when all the relative phenomena are in harmony with it. This is
eminently the case with the nebulous hypothesis, for here the associated
facts cannot be explained on any other supposition. We have seen reason
to conclude that the primary condition of matter was that of a diffused
mass, in which the component molecules were probably kept apart through
the efficacy of heat; that portions of this agglomerated into suns, which
threw off planets; that these planets were at first very much diffused,
but gradually contracted by cooling to their present dimensions. Now, as
to our own globe, there is a remarkable proof of its having been in a
fluid state at the time when it was finally solidifying, in the fact of
its being bulged at the equator, the very form which a soft revolving
body takes, and must inevitably take, under the influence of centrifugal
force. This bulging makes the equatorial exceed the polar diameter as
230 to 229, which has been demonstrated to be precisely the departure
from a correct sphere which might be predicated from a knowledge of the
amount of the mass and the rate of rotation. There is an almost equally
distinct memorial of the original high temperature of the materials, in
the store of heat which still exists in the interior. The immediate
surface of the earth, be it observed, exhibits only the temperature which
might be expected to be imparted to such materials, by the heat of the
sun. There is a point, very short way down, but varying in different
climes, where all effect from the sun’s rays ceases. Then, however,
commences a temperature from an entirely different cause, one which
evidently has its source in the interior of the earth, and which
regularly increases as we descend to greater and greater depths, the rate
of increment being about one degree Fahrenheit for every sixty feet; and
of this high temperature there are other evidences, in the phenomena of
volcanoes and thermal springs, as well as in what is ascertained with
regard to the density of the entire mass of the earth. This, it will be
remembered, is four and a half times the weight of water; but the actual
weight of the principal solid substances composing the outer crust is as
two and a half times the weight of water; and this, we know, if the globe
were solid and cold, should increase vastly towards the centre, water
acquiring the density of quicksilver at 362 miles below the surface, and
other things in proportion, and these densities becoming much greater at
greater depths; so that the entire mass of a cool globe should be of a
gravity infinitely exceeding four and a half times the weight of water.
The only alternative supposition is, that the central materials are
greatly expanded or diffused by some means; and by what means could they
be so expanded but by heat? Indeed, the existence of this central heat,
a residuum of that which kept all matter in a vaporiform chaos at first,
is amongst the most solid discoveries of modern science, {42} and the
support which it gives to Herschel’s explanation of the formation of
worlds is most important. We shall hereafter see what appear to be
traces of an operation of this heat upon the surface of the earth in very
remote times; an effect, however, which has long passed entirely away.
The central heat has, for ages, reached a fixed point, at which it will
probably remain for ever, as the non-conducting quality of the cool crust
absolutely prevents it from suffering any diminution.
THE EARTH FORMED—ERA OF THE
PRIMARY ROCKS.
ALTHOUGH the earth has not been actually penetrated to a greater depth
than three thousand feet, the nature of its substance can, in many
instances, be inferred for the depth of many miles by other means of
observation. We see a mountain composed of a particular substance, with
strata, or beds of other rock, lying against its sloped sides; we, of
course, infer that the substance of the mountain dips away under the
strata which we see lying against it. Suppose that we walk away from the
mountain across the turned up edges of the stratified rocks, and that for
many miles we continue to pass over other stratified rocks, all disposed
in the same way, till by and bye we come to a place where we begin to
cross the opposite edges of the same beds; after which we pass over these
rocks all in reverse order till we come to another extensive mountain
composed of similar material to the first, and shelving away under the
strata in the same way. We should then infer that the stratified rocks
occupied a basin formed by the rock of these two mountains, and by
calculating the thickness right through these strata, could be able to
say to what depth the rock of the mountain extended below. By such
means, the kind of rock existing many miles below the surface can often
be inferred with considerable confidence.
The interior of the globe has now been inspected in this way in many
places, and a tolerably distinct notion of its general arrangements has
consequently been arrived at. It appears that the basis rock of the
earth, as it may be called, is of hard texture, and crystalline in its
constitution. Of this rock, granite may be said to be the type, though
it runs into many varieties. Over this, except in the comparatively few
places where it projects above the general level in mountains, other
rocks are disposed in sheets or strata, with the appearance of having
been deposited originally from water; but these last rocks have nowhere
been allowed to rest in their original arrangement. Uneasy movements
from below have broken them up in great inclined masses, while in many
cases there has been projected through the rents rocky matter more or
less resembling the great inferior crystalline mass. This rocky matter
must have been in a state of fusion from heat at the time of its
projection, for it is often found to have run into and filled up lateral
chinks in these rents. There are even instances where it has been rent
again, and a newer melted matter of the same character sent through the
opening. Finally, in the crust as thus arranged there are, in many
places, chinks containing veins of metal. Thus, there is first a great
inferior mass, composed of crystalline rock, and probably resting
immediately on the fused and expanded matter of the interior: next,
layers or strata of aqueous origin; next, irregular masses of melted
inferior rock that have been sent up volcanically and confusedly at
various times amongst the aqueous rocks, breaking up these into masses,
and tossing them out of their original levels. This is an outline of the
arrangements of the crust of the earth, as far as we can observe it. It
is, at first sight, a most confused scene; but after some careful
observation, we readily detect in it a regularity and order from which
much instruction in the history of our globe is to be derived.
