Henry Alleyne Nicholson.

The Ancient Life History of the Earth A Comprehensive Outline of the Principles and Leading Facts of Palæontological Science online

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submerged, the Carboniferous rocks, or any younger formation,
might be deposited directly upon Silurian strata. From one or
other of these causes, then, or from subsequent disturbances
and denudations, it happens that we can rarely find many of the
primary formations following one another consecutively and in
their regular order.

[Footnote 8: As we have every reason to believe that dry land
and sea have existed, at any rate from the commencement of the
Laurentian period to the present day, it is quite obvious that
no one of the great formations can ever, under any circumstances,
have extended over the entire globe. In other words, no one of
the formations can ever have had a greater geographical extent
than that of the seas of the period in which the formation was
deposited. Nor is there any reason for thinking that the proportion
of dry land to ocean has ever been materially different to what
it is at present, however greatly the areas of sea and land may
have changed as regards their place. It follows from the above,
that there is no sufficient basis for the view that the crust of
the earth is composed of a succession of concentric layers, like
the coats of an onion, each layer representing one formation.]

In no case, however, do we ever find the Devonian resting upon
the Carboniferous, or the Silurian rocks reposing on the Devonian.
We have therefore, by a comparison of many different areas, an
established order of succession of the stratified formations, as
shown in the subjoined ideal section of the crust of the earth
(fig. 17).

The main subdivisions of the stratified rocks are known by the
following names: -

1. Laurentian.
2. Cambrian (with Huronian ?).
3. Silurian.
4. Devonian or Old Red Sandstone.
5. Carboniferous.
6. Permian \_ New Red Sandstone.
7. Triassic /
8. Jurassic or Oolitic.
9. Cretaceous.
10. Eocene.
11. Miocene.
12. Pliocene.
13. Post-tertiary.

[Illustration: Fig. 17. IDEAL SECTION OF THE CRUST OF THE EARTH.]

Of these primary rock divisions, the Laurentian, Cambrian, Silurian,
Devonian, Carboniferous, and Permian are collectively grouped
together under the name of the Primary or _Paloeozoic_ rocks (Gr.
_palaios_, ancient; _zoe_, life). Not only do they constitute the
oldest stratified accumulations, but from the extreme divergence
between their animals and plants and those now in existence, they may
appropriately be considered as belonging to an "Old-Life" period of
the world's history. The Triassic, Jurassic, and Cretaceous systems
are grouped together as the _Secondary_ or _Mesozoic_ formations
(Gr. _mesos_, intermediate; _zoe_, life); the organic remains of
this "Middle-Life" period being, on the whole, intermediate in
their characters between those of the palæozoic epoch and those
of more modern strata. Lastly, the Eocene, Miocene, and Pliocene
formations are grouped together as the _Tertiary_ or _Kainozoic_
rocks (Gr. _kainos_, new; _zoe_, life); because they constitute
a "New-Life" period, in which the organic remains approximate in
character to those now existing upon the globe. The so-called
_Post-Tertiary_ deposits are placed with the Kainozoic, or may
be considered as forming a separate _Quaternary_ system.




CHAPTER IV.

THE BREAKS IN THE GEOLOGICAL AND PALÆONTOLOGICAL RECORD.

The term "contemporaneous" is usually applied by geologists to
groups of strata in different regions which contain the same
fossils, or an assemblage of fossils in which many identical
forms are present. That is to say, beds which contain identical,
or nearly identical, fossils, however widely separated they may
be from one another in point of actual distance, are ordinarily
believed to have been deposited during the same period of the
earth's history. This belief, indeed, constitutes the keystone
of the entire system of determining the age of strata by their
fossil contents; and if we take the word "contemporaneous" in a
general and strictly geological sense, this belief can be accepted
as proved beyond denial. We must, however, guard ourselves against
too literal an interpretation of the word "contemporaneous,"
and we must bear in mind the enormously-prolonged periods of
time with which the geologist has to deal. When we say that two
groups of strata in different regions are "contemporaneous," we
simply mean that they were formed during the same geological
period, and perhaps at different stages of that period, and we
do not mean to imply that they were formed at precisely the same
instant of time.

