CHAPTER VIII

HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS IN GEOLOGICAL HISTORY

BROADLY considered, there are two distinct ways in which Plants and Animals leave their mark upon the surface of the earth. In the first place, they act directly by promoting or arresting the decay of the land, and by forming out of their own remains deposits which are sometimes thick and extensive. In the second place, their remains are transported and entombed in sedimentary accumulations of many different kinds, and furnish important evidence as to the conditions under which these accumulations were formed. Each of these two forms of memorial deserves our careful attention, for, taken together, they comprise the most generally interesting departments of geology, and those in which the history of the earth is principally discussed.1

i. Direct action of living things upon the surface of the globe. This action is often of a destructive kind, both plants and animals taking their part in promoting the general disintegration of rocks and soils. Thus, by their decay they furnish to the soil those organic acids referred to on pp. 16, 24, as so important in increasing the solvent power of water, and thereby promoting the waste of rocks. By thrusting their roots into crevices of cliffs, plants loosen and gradually wedge off pieces of rock, and by sending their roots and rootlets through the soil, they open up the subsoil to be attacked by air and descending moisture (p. 16). The action of the common earthworm in bringing up fine soil to be exposed to the influences of wind and rain was referred to at p. 18. Many burrowing animals also, such as the mole and

1 In the Appendix a Table of the Vegetable and Animal Kingdoms is given, from which the organic grade of the plants and animals referred to in this and subsequent chapters may be understood.

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rabbit, throw up large quantities of soil and subsoil which are liable to be blown or washed away.

On the other hand, the action may be conservative, as, for instance, where, by forming a covering of turf, vegetation protects the soil underneath from being rapidly removed, or where sandloving plants bind together the surface of dunes, and thereby arrest the progress of the sand, or where forests shield a mountain-side from the effects of heavy rains and descending avalanches.

(1) Deposits formed of the remains of Plants.-But it is chiefly by the aggregation of their own remains into more or less extensive deposits that plants and animals leave their most prominent and enduring memorials. As examples of the way in which this is done by plants, reference may be made to peat-bogs, mangroveswamps, infusorial earth, and calcareous sea-weeds.

Peat-bogs. In temperate and arctic countries, marshy vegetation accumulates in peat-bogs over areas from an acre or two to many square miles, and to a depth of sometimes 50 feet. These deposits are largely due to the growth of bog-mosses and other aquatic plants which, dying in their lower parts, continue to grow upward on the same spot. On flat or gently-inclined moors, in hollows between hills, on valley-bottoms, and in shallow lakes, this marshy vegetation accumulates as a wet spongy fibrous mass, the lower portions of which by degrees become a more or less compact dark brown or black pulpy substance, wherein the fibrous texture, so well seen in the upper or younger parts, in large measure disappears. In a thick bed of peat, it is not

infrequently possible to detect a succession of plant remains, showing that one kind of vegetation has given place to another during the accumulation of the mass. In Europe, as already mentioned on p. 4, peat-bogs often rest directly upon fresh-water marl containing remains of lacustrine shells (1 in Fig. 30). In every such case, it is evident that the peat has accumulated on the site of a shallow lake which has FIG. 30.-Section of a peat-bog. been filled up, and converted into a morass by the growth of marsh-plants along its edges and over its floor. The lowest parts of the peat may contain remains of the reeds, sedges, and other aquatic plants which choked up

the lake (2, 3). Higher up, the peat consists almost entirely of the matted fibres of different mosses, especially of the kind known as Bog-moss or Sphagnum (4). The uppermost layers (5, 6) may be full of roots of different heaths which spread over the surface of the bog.

The rate of growth of peat has been observed in different situations in Central Europe to vary from less than a foot to about two feet in ten years; but in more northern latitudes the growth is probably slower. Many thousand square miles of Europe and North America are covered with peat-bogs, those of Ireland being computed to occupy a seventh part of the surface of the island, or upwards of 4000 square miles.

As the aquatic plants grow from the sides toward the centre of a shallow lake, they gradually cover over the surface of the water with a spongy layer of matted vegetation.

