series, are again met with in Auvergne, Cantal,
and Velay, the sites of which may be seen in the annexed map. They
appear to be the monuments of ancient lakes, which, like some of those
now existing in Switzerland, once occupied the depressions in a
mountainous region, and have been each fed by one or more rivers and
torrents. The country where they occur is almost entirely composed of
granite and different varieties of granitic schist, with here and there
a few patches of secondary strata, much dislocated, and which have
probably suffered great denudation. There are also some vast piles
of volcanic matter (see the map), the greater part of which is newer
than the freshwater strata, and is sometimes seen to rest upon them,
while a small part has evidently been of contemporaneous origin.
Of these igneous rocks I shall treat more particularly in another
part of this work.
Before entering upon any details, I may observe, that the study of these
regions possesses a peculiar interest, very distinct in kind from that
derivable from the investigation either of the Parisian or English tertiary
strata. For we are presented in Auvergne with the evidence of a series of
events of astonishing magnitude and grandeur, by which the original form
and features of the country have been greatly changed, yet never so far
obliterated but that they may still, in part at least, be restored in
imagination. Great lakes have disappeared,--lofty mountains have been
formed, by the reiterated emission of lava, preceded and followed by
showers of sand and scoriae,--deep valleys have been subsequently furrowed
out through masses of lacustrine and volcanic origin,--at a still later
date, new cones have been thrown up in these valleys,--new lakes have been
formed by the damming up of rivers,--and more than one creation of
quadrupeds, birds, and plants, Eocene, Miocene, and Pliocene, have followed
in succession; yet the region has preserved from first to last its
geographical identity; and we can still recall to our thoughts its external
condition and physical structure before these wonderful vicissitudes began,
or while a part only of the whole had been completed. There was first a
period when the spacious lakes, of which we still may trace the boundaries,
lay at the foot of mountains of moderate elevation, unbroken by the bold
peaks and precipices of Mont Dor, and unadorned by the picturesque outline
of the Puy de Dome, or of the volcanic cones and craters now covering the
granitic platform. During this earlier scene of repose deltas were slowly
formed; beds of marl and sand, several hundred feet thick, deposited;
siliceous and calcareous rocks precipitated from the waters of mineral
springs; shells and insects imbedded, together with the remains of the
crocodile and tortoise, the eggs and bones of water birds, and the
skeletons of quadrupeds, some of them belonging to the same genera as those
entombed in the Eocene gypsum of Paris. To this tranquil condition of the
surface succeeded the era of volcanic eruptions, when the lakes were
drained, and when the fertility of the mountainous district was probably
enhanced by the igneous matter ejected from below, and poured down upon the
more sterile granite. During these eruptions, which appear to have taken
place after the disappearance of the Eocene fauna, and in the Miocene
epoch, the mastodon, rhinoceros, elephant, tapir, hippopotamus, together
with the ox, various kinds of deer, the bear, hyaena, and many beasts of
prey, ranged the forest, or pastured on the plain, and were occasionally
overtaken by a fall of burning cinders, or buried in flows of mud, such as
accompany volcanic eruptions. Lastly, these quadrupeds became extinct, and
gave place to Pliocene mammalia, and these, in their turn, to species now
existing. There are no signs, during the whole time required for this
series of events, of the sea having intervened, nor of any denudation which
may not have been accomplished by currents in the different lakes, or by
rivers and floods accompanying repeated earthquakes, during which the
levels of the district have in some places been materially modified, and
perhaps the whole upraised relatively to the surrounding parts of France.
_Auvergne._--The most northern of the freshwater groups is situated in the
valley-plain of the Allier, which lies within the department of the Puy de
Dome, being the tract which went formerly by the name of the Limagne
d'Auvergne. It is inclosed by two parallel mountain ranges,--that of the
Forez, which divides the waters of the Loire and Allier, on the east; and
that of the Monts Domes, which separates the Allier from the Sioule, on the
west.[181-A] The average breadth of this tract is about 20 miles; and it is
for the most part composed of nearly horizontal strata of sand, sandstone,
calcareous marl, clay, and limestone, none of which observe a fixed and
invariable order of superposition. The ancient borders of the lake, wherein
the freshwater strata were accumulated, may generally be traced with
precision, the granite and other ancient rocks rising up boldly from the
level country. The actual junction, however, of the lacustrine and granitic
beds is rarely seen, as a small valley usually intervenes between them. The
freshwater strata may sometimes be seen to retain their horizontality
within a very slight distance of the border-rocks, while in some places
they are inclined, and in few instances vertical. The principal divisions
into which the lacustrine series may be separated are the following:--1st,
Sandstone, grit, and conglomerate, including red marl and red sandstone.