A moment's consideration will show us that it is only in the former
sense that we can properly speak of strata being "contemporaneous;"
and that, in point of fact, beds containing the same fossils, if
occurring in widely distant areas, can hardly be "contemporaneous"
in any literal sense; but that the very identity of their fossils
is proof that they were deposited one after the other. If we find
strata containing identical fossils within the limits of a single
geographical region - say in Europe - then there is a reasonable
probability that these beds are strictly contemporaneous, in the
sense that they were deposited at the same time. There is a
reasonable probability of this, because there is no improbability
involved in the idea of an ocean occupying the whole area of
Europe, and peopled throughout by many of the same species of
marine animals. At the present day, for example, many identical
species of animals are found living on the western coasts of
Britain and the eastern coasts of North America, and beds now
in course of deposition off the shores of Ireland and the seaboard
of the state of New York would necessarily contain many of the
same fossils. Such beds would be both literally and geologically
contemporaneous; but the case is different if the distance between
the areas where the strata occur be greatly increased. We find,
for example, beds containing identical fossils (the Quebec or
Skiddaw beds) in Sweden, in the north of England, in Canada,
and in Australia. Now, if all these beds were contemporaneous,
in the literal sense of the term, we should have to suppose that
the ocean at one time extended uninterruptedly between all these
points, and was peopled throughout the vast area thus indicated
by many of the same animals. Nothing, however, that we see at
the present day would justify us in imagining an ocean of such
enormous extent, and at the same time so uniform in its depth,
temperature, and other conditions of marine life, as to allow the
same animals to flourish in it from end to end; and the example
chosen is only one of a long and ever-recurring series. It is
therefore much more reasonable to explain this, and all similar
cases, as owing to the _migration_ of the fauna, in whole or in
part, from one marine area to another. Thus, we may suppose an
ocean to cover what is now the European area, and to be peopled
by certain species of animals. Beds of sediment - clay, sands,
and limestones - will be deposited over the sea-bottom, and will
entomb the remains of the animals as fossils. After this has
lasted for a certain length of time, the European area may undergo
elevation, or may become otherwise unsuitable for the perpetuation
of its fauna; the result of which would be that some or all of the
marine animals of the area would migrate to some more suitable
region. Sediments would then be accumulated in the new area to
which they had betaken themselves, and they would then appear,
for the second time, as fossils in a set of beds widely separated
from Europe. The second set of beds would, however, obviously
not be strictly or literally contemporaneous with the first, but
would be separated from them by the period of time required for
the migration of the animals from the one area into the other.
It is only in a wide and comprehensive sense that such strata
can be said to be contemporaneous.

It is impossible to enter further into this subject here; but it
may be taken as certain that beds in widely remote geographical
areas can only come to contain the same fossils by reason of a
migration having taken place of the animals of the one area to
the other. That such migrations can and do take place is quite
certain, and this is a much more reasonable explanation of the
observed facts than the hypothesis that in former periods the
conditions of life were much more uniform than they are at present,
and that, consequently, the same organisms were able to range over
the entire globe at the same time. It need only be added, that
taking the evidence of the present as explaining the phenomena
of the past - the only safe method of reasoning in geological
matters - we have abundant proof that deposits which _are_ actually
contemporaneous, in the strict sense of the term, _do not contain
the same fossils, if far removed from one another in point of
distance_. Thus, deposits of various kinds are now in process of
formation in our existing seas, as, for example, in the Arctic
Ocean, the Atlantic, and the Pacific, and many of these deposits
are known to us by actual examination and observation with the
sounding-lead and dredge. But it is hardly necessary to add that
the animal remains contained in these deposits - the fossils of some
future period - instead of being identical, are widely different
from one another in their characters.

