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.
Before describing the most modern of the sets of strata enumerated in
the tables given at the end of the last chapter, it will be necessary
to say something generally of the mode of classifying the formations
called tertiary.
The name of tertiary has been given to them, because they are all posterior
in date to the rocks termed "secondary," of which the chalk constitutes the
newest group. These tertiary strata were at first confounded, as before
stated, p. 91., with the superficial alluviums of Europe; and it was long
before their real extent and thickness, and the various ages to which they
belong, were fully recognized. They were observed to occur in patches, some
of freshwater, others of marine origin, their geographical area being
usually small as compared to the secondary formations, and their position
often suggesting the idea of their having been deposited in different bays,
lakes, estuaries, or inland seas, after a large portion of the space now
occupied by Europe had already been converted into dry land.
The first deposits of this class, of which the characters were accurately
determined, were those occurring in the neighbourhood of Paris, described
in 1810 by MM. Cuvier and Brongniart. They were ascertained to consist of
successive sets of strata, some of marine, others of freshwater origin,
lying one upon the other. The fossil shells and corals were perceived to be
almost all of unknown species, and to have in general a near affinity to
those now inhabiting warmer seas. The bones and skeletons of land animals,
some of them of large size, and belonging to more than forty distinct
species, were examined by Cuvier, and declared by him not to agree
specifically and for the most part not even generically, with any hitherto
observed in the living creation.
Strata were soon afterwards brought to light in the vicinity of London, and
in Hampshire, which, although dissimilar in mineral composition, were
justly inferred by Mr. T. Webster to be of the same age as those of Paris,
because the greater number of the fossil shells were specifically
identical. For the same reason rocks found on the Gironde, in the South of
France, and at certain points in the North of Italy, were suspected to be
of contemporaneous origin.
A variety of deposits were afterwards found in other parts of Europe, all
reposing immediately on rocks as old or older than the chalk, and which
exhibited certain general characters of resemblance in their organic
remains to those previously observed near Paris and London. An attempt was
therefore made at first to refer the whole to one period; and when at
length this seemed impracticable, it was contended that as in the Parisian
series there were many subordinate formations of considerable thickness
which must have accumulated one after the other, during a great lapse of
time, so the various patches of tertiary strata scattered over Europe might
correspond in age, some of them to the older, and others to the newer,
subdivisions of the Parisian series.
This error, although most unavoidable on the part of those who made the
first generalizations in this branch of geology, retarded seriously for
some years the progress of classification. A more scrupulous attention to
specific distinctions, aided by a careful regard to the relative position
of the strata containing them, led at length to the conviction that there
were formations both marine and freshwater of various ages, and all newer
than the strata of the neighbourhood of Paris and London.
One of the first steps in this chronological reform was made in 1811, by
an English naturalist, Mr. Parkinson, who pointed out the fact that
certain shelly strata, provincially termed "Crag" in Suffolk, lay
decidedly over a deposit which was the continuation of the blue clay of
London. At the same time he remarked that the fossil testacea in these
newer beds were distinct from those of the blue clay, and that while
some of them were of unknown species, others were identical with species
now inhabiting the British seas.
Another important discovery was soon afterwards made by Brocchi in Italy,
who investigated the argillaceous and sandy deposits replete with shells
which form a low range of hills, flanking the Apennines on both sides, from
the plains of the Po to Calabria. These lower hills were called by him the
Subapennines, and were formed of strata of different ages, all newer than
those of Paris and London.
Another tertiary group occurring in the neighbourhood of Bordeaux and Dax,
in the south of France, was examined by M. de Basterot in 1825, who
described and figured several hundred species of shells, which differed for
the most part both from the Parisian series and those of the Subapennine
hills. It was soon, therefore, suspected that this fauna might belong to a
period intermediate between that of the Parisian and Subapennine strata,
and it was not long before the evidence of superposition was brought to
bear in support of this opinion; for other strata, contemporaneous with
those of Bordeaux, were observed in one district (the Valley of the Loire),
to overlie the Parisian formation, and in another (in Piedmont) to underlie
the Subapennine beds. The first example of these was pointed out in 1829 by
M. Desnoyers, who ascertained that the sand and marl of marine origin
called Faluns, near Tours, in the basin of the Loire, full of sea-shells
and corals, rested upon a lacustrine formation, which constitutes the
uppermost subdivision of the Parisian group, extending continuously
throughout a great table-land intervening between the basin of the Seine
and that of the Loire. The other example occurs in Italy, where strata,
containing many fossils similar to those of Bordeaux, were observed by
Bonelli and others in the environs of Turin, subjacent to strata belonging
to the Subapennine group of Brocchi.
Without pretending to give a complete sketch of the progress of
discovery, I may refer to the facts above enumerated, as illustrating
the course usually pursued by geologists when they attempt to found new
chronological divisions. The method bears some analogy to that pursued
by the naturalist in the construction of genera, when he selects a
typical species, and then classes as congeners all other species of
animals and plants which agree with this standard within certain limits.
