Any organic substance imbedded in a geological formation, or any product
of organic life, as a coprolite or a coin, or any marking which an
organic substance has given to a rock, is regarded as a _fossil_. The
study of fossils, as a branch of practical geology, requires an
acquaintance with the principles and the minute details of botany and
zoology. Without this knowledge, however, many of the general
conclusions to which the study of fossils has led may be understood.
1. _Fossils are preserved in different ways._--When any organic
substance is imbedded in a forming rock, it may itself remain; or it may
be removed by the infiltration of water, or other causes, so gradually
as to leave its form, and even its most delicate markings, in the rock;
or some mineral substance may have been substituted, and fill the space
which the organic substance once occupied; that is, it may be an organic
substance preserved, it may be an impression of it, or it may be a cast
of it.
2. The process by which the substitution in this last case is effected
is called _mineralization_. The mineralizing ingredient is generally
derived from the contiguous rock. In siliceous rocks it is silex. In
calcareous rocks it is carbonate of lime. When iron is diffused through
a rock, it often becomes the mineralizer. The substituted mineral is
generally a very perfect representation of the original fossil. We
cannot therefore suppose that the original substance was entirely
removed before any of the mineral matter was deposited. The substitution
must have taken place particle by particle, as the organic matter was
removed. Fossils are, in fact, often found, in which the mineralization
has been arrested after it had commenced, so that the fossil is in part
an organic and in part a mineral substance. It has been proved, by
direct experiment, that these changes of removal and substitution are
simultaneous. Pieces of wood were placed in a solution of sulphate of
iron. After a few days, the wood was found to be partially mineralized,
and after the remaining ligneous matter had been removed by exposing it
to a red heat, "oxide of iron was found to have taken the form of the
wood so exactly, that even the dotted vessels, peculiar to the species
employed, were distinctly visible under the microscope."
3. As the fossiliferous strata are generally of marine origin, it is to
be presumed that only a small proportion of terrestrial animals are
preserved; and our knowledge of the organic remains which are preserved
is yet so imperfect, that discoveries are constantly making, as
examinations are extended. Still, enough is known to enable us to draw
some satisfactory conclusions as to _the order in which living beings
were created upon the earth_.
Though most of the earlier organic forms which have been preserved are
of animal origin, yet vegetable remains occasionally occur in connection
with them, and we must suppose vegetables to have been produced
abundantly. For all animal food consists of vegetable substances, or of
animal substances which have once existed in the vegetable form. No
animal is capable of effecting those combinations of inorganic matter
upon which its growth and sustenance depend. We may therefore conclude
that _the introduction of animals and vegetables was contemporaneous_.
The greatest development of vegetable life was, however, during the
carboniferous period. The design of this abundant growth was
prospective. It was not produced for the support of animal life, but for
fuel, and stored till man should be introduced, and so far advanced in
civilization as to make this supply of carbonaceous matter subservient
to his wants and happiness.
In the earlier periods, the lower forms of animal life were, beyond all
comparison, the most abundant; yet the four great divisions of the
animal kingdom, Radiated, Articulated, Molluscous, and Vertebrated
animals, were all represented. There is, however, no evidence that any
vertebrated animals, except fishes, were created till after the
carboniferous period. In the next formation, the new red sandstone, we
find the tracks of reptiles and birds, and probably of marsupial
animals. The first evidence of the existence of mammalia in great
numbers is in the tertiary period, when the pachydermata and edentata
were so much more abundant than they have ever been since, and when the
bimana first appear.
But there is no evidence from geology that man existed till after the
close of the tertiary period. The grounds upon which contrary statements
have sometimes been made are untenable. In Ohio a very perfect
impression of a human foot was found on a slab of limestone of the
silurian age. But it was subsequently ascertained to have been common
for the aborigines, in the vicinity of their encampments, to cut in the
rocks, with surprising accuracy, the forms of the tracks of man and
other animals.
There is a human skeleton in the British Museum imbedded in solid
limestone, and another in Paris, both taken from Guadaloupe. It was at
one time supposed, from the degree of solidification of the limestone,
that it must have been formed at an early geological period; but it is
found that the beach-sand of that island now solidifies rapidly, from
the carbonate of lime which the waters there hold in solution. It is
rendered probable that the skeletons found there have not been buried
more than a century and a half.
