These deposits are caused by the infiltrated water charged with carbonate of lime, which, oozing drop by drop through the interstices of the rock, slowly discharge the carbonic acid which held the carbonate of lime in solution, and the salts gradually precipitating form the crystalline or amorphous deposits which constitute these natural columns. The calcareous deposits which spread over the ground of the caverns are called _stalagmite_, and the name of _stalactite_ is given to those which hang down from the roof, forming pendants, natural decorations, and ornaments as of alabaster or marble, producing sometimes the most magnificent effects. Under the stalagmite the largest number of animal bones have been found. This crust, which has been to them a preservatory grave, is so thick and hard that a pick-axe is required in order to break it. Thanks to the protecting cover, the bones have been sheltered from all the various causes of decomposition and destruction. The limestone formed a kind of cement which, uniting clay, mud, sand, flints, bones of men and animals, weapons and utensils into a compact mass, has preserved them for the study and consideration of scientific men in our own days. The soil called _bone-earth_ is, in fact, found under the crystalline bed which covers the ground of the caverns. Fig. 21, which represents a section of the cave of Galeinreuth, in Bavaria, will enable us clearly to understand the position occupied by the bones in most of these caverns. [Illustration: Fig. 21.--The Cave of Galeinreuth, in Bavaria.] Bone-earth consists of a reddish or yellowish clay, often mixed with pebbles, which seem to have come from some distant beds, for they cannot be attributed to the adjacent rocks. This stratum varies considerably in depth; in some spots it is very thin, in others it rises almost to the top of the cavern, to a height of forty or fifty feet. But in this case it is, in reality, composed of several strata belonging to different ages, and explorers ought to note with much attention the exact depth of any of the organic remains found in their mass.