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IN Chapter VII. it was pointed out that subterranean action had played a most important part in the production of certain surface-features. In particular it was shown that depression of the surface has frequently taken place as a result of that action. We have now to consider another kind of action altogether, which, although by no means so important as that just referred to, nevertheless now and again causes the surface in certain regions to subside. Rocks, as we have seen, are very variously acted upon by water —a few are readily soluble, but the great majority are not. The most important of the soluble rocks are rock salt, gypsum, and limestones of every kind. These are all more or less easily removed by meteoric water. Rock salt is so very soluble that it is seldom >r never found cropping out at the surface; any surace-exposure in temperate lands would rapidly disappear. It is only in dry and rainless tracts, therefore, that rock salt can exist as a superficial accumulation. Gypsum is more readily dissolved than limestone, but both rocks become eaten into at the surface, and, according to circumstances, are more or less rapidly washed away. This process of dissolution, it is needless to add, is not confined to the surface. Meteoric water penetrates the ground, and circulates through the crust to considerable depths. After pursuing a shorter or longer course, it reappears at the surface as springs, the waters of which are more or less abundantly charged with dissolved mineral matter, according to the nature of the rocks through which it has passed. In this way enormous quantities of soluble materials are brought up from below; in short, wholesale chemical erosion goes on underground. It follows that in regions where soluble rocks enter largely into the framework of the land the surface must in time subside slowly or suddenly. The copious outpouring of brine-springs gradually reduces beds and sheets of rock salt, and the overlying strata sink down and thus produce depression at the surface. And the same result is brought about by the dissolution of gypsum, limestone, and dolomite. Sometimes the surface slowly subsides, but now and again it collapses suddenly, producing earthquakes, accompanied by much fracturing and shifting of the rocks. Thus it is believed that the earthquakes which disturbed

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the Visp-Thal in Valais during the summer and autumn of 1855 were the result of the caving-in of the rocks consequent on the dissolution and removal of gypsum, for the springs of that district bring to the surface annually over 200 cubic metres of the mineral in solution. Similarly, it can hardly be doubted that many of the larger and deeper depressions of the surface which appear in regions of calcareous rock are the result of sudden collapse due to the removal of material by underground water.

As rock salt and gypsum do not enter largely into the composition of the crust, they are less important from our point of view than limestones. The latter not only attain in many cases a much greater thickness, but they are far more widely distributed, and extend over much broader areas of the earth's surface. It is in regions of calcareous rocks, therefore, where underground water plays the most prominent rSle, and where its action in modifying surface-features is best displayed. In a former chapter reference has been made to the fact that in countries occupied by limestone, the drainage is often largely or even wholly conducted underground. The rocks are so penetrated in all directions by rifts, clefts, and tunnels, that the water which falls at the surface very soon disappears. Concerning the origin of these subterranean spaces there is not much difference of opinion. Geologists recognise that they have been worked out by chemical and mechanical water-erosion. But while some have maintained that the underground water -ias licked and worn out a passage for itself chiefly 3.1ong the normal divisions of the rocks—their joints and bedding-planes—others have held that the main lines of underground drainage have been determined t>y faults or dislocations. Both views are doubtless true: some caves and underground tunnels appear to Have no connection with faults; others, on the contrary, follow these, although many of the channels connected with them have been worked out along joints and bedding:planes.

Underground water usually follows a zigzag and irregular course—now plunging downwards at high angles, or even vertically, through relatively constricted clefts and fissures; now winding through approximately horizontal tunnels, or forming lake-like expansions in broad and lofty halls and chambers; now dividing into more or less numerous torrents and streams, which zigzag downwards to lower and lower levels. In time many changes are effected. Here and there passages are blocked with sediment or by falls from the roof, and become partially or wholly abandoned, the water, dammed back, rising and making its escape by other clefts and hollows. Thus eventually the limestone becomes traversed in all directions by a perfect net-work of intercrossing channels—winding and angulate, low and lofty, broad and narrow—many of which become abandoned by the water as it works its way to lower and lower levels. To what depth from the surface considerable tunnels can be excavated by chemical and mechanical erosion we cannot tell. It is obvious, however, that a limit must be reached when the pressure of the superincumbent and surrounding rocks becomes so great that no vacant spaces can exist. Water descending from the surface must thus eventually be forced by hydrostatic pressure to rise again and escape at lower levels than its source. Large underground channels, therefore, probably descend to no great depth from the surface, and their size is naturally limited by the structure of the rock in which they are excavated. Where this is much jointed and fissured it is obvious that the span of a cavern cannot be great; the disjointed rocks, losing support, tend to collapse. The widest underground chambers do not exceed 100 yards in width.

In course of time the whole surface of a country is gradually lowered by denudation. This change goes on most rapidly no doubt in regions where the superficial rocks are more or less impermeable. But lands composed chiefly of limestone do not escape—corrosion, especially, proceeds more or less rapidly. Ever and anon, too, the surface sinks slowly or suddenly as the case may be, consequent on the withdrawal of rock-material from below. The peculiar deformations caused by such changes are among the most characteristic features of limestone regions. Typical regions of the kind show no regular river-systems; brooks and rivulets are wanting. Water sinks at once into the ground by pipes and swallow-holes, clefts and fissures. In the lower-lying parts of such lands now

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