2dly, Green and white foliated marls. 3dly, Limestone or travertin, often
oolitic. 4thly, Gypseous marls.
1. _a_. _Sandstone and conglomerate._--Strata of sand and gravel, sometimes
bound together into a solid rock, are found in great abundance around the
confines of the lacustrine basin, containing, in different places, pebbles
of all the ancient rocks of the adjoining elevated country; namely,
granite, gneiss, mica-schist, clay-slate, porphyry, and others. But these
strata do not form one continuous band around the margin of the basin,
being rather disposed like the independent deltas which grow at the mouths
of torrents along the borders of existing lakes.
At Chamalieres, near Clermont, we have an example of one of these deltas,
or littoral deposits, of local extent, where the pebbly beds slope away
from the granite, as if they had formed a talus beneath the waters of the
lake near the steep shore. A section of about 50 feet in vertical height
has been laid open by a torrent, and the pebbles are seen to consist
throughout of rounded and angular fragments of granite, quartz, primary
slate, and red sandstone; but without any intermixture of those volcanic
rocks which now abound in the neighbourhood, and which could not have been
there when the conglomerate was formed. Partial layers of lignite and
pieces of wood are found in these beds.
At some localities on the margin of the basin quartzose grits are found;
and, where these rest on granite, they are sometimes formed of separate
crystals of quartz, mica, and felspar, derived from the disintegrated
granite, the crystals having been subsequently bound together by a
siliceous cement. In these cases the granite seems regenerated in a new and
more solid form; and so gradual a passage takes place between the rock of
crystalline and that of mechanical origin, that we can scarcely distinguish
where one ends and the other begins.
In the hills called the Puy de Jussat and La Roche, we have the advantage
of seeing a section continuously exposed for about 700 feet in thickness.
At the bottom are foliated marls, white and green, about 400 feet thick;
and above, resting on the marls, are the quartzose grits, cemented by
calcareous matter, which is sometimes so abundant as to form imbedded
nodules. These sometimes constitute spheroidal concretions 6 feet in
diameter, and pass into beds of solid limestone, resembling the Italian
travertins, or the deposits of mineral springs. This section is close to
the confines of the basin; so that the lake must here have been filled up
near the shore with fine mud, before the coarse superincumbent sand was
introduced. There are other cases where sand is seen below the marl.
1. _b._ _Red marl and sandstone_.--But the most remarkable of the
arenaceous groups is one of red sandstone and red marl, which are identical
in all their mineral characters with the secondary _New Red sandstone_ and
marl of England. In these secondary rocks the red ground is sometimes
variegated with light greenish spots, and the same may be seen in the
tertiary formation of freshwater origin at Coudes, on the Allier. The marls
are sometimes of a purplish-red colour, as at Champheix, and are
accompanied by a reddish limestone, like the well-known "cornstone," which
is associated with the Old Red sandstone of English geologists. The red
sandstone and marl of Auvergne have evidently been derived from the
degradation of gneiss and mica-schist, which are seen _in situ_ on the
adjoining hills, decomposing into a soil very similar to the tertiary red
sand and marl. We also find pebbles of gneiss, mica-schist, and quartz in
the coarser sandstones of this group, clearly pointing to the parent rocks
from which the sand and marl are derived. The red beds, although destitute
themselves of organic remains, pass upwards into strata containing Eocene
fossils, and are certainly an integral part of the lacustrine formation.
From this example the student will learn how small is the value of mineral
character alone, as a test of the relative age of rocks.
2. _Green and white foliated marls._--The same primary rocks of Auvergne,
which, by the partial degradation of their harder parts, gave rise to the
quartzose grits and conglomerates before mentioned, would, by the reduction
of the same materials into powder, and by the decomposition of their
felspar, mica, and hornblende, produce aluminous clay, and, if a sufficient
quantity of carbonate of lime was present, calcareous marl. This fine
sediment would naturally be carried out to a greater distance from the
shore, as are the various finer marls now deposited in Lake Superior. And,
as in the American lake, shingle and sand are annually amassed near the
northern shores, so in Auvergne the grits and conglomerates before
mentioned were evidently formed near the borders.
[Illustration: Fig. 156. _Cypris unifasciata_, a living species,
greatly magnified.
