and sheets of brown iron ore. Throughout them all, remains of the plants and animals of the surrounding land are likely to be entombed and preserved.

Salt lakes leave, as their enduring memorial, beds of rock-salt and gypsum, sometimes carbonate of soda and other salts. Many of them were at first fresh, as is shown by the presence of ordinary fresh-water shells in their upper terraces. But by change of climate and long-continued excess of evaporation over precipitation, the water has gradually become more and more saline, and has sometimes disappeared altogether, leaving behind it deposits of common salt, gypsum, and other chemical precipitates.

F

CHAPTER V.

HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HISTORY.

THE changes made by running water upon the land are not confined to that portion of the rainfall which courses along the surface. Even when it sinks underground and seems to have passed out of the general circulation, the subterranean moisture does not remain inactive. After travelling for a longer or shorter distance through the pores of rocks, or along their joints and other divisional planes, it finds its way once more to daylight and reappears in Springs.1 In this underground journey, it corrodes rocks, somewhat in the same way as rain attacks those that are exposed to the outer air, and it works some curious changes upon the face of the land. Subterranean water thus leaves distinct and characteristic memorials as its contributions to geological history.

There are two aspects in which the work of underground water may be considered here. In the first place, portions of the substance of subterranean rocks are carried up above ground; in the second place, some of these materials are laid down again in a new form and take a conspicuous place among the geological monuments of their time.

Abstraction of Material. In the removal of mineral

substance, water percolating through rocks acts in two distinct ways, mechanical and chemical, each of which shows itself in its own peculiar effects upon the surface. While slowly filtering through porous materials, water tends to remove loose particles and thus to lessen the support of overlying rocks. But even where there is no transport, the water itself, by saturating a porous layer that rests upon a more or less impervious one, loosens the cohesion of that porous layer. The overlying mass of rock is thus made to rest upon a watery and weakened platform, and if from its position it should have a tendency to gravitate in any given direction, it may at last yield to this tendency and slide downwards. Along the sides of sea-cliffs, on the precipitous slopes of valleys or river-gorges, or on the declivities of hills and mountains, the conditions are often extremely favourable for the descent of large masses of rock from higher to lower levels.

Remarkable illustrations of such Landslips, as they are called, have been observed along the south coast of England, where certain porous sandy rocks underlying a thick sheet of chalk rest upon more or less impervious clays, which, by arresting the water in its descent, throw it out along the base of the slopes. After much wet weather, the upper surface of these clays becomes, as it were, lubricated by the accumulation of water, and large slices of the overlying rocks, having their support thereby weakened, break off from the solid cliffs behind and slide down towards the sea. The most memorable example occurred at Christmas time, in the year 1839, on the coast of Devonshire not far from Axmouth. At that locality, the chalk-downs end off in a line of broken cliff some 500 feet above the sea. From the edge of the downs flanked by this cliff a tract about 800 yards long, containing not less than 30 acres of arable land, sank down with all its fields, hedgerows, and pathways. This sunken mass, where it broke away from the upland, left behind it a new cliff, showing along the crest the truncated ends of the fields, of which the continuation was to be found in a chasm more than 200 feet deep. While the ground sank into this defile and was tilted steeply towards the base of the cliff, it was torn up by a long rent running on the whole in the line of the cliff, and by many parallel and transverse fissures. Nearly half a century has passed away since this landslip occurred. The cliff remains much as it was at first, and the sunken fields with their bits of hedgerow still slope steeply down to the bottom of the declivity. But the lapse of time has allowed the influence of the atmosphere to come into play. The outstanding dislocated fragments with their vertical walls and flat tops, showing segments of fields, have been gradually worn into tower-like masses with sloping declivities of debris. The long parallel rent has been widened by rain into a defile with shelving sides. Everywhere the rawness of the original fissures has been softened by the rich tapestry of verdure which the genial climate of that southern coast fosters in every sheltered nook. But the scars have not been healed, and they will no doubt remain still visible for many a year

to come.

Along the south coast of England, many landslips, of which there is no historical record, have produced some of the most picturesque scenery of that region. Masses that have slipped away from the main cliff have so grouped themselves down the slopes that hillocks and hollows succeed each other in endless confusion, as in the well-known