Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/304

Rh 290 mountain sides have been washed bare of their soil. The desiccation of the countries bordering the eastern Mediter- ranean has been ascribed to a similar cause. 5. In mountain districts pine forests exercise also an important conservative function in preventing the formation or arresting the pro- gress of avalanches. In Switzerland some of the forests which cross the lines of frequent snow-falls are carefully preserved. Animals do not exert any important conservative action upon the earth’s surface, save in so far as they form new deposits, as will be immediately referred to. In the prairie regions of Wyoming and other tracts of North America, some interesting minor effects are referable to the herds of roving animals which migrate over these territories. Pro- fessor Comstock describes the trails made by the bison, the elk, and the big-horn or mouiitain-slieep as ﬁrinly-trodden tracks on which vegetation will not grow for many years. All over the region traversed by the bison numerous circular patches of grass are to be seen which have been formed on the hollows where this animal has wallowed. Originally they are shallow depressions formed in great numbers where a herd of bisons has rested for a time. On the advent of the rains they become pools of water ; thereafter grasses spring up luxuriantly, and so bind the soil together that these grassy patches, or “bison—wallows,” may actually become slightly raised above the general level if the surrounding ground becomes parched and degraded by the winds (I».’e- cozmaissrmce of .''.W. W3/orning, 1875, p. 175). III. REPRODUCTIVE ACTIoN.——Botli plants and animals contribute materials to wards new geological formations. Their remains are enclosed in deposits of sand and mud and there preserved. But they form of themselves not unimportant accumulations. Of plant formations the following illustra- tive examples may be given. (1.) Peat-.l[osses.—These are accumulations of marshy vegetation which occur in tem- perate and arctic latitudes, sometimes to a depth of 40 feet or more. In Europe they have been largely formed by plants of the genus SpIza_r/nmn, which, growing as a spongy ﬁbrous mass over wet ground, die in their lower parts and send out new ﬁbres above. It is this lower decaying stratum which forms the peat. Every stage of the process may be seen in a large moss, from the green living plants at the top, through ﬁbrous brown turf full of the scarcely decayed rootlets of the Splzagmzm, down to the compact brown or almost black peat at the bottom. Many peat- mosses were at one time lakes which have been gradually ﬁlled up by the accumulation of marsh-plants. Peat pos- sesses a great antiseptic power ; the bodies of animals which have been entombed in it are sometimes preserved for many centuries. (2.) A1! cm/jrove Swamps. —On the low moist shores and river mouths of tropical countries, the mangrove tree plays an important geological part. It grows in such situations in a dense jungle, sometimes 20 miles broad, which fringes the coast as a green selvage, and runs up if it does not quite occupy creeks and inlets. The mangrove fiourishes in sea-water even down to low-water mark, form- ing.there a dense thicket which, as the trees drop their radicles and take root, grows outward into the sea. It is singular to ﬁnd terrestrial birds nestling in the branches above ‘and crabs and barnacles living among the roots below. _By this network of subaqueous radicles and roots the water is ﬁltered of its sediment, which, retained among the vegeta- tion, helps to turn the spongy jungle into a ﬁrm soil. On the coast of Florida the mangrove swamps stretch for long distances as a belt from 5 to 20 miles broad, which winds round the creeks and inlets. At Bemiuda the mangroves co-operate with grasses and other plants to choke up the creeks and brackish lakes. In these waters calcareous alga: abound, and as their remains are thrown up amidst the sand and vegetation they form a remarkably calcareous soil. (3.) Diatom Jlucl or ]?rtrI/e.—As the minute siliceous plants GEOLOGY [iii. DY.'AMICA L. called diatoms occur both in fresh and salt water, the de- posit formed from their congregated remains is found both on the sites of lakes and on the sea—ﬂoor. “Infusorial” earth and “ tripoli powder” consist mainly of the frustules and fragmentary debris of diatoms which have accumulated on the bottoms of lacustrine areas. Towards the Antarctic circle the “ Challenger” met with 1)z'ulomacea: in abundance, both in the surface waters of the ocean and on the bottom. They form at depths of from 1260 to 1975 fathonis a pale straw-coloured deposit, which when dried is white and very light. Animal formations are chiefly composed of the remains of the lower grades of the animal kingdom, especially of Jlfollusca, Actinozoa, and 1"o7°(miz'1z2fe7'(¢. (1.) In some cases they are calcareous. Lime, chieﬂy in the form of carbonate, is the mineral substance of which the solid parts of animals are mainly built up. Hence the great majority of the accum- ulations formed of animal remains are calcareous. In fresh water they are represented by the marl of lakes—a white, chalky deposit consisting of the nioiilderiiig remains of .-llollusca, 1L'7lt07)l0Sf7‘((C((, and partly of f resli-water alga-. O11 the sea-bottoiii in shallow water they coiisist of beds of shells, such asthe oyster-banl-:s of English seas. The fringing, barrier, and atoll coral--reefs of warm seas are conspicuous examples of wide and thick masses of rock formed from the accumulated growth of animal organisms. The great reef of Australia, for example, is 1:250 miles long, from 10 to 90 miles broad, and more than 1800 feet thick. The coral rock, though formed by the continuous growth of the polyps, gradually loses any distinct organic structure, and acquires an internal crystalline character owing to the inﬁltration of water through its mass, whereby carbonate of lime is carried down and deposited in the pores and crevices as in a grow- ing stalactite. Great quantities of calcareous niud are pro- duced by the breakers which beat upon the outer edge of the reefs. This mud is partly washed up upon the reefs and aids in their consolidation, but in great measure it is swept away by the ocean currents and distributed over many thousands of square miles of the sea—ﬂoor. In deep watrr over the bed of the Atlantic and other oceans a remarkable calcareous ooze occurs which is formed of the remains of Fora7nz'ng'/‘era, and chieﬂy of species of the genus (w'lob2'g«*-rhzrz. It is next in abundance to the red and grey clays of the deep sea. It is a pale-grey marl, sometimes red from peroxide of iron, or brown from peroxide of manganese ; and it usually contains more or less clay, even with occasional fragments of pumice. Siliceous deposits formed fi-uni animal exuviae are illustrated by another of the deep-.-ca formations brought to light by the“ C‘liallenger” researclies. In certain regions of the western and middle Pacific Ocean, the bottom was found to be covered with an ooze consisting almost entirely of Ii’a<Iz'olarir(. These minute organisiiis occur, indeed, more or less abundantly in almost all deep oceanic deposits. From the deepest sounding yet taken (4575 fathonis, or more than 5 miles) a radiolarian ooze was obtained. The spicules of sponges likewise furnish materials towards these siliceous accumulations. (3.) Plzosplzalic deposits, in the great majority of cases, betokcn some of the vertebrate animals, seeing that phosphate of lime enters largely into the composition of their bones and occurs in their excrement. The most typical modern accumu- lations of this nature are the guano beds of rainless islands of? the western coasts of South America and Soutliern Africa. In these regions immense ﬂocks of sea—fowl have in the course of centuries covered the ground with an accumulation of their droppings to a depth of sometimes 30 to 80 feet, or even more. This deposit, consisting chieﬂy of organic matter and ammoniacal salts, with about 20 per cent. of phosphate of lime, has acquired a high value as a manure, and is being rapidly cleared off. It could only have been