[F] _Palaios_, ancient, _onta_, beings, and _logos_, a discourse.
87. _Fossils._--In our description of rock-masses, and again in our
account of geological agencies, we referred to the fact that certain
rocks are composed in large measure, or exclusively, of animal or
vegetable organisms, or of both together; and we saw that analogous
organic formations were being accumulated at the present time. But we
have deferred to this place any special account of the organic remains
which are entombed in rocks. _Fossils_, as these are called, consist
generally of the harder and more durable parts of animals and plants,
such as bones, shells, teeth, seeds, bark, and ligneous tissues, &c. But
it is usual to extend the term fossil to even the _casts_ or
_impressions_ of such remains, and to foot-marks and tracks, whether of
vertebrates, molluscs, crustaceans, or annelids. The organic remains
met with in the rocks have usually undergone some chemical change. They
have become _petrified_ wholly or in part. The gelatine which originally
gave flexibility to some of them has disappeared, and even the carbonate
and phosphate of lime of the harder parts have frequently been replaced
by other mineral matter, by flint, pyrites, or the like. So perfect is
the petrifaction in many cases, that the most minute structures have
been entirely preserved--the original matter having been replaced atom
by atom. As a rule, fossils occur most abundantly and in the best state
in clay-rocks, like shale; while in porous rocks, like sandstone, they
are generally poorly preserved, and not of so frequent occurrence. One
reason for this is, that clay-rocks are much less pervious than
sandstone, and their imbedded fossils have consequently escaped in
greater measure the solvent powers of percolating water. But there are
other reasons for the comparative paucity of fossils in arenaceous
strata, as we shall see presently.
88. _Proofs of varied Physical Conditions._--Organic remains are either
of terrestrial, fresh-water, or marine origin, and they are therefore of
the utmost value to the geologist in deciphering the history of those
great changes which have culminated in the present. But we can go a step
further than this. We know that at the present day the distribution of
animal and vegetable life is due to a variety of causes--to climatic and
physical conditions. The creatures inhabiting arctic and temperate
regions contrast strongly with those that tenant the tropics. So also we
observe a change in animal and vegetable forms as we ascend from the low
grounds of a country to its mountain heights. Similar changes take place
in the sea. The animals and plants of littoral regions differ from those
whose habitat is in deeper water. Now, the fossiliferous strata of our
globe afford similar proofs of varying climatic and physical conditions.
There are littoral deposits and deep-sea accumulations: the former are
generally coarse-grained (conglomerates, grit, and sandstone); the
latter are for the most part finer-grained (clay, shale, limestone,
chalk, &c.); and both inshore and deep-water formations have each their
peculiar organic remains. Again, we know that some parts of the
sea-bottom are not so prolific in life as others--where, for example,
any considerable deposit of sand is taking place, or where sediment is
being constantly washed to and fro upon the bottom, shells and other
creatures do not appear in such numbers as where there is less
commotion, and a finer and more equable deposit is taking place. It is
partly for the same reason that certain rocks are more barren of organic
remains than others.
89. _Fossil Genera and Species frequently extinct._--It might perhaps at
first be supposed that similar rocks would contain similar fossils. For
example, we might expect that formations resembling in their origin
those which are now forming in our coral seas would also, like the
latter, contain corals in abundance, with some commingling of shells,
crustaceans, fish, &c., such as are peculiar to the warm seas in which
corals flourish. And this in some measure holds good. But when we
examined carefully the fossils in certain of the limestones of our own
country, we should find that while the same great orders and classes
were actually present, yet the genera and species were frequently
entirely different; and not only so, but that often none of these were
now living on the earth. Moreover, if we extended our research, we
should soon discover that similar wide differences actually obtained
between many of the limestones themselves and other fossiliferous strata
of our country.
90. _Fossiliferous Strata of Different Ages._--Another fact would also
gradually dawn upon us--this, namely, that in certain rocks the fossils
depart much more widely from analogous living forms, than the organic
remains in certain other rocks do. The cause of this lies in the fact
that the fossiliferous strata are of different ages; they have not all
been formed at approximately the same time. On the contrary, they have
been slowly amassed, as we have seen, during a long succession of eras.
