Vestiges of the Natural History of Creation (Chapter 3)
by Robert Chambers
The Earth formed—Era of the Primary Rocks.
lthough 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 subaqueous 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 laminae 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.[1] 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.
Notes
Delabeche's Geological Researches.
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[ Robert Chambers,
Vestiges of the
Natural History of Creation, 1st edition, 1844; Reprinted in James Secord,
ed., Chicago: University of Chicago Press, 1994, pp. 44-53. ]
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