The genera A. and C. having been founded on these principles, a new
species is afterwards met with, departing widely both from A. and C.,
but in many respects of an intermediate character. For this new type it
becomes necessary to institute the new genus B., in which are included
all species afterwards brought to light, which agree more nearly with B.
than with the types of A. or C. In like manner a new formation is met
with in geology, and the characters of its fossil fauna and flora
investigated. From that moment it is considered as a record of a certain
period of the earth's history, and a standard to which other deposits
may be compared. If any are found containing the same or nearly the same
organic remains, and occupying the same relative position, they are
regarded in the light of contemporary annals. All such monuments are
said to relate to one period, during which certain events occurred, such
as the formation of particular rocks by aqueous or volcanic agency, or
the continued existence and fossilization of certain tribes of animals
and plants. When several of these periods have had their true places
assigned to them in a chronological series, others are discovered
which it becomes necessary to intercalate between those first known;
and the difficulty of assigning clear lines of separation must
unavoidably increase in proportion as chasms in the past history of
the globe are filled up.
Every zoologist and botanist is aware that it is a comparatively easy
task to establish genera in departments which have been enriched with
only a small number of species, and where there is as yet no tendency in
one set of characters to pass almost insensibly, by a multitude of
connecting links, into another. They also know that the difficulty of
classification augments, and that the artificial nature of their
divisions becomes more apparent, in proportion to the increased number
of objects brought to light. But in separating families and genera, they
have no other alternative than to avail themselves of such breaks as
still remain, or of every hiatus in the chain of animated beings which
is not yet filled up. So in geology, we may be eventually compelled to
resort to sections of time as arbitrary, and as purely conventional, as
those which divide the history of human events into centuries. But in
the present state of our knowledge, it is more convenient to use the
interruptions which still occur in the regular sequence of geological
monuments, as boundary lines between our principal groups or periods,
even though the groups thus established are of very unequal value.
The isolated position of distinct tertiary deposits in different parts
of Europe has been already alluded to. In addition to the difficulty
presented by this want of continuity when we endeavour to settle the
chronological relations of these deposits, another arises from the
frequent dissimilarity in mineral character of strata of contemporaneous
date, such, for example, as those of London and Paris before mentioned.
The identity or non-identity of species is also a criterion which often
fails us. For this we might have been prepared, for we have already
seen, that the Mediterranean and Red Sea, although within 70 miles of
each other, on each side of the Isthmus of Suez, have each their
peculiar fauna; and a marked difference is found in the four groups of
testacea now living in the Baltic, English Channel, Black Sea, and
Mediterranean, although all these seas have many species in common. In
like manner a considerable diversity in the fossils of different
tertiary formations, which have been thrown down in distinct seas,
estuaries, bays, and lakes, does not always imply a distinctness in the
times when they were produced, but may have arisen from climate and
conditions of physical geography wholly independent of time. On the
other hand, it is now abundantly clear, as the result of geological
investigation, that different sets of tertiary strata, immediately
superimposed upon each other, contain distinct imbedded species of
fossils, in consequence of fluctuations which have been going on in the
animate creation, and by which in the course of ages one state of things
in the organic world has been substituted for another wholly dissimilar.
It has also been shown that in proportion as the age of a tertiary
deposit is more modern, so is its fauna more analogous to that now in
being in the neighbouring seas. It is this law of a nearer agreement of
the fossil testacea with the species now living, which may often furnish
us with a clue for the chronological arrangement of scattered deposits,
where we cannot avail ourselves of any one of the three ordinary
chronological tests; namely, superposition, mineral character, and the
specific identity of the fossils.
Thus, for example, on the African border of the Red Sea, at the height
of 40 feet, and sometimes more, above its level, a white calcareous
formation has been observed, containing several hundred species of
shells differing from those found in the clay and volcanic tuff of the
country round Naples, and of the contiguous island of Ischia. Another
deposit has been found at Uddevalla, in Sweden, in which the shells do
not agree with those found near Naples. But although in these three
cases there may be scarcely a single shell common to the three different
deposits, we do not hesitate to refer them all to one period (the
Post-Pliocene), because of the very close agreement of the fossil
species in every instance with those now living in the contiguous seas.
To take another example, where the fossil fauna recedes a few steps farther
back from our own times. We may compare, first, the beds of loam and clay
bordering the Clyde in Scotland (called glacial by some geologists),
secondly, others of fluvio-marine origin near Norwich, and, lastly, a third
set often rising to considerable heights in Sicily, and we discover that in
every case more than three-fourths of the shells agree with species still
living, while the remainder are extinct. Hence we may conclude that all
these, greatly diversified as are their organic remains, belong to one and
the same era, or to a period immediately antecedent to the Post-Pliocene,
because there has been time in each of the areas alluded to for an equal or
nearly equal amount of change in the marine testaceous fauna.
Contemporaneousness of origin is inferred in these cases, in spite of the
most marked differences of mineral character or organic contents, from a
similar degree of divergence in the shells from those now living in the
adjoining seas. The advantage of such a test consists in supplying us with
a common point of departure in all countries, however remote.
But the farther we recede from the present times, and the smaller the
relative number of recent as compared with extinct species in the
tertiary deposits, the less confidence can we place in the exact value
of such a test, especially when comparing the strata of very distant
regions; for we cannot presume that the rate of former alterations in
the animate world, or the continual going out and coming in of species,
has been every where exactly equal in equal quantities of time. The
form of the land and sea, and the climate, may have changed more in
one region than in another; and consequently there may have been a
more rapid destruction and renovation of species in one part of the
globe than elsewhere. Considerations of this kind should undoubtedly
put us on our guard against relying too implicitly on the accuracy of
this test; yet it can never fail to throw great light on the
chronological relations of tertiary groups with each other, and with
the Post-Pliocene period.