While they have been accumulating, great vicissitudes in the
distribution of land and sea have taken place, climates have frequently
altered, and the whole organic life of the globe has slowly changed
again and again--successive races of plants and animals flourishing each
for its allotted period, and then becoming extinct for ever.[G] Thus,
strata formed at approximately the same time contain generally the same
fossils; while, on the other hand, sedimentary deposits accumulated at
different periods are charged with different fossils. Fossils in this
way become invaluable to the geologist. They enable him to identify
formations in separate districts, and to assign to them their relative
antiquity.[H] If, for example, we have a series of formations, A, B, C,
piled one on the top of the other, A being the lowest, and C the
highest, and each charged with its own peculiar fossils, we may compare
the fossils met with in other sets of strata with the organic remains
found in A, B, C. Should the former be found to correspond with the
fossil contents of B, we conclude that the rocks in which they occur are
approximately of contemporaneous origin with B, even although the
equivalents of the formations A and C should be entirely wanting.
Further, we soon learn that the order of the series A, B, C, is never
inverted. If A be the lowest, and C the highest stratum in one place, it
is quite certain that the same order of succession will obtain wherever
the equivalents of these strata happen to occur together. But the
succession of strata is not invariably the same all the world over; in
some countries, we may have dozens of separate formations piled one on
the top of the other; in other countries, many members of the series are
absent; in brief, _blanks in the succession_ are of constant occurrence.
But by dovetailing, as it were, all the formations known to us, we are
enabled to form a more or less complete series of rocks arranged in the
order of their age. A little reflection will serve to shew that the
partial mode in which the strata are distributed over the globe arises
chiefly from two causes. We have to remember, _first_, that the deposits
themselves were laid down only here and there in irregular spreads and
patches--opposite the mouths of rivers, at various points along the
ancient coast-lines, and over certain areas in the deeper abysses of the
ocean--the coarser accumulations being of much less extent than those
formed of finer materials. And, _second_, we must not forget the intense
denudation which they have experienced, so that miles and miles of
strata which once existed have been swept away, and their materials
built up into new formations.
[G] To this there are some exceptions. Certain small foraminifers,
for example, met with in some of the oldest formations, do not
seem to differ from species which are still living. The genus
_Lingula_ (Mollusca) has also come down from remotest ages,
having outlived all its earlier associates.
[H] This holds strictly true, however, only in regard to comparatively
limited areas. The student must remember that strata occurring
in widely separate regions of the earth, even although they
contain very much the same assemblage of fossils, are not
necessarily contemporaneous, in the strict meaning of the word;
for the _fauna_ and _flora_ (the animal and plant life) may have
died out, and become replaced by new forms more rapidly in one
place than another. The term 'contemporaneous,' therefore, is a
very lax one, and may sometimes group together deposits which,
for aught that we can tell, may really have been accumulated at
widely separated times.
91. _Gradual Extinction of Species._--When a sufficient number of
fossils has been diligently compared, we discover that those in the
younger strata approach most nearly to the present living forms, and
that the older the strata are, the more widely do their organic remains
depart from existing types of animals and plants. We may notice also,
that when a series of beds graduate up into each other, so that no
strongly marked line separates the overlying from the underlying strata,
there is also a similar gradation amongst the fossils. The fossils in
the highest beds may differ entirely from those in the lowest; but in
the middle beds there is an intermingling of forms. In short, it is
evident that the creatures gradually became extinct, and were just as
gradually replaced by new forms, until a time came when all the species
that were living while the lowest beds were being amassed, at last died
out, and a complete change was effected.
