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 dissolved 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.

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. 6, 61) 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.

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, 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 remains of the very same kinds of creatures that are living there.

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

FIG. 33. Globigerina ooze dredged up by Challenger Expedition from a depth of 1900 fathoms in the North Atlantic (2).

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 islands 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 seabottom 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 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

FIG. 34. Section of a coral-reef. I. 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.

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 seawater, 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 sea-bottom 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.

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