We may derive a conviction of this truth not only from a study of
geological monuments of all ages, but also by reflecting on the tendency
which prevails in the present state of nature to a uniform rate of
simultaneous fluctuation in the flora and fauna of the whole globe. The
grounds of such a doctrine cannot be discussed here, and I have explained
them at some length in the third Book of the Principles of Geology, where
the causes of the successive extinction of species are considered. It will
be there seen that each local change in climate and physical geography is
attended with the immediate increase of certain species, and the limitation
of the range of others. A revolution thus effected is rarely, if ever,
confined to a limited space, or to one geographical province of animals or
plants, but affects several other surrounding and contiguous provinces. In
each of these, moreover, analogous alterations of the stations and
habitations of species are simultaneously in progress, reacting in the
manner already alluded to on the first province. Hence, long before the
geography of any particular district can be essentially altered, the flora
and fauna throughout the world will have been materially modified by
countless disturbances in the mutual relation of the various members of the
organic creation to each other. To assume that in one large area inhabited
exclusively by a single assemblage of species any important revolution in
physical geography can be brought about, while other areas remain
stationary in regard to the position of land and sea, the height of
mountains, and so forth, is a most improbable hypothesis, wholly opposed to
what we know of the laws now governing the aqueous and igneous causes. On
the other hand, even were this conceivable, the communication of heat and
cold between different parts of the atmosphere and ocean is so free and
rapid, that the temperature of certain zones cannot be materially raised or
lowered without others being immediately affected; and the elevation or
diminution in height of an important chain of mountains or the submergence
of a wide tract of land would modify the climate even of the antipodes.
It will be observed that in the foregoing allusions to organic remains, the
testacea or the shell-bearing mollusca are selected as the most useful and
convenient class for the purposes of general classification. In the first
place, they are more universally distributed through strata of every age
than any other organic bodies. Those families of fossils which are of rare
and casual occurrence are absolutely of no avail in establishing a
chronological arrangement. If we have plants alone in one group of strata
and the bones of mammalia in another, we can draw no conclusion respecting
the affinity or discordance of the organic beings of the two epochs
compared; and the same may be said if we have plants and vertebrated
animals in one series and only shells in another. Although corals are more
abundant, in a fossil state, than plants, reptiles, or fish, they are still
rare when contrasted with shells, especially in the European tertiary
formations. The utility of the testacea is, moreover, enhanced by the
circumstance that some forms are proper to the sea, others to the land, and
others to freshwater. Rivers scarcely ever fail to carry down into their
deltas some land shells, together with species which are at once fluviatile
and lacustrine. By this means we learn what terrestrial, freshwater, and
marine species co-existed at particular eras of the past; and having thus
identified strata formed in seas with others which originated
contemporaneously in inland lakes, we are then enabled to advance a step
farther, and show that certain quadrupeds or aquatic plants, found fossil
in lacustrine formations, inhabited the globe at the same period when
certain fish, reptiles, and zoophytes lived in the ocean.
Among other characters of the molluscous animals, which render them
extremely valuable in settling chronological questions in geology, may be
mentioned, first, the wide geographical range of many species; and,
secondly, what is probably a consequence of the former, the great duration
of species in this class, for they appear to have surpassed in longevity
the greater number of the mammalia and fish. Had each species inhabited a
very limited space, it could never, when imbedded in strata, have enabled
the geologist to identify deposits at distant points; or had they each
lasted but for a brief period, they could have thrown no light on the
connection of rocks placed far from each other in the chronological, or, as
it is often termed, vertical series.
Many authors have divided the European tertiary strata into three
groups--lower, middle, and upper; the lower comprising the oldest
formations of Paris and London before-mentioned; the middle those of
Bordeaux and Touraine; and the upper all those newer than the middle group.
When engaged in 1828 in preparing my work on the Principles of Geology, I
conceived the idea of classing the whole series of tertiary strata in four
groups, and endeavouring to find characters for each, expressive of their
different degrees of affinity to the living fauna. With this view, I
obtained information respecting the specific identity of many tertiary and
recent shells from several Italian naturalists, and among others from
Professors Bonelli, Guidotti, and Costa. Having in 1829 become acquainted
with M. Deshayes, of Paris, already well known by his conchological works,
I learnt from him that he had arrived, by independent researches, and by
the study of a large collection of fossil and recent shells, at very
similar views respecting the arrangement of tertiary formations. At my
request he drew up, in a tabular form, lists of all the shells known to him
to occur both in some tertiary formation and in a living state, for the
express purpose of ascertaining the proportional number of fossil species
identical with the recent which characterized successive groups; and this
table, planned by us in common, was published by me in 1833.[110-A] The
number of tertiary fossil shells examined by M. Deshayes was about 3000;
and the recent species with which they had been compared about 5000. The
result then arrived at was, that in the lower tertiary strata, or those of
London and Paris, there were about 3-1/2 per cent. of species identical
with recent; in the middle tertiary of the Loire and Gironde about 17 per
cent.; and in the upper tertiary or Subapennine beds, from 35 to 50 per
cent. In formations still more modern, some of which I had particularly
studied in Sicily, where they attain a vast thickness and elevation above
the sea, the number of species identical with those now living was believed
to be from 90 to 95 per cent. For the sake of clearness and brevity, I
proposed to give short technical names to these four groups, or the periods
to which they respectively belonged. I called the first or oldest of them
Eocene, the second Miocene, the third Older Pliocene, and the last or
fourth Newer Pliocene. The first of the above terms, Eocene, is derived
from +eos+, eos, _dawn_, and +kainos+, cainos, _recent_, because the fossil
shells of this period contain an extremely small proportion of living
species, which may be looked upon as indicating the dawn of the existing
state of the testaceous fauna, no recent species having been detected in
the older or secondary rocks.