92. _Proofs of Cosmical Changes of Climate._--From the preceding remarks
it will be also apparent that fossils teach us much regarding the
climatology of past ages. They tell us how the area of the British
Islands has experienced many vicissitudes of climate, sometimes
rejoicing in a warm or almost tropical temperature, at other times
visited with a climate as severe as is now experienced in arctic and
antarctic regions. Not only so, but we learn from fossils that
Greenland once supported myrtles and other plants which are now only
found growing under mild and genial climatic conditions; while, on the
other hand, remains of arctic mammals are met with in the south of
France. Such great changes of climate are due, according to Mr Croll, to
variations in the eccentricity of the earth's orbit combined with the
precession of the equinox. It is well known that the orbit of our earth
becomes much more elliptical at certain irregularly recurring periods
than it is at present. During a period of extreme ellipticity, the earth
is, of course, much further away from the sun in _aphelion_[I] than it
is at a time of moderate ellipticity, while, in _perihelion_,[J] it is
considerably nearer. Now, let us suppose that, at a time when the
ellipticity is great, the movement known as the precession of the
equinox has changed the incidence of our seasons, so that our summer
happens in perihelion and not in aphelion, while that of the southern
hemisphere occurs in aphelion, and not, as at present, in perihelion.
Under such conditions, the climate of the globe would experience a
complete change. In the northern hemisphere, so long and intensely cold
would the winter be, that all the moisture that fell would fall as rain,
and although the summer would be very warm, it would nevertheless be
very short, and the heat then received would be insufficient to melt the
snow and ice which had accumulated during the winter. Thus gradually
snow and ice would cover all the lands down to temperate latitudes. In
the southern hemisphere, the reverse of all this would obtain. The
winter there would be short and mild, and the summer, although cool,
would be very long. But such changes would bring into action a whole
series of physical agencies, every one of which would tend still further
to increase the difference between the climates of the two hemispheres.
Owing to the vast accumulation of snow and ice in the northern
hemisphere, the difference of temperature between equatorial and
temperate and polar regions would be greater in that hemisphere than in
the southern. Hence the winds blowing from the north would be more
powerful than those coming from the southern and warmer hemisphere, and
consequently the warm water of the tropics would necessarily be impelled
into the southern ocean. This would tend still further to lower the
temperature of our hemisphere, while, at the same time, it would raise
correspondingly the temperature at our antipodes. The general result
would be, that in our hemisphere ice and snow would cover the ground
down to low temperate latitudes--the British Islands being completely
smothered under a great sea of confluent glaciers. In the southern
hemisphere, on the contrary, a kind of perennial summer would reign even
up to the pole. Such conditions would last for some ten or twelve
thousand years, and then, owing to the precession of the equinox, a
complete change would come about--the ice-cap would disappear from the
north, and be replaced by continuous summer, while at the same time an
excessively severe or glacial climate would characterise the south; and
such great changes would occur several times during each prolonged epoch
of great eccentricity. This, in few words, is an outline of Mr Croll's
theory. That theory is at present _sub judice_, but there can be no
doubt that it gives a reasonable explanation of many geological facts
which have hitherto been inexplicable. Of course, it is not maintained
that all changes of climate are due directly or indirectly to
astronomical causes. Local changes of climate--changes affecting limited
regions--may be induced by mutations of land and sea, resulting in the
partial deflection of ocean currents, which are the chief secondary
means employed by nature for the distribution of heat over the globe's
surface.
[I] _Apo_, away from; _helios_, the sun.
[J] _Peri_, round about or near by; _helios_, the sun.
From what has been stated in the foregoing paragraphs, it is clear that
in our endeavours to decipher the geological history of our planet,
palaeontological must go hand in hand with stratigraphical evidence. We
may indeed learn much from the mode of arrangement of the rocks
themselves. But the test of superposition does not always avail us. It
is often hard, and sometimes quite impossible, to tell from
stratigraphical evidence which are the older rocks of a district. In the
absence of fossils we must frequently be in doubt. But physical evidence
alone will often afford us much and varied information. It will shew us
what was land and what sea at some former period; it will indicate to us
the sites of ancient igneous action; it will tell us of rivers, and
lakes, and seas which have long since passed away. Nay, in some cases,
it will even convince us that certain great climatic changes have taken
place, by pointing out to us the markings, and debris, and wandered
blocks which are the sure traces of ice action, whether of glaciers or
icebergs. The results obtained by combining physical and palaeontological
evidence form what is termed Historical Geology.