ferric oxide or ochre is thickest. It encloses leaves, stems, and other organic remains, and preserves moulds or casts of their forms. It also cements the loose sand and shingle of a river-bottom into solid rock.

One other deposit from spring-water may be enumerated here. In volcanic regions, hot springs (geysers) rise to the surface which, besides other mineral ingredients, contain a considerable proportion of silica (p. 157). This substance is deposited as Siliceous Sinter round the vents whence the water is discharged, where it forms a white stone rising into mounds and terraces with fringes and bunches of coral-like growth. Where many springs have risen in the same district, their respective sheets of sinter may unite, and thus extensive tracts are buried under the deposit. In Iceland, for example, one of the sheets is said to be two leagues long, a quarter of a league wide, and a hundred feet thick. In the Yellowstone Park of North America, many valleys are floored over with heaps of sinter, and in New Zealand other extensive accumulations of the same material are to be found. It is obvious that, like travertine, siliceous sinter may readily entomb and preserve a record of the plants and animals that lived at the time of its deposition.

Summary. The underground circulation of water produces changes that leave durable records in geological history. These changes are of two kinds. (1) Landslips are caused, by which the forms of cliffs, hills, and mountains are permanently altered. Vast labyrinths of subterranean tunnels, galleries, and caverns are dissolved out of calcareous rocks, and openings are made from these passages up to the surface whereby rivers are engulfed. Many of the caves thus hollowed out have served as dens of wild beasts and dwelling-places for man, and the relics of these inhabitants have been preserved under the stalagmite of the floors. (2) An enormous quantity of mineral matter is brought up to the surface by springs. Most of these solutions are conveyed ultimately to the sea where they partly supply the substances required by the teeming population of marine plants and animals. But under favourable circumstances, considerable deposits of mineral matter are made by springs, more especially in the form of travertine, siliceous sinter, and ochre. In these deposits the remains of terrestrial vegetation, also of insects, birds, mammals, and other animals are not infrequently preserved, and remain as permanent memorials of the life of the time when they flourished.

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CHAPTER VI.

ICE-RECORDS.

ICE in various ways alters the surface of the land. By eroding even the most durable rocks, and by removing and piling up elsewhere a vast amount of loose materials, it greatly modifies the details of a landscape. As it assumes various forms, so it accomplishes its work with considerable diversity. The action of frost upon soil and bare surfaces of rock has already (p. 18) been described. We have now to consider the action of frozen rivers and lakes, snow and glaciers, which have each their own characteristic style of operation, and leave behind them their distinctive contribution to the geological history of the earth.

Frozen Rivers and Lakes. - In countries with a severe winter climate, the rivers and lakes are frozen over and the cake of ice that covers them may be more than two feet thick. When this cake is broken up in early summer, large masses of it are driven ashore, tearing up the boulders, gravel, sand, or mud, and pushing them to a height of many feet above the ordinary level of the water. When the ice melts, huge heaps of detritus are found to have been piled up by it, which remain as enduring monuments of its power. Not only so, but large fragments of the ice that has been formed along shore and has enclosed blocks of stone, gravel, and sand, are driven away and may travel many miles before they melt and drop their freight of stones. On the St. Lawrence and on the coast of Labrador, there is a constant transportation of boulders by this means. Further, besides freezing over the surface, the water not infrequently forms a loose spongy kind of ice on the bottom (Anchor-ice, Groundice) which encloses stones and gravel, and carries them up to the surface where it joins the cake of ice there. This bottom of ice is formed abundantly on some parts of the Canadian rivers. Swept down by the current, it accumulates against the bars or banks, or is pushed over the upper ice, and from time to time gathers into temporary barriers the bursting of which may cause destructive floods. In the river St. Lawrence, banks and islets have been to a large extent worn down by the grating of successive ice-rafts upon them.

Snow. On level or gently inclined ground, snow disappears where it falls. But while it remains, it exercises a protective influence upon the soil and vegetation, shielding them from the action of frost. On slopes of sufficient declivity, however, the sheet of snow acquires a tendency to descend by gravitation. In many cases, it creeps or slides down the side of a hill or valley, and in so doing moves forward bare soil, loose stones, or other objects lying on the surface. By this means, the debris of weathered rock in exposed situations is gradually pushed down-hill and the rock is bared for further disintegration. But it is where the declivities are steep enough to allow the snow to break off in large sheets and to rush rapidly down that the most striking changes are observable. Such descending masses are known as Avalanches. Varying from 10 to 50 feet or more in thickness and several hundred yards broad and long, they sweep down the mountain sides with terrific force, carrying away trees, soil, houses, and even large blocks of rock. The winter of 1884-85 was especially remarkable for the number of avalanches in the valleys of the Alps, and for the enormous loss of life and property which they caused. Not only are the declivities bared of their trees, soil, and boulders, but huge mounds of debris are piled up on the valley below. Frequently, also, such a quantity of snow, ice, and rubbish is thrown across the course of a stream as to dam back the water, which accumulates until it overflows or sweeps away the barrier. In another but indirect way, snow may powerfully affect the surface of a district where, by rapid melting, it so swells the rivers as to give rise to destructive floods.

While, therefore, the influence of snow is on the whole to protect the surface of the land, it shows itself in mountainous regions singularly destructive, and leaves as chief memorials of this destructiveness the mounds and rough heaps of earth and stones that mark where the down-rushing avalanches have come to rest.

Glaciers and Ice-Sheets leave their record in characters so distinct that they cannot usually be confounded with those of any other kind of geological agent. The changes which they produce on the surface of the land may be divided into two parts: (1) the transport of materials from the high grounds to lower levels, and (2) the erosion of their beds. As a glacier descends its valley, it receives upon its surface the earth, sand, mud, gravel, boulders, and blocks of rock that roll or are washed down from the slopes on either side. Most of this rubbish accumulates on the edges of the glacier, where it is slowly borne to lower levels as the ice creeps downwards. But some of it falls into the