The caverns hollowed in calcareous rocks present features by which they are distinguished from any others. They open, for the most part, on the abrupt sides of valleys and ravines at various levels, being arranged round the main axis of erosion just as branches are arranged round the trunk of a tree--as, for example, in Cheddar Pass. The transition in some cases from the valley to the ravine, and from the ravine to the cave, is so gradual, that it is impossible to deny that all three are due to the same cause. The caves themselves ramify in the same irregular fashion as the valleys, and are to be viewed merely as the capillaries in the general valley system, through which the rainfall passes to join the main channels. Very frequently, however, the drainage has found an outlet at a lower level, and its ancient passage is left dry; but in all cases unmistakeable proof of the erosive action of water is to be seen in the sand, gravel, and clay which compose the floor, as well as in the worn surfaces of the sides and the bottom. In all districts in which caves occur are funnel-shaped cavities of various sizes, known as “pot-holes” or “swallow-holes” in Britain, as “betoires,” “chaldrons du diable,” “marmites de géants,” in France, and as “kata-vothra” in Greece, in which the rainfall is collected before it finally disappears in the subterranean passages. They are to be seen in all stages; sometimes being mere shallow funnels, that only contain water after excessive rain, and at others as profound vertical shafts, into which the water is continually falling, as in Helln Pot, in Yorkshire. The cirques, also, described by M. Desnoyers, belong to the same class of cavities, although all those which are mentioned by the Rev. T. G. Bonney,[24] at the head of valleys, and in some cases hollowed in shale and igneous rocks, are most probably to be referred to the vertical, chisel-like action of streams flowing under physical conditions, that resemble those under which the cañons of the Colorado, or of the Zambesi, are being excavated, and in which frost, ice, and snow have played a very subordinate part. The intimate relation between pot-holes, caves, ravines, and valleys will be discussed in the rest of this chapter, and illustrated by English examples; and then we shall proceed to show that the chemical action of the carbonic acid in the rain-water, and the mechanical friction of the sand and gravel, set in motion by the water, by which Professor Phillips explains the origin of caves, will equally explain the pot-holes and ravines by which they are invariably accompanied. _The Water-Cave of Wookey Hole, near Wells, Somerset._