according as the currents vary in direction and force. On the other hand, they may be gradually cemented into a solid calcareous mass, as has been observed off the coast of Florida, where they form on the sea-bottom a sheet of limestone, made up of their remains.

Ooze. From observations made during the great expedition of the Challenger, it has been estimated that in a square mile of the tropical ocean down to a depth of 100 fathoms there are more than 16 tons of carbonate of lime in the form of living animals. A continual rain of dead calcareous organisms is falling to the bottom, where their remains accumulate as a soft chalky ooze.

Wide tracts of the ocean-floor are covered with a pale grey ooze of this nature, composed mainly of the remains of the shells of the foraminifer Globigerina (Fig. 33). In the north Atlantic this deposit probably extends not less than 1300 miles from east to west, and several hundred miles from north to south.

Here and there, especially among volcanic islands, portions of the sea-bed have been raised up into land, and masses of modern limestone have thereby been exposed to view. Though they are full of the same kind of shells as are still living in the neighbouring sea, they have been cemented into compact and even somewhat crystalline rock, which has been eaten into caverns by percolating water, like limestones of much older date. This cementation, as above remarked, is due to water permeating the stone, dissolving from its outer parts the calcareous matter of shells, corallines, and other organic remains, and redepositing it again lower down, so as to cement the organic detritus into a compact stone.

Coral-reefs offer an impressive example of how extensive masses of solid rock may be built up entirely of the aggregated remains of animals. In some of the warmer seas of the globe, and notably in the track of the great ocean-currents, where marine life is so abundant, various kinds of coral take root upon the edges and summits of submerged ridges and peaks, as well as on the shelving sea-bottom facing continents or encircling islands (1 in Fig. 34). These creatures do not appear to flourish at a greater depth than 15 or 20 fathoms, and they are killed by exposure to sun and air. The vertical space within which they live may therefore be stated broadly as about 100 feet. They grow in colonies, each composed of many individuals, but all united into one mass, which at first may be merely a little solitary clump on the sea-floor, but which, as it grows, joins other similar clumps to form what is

FIG. 34.-Section of a coral-reef. 1. Top of the submarine ridge or bank on which the corals begin to build. 2. Coral-reef. 3. Talus of large blocks of coral-rock on which the reef is built outward. 4. Fine coral sand and mud produced by the grinding action of the breakers on the edge of the reef. 5. Coral sand thrown up by the waves and gradually accumulating above their reach to form dry ground.

known as a reef. Each individual secretes from the sea-water a hard calcareous skeleton inside its transparent jelly-like body, and when it dies, this skeleton forms part of the platform upon which the next generation starts. Thus the reef is gradually built upward as a mass of calcareous rock (2), though only its upper surface is covered with living corals. These creatures continue to work upward until they reach low-water mark, and then their further upward progress is checked. But they are still able to grow outward. On the outer edges of the reef they flourish most vigorously, for there, amid the play of the breakers, they find the food that is brought to them by the ocean-currents. From time to time fragments are torn off by breakers from the reef and roll down its steep front (3). There, partly by the chemical action of the sea-water, and partly by the fine calcareous mud and sand (4), produced by the grinding action of the waves and washed into their crevices, these loose blocks are cemented into a firm, steep

slope, on the top of which the reef continues to grow outwards. Blocks of coral and quantities of coral-sand are also thrown up on the surface of the reef, where by degrees they form a belt of low land above the reach of the waves (5). On the inside of the reef, where the corals cannot find the abundant food-supply afforded by the open water outside, they dwindle and die. Thus the tendency of all reefs must be to grow seawards, and to increase in breadth. Perhaps their breadth may afford some indication of their relative age.

Where a reef has started on a shelving sea-bottom near the coast of a continent, or round a volcanic island, the space of water inside is termed the Lagoon Channel. Where the reef has been built up on some submarine ridge or peak, and there is consequently no land inside, the enclosed space of water is called a Lagoon, and the circular reef of coral is known as an Atoll. If no subsidence of the sea-bottom takes place, the maximum thickness of a reef must be limited by the space within which the corals can thrive—that is, a vertical depth of about 100 feet from the surface of the sea. But the effect of the destruction of the ocean-front of the reef, and the piling up of a slope of its fragments on the seabottom outside, will be to furnish a platform of the same materials on which the reef itself may grow outward, so that the united mass of calcareous rock may attain a very much greater thickness than 100 feet. On the other hand, if the sea-bottom were to sink at so slow a rate that the reef-building corals could keep pace with the subsidence, a mass of calcareous rock many thousand feet thick might obviously be formed by them. It is a disputed question in which of these two ways atolls have been formed.

