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-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 1oo 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 and again rivers suddenly emerge at the surface, and after usually a short course may again disappear below ground. In the rainy season water often rises through the apertures by which the surface is more or less abundantly pierced, and dry valleys and wide basin-shaped depressions become flooded. Of course when the supply fails the water again returns to the depths from which it was discharged.
In the karst-regions of Carinthia and Illyria these phenomena are very well displayed. The funnelshaped depressions communicating with underground galleries, which with us are termed swallow-holes, are known in Carinthia as dolinas. These vary in width and depth from a few yards up to half a mile in width, and from ioo to 200 yards and more in depth. Most of them, however, are small—40 or 50 yards across, and about 30 yards or so in depth. Their bottom is somewhat flat, and often covered with loam or clay. The larger ones are relatively shallower in proportion to their width than the others. Not less characteristic features of the karst-lands are the so-called blindvalleys and dry-valleys. Through the former a river flows to disappear into a tunnel at the closed or blind end. The dry-valleys have no river; the bottom is usually irregular and often pitted with dolinas. Besides these land-forms, geographers recognise another kind of depression, the so-called "kettle-valleys," which are trough-like or dish-shaped basins of variable extent, some of them having an area of several hundred square miles. Not infrequently the smaller
ones run in parallel zones following the direction of the strike of the strata. All these surf ace-features are for the most part the result of underground erosion. Some of the dolinas may have been eroded by water descending through fissures from the surface; but probably the greater number, and certainly all the larger ones, have been caused by the caving-in of underground tunnels. So, again, the blind-valleys and dry-valleys appear in most cases to form part of the subterranean drainage-system, now exposed by collapse of roof and the general degradation of the surface. The natural bridges or arches which are seen often enough in such regions are simply the relics of old underground tunnels and waterways, the ruins of which often cumber the depressions of the surface. It is hardly worth while adding that the numerous limestone caverns in which geologists have hunted so successfully for remains of primeval man and his associates are merely the abandoned courses of ancient underground streams and rivers. Almost everywhere, indeed, in great limestone-regions one may' trace at the surface evidence of the effects produced by subterranean erosion. The trough-shaped basins (kettle-valleys) referred to above seem to owe their origin in the first place to determinate fissures. These are widened by the action of the surface-water as it passes underground, and the depression at the surface increases as the rock becomes undermined, collapse taking place from time to time. If the collapse be recent the bottom of the kettle-valley is
trewn with broken rock-ddbrt■s. Not a few kettlealleys in limestone-plateaux, however, may have been martially excavated by superficial water-action before :he system of underground drainage was established, but by the action of the latter they have since been more or less modified. It may be taken as generally true that most of the depressions or basins, great and small, which are so characteristic of karst-lands, are either largely or wholly due to the corrosive and erosive action of underground water.
Lakes, as we have seen, often appear periodically in these regions. Some are very regular in their coming and going, others only show at intervals after unusually heavy rain or long-continued wet weather. One of the best-known examples is the Lake of Jessero, or Zirknitz, in Carniola, which appears now and then in the broad valley of the Planina. This river, after flowing underground for a long distance, returns to the surface, and shortly afterwards winds through a wide plain encircled by high cliffs of limestone. The plain is pierced by hundreds of dolinas, from which, after excessive or continuous rain, the water wells and rushes until the whole wide area is transformed into a lake. The extent and depth and the duration of this temporary lake vary; and the intervals between its successive appearances are likewise inconstant; sometimes only a year, or two or three years may elapse, but intervals of ten and even of thirty years have been experienced. Not a few depressions in the surface of calcareous tracts may be 18