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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 petrifications, and partly being conveyed down to the sea to be ultimately secreted in the tissues of the Mollusca, Echinodermata and Foraminifera. 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 neverending 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 was formed out of the remains of the animals and plants 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 .2946 in. per annum. It is obvious, therefore, that the vast antiquity of deposits containing 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 the 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 Weathercote, 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 waste, or by the grinding of glaciers through it during the glacial stage of the Pleistocene period.

For further details as to the physical history of caverns we must refer the reader to the works quoted at the end of this article, by E. A. Martel, the intrepid explorer of most of the large European caves, including those of Great Britain and Ireland. The history of the Glacières or Ice-caves will be found in Browne’s Ice Caves in France and Switzerland.

Classification.—The caves which have offered shelter to the mammalia are classified according to their contents, and are of various ages, ranging from the Pliocene to the present day. (1) Those containing the Pliocene mammalia belong to that age. (2) Those with the remains of the mammoth, woolly rhinoceros and other extinct species, or with paleolithic man (see Archaeology), are termed Pleistocene. These are sometimes called Quaternary, under the mistaken idea that they belong to an age succeeding the Tertiary period. (3) Those which contain the remains of the domestic animals in association with the remains of man either in the Neolithic, Bronze or Iron stages of civilization are termed Prehistoric. (4) The fourth group consists of those which can be brought into relation with the historic period, and are therefore termed Historic.

The Pliocene Caves.—It is a singular fact, only to be explained by the vast denudation of the earth’s surface since the Pliocene Age, that only one cave referable to that age has as yet been discovered, that at Doveholes near Buxton, Derbyshire, described by Boyd Dawkins in 1903 (Quart. Journ. Geol. Soc.). The cave consists of a large horizontal chamber and a small passage, connected with a swallow-hole close by, and exposed in the working face of a quarry in 1901, at a depth of about 40 ft. from the surface. The locality is in the limestone plateau, 1158 ft. high, which forms the divide between the waters flowing into the Mersey on the west and the Humber on the east. Both swallow-hole and cave were completely blocked up with débris, and the latter was filled with red and yellow clay, horizontally stratified and containing pebbles of sandstone from the neighbouring ridge of Axe Edge, and bones and teeth of fossil mammals, some waterworn and others without traces of transport by water. All the mammals belong to well-known species found in the Pliocene strata of East Anglia, and in Auvergne and Italy. Among them were the sabre-toothed lion (Machairodus crenatidens), the hyena of Auvergne, the mastodon, and the southern elephant (E. meridionalis), and rhinoceros (R. Etruscus), and Steno’s horse. Most of the bones had evidently been gnawed by hyenas and accumulated in one of their dens, and had afterwards been carried by water into the chambers deep down in the rock, where they were found. Since that time the general level of the district has been lowered by denudation to an extent of more than 230 ft., and all the hyena dens destroyed with the Pliocene surface not only in this district but generally over the world. In this case a covering of limestone some 270 ft. thick, including the depth from the present surface, protected the remains from the denuding forces.

The Pleistocene Caves.—The search after ebur fossile or unicorns' horn, or in other words the fossil bones which ranked high in the materia medica of the 16th and 17th centuries, led to the discovery of the ossiferous caverns of the Harz Mountains, and of Hungary and Franconia. The famous cave of Gailenreuth in the last of these districts was explored by Goldfuss in 1810. The bones of the hyena, lion, wolf, fox and stag, which it contained, were identified by Baron Cuvier, and some of the skulls have been proved by Busk to belong to the grizzly bear. They were associated with the bones of the reindeer, horse and bison, as well as with those of the great cave bear. These discoveries were of very great interest, because they established the fact that the above animals had lived in Germany in ancient times. The first bone cave systematically explored in England was one at Oreston near Plymouth in 1816, which proved that an extinct species of rhinoceros (R. leptorhinus) lived in that district. Four years later the famous hyena den at Kirkdale in Yorkshire was explored by Buckland. He brought forward proof that it had been inhabited by hyenas, and that the broken and gnawed bones of the mammoth, rhinoceros, stag, bison and horse belonged to animals which had been dragged in for food. He pointed out that all these animals had lived in Yorkshire in ancient times, and that it was impossible for the carcases of the rhinoceros, hyena and mammoth to have been floated from tropical regions into the places where he found their bones. He subsequently investigated bone caves in Derbyshire, South Wales and Somerset, as well as in Germany, and published his Reliquiae Diluvianae in 1822, a work which laid the foundations of the new science of cavehunting in this country. The well-known cave of Kent’s Hole near Torquay furnished McEnery, between the years 1825 and 1841, with the first flint implements discovered in intimate association with the bones of extinct animals. He recognized the fact that they proved the existence of man in Devonshire while those animals were alive, but the idea was too novel to be accepted by his contemporaries. His discoveries have since been verified by the subsequent investigations carried on by Godwin Austen, and ultimately by the committee of the British Association, which worked for several years under the guidance of Pengelly. There are four distinct strata in the cave. 1st, The surface is composed of dark earth, and contains medieval remains, Roman pottery and articles which prove that it was in use during the Iron, Bronze and Neolithic Ages. 2nd, Below this is a stalagmite floor, varying in thickness from 1 to 3 ft., and covering (3rd) the red earth, which contained bones of the hyena, lion, mammoth, rhinoceros and other animals, in association with flint implements and an engraved antler, which proved man to have been an inhabitant of the cavern during the time of its deposition. 4th, Filling the bottom of the cave is a hard breccia, with the remains of bears and flint implements, in the main ruder than those found above; in some places it was no less than 12 ft. thick. The most remarkable animal found in Kent’s Hole is the sabre-toothed carnivore, Machairodus latidens of Owen. While the value of McEnery’s discoveries was in dispute the