In this chapter we shall treat of the origin of caves and of their place in physical geography. The most obvious agent in hollowing out caves is the sea. The set of the current, the tremendous force of the breakers, and the grinding of the shingle, inevitably discover the weak places in the cliff, and leave caves as the results of their work, modified in each case by the local conditions of the rock. Caves formed in this manner have certain characters which are easily recognized. Their floors are very rarely much out of the horizontal, their outlook is over the sea, and they very seldom penetrate far into the cliff. A general parallelism is also to be observed in a group in the same district, and their entrances are all in the same horizontal plane, or in a succession of horizontal and parallel planes. In some cases they are elevated above the present reach of the waves, and mark the line at which the sea formerly stood. From their generally inaccessible position sea-caves have very rarely been occupied by man, and the history of their formation is so obvious that it requires no further notice. Among them the famous Fingal’s Cave, off the north coast of Ireland, and that of Staffa, on the opposite shore of Scotland, hollowed out of columnar basalt, are perhaps the most remarkable in Europe. In volcanic regions also there are caves formed by the passage of lava to the surface of the ground, or by the imprisoned steam and gases in the lava while it was in a molten state: but these are of comparatively little importance so far as relates to the general question of caves, from the very small areas which are occupied by active volcanoes in Europe. They have been observed in Vesuvius, Etna, Iceland, and Teneriffe. _Caves in Arenaceous Rocks._ Caves also occur sometimes in sandstones, in which case they are the result of the erosion of the lines of the joints by the passage of subaërial water, and if the joints happen to traverse a stratum less compacted than the rest, the weak point is discovered, and a hollow is formed extending laterally from the original fissure. The massive millstone grit of Derbyshire and Yorkshire present many examples of this, as for instance in Kinderscout in the former county. The rocks at Tunbridge Wells also show to what extent the joints in the Wealden sandstones may become open fissures, more or less connected with caves, on a small scale, by the mere mechanical action of water. M. Desnoyers gives instances of the same kind in the Tertiary sandstones of the Paris basin, which have furnished remains of rhinoceros, reindeer, hyæna, and bear. Caverns, however, in the sandstone are rarely of great extent, and may be passed over as being of small importance in comparison with those in the calcareous rocks. _Caves in Calcareous Rocks of various ages._