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 Undercliff of the Isle of Wight. Some of the tumbled rocks are still fresh enough to show that they have fallen at no very remote period, or even that the slipping still continues; others, again, have yielded so much to the weather that their date doubtless goes far back into the past, and some of them are crowned with what are now venerable ruins.

The most stupendous landslips on record have occurred in mountainous countries. Upwards of 150 destructive examples have been chronicled in Switzerland. Of these, one of the most memorable was that of the Rossberg, a mountain lying behind the Rigi, and composed of thick masses of hard red sandstone and conglomerate so arranged as to slope down into the valley of Goldau. The summer of the year 1806 having been particularly wet, so large an amount of water had collected in the more porous layers of rock as to weaken the support of the overlying mass; consequently a large part of the side of the mountain suddenly gave way and rushed down into the valley, burying under the debris about a square German mile of fertile land, four villages containing 330 cottages and outhouses, and 457 inhabitants. this day, huge angular blocks of sandstone lying on the farther side of the valley bear witness to the destruction caused by this landslip, and the scar on the mountain-slope whence the fallen masses descended is still fresh.

To

(2) Chemical Action-(a) Solution.-But it is by its chemical action on the rocks through which it flows that subterranean water removes by far the largest amount of mineral matter, and produces the greatest geological change. Even pure water will dissolve a minute quantity of the substance of many rocks. But rain is far from being chemically pure water. In previous chapters it has been described as taking oxygen and carbonic acid out of the air in its descent, and abstracting organic acids and carbonic acid from the soil through which it sinks. By help of these ingredients, it is enabled to attack even the most durable rocks, and to carry some of their dissolved substance up to the surface of the ground.

One of the substances most readily attacked and removed even by pure water is the mineral known as carbonate of lime. Among

other impurities, natural waters generally contain carbonic acid, which may be derived from the air or from the soil; occasionally from some deeper subterranean source. The presence of this acid gives the water greatly increased solvent power, enabling it readily to attack carbonate of lime, whether in the form of limestone, or diffused through rocks composed mainly of other substances. Even lime, which is not in the form of carbonate, but is united with silica in various crystalline minerals (silicates, p. 130), may by this means be decomposed and combined with carbonic acid. It is then removed in solution as carbonate. So long as the water retains enough of free carbonic acid, it can keep the carbonate of lime in solution and carry it onward.

Limestone is a rock almost entirely composed of carbonate of lime. It occurs in most parts of the world, covering sometimes tracts of hundreds or thousands of square miles, and often rising into groups of hills, or even into ranges of mountains (see pp. 154, 158). The abundance of this rock affords ample opportunity for the display of the solvent action of subterranean water. Trickling down the vertical joints and along the planes between the limestone beds, the water dissolves and removes the stone, until in the course of centuries these passages are gradually enlarged into clefts, tunnels, and caverns. The ground becomes honeycombed with openings into dark subterranean chambers, and running streams fall into these openings and continue their course underground.

Every country which possesses large limestone tracts furnishes examples of the way in which such labyrinthine tunnels and systems of caverns are excavated. In England, for example, the Peak Cavern of Derbyshire is believed to be 2300 feet long, and in some places 120 feet high. On a much more magnificent scale are the caverns of Adelsberg near Trieste, which have been explored to a distance of between four and five miles, but are probably still more extensive. The river Poik has broken into one part of the labyrinth of chambers, through which it rushes before emerging again to the light. Narrow tunnels expand into spacious halls, beyond which egress is again afforded by low passages into other lofty recesses. The most stupendous chamber measures 669 feet in length, 630 feet in breadth, and III feet in height. From the roofs hang pendent white stalactites (p. 55), which, uniting with the floor, form pillars of endless varieties of form and size. Still more gigantic is the system of subterranean passages in the Mammoth Cave of Kentucky, the accessible parts of which are

The

believed to have a combined length of about 150 miles. largest cavern in this vast labyrinth has an area of two acres, and is covered by a vault 125 feet high.

Of the mineral matter dissolved by permeating water out of the rocks underground, by far the larger part is discharged by springs into rivers, and ultimately finds its way to the sea. The total amount of material thus supplied to the sea every year must be enormous. Much of it, indeed, is abstracted from ocean-water by the numerous tribes of marine plants and animals. In particular, the lime, silica, and organic matter are readily seized upon to build up the framework and furnish the food of these creatures. But probably more mineral matter is supplied in solution than is re

FIG. 19.-Section of cavern with stalactites and stalagmite.

quired by the organisms of the sea, in which case the water of the sea must be gradually growing heavier and salter.

(b) Deposition. But it is the smaller proportion of the material not conveyed into the sea that specially demands attention. Every spring, even the purest and most transparent, contains mineral solutions in sufficient quantity to be detected by chemical analysis. Hence all plants and animals that drink the water of springs and rivers necessarily imbibe these solutions which, indeed, supply some of the mineral salts whereof the harder parts both of plants and animals are constructed. springs, however, contain so large a proportion of mineral matter, that when they reach the surface and begin to evaporate, they drop their solutions as a precipitate, which settles down upon the bottom or on objects within reach of the water. After years of undisturbed continuance, extensive sheets of mineral material may in this manner be accumulated, which remain as enduring monu

Many

ments of the work of underground water, even long after the springs that formed them have ceased to flow.

Calcareous Springs.-Among the accumulations of this nature by far the most frequent and important are those formed by what are called Calcareous Springs. In regions abounding in limestone or rocks containing much carbonate of lime, the subterranean waters which, as we have seen, gradually erode such vast systems of tunnels, clefts, and caverns, carry away the dissolved rock, and retain it in solution only so long as they can keep their carbonic acid. As soon as they begin to evaporate and to lose some of this acid, they lose also the power of retaining so much carbonate of lime in solution. This substance is accordingly dropped as a fine white precipitate, which gathers on the surfaces over which the water trickles or flows.

The most familiar example of this process is to be seen under the arches of bridges and vaults. Long pendent white stalks or stalactites hang from between the joints of the masonry, while wavy ribs of the same substance run down the piers or walls, and even collect upon the ground (stalagmite). A few years may suffice to drape an archway with a kind of fringe of these pencil-like icicles of stone. Percolating from above through the joints between the stones of the masonry, the rain-water, armed with its minute proportion of carbonic acid at once attacks the lime of the mortar and forms carbonate of lime, which is carried downward in solution. Arriving at the surface of the arch, the water gathers into a drop, which remains hanging there for a brief interval before it falls to the ground. That interval suffices to allow some of the carbonic acid to escape, and some of the water to evaporate. Consequently, round the outer rim of the drop a slight precipitation of white chalky carbonate of lime takes place. This circular pellicle, after the drop falls, is increased by a similar deposit from the next drop, and thus drop by drop the original rim or ring is gradually lengthened into a tube which may eventually be filled up inside, and may be thickened irregularly outside by the trickle