The deposition of the aqueous rocks, and the projection of the volcanic,
have unquestionably taken place since the settlement of the earth in its
present form. They are indeed of an order of events which we see going
on, under the agency of more or less intelligible causes, even down to
the present day. We may therefore consider them generally as
comparatively recent transactions. Abstracting them from the
investigations before us, we arrive at the idea of the earth in its first
condition as a globe of its present size—namely, as a mass, externally at
least, consisting of the crystalline kind of rock, with the waters of the
present seas and the present atmosphere around it, though these were
probably in considerably different conditions, both as to temperature and
their constituent materials, from what they now are. We are thus to
presume that that crystalline texture of rock which we see exemplified in
granite is the condition into which the great bulk of the solids of our
earth were agglomerated directly from the nebulous or vaporiform state.
It is a condition eminently of combination, for such rock is invariably
composed of two or more of four substances—silica, mica, quartz, and
hornblende—which associate in it in the form of grains or crystals, and
which are themselves each composed of a group of the simple or elementary
substances.
Judging from the results and from still remaining conditions, we must
suppose that the heat retained in the interior of the globe was more
intense, or had greater freedom to act, in some places than in others.
These became the scenes of volcanic operations, and in time marked their
situations by the extrusion of traps and basalts from below—namely, rocks
composed of the crystalline matter fused by intense heat, and developed
on the surface in various conditions, according to the particular
circumstances under which it was sent up; some, for example, being thrown
up under water, and some in the open air, which conditions are found to
have made considerable difference in its texture and appearance. The
great stores of subterranean heat also served an important purpose in the
formation of the aqueous rocks. These rocks might, according to Sir John
Herschel, become subject to heat in the following manner:—While the
surface of a particular mass of rock forms the bed of the sea, the heat
is kept at a certain distance from that surface by the contact of the
water; philosophically speaking, it radiates away the heat into the sea,
and (to resort to common language) is cooled a good way down. But when
new sediment settles at the bottom of that sea, the heat rises up to what
was formerly the surface; and when a second quantity of sediment is laid
down, it continues to rise through the first of the deposits, which then
becomes subjected to those changes which heat is calculated to produce.
This process is precisely the same as that of putting additional coats
upon our own bodies; when, of course, the internal heat rises through
each coat in succession, and the third (supposing there is a fourth above
it) becomes as warm as perhaps the first originally was.
In speaking of sedimentary rocks, we may be said to be anticipating. It
is necessary, first, to shew how such rocks were formed, or how
stratification commenced.
Geology tells us as plainly as possible, that the original crystalline
mass was not a perfectly smooth ball, with air and water playing round
it. There were vast irregularities in the surface,—irregularities
trifling, perhaps, compared with the whole bulk of the globe, but
assuredly vast in comparison with any which now exist upon it. These
irregularities might be occasioned by inequalities in the cooling of the
substance, or by accidental and local sluggishness of the materials, or
by local effects of the concentrated internal heat. From whatever cause
they arose, there they were—enormous granitic mountains, interspersed
with seas which sunk to a depth equally profound, and by which, perhaps,
the mountains were wholly or partially covered. Now, it is a fact of
which the very first principles of geology assure us, that the solids of
the globe cannot for a moment be exposed to water, or to the atmosphere,
without becoming liable to change. They instantly begin to wear down.
This operation, we may be assured, proceeded with as much certainty in
the earliest ages of our earth’s history, as it does now, but upon a much
more magnificent scale. There is the clearest evidence that the seas of
those days were not in some instances less than a hundred miles in depth,
however much more. The sub-aqueous mountains must necessarily have been
of at least equal magnitude. The system of disintegration consequent
upon such conditions would be enormous. The matters worn off, being
carried into the neighbouring depths, and there deposited, became the
components of the earliest stratified rocks, the first series of which is
the _Gneiss and Mica Slate System_, or series, examples of which are
exposed to view in the Highlands of Scotland and in the West of England.