_a._ Upper part.
_b._ Side view of the same.]
[Illustration: Fig. 157. _Cypris vidua_, a living species,
greatly magnified.[183-A]]
The entire thickness of these marls is unknown; but it certainly exceeds,
in some places, 700 feet. They are, for the most part, either light-green
or white, and usually calcareous. They are thinly foliated,--a character
which frequently arises from the innumerable thin shells, or
carapace-valves, of that small animal called _Cypris_; a genus which
comprises several species, of which some are recent, and may be seen
swimming swiftly through the waters of our stagnant pools and ditches. The
antennae, at the end of which are fine pencils of hair, are the principal
organs of motion, and are seen to vibrate with great rapidity. This animal
resides within two small valves, not unlike those of a bivalve shell, and
moults its integuments periodically, which the conchiferous mollusks do
not. This circumstance may partly explain the countless myriads of the
shells of _Cypris_ which were shed in the ancient lakes of Auvergne, so as
to give rise to divisions in the marl as thin as paper, and that, too, in
stratified masses several hundred feet thick. A more convincing proof of
the tranquillity and clearness of the waters, and of the slow and gradual
process by which the lake was filled up with fine mud, cannot be desired.
But we may easily suppose that, while this fine sediment was thrown down in
the deep and central parts of the basin, gravel, sand, and rocky fragments
were hurried into the lake, and deposited near the shore, forming the group
described in the preceding section.
Not far from Clermont, the green marls, containing the _Cypris_ in
abundance, approach to within a few yards of the granite which forms the
borders of the basin. The occurrence of these marls so near the ancient
margin may be explained by considering that, at the bottom of the
ancient lake, no coarse ingredients were deposited in spaces
intermediate between the points where rivers and torrents entered, but
finer mud only was drifted there by currents. The _verticality_ of some
of the beds in the above section bears testimony to considerable local
disturbance subsequent to the deposition of the marls; but such inclined
and vertical strata are very rare.
[Illustration: Fig. 158. Vertical strata of marl, at Champradelle,
near Clermont.
A. Granite.
B. Space of 60 feet, in which no section is seen.
C. Green marl, vertical and inclined.
D. White marl.]
3. _Limestone, travertin, oolite._--Both the preceding members of the
lacustrine deposit, the marls and grits, pass occasionally into
limestone. Sometimes only concretionary nodules abound in them; but
these, where there is an increase in the quantity of calcareous matter,
unite into regular beds.
On each side of the basin of the Limagne, both on the west at Gannat, and
on the east at Vichy, a white oolitic limestone is quarried. At Vichy, the
oolite resembles our Bath stone in appearance and beauty; and, like it, is
soft when first taken from the quarry, but soon hardens on exposure to the
air. At Gannat, the stone contains land-shells and bones of quadrupeds,
resembling those of the Paris gypsum. At Chadrat, in the hill of La Serre,
the limestone is pisolitic, the small spheroids combining both the radiated
and concentric structure.
_Indusial limestone._--There is another remarkable form of freshwater
limestone in Auvergne, called "indusial," from the cases, or _indusiae_,
of caddis-worms (the larvae of _Phryganea_); great heaps of which have
been incrusted, as they lay, by carbonate of lime, and formed into a
hard travertin. The rock is sometimes purely calcareous, but there is
occasionally an intermixture of siliceous matter. Several beds of it are
frequently seen, either in continuous masses, or in concretionary
nodules, one upon another, with layers of marl interposed. The annexed
drawing (fig. 159.) will show the manner in which one of these indusial
beds (_a_) is laid open at the surface, between the marls (_b b_), near
the base of the hill of Gergovia; and affords, at the same time, an
example of the extent to which the lacustrine strata, which must once
have filled a hollow, have been denuded, and shaped out into hills and
valleys, on the site of the ancient lakes.
[Illustration: Fig. 159. Bed of indusial limestone, interstratified with
freshwater marl, near Clermont (Kleinschrod.)]
[Illustration: Fig. 160. Larva of recent Phryganea.[185-A]]
[Illustration: Fig. 161.
_a_. Indusial limestone of Auvergne.
_b_. Fossil _Paludina_ magnified.]
