We turn now to the consideration of the _organic_ agencies. And I will
merely allude in passing to the vast importance of the fossil organic
remains found in the stratified rocks as marks by which to determine the
relative ages of the formations.
As regards the _destruction_ of rocks—_erosion_—plants and animals are
almost powerless; but in the role of _rock-makers_ they play a very
important part, being very efficient agents of _deposition_.
FORMATION OF COALS AND BITUMENS.—Specimen No. 8 is an example of peat
from the vicinity of Boston; but just as good specimens may be obtained
in thousands of places in this and other States.
The general physical conditions under which peat is formed are familiar
facts. We require simply low, level land, covered with a thin sheet of
water and abundant vegetation; in other words, a marsh or swamp. If
plants decay on the dry land, the decomposition is complete; they are
burned up by the oxygen of the air to _carbon dioxide_ and _water_ just
as surely as if they had been thrown into a furnace, though less
rapidly, and nothing is returned to the soil but what had been taken
from it by the plants during their growth. But if the plants decay under
water, as in a peat-marsh or bog, the decay is incomplete, and most of
the carbon of the wood is left behind. Now, if this incomplete
combustion of vegetable tissues takes place in a charcoal-pit, where the
wood is out of contact with air from being covered with earth, we call
the carbonaceous product charcoal; but if under the water of a marsh, in
Nature’s laboratory, we call the product peat. Peat is simply a natural
charcoal; and, just as in ordinary charcoal, its vegetable origin is
always perfectly evident. But when the deposit becomes thicker, and
especially when it is buried under thick formations of other rocks, like
sand and clay, the great pressure consolidates the peat; it becomes
gradually more mineralized and shining, shows the vegetable tissues less
distinctly, becomes more nearly pure carbon, and we call it in
succession lignite, bituminous coal, and anthracite.
This is, briefly, the way in which all varieties of coal, as well as the
more solid kinds of bitumen, like asphaltum, are formed. But the lighter
forms of bitumen, such as petroleum and naphtha, are derived mainly, if
not entirely, from the partial decomposition of animal tissues. These,
it is well known, decay much more readily than vegetable tissues; and
the water of an ordinary marsh or lake contains sufficient oxygen for
their complete and rapid decomposition. In the deeper parts of the
ocean, however, the conditions are very different, for recent researches
have shown, contrary to the old idea, that the deep sea holds an
abundant fauna. All grades of animal life, from the highest to the
lowest, have need of a constant supply of oxygen. On the land vegetation
is constantly returning to the air the oxygen consumed by animals, but
in the abysses of the ocean vegetable life is scarce or wanting; and
hence it must result that over these greater than continental areas
countless myriads of animals are living habitually on short rations of
oxygen, and in water well charged with carbon dioxide, the product of
animal respiration. As a consequence, when these animals die their
tissues do not find the oxygen essential for their perfect
decomposition, and in the course of time become buried, in a
half-decayed state, in the ever-increasing sediments of the ocean-floor.
It is important to observe that an abundance of organic matter decaying
under water is not the only condition essential to the formation of beds
of coal and bitumen; for this condition is realized in the luxuriant
growth of sea-weeds fringing the coast in every quarter of the globe;
and yet coals and bitumens are rarely of sea-shore origin. These organic
products, even under the most favorable circumstances, accumulate with
extreme slowness; far more slowly, as a rule, than the ordinary
mechanical sediments, like sand and clay, with which they are mixed, and
in which they are often completely lost. Consequently, although the
deposition of the carbonized remains of plants and animals is taking
place in nearly all seas, lakes, and marshes, it is only in those places
where there is little or no mechanical sediment that they can
predominate so as to build up beds pure enough to be called coal or
bitumen. In all other cases we get merely more or less carbonaceous sand
or clay. Now these especially favorable localities will manifestly not
be often found along the seashore, where we have strewn the sand and
clay brought down by rivers or washed off the land directly by the
ever-active surf; but they must exist in the central portions of the
ocean, where there is almost no mechanical sediment and yet an abundance
of life, and in swamps and marshes, where there is scarcely sufficient
water to cover the vegetation, and no waves or currents to wash down the
soil from the surrounding hills.
FORMATION OF IRON-ORES.—The iron-ores are another class of rocks which
are formed only through the agency of organic matter. Iron is an
abundant and wide-spread element in the earth’s crust, and, but for the
intervention of life, we might say that, while there is iron everywhere,
there is not much of it in any one place, since it is originally very
thinly diffused. All rocks and soils contain iron, but it is mainly in
the form of the peroxide, in which state it is entirely insoluble, and
hence cannot be soaked out of the soil by the rain-water and
concentrated by the evaporation of the water at lower levels in ponds
and marshes, as a soluble substance like salt would be. If carried off
with the sand and clay, by the mechanical action of water, it remains
uniformly mixed with them, and there is no tendency to its separation
and concentration so as to form a true iron-ore.
