72. _Plants._--The disintegration of rocks is often aided by the
action of plants, which force their roots into joints and crevices,
and thus loosen blocks and fragments. Carbonic acid, derived from the
decay of plants, being absorbed by rain-water, acts chemically upon
many rocks, as in the case of limestone (see 59, 60, 61). In temperate
regions, vegetation frequently accumulates, under certain conditions,
to form very considerable masses. Of such a nature is _peat_, which,
as is well known, covers many thousands of acres in the British
Islands. This substance is composed fundamentally of the bog-moss
(_Sphagnum palustre_), with which, however, are usually associated
many other marsh-loving plants. The lower parts of bog-moss die and
decay while its upper portions continue to flourish, and thus, in
process of time, a thickness of peat is accumulated to the extent of
six, twelve, twenty-four, or even forty feet. Many of the hill-tops
and hill-slopes in Scotland and Ireland are covered with a few feet of
peat, but it is only in valleys and hollows where the peat-bogs attain
their greatest depth. In not a few cases, the bogs seem to occupy the
sites of ancient lakes, shell-marl often occurring at the bottom of
these. The trunks and roots of trees are also commonly met with
underneath peat, and occasionally the remains of land animals.
Frequently, indeed, it would seem as if the overthrow of the trees, by
obstructing the drainage of the country, had given rise to a marsh,
and the consequent formation of peat. Some of the most valuable peat
closely resembles lignite, and makes a good fuel. In tropical
countries, the rapidity with which vegetation decays prevents, as a
rule, any great accumulation taking place; but the mangrove swamps are
exceptions.
73. _Animals._--The action of animal life is for the most part
conservative and reconstructive. Considerable accumulations of
shell-marl take place in fresh-water lakes, and the flat bottoms which
mark the sites of lakes which have been drained are frequently dug to
obtain this material. But by far the most conspicuous formations due
to the action of animal life accumulate in the sea. Molluscs,
crustaceans, corals, and the like, secrete from the ocean the
carbonate of lime of which their hard shells and skeletons are
composed, and these hard parts go to the formation of limestone. The
most remarkable masses of modern limestone occur within intertropical
regions. These are the coral reefs of the Pacific and Indian Oceans.
[Illustration: Fig. 24.--Formation of Coral Reefs.]
74. _Coral_ is the calcareous skeleton of certain small soft-bodied
gelatinous animals called _actinozoa_. These zoophytes flourish only in
clear water, the temperature of which is not below 66 deg. F., and they
cannot live at greater depths than one hundred feet. There are three
kinds of coral reef--namely, _fringing_ reefs, _barrier_ reefs, and
_atolls_. Fringing reefs occur, as a rule, near to the shore; but if
this latter be gently sloping, they may extend for one or even two miles
out to sea; as far, indeed, as the depth of water is not too great for
the actinozoa. Barrier reefs are met with at greater distances from the
land, and often rise from profound depths. The barrier reef which
extends along the north-east coast of Australia, often at a distance
from the land of fifty or sixty miles, stretches, with interruptions,
for about 1250 miles, with a breadth varying from ten to ninety miles.
