The caves which we have been considering in England are all in limestone strata, and have been formed by streams of water which have enlarged some natural fissures both by mechanical action in wearing away the rocks, and by chemical action in dissolving them. Through the lowering of the main line of drainage, caverns with a dry floor are at length left, offering shelter and protection both to man and beast. Oftentimes, but not always, some idea of the age of these caverns may be obtained by observing the depth to which the main channel of drainage to which they were tributary has been lowered since their formation. But to this subject also we will return when we come specifically to discuss the chronological question. _The Continent._ Systematic explorations in the caves of Belgium were begun in 1833 by Dr. Schmerling, in the valley of the Meuse, near his residence in Liége. The Meuse is here bordered by limestone precipices 200 or more feet in height. Opening out from these rocky walls are the entrances to the numerous caverns which have rendered the region so famous. To get access to the most important of these, Dr. Schmerling had to let himself down over a precipice by a rope tied to a tree, and then to creep along on all-fours through intricate channels to reach the larger chambers which it was his object to explore. In the cave at Engis, on the left bank of the Meuse, about eight miles above Liége, he found a human skull deeply buried in breccia in company with many bones of the extinct animals previously stated to have been associated with man during the Glacial period. This so-called "Engis skull" was by no means apelike in its character, but closely resembled that of the average Caucasian man. But this established the association upon the Continent of man with some of the extinct animals of the Glacial period. [Illustration: Fig. 79.--Engis skull, reduced (after Lyell.)] The vicinity of Liége has also furnished us another cavern whose contents are of the highest importance, ranking indeed as perhaps the most significant single discovery yet made. The cave referred to is on the property of the Count of Beauffort, in the commune of Spy, in the province of Namur in Belgium. For the facts relating to it we are indebted to Messrs: Lohest and Fraipont, the former Professor of Geology and the latter of Anatomy in the University of Liége. The exploration of the cave was made in 1886, and the full report with illustrations published in the following year in Archives de Biologie.[DF] The significance of this discovery is enhanced by the light it sheds upon and the confirmation it brings to the famous Neanderthal skull and others of similar character, which for a long time had been subjects of vigorous discussion. Before describing it, therefore, we will give a brief account of the previous discoveries. [Footnote DF: See pp. 587, 757.]