The term Miocene (from +meion+, meion, _less_, and +kainos+, cainos,
_recent_) is intended to express a minor proportion of recent species (of
testacea), the term Pliocene (from +pleion+, pleion, _more_, and +kainos+,
cainos, _recent_) a comparative plurality of the same. It may assist the
memory of students to remind them, that the _Mi_ocene contain a _mi_nor
proportion, and _Pl_iocene a comparative _pl_urality of recent species; and
that the greater number of recent species always implies the more modern
origin of the strata.
It has sometimes been objected to this nomenclature that certain species of
infusoria found in the chalk are still existing, and, on the other hand,
the Miocene and Older Pliocene deposits often contain the remains of
mammalia, reptiles, and fish, exclusively of extinct species. But the
reader must bear in mind that the terms Eocene, Miocene, and Pliocene were
originally invented with reference purely to conchological data, and in
that sense have always been and are still used by me.
The distribution of the fossil species from which the results before
mentioned were obtained in 1830 by M. Deshayes was as follows:--
In the formations of the Pliocene periods, older and newer 777
In the Miocene 1021
In the Eocene 1238
----
3036
----
Since the year 1830 the progress of conchological science has been most
rapid, and the number of living species obtained from different parts of
the globe has been raised from about 5000 to more than 10,000. New
fossil species have also been added to our collections in great
abundance; and at the same time a more copious supply of individuals
both of fossil and recent species, some of which were previously very
rare, have been procured, affording more ample data for determining the
specific character. Besides the reforms introduced in consequence of
these new zoological facilities, other errors of a geological nature
have been in many instances removed.
POST-PLIOCENE FORMATIONS.
I have adopted the term Post-Pliocene for those strata which are sometimes
called post-tertiary or modern, and which are characterized by having all
the imbedded fossil shells identical with species now living, whereas even
the Newer Pliocene, or newest of the tertiary deposits above alluded to,
contain always some small proportion of shells of extinct species.
These modern formations, thus defined, comprehend not only those strata
which can be shown to have originated since the earth was inhabited by man,
but also deposits of far greater extent and thickness, in which no signs of
man or his works can be detected. In some of these, of a date long anterior
to the times of history and tradition, the bones of extinct quadrupeds have
been met with of species which probably never co-existed with the human
race, as, for example, the mammoth, mastodon, megatherium, and others, and
yet the shells are the same as those now living.
That portion of the post-pliocene group which belongs to the human epoch,
and which is sometimes called _Recent_, forms a very unimportant feature in
the geological structure of the earth's crust. I have shown, however, in
"The Principles," where the recent changes of the earth illustrative of
geology are described at length, that the deposits accumulated at the
bottom of lakes and seas within the last 4000 or 5000 years can neither be
insignificant in volume or extent. They lie hidden, for the most part, from
our sight; but we have opportunities of examining them at certain points
where newly-gained land in the deltas of rivers has been cut through during
floods, or where coral reefs are growing rapidly, or where the bed of a sea
or lake has been heaved up by subterranean movements and laid dry. Their
age may be recognized either by our finding in them the bones of man in a
fossil state, that is to say, imbedded in them by natural causes, or by
their containing articles fabricated by the hands of man.
Thus at Puzzuoli, near Naples, marine strata are seen containing
fragments of sculpture, pottery, and the remains of buildings, together
with innumerable shells retaining in part their colour, and of the same
species as those now inhabiting the Bay of Baiae. The uppermost of these
beds is about 20 feet above the level of the sea. Their emergence can be
proved to have taken place since the beginning of the sixteenth
century.[112-A] Now here, as in almost every instance where any
alterations of level have been going on in historical periods, it is
found that rocks containing shells, all, or nearly all, of which still
inhabit the neighbouring sea, may be traced for some distance into the
interior, and often to a considerable elevation above the level of the
sea. Thus, in the country round Naples, the post-pliocene strata,
consisting of clay and horizontal beds of volcanic tuff, rise at certain
points to the height of 1500 feet. Although the marine shells are
exclusively of living species, they are not accompanied like those on
the coast at Puzzuoli by any traces of man or his works. Had any such
been discovered, it would have afforded to the antiquary and geologist
matter of great surprise, since it would have shown that man was an
inhabitant of that part of the globe, while the materials composing
the present hills and plains of Campania were still in the progress
of deposition at the bottom of the sea; whereas we know that for
nearly 3000 years, or from the times of the earliest Greek colonists,
no material revolution in the physical geography of that part of
Italy has occurred.
