The very remarkable glacières, or caves containing ice instead of water, in the Jura, Pyrenees, in Teneriffe, Iceland, and other districts of high altitude and low temperature, in which the temperature even in summer does not rise much above freezing-point, may be explained by the theory advanced independently by De Luc and the Rev. G. F. Browne. “The heavy cold air of winter,” writes the latter, “sinks down into the glacières, and the lighter, warm air of summer cannot on ordinary principles dislodge it, so that heat is very slowly spread in the caves; and even when some amount of heat does reach the ice, the latter melts but slowly, since a kilogramme of ice absorbs 79° C. of heat in melting; and thus when ice is once formed, it becomes a material guarantee for the permanence of cold in the cave. For this explanation to hold good it is necessary that the level at which the ice is found should be below the level of the entrance to the cave; otherwise the mere weight of the cold air would cause it to leave its prison as soon as the spring warmth arrived.” It is also necessary that the cave should be protected from direct radiation and from the action of wind. These conditions are satisfied by all the glacières explored by Mr. Browne.[39] The apparent anomaly that one only out of a group of caves exposed to the same temperatures should be a glacière, may be explained by the fact that these conditions are found in combination but rarely, and if one were absent there would be no accumulation of perpetual ice. It is very probable that the store of cold laid up in these caves, as in an ice-house, has been ultimately derived from the great refrigeration of climate in Europe in the Glacial Period.
Conclusion.
In this chapter we have examined the physical history of caves, their formation, and their relation to pot-holes, cirques, and ravines; and we have seen that they are not the result of subterranean disturbance, but of the mechanical action of rain-water and the chemical action of carbonic acid, both operating from above. We have seen that cave-hunting is not merely an adventurous amusement, but also a quest that brings us into a great laboratory, so to speak, in which we can see the natural agents at work that have carved out the valleys and gorges, and shaped the hills wherever the calcareous rocks are to be found.
The rest of this treatise will be devoted to the evidence which they offer as to the former inhabitants, both men and animals, of Europe.
CHAPTER III.
HISTORIC CAVES IN BRITAIN.
Definition of Historic Period.—Wild Animals in Britain during the Historic Period.—Animals living under the care of Man.—Classificatory value of Historic Animals.—The Victoria Cave, Settle, Yorkshire.—History of Discovery.—The Romano-Celtic or Brit-Welsh Stratum.—The Bones of the Animals.—Miscellaneous Articles.—The Coins.—The Jewelry, and its Relation to Irish Art.—Similar Remains in other Caves in Yorkshire.—These Caves used as Places of Refuge.—The evidence of History as to Date.—Britain under the Romans.—The Inroads of the Picts and Scots.—The English Conquest.—The Neolithic Stratum.—The approximate Date of the Neolithic Occupation.—The Grey Clays.—The Pleistocene Occupation by the Hyænas.—The probable Preglacial Age of the Pleistocene Stratum.—The Kirkhead Cave.—Poole’s Cave, near Buxton.—Thor’s Cave, near Ashbourne.—Historic value of Brit-Welsh Group of Caves.—Principal Animals and Articles.—The use of Horse-flesh.—The Cave of Long-berry Bank.