But what water cannot do alone is accomplished very readily when the
water is aided by decaying organic matter, which is always hungry for
oxygen, being, in the language of the chemist, a powerful reducing
agent. The soil, in most places, has a superficial stratum of vegetable
mould or half-decayed vegetation. The rainwater percolates through this
and dissolves more or less of the organic matter, which is thus carried
down into the sand and clay beneath and brought in contact with the
ferric oxide, from which it takes a certain proportion of oxygen,
reducing the ferric to the ferrous oxide. At the same time the
vegetation is burned up by the oxygen thus obtained, forming carbon
dioxide, which immediately combines with the ferrous oxide, forming
carbonate of iron, which, being soluble under these conditions, is
carried along by the water as it gradually finds its way by subterranean
drainage to the bottom of the valley and emerges in a swamp or marsh.
Here one of two things will happen: If the marsh contains little or no
decaying vegetation, then as soon as the ferrous carbonate brought down
from the hills is exposed to the air it is decomposed, the carbon
dioxide escapes, and the iron, taking on oxygen from the air, returns to
its original ferric condition; and being then quite insoluble, it is
deposited as a loose, porous, earthy mass, commonly known as
bog-iron-ore, which becomes gradually more solid and finally even
crystalline through the subsequent action of heat and pressure. When
first deposited, the ferric oxide is combined with water or hydrated,
and is then known as limonite (specimen No. 12); at a later period the
water is expelled, and we call the ore hematite (specimen No. 13); and
at a still later age it loses part of its oxygen, becomes magnetic and
more crystalline, and is then known as magnetite (specimen No. 14). Thus
it is seen that the iron-ores, as we pass from bog-limonite to
magnetite, form a natural series similar to and parallel with that
afforded by the coals as we pass from peat to graphite.
If the drainage from the hills is into a marsh containing an abundance
of decaying vegetation, _i.e._, if peat is forming there, the ferrous
carbonate, in the presence of the more greedy organic matter, will be
unable to obtain oxygen from the air; and as the evaporation of the
water goes on, it will sooner or later become saturated with this salt,
and the latter will be deposited. Here we find an explanation of a fact
often observed by geologists, viz., that the carbonate iron-ores are
usually associated with beds of coal.
The formation of the iron-ores, like that of the coals and bitumens, is
a slow process; and the ores, like the coals, etc., will be pure only
where there is a complete absence of mechanical sediment, a condition
that is realized most nearly in marshes.
FORMATION OF LIMESTONE, DIATOMACEOUS EARTH, ETC.—Marine animals take
from the sea-water certain mineral substances, especially silica and
carbonate of calcium, to form their skeletons. Silica is used only by
the lowest organisms, such as Radiolaria, Sponges, and the minute
unicellular plants, Diatoms. The principal animals secreting carbonate
of calcium are Corals and Mollusks. These hard parts of the organisms
remain undissolved after death; and over portions of the ocean-floor
where there is but little of other kinds of sediment they form the main
part of the deposits, and in the course of ages build up very extensive
formations which we call diatomaceous earth or tripolite, if the
organisms are siliceous, or limestone if they are calcareous. A very
satisfactory account of the formation of limestone on a stupendous scale
by the polyps in coral reefs and islands is contained in No. IV. of this
series of guides.
The rocks here considered may be, and, as we have already seen,
sometimes are, deposited in a purely chemical way, without the aid of
life; and it is important to observe that in no case do the organisms
make the silica and carbonate of calcium of their skeletons, but they
simply appropriate and reduce to the solid state what exists ready made
in solution in the sea-water. These minerals, and others, as we know,
are produced by the decomposition of the rocks of the land, and are
being constantly carried into the sea by rivers; and, if there were no
animals in the sea, these processes would still go on until the
sea-water became saturated with these substances, when their
precipitation as limestone, etc., would necessarily follow. Hence it is
clear that all the animals do is to effect the precipitation of certain
minerals somewhat sooner than it would otherwise occur; so that from a
geological standpoint the differences between chemical and organic
deposition are not great.
This section of our subject may be summarized as follows: Animals and
plants contribute to the formation of rocks in three distinct ways:—
1. During their growth they deoxidize carbon dioxide and water,
and reduce to the solid state in their tissues carbon and the
permanent gases oxygen, hydrogen, and nitrogen; and after death,
through the accumulation of the half-decayed tissues in favorable
localities,—marshes, etc.,—these elements are added to the solid
crust of the earth in the form of coal and bitumen.
2. During the decomposition, _i.e._, oxidation, of the organic tissues,
the iron existing everywhere in the soil is partially deoxidized, and,
being thus rendered soluble, is removed by rain-water and concentrated
in low places, forming beds of iron-ore.
3. Through the agency of marine organisms, certain mineral substances
are being constantly removed from the sea-water and deposited upon the
ocean floor, forming various calcareous and siliceous rocks.
I now bring our study of the aqueous or superficial agencies to a
conclusion by noting once more that the great geological results
accomplished by _air_, _water_, and _organic matter_ or _life_ are: (1)
_Erosion_, or the wearing away of the surface of the land; and (2)
_Deposition_, or the formation from the débris of the eroded land of two
great classes of stratified rocks,—the mechanically formed or fragmental
rocks, and the chemically and organically formed rocks.