In Ischia, a small island near Naples, composed in like manner of marine
and volcanic formations, Dr. Philippi collected in the stratified tuff and
clay ninety-two species of shells of existing species. In the centre of
Ischia, the lofty hill called Epomeo, or San Nicola, is composed of
greenish indurated tuff, of a prodigious thickness, interstratified in some
parts with marl, and here and there with great beds of solid lava. Visconti
ascertained by trigonometrical measurement that this mountain was 2605 feet
above the level of the sea. Not far from its summit, at the height of about
2000 feet, as also near Moropano, a village only 100 feet lower, on the
southern declivity of the mountain, I collected, in 1828, many shells of
species now inhabiting the neighbouring gulf. It is clear, therefore, that
the great mass of Epomeo was not only raised to its present height, but was
also _formed_ beneath the waters, within the post-pliocene period.
It is a fact, however, of no small interest, that the fossil shells from
these modern tuffs of the volcanic region surrounding the Bay of Baiae,
although none of them extinct, indicate a slight want of correspondence
between the ancient fauna and that now inhabiting the Mediterranean.
Philippi informs us that when he and M. Scacchi had collected ninety-nine
species of them, he found that only one, _Pecten medius_, now living in the
Red Sea, was absent from the Mediterranean. Notwithstanding this, he adds,
"the condition of the sea when the tufaceous beds were deposited must have
been considerably different from its present state; for _Tellina striata_
was then common, and is now rare; _Lucina spinosa_ was both more abundant
and grew to a larger size; _Lucina fragilis_, now rare, and hardly
measuring 6 lines, then attained the enormous dimensions of 14 lines, and
was extremely abundant; and _Ostrea lamellosa_, Broc., no longer met with
near Naples, existed at that time, and attained a size so large that one
lower valve has been known to measure 5 inches 9 lines in length, 4 inches
in breadth, 1-1/2 inch in thickness, and weighed 26-1/2 ounces."[113-A]
There are other parts of Europe where no volcanic action manifests itself
at the surface, as at Naples, whether by the eruption of lava or by
earthquakes, and yet where the land and bed of the adjoining sea are
undergoing upheaval. The motion is so gradual as to be insensible to the
inhabitants, being only ascertainable by careful scientific measurements
compared after long intervals. Such an upward movement has been proved to
be in progress in Norway and Sweden throughout an area about 1000 miles N.
and S., and for an unknown distance E. and W., the amount of elevation
always increasing as we proceed towards the North Cape, where it may equal
5 feet in a century. If we could assume that there had been an average rise
of 2-1/2 feet in each hundred years for the last fifty centuries, this
would give an elevation of 125 feet in that period. In other words, it
would follow that the shores, and a considerable area of the former bed of
the Baltic and North Sea, had been uplifted vertically to that amount, and
converted into land in the course of the last 5000 years. Accordingly, we
find near Stockholm, in Sweden, horizontal beds of sand, loam, and marl
containing the same peculiar assemblage of testacea which now live in the
brackish waters of the Baltic. Mingled with these, at different depths,
have been detected various works of art implying a rude state of
civilization, and some vessels built before the introduction of iron, the
whole marine formation having been upraised, so that the upper beds are now
60 feet higher than the surface of the Baltic. In the neighbourhood of
these recent strata, both to the north-west and south of Stockholm, other
deposits similar in mineral composition occur, which ascend to greater
heights, in which precisely the same assemblage of fossil shells is met
with, but without any intermixture of human bones or fabricated articles.
On the opposite or western coast of Sweden, at Uddevalla, post-pliocene
strata, containing recent shells, not of that brackish water character
peculiar to the Baltic, but such as now live in the northern ocean, ascend
to the height of 200 feet; and beds of clay and sand of the same age attain
elevations of 300 and even 700 feet in Norway, where they have been usually
described as "raised beaches." They are, however, thick deposits of
submarine origin, spreading far and wide, and filling valleys in the
granite and gneiss, just as the tertiary formations, in different parts of
Europe, cover or fill depressions in the older rocks.
It is worthy of remark, that although the fossil fauna characterizing these
upraised sands and clays consists exclusively of existing northern species
of testacea, yet, according to Loven (an able living naturalist of Norway),
the species do not constitute such an assemblage as now inhabits
corresponding latitudes in the German Ocean. On the contrary, they
decidedly represent a more arctic fauna.[114-A] In order to find the same
species flourishing in equal abundance, or in many cases to find them at
all, we must go northwards to higher latitudes than Uddevalla in Sweden, or
even nearer the pole than Central Norway.
Judging by the uniformity of climate now prevailing from century to
century, and the insensible rate of variation in the organic world in our
own times, we may presume that an extremely lengthened period was required
even for so slight a modification of the molluscous fauna, as that of which
the evidence is here brought to light. On the other hand, we have every
reason for inferring on independent grounds (namely, the rate of upheaval
of land in modern times) that the antiquity of the deposits in question
must be very great. For if we assume, as before suggested, that the mean
rate of continuous vertical elevation has amounted to 2-1/2 feet in a
century (and this is probably a high average), it would require 27,500
years for the sea-coast to attain the height of 700 feet, without making
allowance for any pauses such as are now experienced in a large part of
Norway, or for any oscillations of level.