The vast thickness of these beds, in some instances, is what attests the
profoundness of the primeval oceans in which they were formed; the
Pensylvanian grawacke, a member of the next highest series, is not less
than a hundred miles in direct thickness. We have also evidence that the
earliest strata were formed in the presence of a stronger degree of heat
than what operated in subsequent stages of the world, for the laminæ of
the gneiss and of the mica and chlorite schists are contorted in a way
which could only be the result of a very high temperature. It appears as
if the seas in which these deposits were formed, had been in the troubled
state of a caldron of water nearly at boiling heat. Such a condition
would probably add not a little to the disintegrating power of the ocean.
The earliest stratified rocks contain no matters which are not to be
found in the primitive granite. They are the same in material, but only
changed into new forms and combinations; hence they have been called by
Mr. Lyell, metamorphic rocks. But how comes it that some of them are
composed almost exclusively of one of the materials of granite; the mica
schists, for example, of mica—the quartz rocks, of quartz, &c.? For this
there are both chemical and mechanical causes. Suppose that a river has
a certain quantity of material to carry down, it is evident that it will
soonest drop the larger particles, and carry the lightest farthest on.
To such a cause is it owing that some of the materials of the worn-down
granite have settled in one place and some in another. {52} Again, some
of these materials must be presumed to have been in a state of chemical
solution in the primeval seas. It would be, of course, in conformity
with chemical laws, that certain of these materials would be precipitated
singly, or in modified combinations, to the bottom, so as to form rocks
by themselves.
The rocks hitherto spoken of contain none of those petrified remains of
vegetables and animals which abound so much in subsequently formed rocks,
and tell so wondrous a tale of the past history of our globe. They
simply contain, as has been said, mineral materials derived from the
primitive mass, and which appear to have been formed into strata in seas
of vast depth. The absence from these rocks of all traces of vegetable
and animal life, joined to a consideration of the excessive temperature
which seems to have prevailed in their epoch, has led to the inference
that no plants or animals of any kind then existed. A few geologists
have indeed endeavoured to shew that the absence of organic remains is no
proof of the globe having been then unfruitful or uninhabited, as the
heat to which these rocks have been subjected at the time of their
solidification, might have obliterated any remains of either plants or
animals which were included in them. But this is only an hypothesis of
negation; and it certainly seems very unlikely that a degree of heat
sufficient to obliterate the remains of plants or animals when dead,
would ever allow of their coming into or continuing in existence.
COMMENCEMENT OF ORGANIC LIFE—
SEA PLANTS, CORALS, ETC.
WE can scarcely be said to have passed out of these rocks, when we begin
to find new conditions in the earth. It is here to be observed that the
subsequent rocks are formed, in a great measure, of matters derived from
the substance of those which went before, but contain also beds of
limestone, which is to no small extent composed of an ingredient which
has not hitherto appeared. Limestone is a carbonate of lime, a secondary
compound, of which one of the ingredients, carbonic acid gas, presents
the element _carbon_, a perfect novelty in our progress. Whence this
substance? The question is the more interesting, from our knowing that
carbon is the main ingredient in organic things. There is reason to
believe that its primeval condition was that of a gas, confined in the
interior of the earth, and diffused in the atmosphere. The atmosphere
still contains about a two-thousandth part of carbonic acid gas, forming
the grand store from which the substance of each year’s crop of herbage
and grain is derived, passing from herbage and grain into animal
substance, and from animals again rendered back to the atmosphere in
their expired breath, so that its amount is never impaired. Knowing
this, when we hear of carbon beginning to appear in the ascending series
of rocks, we are unavoidably led to consider it as marking a time of some
importance in the earth’s history, a new era of natural conditions, one
in which organic life has probably played a part.
It is not easy to suppose that, at this period, carbon was adopted
directly in its gaseous form into rocks; for, if so, why should it not
have been taken into earlier ones also? But we know that plants take it
in, and transform it into substance; and we also know that there are
classes of animals (marine polypes) which are capable of appropriating
it, in connexion with lime, (carbonate of lime,) from the waters of the
ocean, provided it be there in solution; and this substance do these
animals deposit in masses (coral reefs) equal in extent to many strata.
It has even been suggested, on strong grounds of probability, that a
class of limestone beds are simply these reefs subjected to subsequent
heat and pressure.
The appearance, then, of limestone beds in the early part of the
stratified series, may be presumed to be connected with the fact of the
commencement of organic life upon our planet, and, indeed, a consequent
and a symptom of it.