We may often observe in our ponds the _Phryganea_ (or Caddis-fly), in its
caterpillar state, covered with small freshwater shells, which they have
the power of fixing to the outside of their tubular cases, in order,
probably, to give them weight and strength. The individual figured in the
annexed cut, which belongs to a species very abundant in England, has
covered its case with shells of a small _Planorbis_. In the same manner a
large species of caddis-worm, which swarmed in the Eocene lakes of
Auvergne, was accustomed to attach to its dwelling the shells of a small
spiral univalve of the genus _Paludina_. A hundred of these minute shells
are sometimes seen arranged around one tube, part of the central cavity of
which is often empty, the rest being filled up with thin concentric layers
of travertin. The cases have been thrown together confusedly, and often
lie, as in fig. 161., at right angles one to the other. When we consider
that ten or twelve tubes are packed within the compass of a cubic inch, and
that some single strata of this limestone are 6 feet thick, and may be
traced over a considerable area, we may form some idea of the countless
number of insects and mollusca which contributed their integuments and
shells to compose this singularly constructed rock. It is unnecessary to
suppose that the _Phryganeae_ lived on the spots where their cases are now
found; they may have multiplied in the shallows near the margin of the
lake, or in the streams by which it was fed, and their cases may have been
drifted by a current far into the deep water.
In the summer of 1837, when examining, in company with Dr. Beck, a small
lake near Copenhagen, I had an opportunity of witnessing a beautiful
exemplification of the manner in which the tubular cases of Auvergne were
probably accumulated. This lake, called the Fuure-Soe, occurring in the
interior of Seeland, is about twenty English miles in circumference, and in
some parts 200 feet in depth. Round the shallow borders an abundant crop of
reeds and rushes may be observed, covered with the indusiae of the
_Phryganea grandis_ and other species, to which shells are attached. The
plants which support them are the bullrush, _Scirpus lacustris_, and common
reed, _Arundo phragmitis_, but chiefly the former. In summer, especially in
the month of June, a violent gust of wind sometimes causes a current by
which these plants are torn up by the roots, washed away, and floated off
in long bands, more than a mile in length, into deep water. The _Cypris_
swarms in the same lake; and calcareous springs alone are wanting to form
extensive beds of indusial limestone, like those of Auvergne.
4. _Gypseous marls._--More than 50 feet of thinly laminated gypseous marls,
exactly resembling those in the hill of Montmartre, at Paris, are worked
for gypsum at St. Romain, on the right bank of the Allier. They rest on a
series of green cypriferous marls which alternate with grit, the united
thickness of this inferior group being seen, in a vertical section on the
banks of the river, to exceed 250 feet.
_General arrangement, origin, and age of the freshwater formations of
Auvergne._--The relations of the different groups above described cannot be
learnt by the study of any one section; and the geologist who sets out with
the expectation of finding a fixed order of succession may perhaps complain
that the different parts of the basin give contradictory results. The
arenaceous division, the marls, and the limestone, may all be seen in some
places to alternate with each other; yet it can, by no means, be affirmed
that there is no order of arrangement. The sands, sandstone, and
conglomerate, constitute in general a littoral group; the foliated white
and green marls, a contemporaneous central deposit; and the limestone is
for the most part subordinate to the newer portions of both. The uppermost
marls and sands are more calcareous than the lower; and we never meet with
calcareous rocks covered by a considerable thickness of quartzose sand or
green marl. From the resemblance of the limestones to the Italian
travertins, we may conclude that they were derived from the waters of
mineral springs,--such springs as even now exist in Auvergne, and which may
be seen rising up through the granite, and precipitating travertin. They
are sometimes thermal, but this character is by no means constant.
It seems that, when the ancient lake of the Limagne first began to be
filled with sediment, no volcanic action had yet produced lava and
scoriae on any part of the surface of Auvergne. No pebbles, therefore, of
lava were transported into the lake,--no fragments of volcanic rocks
embedded in the conglomerate. But at a later period, when a considerable
thickness of sandstone and marl had accumulated, eruptions broke out,
and lava and tuff were deposited, at some spots, alternately with the
lacustrine strata. It is not improbable that cold and thermal springs,
holding different mineral ingredients in solution, became more numerous
during the successive convulsions attending this development of volcanic
agency, and thus deposits of carbonate and sulphate of lime, silex, and
other minerals were produced. Hence these minerals predominate in the
uppermost strata. The subterranean movements may then have continued
until they altered the relative levels of the country, and caused the
waters of the lakes to be drained off, and the farther accumulation of
regular freshwater strata to cease.