In England, buried ships have been found in the ancient and now deserted
channels of the Rother in Sussex, of the Mersey in Kent, and the Thames
near London. Canoes and stone hatchets have been dug up, in almost all
parts of the kingdom, from peat and shell-marl; but there is no evidence,
as in Sweden, Italy, and many other parts of the world, of the bed of the
sea, and the adjoining coast, having been uplifted bodily to considerable
heights within the human period. Recent strata have been traced along the
coasts of Peru and Chili, inclosing shells in abundance, all agreeing
specifically with those now swarming in the Pacific. In one bed of this
kind, in the island of San Lorenzo, near Lima, Mr. Darwin found, at the
altitude of 85 feet above the sea, pieces of cotton-thread, plaited rush,
and the head of a stalk of Indian corn, the whole of which had evidently
been imbedded with the shells. At the same height on the neighbouring
mainland, he found other signs corroborating the opinion that the ancient
bed of the sea had there also been uplifted 85 feet, since the region was
first peopled by the Peruvian race.[115-A] But similar shelly masses are
also met with at much higher elevations, at innumerable points between the
Chilian and Peruvian Andes and the sea-coast, in which no human remains
were ever, or in all probability ever will be, discovered.
In the West Indies, also, in the island of Guadaloupe, a solid limestone
occurs, at the level of the sea-beach, enveloping human skeletons. The
stone is extremely hard, and chiefly composed of comminuted shell and
coral, with here and there some entire corals and shells, of species now
living in the adjacent ocean. With them are included arrow-heads, fragments
of pottery, and other articles of human workmanship. A limestone with
similar contents has been formed, and is still forming, in St. Domingo. But
there are also more ancient rocks in the West Indian Archipelago, as in
Cuba, near the Havanna, and in other islands, in which are shells identical
with those now living in corresponding latitudes; some well-preserved,
others in the state of casts, all referable to the post-pliocene period.
I have already described in the seventh chapter, p. 84., what would be
the effects of oscillations and changes of level in any region drained
by a great river and its tributaries, supposing the area to be first
depressed several hundred feet, and then re-elevated. I believe that
such changes in the relative level of land and sea have actually
occurred in the post-pliocene era in the hydrographical basin of the
Mississippi and in that of the Rhine. The accumulation of fluviatile
matter in a delta during a slow subsidence may raise the newly gained
land superficially at the same rate at which its foundations sink, so
that these may go down hundreds or thousands of feet perpendicularly,
and yet the sea bordering the delta may always be excluded, the whole
deposit continuing to be terrestrial or freshwater in character. This
appears to have happened in the deltas both of the Po and Ganges, for
recent artesian borings, penetrating to the depth of 400 feet, have
there shown that fluviatile strata, with shells of recent species,
together with ancient surfaces of land supporting turf and forests, are
depressed hundreds of feet below the sea level.[116-A] Should these
countries be once more slowly upraised, the rivers would carve out
valleys through the horizontal and unconsolidated strata as they rose,
sweeping away the greater portion of them, and leaving mere fragments in
the shape of terraces skirting newly-formed alluvial plains, as
monuments of the former levels at which the rivers ran. Of this nature
are "the bluffs," or river cliffs, now bounding the valley of the
Mississippi throughout a large portion of its course. Thus let _a b_,
fig. 106., represent the alluvial plain of the Mississippi, a plain
which, at the point alluded to, is more than 30 miles broad, and is
truly a prolongation of the modern delta of that river. It is bounded by
bluffs, the upper portions of which consist, both on the east and west
side, of shelly loam, No. 2. rising from 100 to 200 feet above the level
of the plain, and containing land and freshwater shells of the genera
_Helix_, _Pupa_, _Succinea_, and _Lymnea_, of the same species as those
now inhabiting the neighbouring forests and swamps. In the same loam
also, No. 2., are found the bones of the Mastodon, Elephant, Megalonyx,
and other extinct quadrupeds.
[Illustration: Fig. 106. Valley of the Mississippi.
1. Alluvium.
2. Loess.
3. _f_. Eocene.