Animals, and

man himself, venturing on this treacherous surface sink through it, and may be drowned in the black peaty mire underneath. Long afterwards, when the morass has become firm ground, and openings are made in it for digging out the peat to be used as fuel, their bodies may be found in an excellent state of preservation. The peaty water so protects them from decay that the very skin and hair sometimes remain. In Ireland, numerous skeletons of the great Irish elk have been obtained from the bogs, though the animal itself has been extinct since before the beginning of the authentic history of the country.

Mangrove-swamps.-Along the flat shores of tropical lands, the mangrove trees grow out into the salt water, forming a belt of jungle which runs up or completely fills the creeks and bays. So dense is the vegetation that the sand and mud, washed into the sea from the land, are arrested among the roots and radicles of the trees, and thus the sea is gradually replaced by firm ground. The coast of Florida is fringed with such mangrove-swamps for a breadth of from 5 to 20 miles. In such regions, not only does the growth of these swamps add to the breadth of the land, but the sea is barred back, and prevented from attacking the newlyformed ground inside.

Infusorial earth.-A third kind of vegetable deposit to be referred to here is that known by the names of infusorial earth, diatom-earth, and tripoli-powder. It consists almost entirely of the minute frustules of microscopic plants called diatoms, which are found abundantly in lakes and likewise in some regions of the ocean (Fig. 31). These lowly organisms are remarkable for

secreting silica in their structure. As they die, their singularly durable siliceous remains fall like a fine dust on the bottom of the water, and accumulate there as a pale gray or straw-coloured deposit, which, when dry, is like flour, and in its pure varieties is made almost entirely of silica (90 to 97 per cent). Underneath the peat-bogs of Britain a layer of this material is sometimes met with. One of the most famous examples is that of Richmond, Virginia, where a bed of it occurs 30 feet thick. At Bilin in Bohemia also an important bed has long been known. The bottom of some parts of the Southern Ocean is covered with a diatom-ooze made up mainly of siliceous diatoms, but containing

also other siliceous organisms (radiolarians) and calcareous foraminifera (Fig. 31).

Accumulations of sea-weeds. Yet one further illustration of plant-action in the building up of solid rock may be given. As a rule the plants of the sea form no permanent accumulations, though here and there under favourable conditions, such as in bays and estuaries, they may be thrown up and buried under sand so as eventually to be compressed into a kind of peat. Some sea-weeds, however, abstract from sea-water carbonate of lime, which they secrete to such an extent as to form a hard stony structure, as in the case of the common nullipore. When the plants die, their remains are thrown ashore and pounded up by the waves, and being durable they form a white calcareous sand. By the action of the wind, this sand is blown inland and may accumulate into dunes. But unlike ordinary sand, it is liable to be slightly dissolved by rain-water, and as the portion so dis

solved is soon redeposited by the evaporation of the moisture, the little sand-grains are cemented together, and a hard crust is formed which protects the sand underneath from being blown away. Meanwhile rain-water percolating through the mounds gradually solidifies them by cementing the particles of sand to each other, and thick masses of solid white stone are thus produced. Changes of this kind have taken place on a great scale at Bermuda, where all the dry land consists of limestone formed of compacted calcareous sand, mainly the detritus of sea-weeds.

(2) Deposits formed of the remains of Animals.—Animals are, on the whole, far more successful than plants in leaving enduring

memorials of their life and work. They secrete hard outer shells and internal skeletons endowed with great durability, and capable of being piled up into thick and extensive deposits which may be solidified into compact and enduring stone. On land, we have an example of this kind of accumulation in the lacustrine marl already (pp. 4, 46) described as formed of the congregated remains of various shells. But it is in the sea that animals, secreting carbonate of lime, build up thick masses of rock, such as shell-banks, ooze, and coral reefs (see Chapter XI, p. 158).

Shell-banks. Some molluscs, such as the oyster, live in populous communities upon submarine banks. In the course of generations, thick accumulations of their shells are formed on these banks. By the action of currents also large quantities of broken shells are drifted to various parts of the sea-bottom not far from land. Such deposits of shells, in situ or transported, may be more or less mixed with or buried under sand and silt,