120 THE ANTIQUITY OP MAN. 



They are inclined at angles of 30° and 40°, their walls being 

 generally coated with stalactite, pieces of which have here and 

 there been broken off and mingled with the contents of the rents, 

 thus helping to explain why we so often meet with detached 

 pieces of that substance in the mud and breccia of the Belgian 

 caves. It is not easy to conceive that a solid horizontal floor of 

 hard stalagmite should, after its formation, be broken up by run- 

 ning water ; but when the walls of steep and tortuous rents, serv- 

 ing as feeders to the principal fissures, and to inferior vaults and 

 galleries, are encrusted with stalagmite, some of the incrustation 

 may readily be torn up when heavy fragments of rock are hurried 

 by a flood through passages inclined at angles of 30° or 40°. 



" The decay and decomposition of the fossil bones seem to have 

 been arrested in most of the caves by a constant supply of water 

 charged with carbonate of lime, which dripped from the roofs 

 while the caves were becoming gradually filled up. By similar 

 agency the mud, sand, and pebbles were usually consolidated. 



" The following explanation of this phenomenon has been sug- 

 gested by the eminent chemist Liebig. On the surface of Fran- 

 conia, where the limestone abounds in caverns, is a fertile soil in 

 which vegetable matter is continually decaying. This mould or 

 humus, being acted on by moisture and air, evolves carbonic acid, 

 which is dissolved by rain.' The rain-water, thus impregnated, 

 permeates the porous limestone, dissolves a portion of it ; and 

 afterwards, when the excess of carbonic acid evaporates in the 

 caverns, parts with the calcareous matter and forms stalactite. 

 So long as water flows, even occasionally, through a suite of caverns 

 no layer of pure stalagmite can be produced ; hence the forma- 

 tion of such a layer, is generally an event posterior in date to the 

 cessation of the old system of drainage ; an event which might 

 be brought about by an earthquake causing new fissures, or by 

 the river wearing its way down to a lower level, and thenceforth 

 running in a new channel. 



" In all the subterranean cavities, more than forty in number, 

 explored by Schmerling, he only observed one cave, namely that 

 of Chokier, where there were two regular layers of stalagmite, 

 divided by fossiliferous cave-mud. In this instance, we may sup- 

 pose that the stream, after flowing for a long period at one level, 

 cut its way down to an inferior suite of caverns, and, flowing 

 through them for centuries, choked them up with debris; after 

 which it rose once more to its original higher level : just as in 