It may not be out of place here to remark, that carbon is presumed to
exist largely in the interior of the earth, from the fact of such
considerable quantities of it issuing at this day, in the form of
carbonic acid gas, from fissures and springs. The primeval and
subsequent history of this element is worthy of much attention, and we
shall have to revert to it as a matter greatly concerning our subject.
Delabeche estimates the quantity of carbonic acid gas locked up in every
cubic yard of limestone, at 16,000 cubic feet. The quantity locked up in
coal, in which it forms from 64 to 75 per cent., must also be enormous.
If all this were disengaged in a gaseous form, the constitution of the
atmosphere would undergo a change, of which the first effect would be the
extinction of life in all land animals. But a large proportion of it
must have at one time been in the atmosphere. The atmosphere would then,
of course, be incapable of supporting life in land animals. It is
important, however, to observe that such an atmosphere would not be
inconsistent with a luxuriant land vegetation; for experiment has proved
that plants will flourish in air containing _one-twelfth_ of this gas, or
166 times more than the present charge of our atmosphere. The results
which we observe are perfectly consistent with, and may be said to
presuppose an atmosphere highly charged with this gas, from about the
close of the primary non-fossiliferous rocks to the termination of the
carboniferous series, for there we see vast deposits (coal) containing
carbon as a large ingredient, while at the same time the leaves of the
_Stone Book_ present no record of the contemporaneous existence of land
animals.
The hypothesis of the connexion of the first limestone beds with the
commencement of organic life upon our planet is supported by the fact,
that in these beds we find the first remains of the bodies of animated
creatures. My hypothesis may indeed be unsound; but, whether or not, it
is clear, taking organic remains as upon the whole a faithful chronicle,
that the deposition of these limestone beds was coeval with the existence
of the earliest, or all but the earliest, living creatures upon earth.
And what were those creatures? It might well be with a kind of awe that
the uninstructed inquirer would wait for an answer to this question. But
nature is simpler than man’s wit would make her, and behold, the
interrogation only brings before us the unpretending forms of various
zoophytes and polypes, together with a few single and double-valved
shell-fish (mollusks), all of them creatures of the sea. It is rather
surprising to find these before any vegetable forms, considering that
vegetables appear to us as forming the necessary first link in the chain
of nutrition; but it is probable that there were sea plants, and also
some simpler forms of animal life, before this period, although of too
slight a substance to leave any fossil trace of their existence.
The exact point in the ascending stratified series at which the first
traces of organic life are to be found is not clearly determined. Dr.
M’Culloch states that he found fossil orthocerata (a kind of shell-fish)
so early as the gneiss tract of Loch Eribol, in Sutherland; but Messrs.
Sedgwick and Murchison, on a subsequent search, could not verify the
discovery. It has also been stated, that the gneiss and mica tract of
Bohemia contains some seams of grawacke, in which are organic remains;
but British geologists have not as yet attached much importance to this
statement. We have to look a little higher in the series for indubitable
traces of organic life.
Above the gneiss and mica slate system, or group of strata, is the _Clay
Slate and Grawacke Slate System_; that is to say, it is higher in the
_order of supraposition_, though very often it rests immediately on the
primitive granite. The sub-groups of this system are in the following
succession upwards:—1, hornblende slate; 2, chiastolite slate; 3, clay
slate; 4, Snowdon rocks, (grawacke and conglomerates;) 5, Bala limestone;
6, Plynlymmon rocks, (grawacke and grawacke slates, with beds of
conglomerates.) This system is largely developed in the west and north
of England, and it has been well examined, partly because some of the
slate beds are extensively quarried for domestic purposes. If we
overlook the dubious statements respecting Sutherland and Bohemia, we
have in this “system” the first appearances of life upon our planet. The
animal remains are chiefly confined to the slate beds, those named from
Bala, in Wales, being the most prolific. _Zoophyta_, _polyparia_,
_crinoidea_, _conchifera_, and _crustacea_, {60} are the orders of the
animal kingdom thus found in the earliest of earth’s sepulchres. The
_orders_ are distinguished without difficulty, from the general
characters of the creatures whose remains are found; but it is only in
this general character that they bear a general resemblance to any
creatures now existing. When we come to consider specific characters, we
see that a difference exists—that, in short, the species and even genera
are no longer represented upon earth. More than this, it will be found
that the earliest species comparatively soon gave place to others, and
that they are not represented even in the next higher group of rocks.
One important remark has been made, that a comparatively small variety of
species is found in the older rocks, although of some particular ones the
remains are very abundant; as, for instance, of a species of asaphus,
which is found between the laminæ of some of the slate rocks of Wales,
and the corresponding rocks of Normandy and Germany in enormous
quantities.