We have seen, then, that the entire stratified series is capable of
subdivision into a number of definite rock-groups or "formations,"
each possessing a peculiar and characteristic assemblage of fossils,
representing the "life" of the "period" in which the formation
was deposited. We have still to inquire shortly how it came to
pass that two successive formations _should_ thus be broadly
distinguished by their life-forms, and why they should not rather
possess at any rate a majority of identical fossils. It was
originally supposed that this could be explained by the hypothesis
that the close of each formation was accompanied by a general
destruction of all the living beings of the period, and that
the commencement of each new formation was signalised by the
creation of a number of brand-new organisms, destined to figure
as the characteristic fossils of the same. This theory, however,
ignores the fact that each formation - as to which we have any
sufficient evidence - contains a few, at least, of the life-forms
which existed in the preceding period; and it invokes forces
and processes of which we know nothing, and for the supposed
action of which we cannot account. The problem is an undeniably
difficult one, and it will not be possible here to give more than
a mere outline of the modern views upon the subject. Without
entering into the at present inscrutable question as to the manner
in which new life-forms are introduced upon the earth, it may be
stated that almost all modern geologists hold that the living
beings of any given formation are in the main modified forms of
others which have preceded them. It is not believed that any
general or universal destruction of life took place at the
termination of each geological period, or that a general introduction
of new forms took place at the commencement of a new period.
It is, on the contrary, believed that the animals and plants
of any given period are for the most part (or exclusively) the
lineal but modified descendants of the animals and plants of
the immediately preceding period, and that some of them, at any
rate, are continued into the next succeeding period, either
unchanged, or so far altered as to appear as new species. To
discuss these views in detail would lead us altogether too far,
but there is one very obvious consideration which may advantageously
receive some attention. It is obvious, namely, that the great
discordance which is found to subsist between the animal life of
any given formation and that of the next succeeding formation,
and which no one denies, would be a fatal blow to the views just
alluded to, unless admitting of some satisfactory explanation.
Nor is this discordance one purely of life-forms, for there is
often a physical break in the successions of strata as well.
Let us therefore briefly consider how far these interruptions
and breaks in the geological and palæontological record can be
accounted for, and still allow us to believe in some theory of
continuity as opposed to the doctrine of intermittent and occasional
action.

In the first place, it is perfectly clear that if we admit the
conception above mentioned of a continuity of life from the
Laurentian period to the present day, we could never _prove_ our
view to be correct, unless we could produce in evidence fossil
examples of _all_ the kinds of animals and plants that have lived
and died during that period. In order to do this, we should require,
to begin with, to have access to an absolutely unbroken and perfect
succession of all the deposits which have ever been laid down
since the beginning. If, however, we ask the physical geologist
if he is in possession of any such uninterrupted series, he will
at once answer in the negative. So far from the geological series
being a perfect one, it is interrupted by numerous gaps of unknown
length, many of which we can never expect to fill up. Nor are
the proofs of this far to seek. Apart from the facts that we
have hitherto examined only a limited portion of the dry land,
that nearly two-thirds of the entire area of the globe is
inaccessible to geological investigation in consequence of its
being covered by the sea, that many deposits can be shown to
have been more or less completely destroyed subsequent to their
deposition, and that there may be many areas in which living
beings exist where no rock is in process of formation, we have
the broad fact that rock-deposition only goes on to any extent
in water, and that the earth must have always consisted partly of
dry land and partly of water - at any rate, so far as any period
of which we have geological knowledge is concerned. There _must_,
therefore, always have existed, at some part or another of the
earth's surface, areas where no deposition of rock was going on,
and the proof of this is to be found in the well-known phenomenon
of "_unconformability_." Whenever, namely, deposition of sediment
is continuously going on within the limits of a single ocean, the
beds which are laid down succeed one another in uninterrupted
and regular sequence. Such beds are said to be "conformable," and
there are many rock-groups known where one may pass through fifteen
or twenty thousand feet of strata without a break - indicating
that the beds had been deposited in an area which remained
continuously covered by the sea. On the other hand, we commonly
find that there is no such regular succession when we pass from
one great formation to another, but that, on the contrary, the
younger formation rests "unconformably," as it is called, either
upon the formation immediately preceding it in point of time,
or upon some still older one. The essential physical feature of
this unconformability is that the beds of the younger formation
rest upon a worn and eroded surface formed by the beds of the
older series (fig. 18); and a moment's consideration will show
us what this indicates. It indicates, beyond the possibility of
misconception, that there was an interval between the deposition
of the older series and that of the newer series of strata; and
that during this interval the older beds were raised above the
sea-level, so as to form dry land, and were subsequently depressed
again beneath the waters, to receive upon their worn and wasted
upper surface the sediments of the later group. During the interval
thus indicated, the deposition of rock must of necessity have
been proceeding more or less actively in other areas. Every
unconformity, therefore, indicates that at the spot where it
occurs, a more or less extensive series of beds must be actually
missing; and though we may sometimes be able to point to these
missing strata in other areas, there yet remains a number of
unconformities for which we cannot at present supply the deficiency
even in a partial manner.

