ton to every four and a half acres. This serves to show how great are the solvent powers of water charged with carbonic acid, and the extent to which, in the course of centuries, it might remove the calcareous rocks with which it came in contact. But when once by this action a channel had been excavated sufficiently large to admit of the rapid passage of a stream of water through it, and the circumstances of the case allowed of such a stream, its enlargement would probably become more rapid, as the water would be liable to be charged with sand and small pebbles, the friction of which would materially conduce to the removal of the rock, the varying hardness of which, combined with the intersection of other channels and fissures, would probably lead to the formation of chambers of various sizes along the course of the channel. In some caverns, we find the streams of water, to which probably they owe their existence, still flowing through them; but in others, the external features of the surrounding country have so much changed since their formation, that the gathering grounds for such streams have been removed by denudation, and water now only finds its way into them by slow percolation through the rock which forms their roof and walls.
It is this same process of denudation which, by removing some portion of the rock in which the caverns were originally formed, has brought them in communication with the outer world, and has thus rendered them accessible to man.
Leaving out of the question the blocks and fragments of stone falling in from the ceiling of the caverns, the methods by which the ossiferous deposits in them may have been formed, are various. The bones may be those of animals which have died in the caverns, or they may have been brought there by beasts of prey, or by man, or by running water, or possibly by several of these agencies combined.
In the case of the caves and rock-shelters of the Dordogne, and many of those in Belgium, the deposits are almost exclusively neither more nor less than refuse heaps, containing the bones, fractured and unfractured, of animals which have served for human food, mixed with which are the lost and waste tools, utensils, and weapons, and even the cooking-hearths of the early cave-dwellers; so that in character they closely resemble the kjökken-möddings of the Danish coasts; though, from their position being usually inland, the marine shells in which these latter abound are, for the most part, absent. The object in resorting to the caves was, no