4. Cretaceous.]
I have endeavoured to show that the deposits forming the delta and alluvial
plain of the Mississippi consist of sedimentary matter, extending over an
area of 30,000 square miles, and known in some parts to be several hundred
feet deep. Although we cannot estimate correctly how many years it may have
required for the river to bring down from the upper country so large a
quantity of earthy matter--the data for such a computation being as yet
incomplete--we may still approximate to a minimum of the time which such an
operation must have taken, by ascertaining experimentally the annual
discharge of water by the Mississippi, and the mean annual amount of solid
matter contained in its waters. The lowest estimate of the time required
would lead us to assign a high antiquity, amounting to many tens of
thousands of years to the existing delta, the origin of which is
nevertheless an event of yesterday when contrasted with those terraces,
_c_, and _d e_, fig. 106., formed of the loam No. 2. above mentioned. These
materials of the bluffs _a_ and _d_ were produced, the reader will observe,
during the first part of that great oscillation of level which depressed to
a depth of 200 feet a larger area than the modern delta and plain of the
Mississippi, and then restored the region to its former position.[117-A]
_Loess of the Valley of the Rhine._--A similar succession of geographical
changes, attended by the production of a fluviatile formation, singularly
resembling that which bounds the great plain of the Mississippi, seems to
have occurred in the hydrographical basin of the Rhine, since the time when
that basin had already acquired its present outline of hill and valley. I
allude to the deposit provincially termed _loess_ in part of Germany, or
_lehm_ in Alsace, filled with land and freshwater shells of existing
species. It is a finely comminuted sand or pulverulent loam of a yellowish
grey colour, consisting chiefly of argillaceous matter combined with a
sixth part of carbonate of lime, and a sixth of quartzose and micaceous
sand. It often contains calcareous sandy concretions or nodules, rarely
exceeding the size of a man's head. Its entire thickness amounts, in some
places, to between 200 and 300 feet; yet there are often no signs of
stratification in the mass, except here and there at the bottom, where
there is occasionally a slight intermixture of drifted materials derived
from subjacent rocks. Unsolidified as it is, and of so perishable a nature,
that every streamlet flowing over it cuts out for itself a deep gully, it
usually terminates in a vertical cliff, from the surface of which land
shells are seen here and there to project in relief. In all these features
it presents a precise counterpart to the loess of the Mississippi. It is so
homogeneous as generally to exhibit no signs of stratification, owing,
probably, to its materials having been derived from a common source, and
having been accumulated by a uniform action. Yet it displays in some few
places decided marks of successive deposition, where coarser and finer
materials alternate, especially near the bottom. Calcareous concretions,
also enclosing land-shells, are sometimes arranged in horizontal layers. It
is a remarkable deposit, from its position, wide extent, and thickness, its
homogeneous mineral composition, and freshwater origin. Its distribution
clearly shows that after the great valley of the Rhine, from Schaffhausen
to Bonn, had acquired its present form, having its bottom strewed over with
coarse gravel, a period arrived when it became filled up from side to side
with fine mud, which was also thrown down in the valleys of the principal
tributaries of the Rhine.
Thus, for example, it may be traced far into Wuertemberg, up the valley of
the Neckar, and from Frankfort, up the valley of the Main, to above
Dettelbach. I have also seen it spreading over the country of Mayence,
Eppelsheim, and Worms, on the left bank of the Rhine, and on the opposite
side on the table-land above the Bergstrasse, between Wiesloch and
Bruchsal, where it attains a thickness of 200 feet. Near Strasburg, large
masses of it appear at the foot of the Vosges on the left bank, and at the
base of the mountains of the Black Forest on the right bank. The
Kaiserstuhl, a volcanic mountain which stands in the middle of the plain of
the Rhine near Freiburg, has been covered almost everywhere with this loam,
as have the extinct volcanos between Coblentz and Bonn. Near Andernach, in
the Kirchweg, the loess containing the usual shells alternates with
volcanic matter; and over the whole are strewed layers of pumice, lapilli,
and volcanic sand, from 10 to 15 feet thick, very much resembling the
ejections under which Pompeii lies buried. There is no passage at this
upper junction from the loess into the pumiceous superstratum; and this
last follows the slope of the hill, just as it would have done had it
fallen in showers from the air on a declivity partly formed of loess.
But, in general, the loess overlies all the volcanic products, even those
between Neuwied and Bonn, which have the most modern aspect; and it has
filled up in part the crater of the Roderberg, an extinct volcano near
Bonn. In 1833 a well was sunk at the bottom of this crater, through 70 feet
of loess, in part of which were the usual calcareous concretions.
The interstratification above alluded to, of loess with layers of pumice
and volcanic ashes, has led to the opinion that both during and since
its deposition some of the last volcanic eruptions of the Lower Eifel
have taken place. Should such a conclusion be adopted, we should be
called upon to assign a very modern date to these eruptions. This
curious point, therefore, deserves to be reconsidered; since it may
possibly have happened that the waters of the Rhine, swollen by the
melting of snow and ice, and flowing at a great height through a valley
choked up with loess, may have swept away the loose superficial scoriae
and pumice of the Eifel volcanos, and spread them out occasionally over
the yellow loam. Sometimes, also, the melting of snow on the slope of
small volcanic cones may have given rise to local floods, capable of
sweeping down light pumice into the adjacent low grounds.
The first idea which has occurred to most geologists, after examining
the loess between Mayence and Basle, is to imagine that a great lake
once extended throughout the valley of the Rhine between those two
places. Such a lake may have sent off large branches up the course of
the Main, Neckar, and other tributary valleys, in all of which large
patches of loess are now seen. The barrier of the lake might be placed
somewhere in the narrow and picturesque gorge of the Rhine between
Bingen and Bonn. But this theory fails altogether to explain the
phenomena; when we discover that that gorge itself has once been filled
with loess, which must have been tranquilly deposited in it, as also in
the lateral valley of the Lahn, communicating with the gorge. The loess
has also overspread the high adjoining platform near the village of
Plaidt above Andernach. Nay, on proceeding farther down to the north, we
discover that the hills which skirt the great valley between Bonn and
Cologne have loess on their flanks, which also covers here and there the
gravel of the plain as far as Cologne, and the nearest rising grounds.
Besides these objections to the lake theory, the loess is met with near
Basle, capping hills more than 1200 feet above the sea; so that a barrier
of land capable of separating the supposed lake from the ocean would
require to be, at least, as high as the mountains called the Siebengebirge,
near Bonn, the loftiest summit of which, the Oehlberg, is 1209 feet above
the Rhine and 1369 feet above the sea. It would be necessary, moreover, to
place this lofty barrier somewhere below Cologne, or precisely where the
level of the land is now lowest.