In some places, the depth of the sea immediately outside of this reef
exceeds 1800 feet. Sometimes barrier reefs completely encircle an island
or islands, which are usually mountainous, as in the case of Pouynipete,
an island in the Caroline Archipelago, and the Gambier Islands in the
Low Archipelago. _Atolls_ are more or less irregular ring-shaped reefs
inclosing a lagoon of quiet water. They usually rise from profound
depths; Keeling Atoll, in the Indian Ocean, is a good example. The upper
surface of atolls and barrier reefs often peers at separate points above
the level of the sea, so as to form low-lying islets. In some cases, the
land thus formed is almost co-extensive with the reef, and being clothed
with palms and tropical verdure, resembles a beautiful chaplet floating,
as it were, in mid-ocean. The rock of a coral reef is a solid white
limestone, similar in composition to that of the limestones occurring in
this country. In some places, it is quite compact, shewing few or no
inclosed shells or other animal remains; in other places, it is made up
of broken and comminuted corals cemented together, or of masses of coral
standing as they slowly grew, with the spaces between the separate
clumps filled up with coral sand and triturated fragments and grit of
coral and shell. The thickness of the reefs is often very great,
reaching in many cases to thousands of feet. At the Fijis, the reef can
hardly be less than 2000 or 3000 feet thick. Below a depth of one
hundred feet, all the coral rock is dead, and since the coral zoophytes
do not live at greater depths than this, it follows that the bed of the
sea in which coral reefs occur must have slowly subsided during a long
course of ages. Mr Darwin was the first to give a reasonable explanation
of the origin of coral reefs. Briefly stated, his explanation is as
follows: The corals began to grow first in water not exceeding one
hundred feet in depth, and built up to the surface of the sea, thus
forming a fringing reef at no great distance from the land. This initial
step is shewn at A, B, in the accompanying section across a coral
island. A, A, are the outer edges of the fringing reef; B, B, the shores
of the island; and S1 the level of the sea. Subsidence ensuing, the
island and the sea-bottom sink slowly down, while the coral animals
continue to grow to the surface--the building of the reef keeping pace
with the subsidence. By and by the island sinks to the level S2, when
B', B', represent the shores of the now diminished island, and A', A',
the outer edges of the reef, which has become a barrier reef; C, C,
being the lagoon between the reef and the central island. We have now
only to suppose a continuance of the submergence to the level S3, when
the island disappears, its site being occupied by a lagoon, C'--the
reef, which has at the same time become an atoll, being shewn at A'',
A''.
75. In extra-tropical latitudes, great accumulations of carbonate of
lime are also taking place. The bottom of the Atlantic has been found to
be covered, over vast areas, by a fine calcareous sticky deposit called
_ooze_, which would appear to consist for the most part of the skeletons
of minute animal organisms, called Foraminifera. This accumulation, when
dried, closely resembled chalk, and there can be no doubt that in the
deep recesses of the Atlantic we have thus a gradually increasing
deposit of carbonate of lime, which rivals, if it does not exceed, in
extent the most widely spread calcareous rocks with which we are
acquainted. A small percentage of siliceous materials occurs in the
ooze, made up partly of granules of quartz, and partly of the skeletons
and coverings of minute animal and vegetable organisms. When in process
of time the chemical forces begin to act upon the siliceous matter
diffused through the Atlantic ooze, _segregation_, or the gathering
together of the particles, may take place, and nodules of flint will be
the result, similar to the flint nodules which occur in chalk, and the
cherty concretions in limestones. Animalcules with siliceous envelopes
and skeletons are by no means so abundant as those that secrete
carbonate of lime, but they are very widely diffused through the oceans,
and in favourable places are so abundant that they may well give rise
eventually to extensive beds of flint. Ehrenberg calculated that 17,946
cubic feet of these organisms were formed annually in the muddy bottom
of the harbour at Wismar, in the Baltic.
It would appear from recent observations (_Challenger_ expedition) that
the calcareous ooze at the bottom of the Atlantic and Southern Oceans,
which occurs at a mean depth of 2250 fathoms, passes gradually as the
ocean deepens into a gray ooze, which is less calcareous, and which
occurs at a mean depth of 2400 fathoms. At still greater depths this
gray ooze also disappears, and is replaced by red clay at a mean depth
of 2700 fathoms. The minute creatures (foraminifera and pelagic
mollusca chiefly) whose shells go to form the calcareous ooze, live for
the most part on the surface, and swarm all over the areas in which ooze
and red clay occur at the bottom. Hence it seems probable that the clay
is merely the insoluble residue or _ash_, as it were, of the
organisms--the delicate shells, as they slowly sink to the more profound
depths, being dissolved by the free carbonic acid, which, as
observations would seem to shew, occurs rather in excess at great
depths. Thus we see how the organic forces may give rise to extensive
accumulations of inorganic matter, closely resembling the finest silt or
mud which is carried down to the sea by rivers, and distributed far and
wide by ocean currents.