[Illustration: Fig. 18. - Section showing strata of Tertiary age
(a) resting upon a worn and eroded surface of White Chalk (b),
the stratification of which is marked by lines of flint.]

It follows from the above that the series of stratified deposits
is to a greater or less extent irremediably imperfect; and in
this imperfection we have one great cause why we can never obtain
a perfect series of all the animals and plants that have lived
upon the globe. Wherever one of these great physical gaps occurs,
we find, as we might expect, a corresponding break in the series
of life-forms. In other words, whenever we find two formations
to be unconformable, we shall always find at the same time that
there is a great difference in their fossils, and that many of
the fossils of the older formation do not survive into the newer,
whilst many of those in the newer are not known to occur in the
older. The cause of this is, obviously, that the lapse of time,
indicated by the unconformability, has been sufficiently great
to allow of the dying out or modification of many of the older
forms of life, and the introduction of new ones by immigration.

Apart, however, altogether, from these great physical breaks
and their corresponding breaks in life, there are other reasons
why we can never become more than partially acquainted with the
former denizens of the globe. Foremost amongst these is the fact
that an enormous number of animals possess no hard parts of the
nature of a skeleton, and are therefore incapable, under any
ordinary circumstances, of leaving behind them any traces of
their existence. It is true that there are cases in which animals
in themselves completely soft-bodied are nevertheless able to leave
marks by which their former presence can be detected: Thus every
geologist is familiar with the winding and twisting "trails" formed
on the surface of the strata by sea-worms; and the impressions
left by the stranded carcases of Jelly-fishes on the fine-grained
lithographic slates of Solenhofen supply us with an example of how
a creature which is little more than "organised sea-water" may
still make an abiding mark upon the sands of time. As a general
rule, however, animals which have no skeletons are incapable of
being preserved as fossils, and hence there must always have
been a vast number of different kinds of marine animals of which
we have absolutely no record whatever. Again, almost all the
fossiliferous rocks have been laid down in water; and it is a
necessary result of this that the great majority of fossils are
the remains of aquatic animals. The remains of air-breathing
animals, whether of the inhabitants of the land or of the air
itself, are comparatively rare as fossils, and the record of
the past existence of these is much more imperfect than is the
case with animals living in water. Moreover, the fossiliferous
deposits are not only almost exclusively aqueous formations, but
the great majority are marine, and only a comparatively small
number have been formed by lakes and rivers. It follows from the
foregoing that the palæontological record is fullest and most
complete so far as sea-animals are concerned, though even here we
find enormous gaps, owing to the absence of hard structures in
many great groups; of animals inhabiting fresh waters our knowledge
is rendered still further incomplete by the small proportion
that fluviatile and lacustrine deposits bear to marine; whilst
we have only a fragmentary acquaintance with the air-breathing
animals which inhabited the earth during past ages.

