266 CAVE the cave, is so intimate that it is impossible to deny that all three have been produced by the same causes. The caves themselves ramify in the same irregular fashion a.3 the valleys, and are to be viewed merely as the capillaries in the general valley system through which the rain passes to join the main channels. Sometimes, as in the famous caves of Adelsberg, Kentucky, Wookey Hole in Somerset shire, the Peak in Derbyshire, and in many in the Jura, they are still the passages of subterranean streams ; but very frequently the drainage has found an outlet at a lower level, and the ancient watercourses have been deserted. These in every case present unmistakable proof that they have been traversed by water in the sand, gravel, and clay which they contain, as well as in the worn surfaces of the sides and bottom. In all districts where there are caves there are funnel-shaped depressions of various sizes called pot-holes or swallow-holes, or betoires, "chaldrons du diable," "marmites des grants," or "kata- vathra," in which the rain is collected before it disappears into the subterranean passages. They are to be seen in all stages, some being mere hollows which only contain water after excessive rain, while others are profound vertical shafts into which the water is continually falling. That known as Helm Pot in Yorkshire, 300 feet deep, is an ex ample of the latter class. The cirques described by M. Desnoyers belong to the same class as the swallow-holes. The history of swallow-holes, caves, ravines, and valleys in calcareous strata may be summed up as follows. The calcareous rocks are invariably traversed by joints or lines of shrinkage, which are lines of weakness by which the direction of the drainage is determined ; and they are composed to a large extent of carbonate of lime, which is readily exchanged into soluble bicarbonate by the addition of carbonic acid. The rain in its passage through the air takes up carbonic acid, and it is still further charged with it in percolating through the surface soil in which there is decomposing vegetable matter. As the rain drops converge towards some one point, determined by some local accident on the surface, and always in a line of joint, the carbonic acid attacks the carbonate of lime with which it comes into contact, and thus a funnel is gradually formed ending in the vertical joint below. Both funnel and vertical joint below are being continually enlarged by this process. This chemical action goes on until the free carbonic acid is used up. The subterranean passages are enlarged in this manner, and what was originally an in significant net-work of fissures is developed into a series of halls, sometimes as much as from 80 to 100 feet high. These results are considerably furthered by the mechanical friction of the pebbles and sand hurried along by the current, and by falls of rock from the roof produced by the removal of the underlying strata. In many cases the results of this action have produced a regular subterranean river system. The thick limestones of Kentucky, for example, are traversed by subterranean waters which collect in large rivers, and ultimately appear at the surface in full power. The River Axe, near Wells, the stream flowing out of the Peak Cavern at Castleton, Derbyshire, that at Adelsberg (see ADELSBERG, vol. i. p. 151) in Carniola, flow out of caverns in full volume. The River Styx and the waters of Acheron disappear in a series of caverns which were supposed to lead down to the infernal regions. If the direction of the drainage in the rock has been altered, either by elevations such as those with which the geologist is familiar, or by the opening out of new passages at a lower level, these watercourses become dry, and present us with the caves which have afforded shelter to man and the wild animals from the remotest ages, sometimes high up on the side of a ravine, at other times close to the level of the stream at the bottom. Caves, as a general rule, are as little effected by dis turbances of the rock as the ravines and valleys, which have been formed, in the main, irrespective of the lines of fault or dislocation. We must now examine what happens to the bicarbonate of lime which has been formed by the action of the acid on the limestone. If a current of air play upon the surface of the water, the carbonic acid, which floats up the lime, so to speak, is given off and the insoluble carbonate is deposited, and as a result of this action we have the elaborate and fantastic stony incrustations termed stalactites and stalagmites. The water percolating through the rock covers the sides of the cavern with a stalactitic drapery, and if a line of drops persistently falls from the same point to the floor, the calcareous deposit gradually descends from the roof, forming in some cases stony tassels, and in others long columns which are ultimately united to the calcareous boss, formed by the plash of the water on the floor. The surface also of the pools is sometimes covered over with an ice-like sheet of stalagmite, which shoots from the sides, and sometimes forms a solid and firm floor when the water on which it was supported has disappeared. Sometimes the drops form a little calcareous basin, beautifully polished inside, which contains small pearl-like particles of carbonate of lime, polished by friction one against the other. The most beautiful stalactitic caves in Great Britain are those of Cheddar in Somerset, Caldy Island, and Poole s Cavern at Buxton. A portion only of the carbonate of lime is thus deposited in the hollows of the rock from which it was taken ; the rest is carried into the open air by the streams, in part deposited on the sides and bottom, forming tufa and the so-called petrifactions, and partly being conveyed down to the sea to be ultimately secreted in the tissues of the Mollusca, Echinodermata, and Foramiuifera. Through these it is again collected in a solid form, and in the long course of ages it is again lifted up above the level of the water as limestone rock, and again undergoes the same series of changes. Thus the cycle of carbonate of lime is a never-ending one from the land to the ocean, from the ocean to the land, and so it has been ever since the first stratum of limestone w r as formed out of the exuviae of the inhabitants of the sea. The rate of the accumulation of stalagmite in caverns is necessarily variable, since it is determined by the presence of varying currents of air. In the Ingleborough cavern a stalagmite, measured in 1839 and in 1873, is growing at the rate of 294G inches per annum. It ia obvious, therefore, that the vast antiquity of deposits con taining remains of man underneath layers of stalagmite cannot be inferred from a thickness of a few inches or even of a few feet. The intimate relation which exists between caves and ravines renders it extremely probable that many of the latter have been originally subterranean watercourses, which have been unroofed by the degradation of the rock. In all limestone districts ravines are to be found continued in tho same direction as the caves, and the process of atmospheric erosion may be seen in the fallen blocks of stone which generally are to be met with at the mouths of the caverns. In illustration of this the valley and caves of Weather- cote, in Yorkshire, may be quoted, or the source of the Axe at Wookey ; and the ravine formed in this way has very frequently been widened out into a valley by the action of subaerial w r aste, or by the grinding of glaciers through it during the glacial stage of the Pleistocene period. Pleistocene Caves in Europe. The caves which have offered shelter to man and the wild animals are classified according to their contents. 1st, Those containing the extinct animals, such as the mammoth, woolly rhinoceros,
or Palaeolithic man (see ARCHAEOLOGY), are termed