Instead, therefore, of supposing one continuous lake of sufficient extent
and depth to allow of the simultaneous accumulation of the loess, at
various heights, throughout the whole area where it now occurs, I formerly
suggested that, subsequently to the period when the countries now drained
by the Rhine and its tributaries had nearly acquired their actual form and
geographical features, they were again depressed gradually by a movement
like that now in progress on the west coast of Greenland.[119-A] In
proportion as the whole district was lowered, the general fall of the
waters between the Alps and the ocean was lessened; and both the main and
lateral valleys, becoming more subject to river inundations, were partially
filled up with fluviatile silt, containing land and freshwater shells. When
a thickness of many hundred feet of loess had been thrown down slowly by
this operation, the whole region was once more upheaved gradually. During
this upward movement most of the fine loam would be carried off by the
denuding power of rains and rivers; and thus the original valleys might
have been re-excavated, and the country almost restored to its pristine
state, with the exception of some masses and patches of loess such as still
remain, and which, by their frequency and remarkable homogeneousness of
composition and fossils, attest the ancient continuity and common origin of
the whole. By imagining these oscillations of level, we dispense with the
necessity of erecting and afterwards removing a mountain barrier
sufficiently high to exclude the ocean from the valley of the Rhine during
the period of the accumulation of the loess.
The proportion of land shells of the genera _Helix_, _Pupa_, and _Bulimus_,
is very large in the loess; but in many places aquatic species of the
genera _Lymnea_, _Paludina_, and _Planorbis_ are also found. These may have
been carried away during floods from shallow pools and marshes bordering
the river; and the great extent of marshy ground caused by the wide
overflowings of rivers above supposed would favour the multiplication of
amphibious mollusks, such as the _Succinea_ (fig. 107.), which is almost
everywhere characteristic of this formation, and is sometimes accompanied,
as near Bonn, by another species, _S. amphibia_ (fig. 34. p. 29.). Among
other abundant fossils are _Helix plebeium_ and _Pupa muscorum_. (See
Figures.) Both the terrestrial and aquatic shells preserved in the loess
are of most fragile and delicate structure, and yet they are almost
invariably perfect and uninjured. They must have been broken to pieces had
they been swept along by a violent inundation. Even the colour of some of
the land shells, as that of _Helix nemoralis_, is occasionally preserved.
[Illustration: Fig. 107. _Succinea elongata._]
[Illustration: Fig. 108. _Pupa muscorum._]
[Illustration: Fig. 109. _Helix plebeium._]
Bones of vertebrated animals are rare in the loess, but those of the
mammoth, horse, and some other quadrupeds have been met with. At the
village of Binningen, and the hills called Bruderholz, near Basle, I found
the vertebrae of fish, together with the usual shells. These vertebrae,
according to M. Agassiz, belong decidedly to the Shark family, perhaps to
the genus _Lamna_. In explanation of their occurrence among land and
freshwater shells, it may be stated that certain fish of this family ascend
the Senegal, Amazon, and other great rivers, to the distance of several
hundred miles from the ocean.[120-A]
At Cannstadt, near Stuttgart, in a valley also belonging to the
hydrographical basin of the Rhine, I have seen the loess pass downwards
into beds of calcareous tuff and travertin. Several valleys in northern
Germany, as that of the Ilm at Weimar, and that of the Tonna, north of
Gotha, exhibit similar masses of modern limestone filled with recent shells
of the genera _Planorbis_, _Lymnea_, _Paludina_, &c., from 50 to 80 feet
thick, with a bed of loess much resembling that of the Rhine, occasionally
incumbent on them. In these modern limestones used for building, the bones
of _Elephas primigenius_, _Rhinoceros tichorinus_, _Ursus spelaeus_, _Hyaena
spelaea_, with the horse, ox, deer, and other quadrupeds, occur; and in 1850
Mr. H. Credner and I obtained in a quarry at Tonna, at the depth of 15
feet, inclosed in the calcareous rock and surrounded with dicotyledonous
leaves and petrified leaves, four eggs of a snake of the size of the
largest European Coluber, which, with three others, had been found lying
in a series, or string.
They are, I believe, the first reptilian remains which have been met with
in strata of this age.
The agreement of the shells in these cases with recent European species
enables us to refer to a very modern period the filling up and
re-excavation of the valleys; an operation which doubtless consumed
a long period of time, since which the mammiferous fauna has undergone
a considerable change.
FOOTNOTES:
[110-A] See Princ. of Geol. vol. iii. 1st ed.
[112-A] See Principles, Index, "Serapis."
[113-A] Geol. Quart. Journ. vol. ii. Memoirs, p. 15.
[114-A] Quart. Geol. Journ. 4 Mems. p. 48.
[115-A] Journal, p. 451.
[116-A] See Principles, 8th ed. pp. 260-268.
[117-A] Lyell's Second Visit to the United States, vol. ii. chap. xxxiv.
[119-A] Princ. of Geol. 3d edition, 1834, vol. iii. p. 414.
[120-A] Proceedings Geol. Soc. No. 43. p. 222.