4. As many parts of the bed of the present seas, which are probably
receiving detrital matter constantly, are unfavorable for the
development of animal life, while other parts are highly favorable, it
might be presumed that animal life would be equally scanty in particular
localities while the earlier rocks were forming, and in other localities
very abundant. Hence some strata, for hundreds of feet in thickness, are
composed almost entirely of fossils, while other strata are nearly or
quite destitute of them. The same member of a formation may in one place
be full of fossils, and in another without them. _The distribution of
fossils_ is therefore subject to no general law; at least, none of which
we can avail ourselves, in the search for them.
5. The value of fossils in geology consists in the use which is made of
them in determining _the origin and age of strata_.
As the animal species which inhabit bodies of fresh water are always
different from those found in the sea, their remains constitute the best
means of determining whether a formation is of _fresh water_ or _marine
origin_. In order to decide this point, it may, in some cases, be
necessary to be acquainted with the habits of particular species. In
most cases, however, it will be sufficient to remember that in
fresh-water formations, first, there are no sponges, corals, or
chambered shells; second, the univalves all have entire mouths (Fig.
44). Third, the bivalves are all bimuscular (Fig. 47). If, therefore, a
formation is found to contain sponge, coral, a chambered shell, a
univalve with a deeply notched mouth (Fig. 45), or a unimuscular bivalve
(Fig. 46), it must be considered a marine formation.
[Illustration: Fig. 44.]
[Illustration: Fig. 45.]
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
We have seen that the same formation, as exhibited in different places,
differs in its thickness, composition and degree of solidification. If
we could trace the strata through all the intermediate space, we might
be certain of their being the same formation, notwithstanding the change
in lithological characters. But this can seldom be done, even for a few
miles in extent. Sections of the strata are obtained only occasionally,
where rivers have cut through them, or where, over limited areas, the
soil has been removed from the outcropping edges. It is also frequently
the case that the strata are so much disturbed that their position will
furnish no aid in determining their age. When folded axes occur (as here
represented), the older strata are often the uppermost. There is an
instance in the Alps in which strata of vast thickness have been
inverted during the process of upheaval, and now rest on a bed of rock
formed from the debris which they had supplied.
[Illustration]
And yet it is important to determine what formations are of the same
age, notwithstanding their displacements, difference in lithological
characters, and separation by great distances and by mountains or
oceans. This determination can be made only by a comparison of the
imbedded fossils. It is found that every formation, and every important
member of a formation, contains an assemblage of fossils peculiar to
itself. When very widely separated, the species of fossils may not be
identical, but so very similar that they are regarded as equivalent
species. _The identification of formations_ consists in the
identification of fossils. It is for this purpose mainly that fossils
are regarded as of so great importance.
6. If each formation is characterized by the presence of new species, it
follows that _the work of creation was a progressive one_, continued
through long periods of time. The latest creation of which we have any
geological evidence is that of man. And if the leading design of the
existence of this earth was as a theatre for the development of moral
character, it is to be presumed that the work of creation ceased when a
species possessing moral capacities had been introduced.
It follows also, from what has been said, that there has been a constant
disappearance, a death, of species. It would seem that each species has
a life assigned to it, which is to be completed and surrendered. Though
its continuance is many times longer than the life of any individual of
the species, yet _it is the course of nature that species should
disappear_.
There may be something in the constitution of each species by which its
continuance is limited, making an old age and death necessary, as it is
in individuals. But there are other causes by which the duration of
species may often be terminated. The subsidence of New Holland would
cause the destruction of a large number of species. The preservation of
the human species was at one time effected only by a special and
miraculous interference. Slowly operating causes are now at work, by
which many species, such as the elephant, wolf and tiger, will at length
become extinct. Their existence in a natural state cannot long be
continued in a civilized country. The forest, their natural abode,
disappears, and some are intentionally destroyed, because they render
life and property unsafe. Under the operation of these causes, the Irish
elk (cervus giganteus) has become extinct, probably within the human
era. The Dodo, a gallinaceous bird, found living when maritime
communication between Europe and the East Indies was first established,
is now extinct. The Apteryx, a bird belonging to New Zealand, has
probably become extinct since the commencement of the present century.