We may easily conceive a similar series of events to give rise to
analogous results in any modern basin, such as that of Lake Superior,
for example, where numerous rivers and torrents are carrying down the
detritus of a chain of mountains into the lake. The transported
materials must be arranged according to their size and weight, the
coarser near the shore, the finer at a greater distance from land; but
in the gravelly and sandy beds of Lake Superior no pebbles of modern
volcanic rocks can be included, since there are none of these at present
in the district. If igneous action should break out in that country, and
produce lava, scoriae, and thermal springs, the deposition of gravel,
sand, and marl might still continue as before; but, in addition, there
would then be an intermixture of volcanic gravel and tuff, and of rocks
precipitated from the waters of mineral springs.
Although the freshwater strata of the Limagne approach generally to a
horizontal position, the proofs of local disturbance are sufficiently
numerous and violent to allow us to suppose great changes of level since
the lacustrine period. We are unable to assign a northern barrier to the
ancient lake, although we can still trace its limits to the east, west,
and south, where they were formed of bold granite eminences. Nor need we
be surprised at our inability to restore entirely the physical geography
of the country after so great a series of volcanic eruptions; for it is
by no means improbable that one part of it, the southern, for example,
may have been moved upwards bodily, while others remained at rest, or
even suffered a movement of depression.
Whether all the freshwater formations of the Limagne d'Auvergne belong to
one period, I cannot pretend to decide, as large masses both of the
arenaceous and marly groups are often devoid of fossils. Much light has
been thrown on the mammiferous fauna by the labours of MM. Bravard and
Croizet, and by those of M. Pomel. The last-mentioned naturalist has
pointed out the specific distinction of all, or nearly all, the species of
mammalia, from those of the gypseous series near Paris. Nevertheless, many
of the forms are analogous to those of Eocene quadrupeds. The
_Cainotherium_, for example, is not far removed from the _Anoplotherium_,
and is, according to Waterhouse, the same as the genus _Microtherium_ of
the Germans. There are two species of marsupial animals allied to
_Didelphys_, a genus also found in the Paris gypsum. The _Amphitragulus
elegans_ of Pomel, has been identified with a Rhenish species from
Weissenau near Mayence, called by Kaup _Dorcatherium nanum_; and other
Auvergne fossils, e.g., _Microtherium Reuggeri_, and a small rodent,
_Titanomys_, are specifically the same with mammalia of the Mayence basin.
_Cantal._--A freshwater formation, very analogous to that of Auvergne,
is situated in the department of Haute Loire, near the town of Le Puy,
in Velay, and another occurs near Aurillac, in Cantal. The leading
feature of the formation last mentioned, as distinguished from those of
Auvergne and Velay, is the immense abundance of silex associated with
calcareous marls and limestone.
The whole series may be separated into two divisions; the lower, composed
of gravel, sand, and clay, such as might have been derived from the wearing
down and decomposition of the granitic schists of the surrounding country;
the upper system, consisting of siliceous and calcareous marls, contains
subordinately gypsum, silex, and limestone.
The resemblance of the freshwater limestone of the Cantal, and its
accompanying flint, to the upper chalk of England, is very instructive, and
well calculated to put the student upon his guard against relying too
implicitly on mineral character alone as a safe criterion of relative age.
When we approach Aurillac from the west, we pass over great heathy plains,
where the sterile mica-schist is barely covered with vegetation. Near
Ytrac, and between La Capelle and Viscamp, the surface is strewed over with
loose broken flints, some of them black in the interior, but with a white
external coating; others stained with tints of yellow and red, and in
appearance precisely like the flint gravel of our chalk districts. When
heaps of this gravel have thus announced our approach to a new formation,
we arrive at length at the escarpment of the lacustrine beds. At the bottom
of the hill which rises before us, we see strata of clay and sand, resting
on mica-schist; and above, in the quarries of Belbet, Leybros, and Bruel, a
white limestone, in horizontal strata, the surface of which has been
hollowed out into irregular furrows, since filled up with broken flint,
marl, and dark vegetable mound. In these cavities we recognize an exact
counterpart to those which are so numerous on the furrowed surface of our
own white chalk. Advancing from these quarries along a road made of the
white limestone, which reflects as glaring a light in the sun, as do our
roads composed of chalk, we reach, at length, in the neighbourhood of
Aurillac, hills of limestone and calcareous marl, in horizontal strata,
separated in some places by regular layers of flint in nodules, the coating
of each nodule being of an opaque white colour, like the exterior of the
flinty nodules of our chalk.