Lastly, the imperfection of the palæontological record, due to
the causes above enumerated, is greatly aggravated, especially
as regards the earlier portion of the earth's history, by the
fact that many rocks which contained fossils when deposited have
since been rendered barren of organic remains. The principal cause
of this common phenomenon is what is known as "metamorphism" - that
is, the subjection of the rock to a sufficient amount of heat to
cause a rearrangement of its particles. When at all of a pronounced
character, the result of metamorphic action is invariably the
obliteration of any fossils which might have been originally
present in the rock. Metamorphism may affect rocks of any age,
though naturally more prevalent in the older rocks, and to this
cause must be set down an irreparable loss of much fossil evidence.
The most striking example which is to be found of this is the
great Laurentian series, which comprises some 30,000 feet of
highly-metamorphosed sediments, but which, with one not wholly
undisputed exception, has as yet yielded no remains of living
beings, though there is strong evidence of the former existence
in it of fossils.

Upon the whole, then, we cannot doubt that the earth's crust, so
far as yet deciphered by us, presents us with but a very imperfect
record of the past. Whether the known and admitted imperfections
of the geological and palæontological records are sufficiently
serious to account satisfactorily for the deficiency of direct
evidence recognisable in some modern hypotheses, may be a matter
of individual opinion. There can, however, be little doubt that
they are sufficiently extensive to throw the balance of evidence
decisively in favour of some theory of _continuity_, as opposed
to any theory of intermittent and occasional action. The apparent
breaks which divide the great series of the stratified rocks
into a number of isolated formations, are not marks of mighty
and general convulsions of nature, but are simply indications
of the imperfection of our knowledge. Never, in all probability,
shall we be able to point to a complete series of deposits, or a
complete succession of life linking one great geological period
to another. Nevertheless, we may well feel sure that such deposits
and such an unbroken succession must have existed at one time.
We are compelled to believe that nowhere in the long series of
the fossiliferous rocks has there been a total break, but that
there must have been a complete continuity of life, and a more
or less complete continuity of sedimentation, from the Laurentian
period to the present day. One generation hands on the lamp of
life to the next, and each system of rocks is the direct offspring
of those which preceded it in time. Though there has not been
continuity in any given area, still the geological chain could
never have been snapped at one point, and taken up again at a
totally different one. Thus we arrive at the conviction that
_continuity_ is the fundamental law of geology, as it is of the
other sciences, and that the lines of demarcation between the
great formations are but gaps in our own knowledge.




CHAPTER V.

CONCLUSIONS TO BE DRAWN FROM FOSSILS.

We have already seen that geologists have been led by the study
of fossils to the all-important generalisation that the vast
series of the Fossiliferous or Sedimentary Rocks may be divided
into a number of definite groups or "formations," each of which is
characterised by its organic remains. It may simply be repeated here
that these formations are not properly and strictly characterised
by the occurrence in them of any one particular fossil. It may be
that a formation contains some particular fossil or fossils not
occurring out of that formation, and that in this way an observer
may identify a given group with tolerable certainty. It very often
happens, indeed, that some particular stratum, or sub-group of a
series, contains peculiar fossils, by which its existence may
be determined in various localities. As before remarked, however,
the great formations are characterised properly by the association
of certain fossils, by the predominance of certain families or
orders, or by an _assemblage_ of fossil remains representing
the "life" of the period in which the formation was deposited.

Fossils, then, enable us to determine the _age_ of the deposits
in which they occur. Fossils further enable us to come to very
important conclusions as to the mode in which the fossiliferous
bed was deposited, and thus as to the condition of the particular
district or region occupied by the fossiliferous bed at the time
of the formation of the latter. If, in the first place, the bed
contain the remains of animals such as now inhabit rivers, we
know that it is "fluviatile" in its origin, and that it must at
one time have either formed an actual riverbed, or been deposited
by the overflowing of an ancient stream. Secondly, if the bed



Online LibraryHenry Alleyne NicholsonThe Ancient Life History of the Earth A Comprehensive Outline of the Principles and Leading Facts of Palæontological Science → online text (page 6 of 36)