Living Organisms have much to do with geological changes. Plants and animals co-operate with inorganic agents in promoting the degradation of the land, and on the other hand, they protect rocks from decay. Plants keep the surface of rocks moist, and promote mechanical and chemical. dissolution. In their decay they produce acids which are potent in decomposing rocks and in disintegrating soils. Of the destructive influence of animal life numerous illustrations may be given. 1. The composition and arrangement of soil are affected. Worms are continually engaged in bringing up the lower portions of the soil to the surface, thus increasing its fertility and its capa bility of being washed away by rain. Burrowing animals, by throwing up the soil and subsoil, expose these to be dried and blown away by the wind. At the same time their subterranean passages serve to drain off the superficial water and to injure the stability of the surface of the ground above them. In Britain, the mole and rabbit are familiar examples. In North America the prairie dog and the gopher have driven excavations under extensive tracts of pasture land in the west. In.Cape Colony wide areas of open country seem to be in a constant state of eruption from the burrowing Operations of multitudes of Bath,gergi and Ohrysochloris—small mole-like animals which bring up the soil and bury the grassy vegetation under it. 2. The flow of streams is sometimes interfered with, or even diverted, by the operations of animals. Thus the beaver, by constructing dams, checks the current of water-courses, intercepts floating materials, and sometimes even diverts the water into new channels. This action is typically dis played in Canada, and other parts of North America. The embankments of the Missis sippi are sometimes weakened to such an extent by the burrowing of the crayfish, as to give way, and allow the river to inundate the surrounding country. Similar results have happened in Europe from the subterranean operations of rats. 3. Some mullusea bore into stone or wood, and by the number of contiguous perforations, greatly weaken the material. Pieces of drift-wood are soon with long,holes by the teredo, while wooden piers, and the bottoms of wooden ships are often rapidly perforated. The sax icavous shells, by piercing rocks and opening cavities for rain and sea-water to fill, promote the decay of the stone. 4. Many animals exercise a ruinously destructive influence upon vegetation. Of the many insect plagues of that kind it will be enough to mention the locust, phylloxera, and Colorado beetle. The pasture in some parts of the s. of Scotland has in recent years been much damaged by mice, which have increased in numbers owing to the indiscriminate shooting and trapping of owls, hawks, and other predaceous creatures. Grasshoppers cause the destruction of vegetation in sonic parts of Wyoming and other western territories of the United States. The way in which animals destroy each other, often on a great scale, may likewise be included among the 'geological operations now under description.
Against these of destruction may be placed forms of conservative or repro ductive action, shown chiefly in Vegetation. The slightest vegetation, to some extent, protccts the surface from erosion, gives it solidity, and induces further' vegetation. Vast sandy tracts have been to some extent redeemed by the judicious planting of trees that broke the force of the wind. In a similar way marine plants protect rocks along the shores. In mountain districts pine forests exercise an important influence in pre venting the formation or arresting the progress of avalanches. Both plants and animals contribute materials towards new geological formations. Their remains are inclosed in deposits of sand and mud and there preserved. But they form of themselves not unim. portant accumulations. Of plant formations the following illustrative examples may be given. 1. Peat-mosses are accumulations of marshy vegetation which occur in temper ate and arctic latitudes, sometimes to a depth of 40 ft. or more. In Europe they have been largely formed by plants of the genus Sphagnum, which, growing as a spongy fibrous mass over wet ground, die in their lower parts and send out new fibers 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 Sphagnum 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 possesses a great antiseptic power; the bodies of animals which have been entombed in it are some times preserved for many centuries. 2. Mangrove swamps are found on the low moist shores and river mouths of tropical countries, and the mangrove tree plays an important geological part. It grows in such situations in a dense jungle, sometimes 20 in. broad, which fringes the coast as a green selvage, and runs up or quite occupies creeks and inlets. The mangrove flourishes 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 vegetation, helps to turn the spongy jungle into a firm soil. On the coast of Florida the mangrove swamps stretch for a long distance as a belt from 5 to 20 m.• broad, which winds round the creeks and inlets. At Bermuda the mangroves co-operate with the 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, mud or earth, the minute siliceous plants called diatoms occur both in fresh and salt water, deposit their congregated remains both on the site of lakes and on the sea-floor. " Infusorial " earth and " tripoli powder" consist mainly of the fraginentary debris of diatoms which have accumulated on the bottoms of lacustrine areas. Towards the antarctic circle the " Challenger" met with diatomacece in abundance, both in the surface waters of the ocean and on the bottom. At depths of from 1260 to 1975 fathoms they form a pale straw-colored 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 malletsca, actinozoa, and forminifera. 1. lu 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 accumulations of animal remains are calcareous. • In fresh water they are represented by the marl of lakes—a white, chalky deposit of the moldering remains of Hollow, Entomostraca, and partly of fresh-water Alp. On the sea-bottom in shallow water they consist of beds of shells, such as the oyster-banks of English seas. The fringing barriers found at all 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 1250 m. long, from 10 to 90 broad, and more than 1800 ft. 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 growing stalactite. Great quantities of calcareous mud are produced by the breakers which beat upon the outer edge of the reefs. This mud is partly washed up on the reefs and aids in their consolidation, but in great measure is swept away by the ocean currents and distributed over many thou sands of square m. of the sea-floor. In deep water over the bed of the Atlantic and many other oceans a remarkable calcareous ooze occurs which is formed of the remains of Foranzinzfera, and chietlyof species of the genus Globigerina. It is next in abundance to the red and gray clays of the deep sea. It is a pale-gray marl, some-. times 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. 2. Siliceous deposits formed from animal exuvim are illustrated by another of the deep sea formations brought to. light by the " Challenger " researches. In certain regions of the western and middle Pacific ocean, the bottom was found to be covered with an ooze consisting almost entirely of Radiolaria. These minute organisms occur, indeed, more or less abundantly in almost all deep oceanic deposits. From the deepest sounding yet taken (4,575 fathoms, or more than 5 miles) a radiolarian ooze was obtained. The apicules of sponges likewise furnish materials towards these silicious accumulations. 3. Phosphatic deposits, in the great majority of cases, betoken sonic 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 accumulations of this nature are the guano beds of rainless islands off the western coast of South America, and Southern 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, in some places, 30 to 80 feet, or even more. This deposit, consisting chiefly of organic matter and ammo niacal salts with about 20 per et. of phosphate of lime, has acquired a high value as a manure, and is being rapidly clearedoff. It could have been preserved only in a rainless or almost rainless climate. On the west of Europe isolated stacks and rocky islands in the sea are often seen to be white from the dropping of seabirds; but it is merely a thin crust, gaining no great depth in a climate where rains are frequent and heavy.