It will be remembered that the siliceous stone of Bilin, called _tripoli_,
is a freshwater deposit, and has been shown, by Ehrenberg, to be of
infusorial origin (see p. 24.). What is true of the Bohemian flint and
opal, where the beds attain a thickness of 14 feet, may also, perhaps, be
found to hold good respecting the silex of Aurillac, which may also have
been immediately derived from the minute cases of microscopic animalcules.
But even if this conclusion be established, the abundant supply both of
siliceous, calcareous, and gypseous matter, which the ancient lakes of
France received, may have been connected with the subterranean volcanic
agency of which those regions were so long the theatre, and which may have
impregnated the springs with mineral matter, even before the great outbreak
of lava. It is well known that the hot springs of Iceland, and many other
countries, contain silex in solution; and it has been lately affirmed, that
steam at a high temperature is capable of dissolving quartzose rocks
without the aid of any alkaline or other flux.[189-A]
Travellers not unfrequently mention, in their accounts of India, Australia,
and other distant lands, that they have seen chalk with flints, which they
have assumed to be of the same age as the Cretaceous system of Europe. A
hasty observation of the white limestone and flint of Aurillac might convey
the same idea; but when we turn from the mineral aspect and composition to
the organic remains, we find in the flints of the Cantal the seed-vessels
of the freshwater _Chara_, instead of the marine zoophytes so abundantly
imbedded in chalk flints; and in the limestone we meet with shells of
_Limnea_, _Planorbis_, and other lacustrine genera, instead of the oyster,
terebratula, and echinus of the Cretaceous period.
_Proofs of gradual deposition_.--Some sections of the foliated marls in the
valley of the Cer, near Aurillac, attest, in the most unequivocal manner,
the extreme slowness with which the materials of the lacustrine series were
amassed. In the hill of Barrat, for example, we find an assemblage of
calcareous and siliceous marls; in which, for a depth of at least 60 feet,
the layers are so thin, that thirty are sometimes contained in the
thickness of an inch; and when they are separated, we see preserved in
every one of them the flattened stems of _Charae_, or other plants, or
sometimes myriads of small _Paludinae_ and other freshwater shells. These
minute foliations of the marl resemble precisely some of the recent
laminated beds of the Scotch marl lakes, and may be compared to the pages
of a book, each containing a history of a certain period of the past. The
different layers may be grouped together in beds from a foot to a foot and
a half in thickness, which are distinguished by differences of composition
and colour, the tints being white, green, and brown. Occasionally there is
a parting layer of pure flint, or of black carbonaceous vegetable matter,
about an inch thick, or of white pulverulent marl. We find several hills in
the neighbourhood of Aurillac composed of such materials, for the height of
more than 200 feet from their base, the whole sometimes covered by rocky
currents of trachytic or basaltic lava.[190-A]
Thus wonderfully minute are the separate parts of which some of the most
massive geological monuments are made up! When we desire to classify, it
is necessary to contemplate entire groups of strata in the aggregate;
but if we wish to understand the mode of their formation, and to explain
their origin, we must think only of the minute subdivisions of which
each mass is composed. We must bear in mind how many thin leaf-like
seams of matter, each containing the remains of myriads of testacea and
plants, frequently enter into the composition of a single stratum, and
how vast a succession of these strata unite to form a single group! We
must remember, also, that piles of volcanic matter, like the Plomb du
Cantal, which rises in the immediate neighbourhood of Aurillac, are
themselves equally the result of successive accumulation, consisting of
reiterated sheets of lava, showers of scoriae, and ejected fragments of
rock.--Lastly, we must not forget that continents and mountain-chains,
colossal as are their dimensions, are nothing more than an assemblage of
many such igneous and aqueous groups, formed in succession during an
indefinite lapse of ages, and superimposed upon each other.
FOOTNOTES:
[175-A] Bulletin des Sci. de la Soc. Philom., May, 1825, p. 74.
[176-A] Hebert. Bulletin. 1849, vol. vi. 2d series, p. 459.
[181-A] Scrope, Geology of Central France, p. 15.
[183-A] See Desmarest's Crustacea, plate 55.
[185-A] I believe that the British specimen here figured is
P. _rhombica_, Linn.
[189-A] See Proceedings of Roy. Soc., No. 44. p. 233.
[190-A] Lyell and Murchison, sur les Depots Lacust. Tertiaries du Cantal,
&c. Ann. des Sci. Nat. Oct. 1829.