It is remarkable how rapidly and completely the structure of the coral-skeleton is effaced from the coral-rock, and a more or less crystalline and compact texture is put in its place. The change is brought about partly by the action of both sea-water and rain-water in dissolving and redepositing carbonate of lime among the minute interstices of the rock, and partly also by the abundant mud and sand produced by the pounding action of the breakers on the reef, and washed into the crevices. On the portion of a reef laid dry at low water, the coral-rock looks in many places as solid and old as some of the ancient white limestones and marbles of the land. There, in pools where a current or ripple of water keeps the grains of coral-sand in motion, each grain may be seen to have taken a spherical form unlike that of the ordinary irregularly rounded or angular particles. This arises

because carbonate of lime in solution in the water is deposited round each grain as it moves along. A mass of such grains aggregated together is called oolite, from its resemblance to fishIn many limestones, now forming wide tracts of richly cultivated country, this oolitic structure is strikingly exhibited. There can be no doubt that in these cases it was produced in a similar way to that now in progress on coral-reefs (see pp. 141, 155).

roe.

In the coral tracts of the Pacific Ocean there are nearly 300 coral islands, besides extensive reefs round volcanic islands. Others occur in the Indian Ocean. Coral-reefs abound in the West Indian Seas, where, on many of the islands, they have been upraised into dry land, in Cuba to a height of 1100 feet above sea-level. The Great Barrier Reef that fronts the north-eastern coast of Australia is 1250 miles long, and from 10 to 90 miles broad.

There are other ways in which the aggregation of animal remains forms more or less extensive and durable rocks. Το some of these reference will be made in later chapters. Enough has been said here to show that by the accumulation of their hard parts animals leave permanent records of their presence both on land and in the sea.

ii. Preservation of remains of Plants and Animals in sedimentary deposits. But it is not only in rocks formed out of their remains that living things leave their enduring records. These remains may be preserved in almost every kind of deposit, under the most wonderful variety of conditions. And as it is in large measure from their occurrence in such deposits that the geologist derives the evidence that successive tribes of plants and animals have peopled the globe, and that the climate and geography of the earth have greatly varied at different periods, we shall find it useful to observe the different ways in which the remains both of plants and animals are at this moment being entombed and preserved upon the land and in the sea. With the knowledge thus gained, it will be easier to understand the lessons taught by the organic remains that lie among the various solid rocks around us.

It is evident that in the vast majority of cases, the plants and animals of the land leave no perceptible trace of their presence. Of the forests that once covered so much of Central and Northern Europe, which is now bare ground, most have disappeared, and unless authentic history told that they had once flourished, we should never have known anything about them. There were also

herds of wild oxen, bears, wolves, and other denizens contemporaneous with the vanished forests. But they too have passed away, and we might ransack the soil in vain for any trace of them.

If the remains of terrestrial vegetation and animals are anywhere preserved it must obviously be only locally, but the favourable circumstances for their preservation, although not everywhere to be found, do present themselves in many places if we seek for them. The fundamental condition is that the relics should, as soon as possible after death, be so covered up as to be protected from the air and from too rapid decomposition. Where this condition is fulfilled, the more durable of them may be preserved for an indefinite series of ages.

(a) On the Land there are various places where the remains both of plants and animals are buried and shielded from decay. To some of these reference has already been made. Thus amid the fine silt, mud, and marl gathering on the floors of lakes, leaves, fruits, and branches, or tree-trunks, washed from the neighbouring shores, may be imbedded, together with insects, birds, fishes, lizards, frogs, field-mice, rabbits, and other inhabitants. These remains may of course often decay on the lake-bottom, but where they sink into or are quickly covered up by the sediment, they may be effectually preserved from obliteration. They undergo a change, indeed, being gradually turned into stone, as will be described in Chapter XV. But this conversion may be effected so gently as to retain the finest microscopic textures of the original organisms.

In peat-bogs also, as already stated (p. 83), wild animals are often engulfed, and their soft parts are occasionally preserved as well as their skeletons. The deltas of river-mouths must receive abundantly the remains of animals swept off by floods. As the carcases float seawards, they begin to fall to pieces and the separate bones sink to the bottom, where they are soon buried in the silt. Among the first bones to separate from the rest of the skeleton are the lower jaws (pp. 308, 311). We should therefore expect that in excavations made in a delta these bones would occur most frequently. The rest of the skeleton is apt to be carried farther out to sea before it can find its way to the bottom. The stalagmite floor of caverns has already been referred to (p. 56) as an admirable material for enclosing and preserving organic remains. The animals that fell into these recesses, or used them as dens in which they lived or into which they dragged their prey, have left their bones on the floors, where,