122 CAVERNS IN LIMESTONE. [Ch. X. 



shells, the Mytilus edulis for example, washed out of the older 

 formation, have been mingled in the fissures with bones of the extinct 

 quadrupeds.* 



There are examples in Europe of marine strata characterized in like 

 manner by embedded shells of living species which reach elevations 

 far exceeding those of Cagliari, but in which no human bones or works 

 of art have yet been discovered. 



CAVERN DEPOSITS CONTAINING HUMAN REMAINS AND BONES 05 

 EXTINCT ANIMALS. 



In England, and in almost all countries where limestone rocks abound 

 caverns are found, usually consisting of cavities of large dimensions, 

 connected together by low, narrow, and sometines tortuous galleries or 

 tunnels. These subterranean vaults are usually filled in part with mud, 

 pebbles, and breccia, in which bones occur belonging to the same 

 assemblage of animals as those characterizing the post-pliocene alluvia 

 above described. Some of these bones are referable to extinct and 

 others to living species, and they are occasionally intermingled, as in 

 the valley gravels, with implements of one or other of the great 

 divisions of the stone age, and these are not unfrequently accompanied 

 by human bones, which are much more common in cavern deposits 

 than in valley alluvium. 



Each suite of caverns, and the passages by which they communi- 

 cate the one with the other, afford memorials to the geologists of at 

 least three successive phases through which the physical geography 

 of the country where they occur must have passed. First there was 

 a period when limestone rocks were dissolved on a great scale, and 

 when the carbonate of lime was carried out gradually by springs from 

 the interior of the earth ; secondly, an era when engulfed rivers 

 or occasional floods swept organic and inorganic debris into the 

 subterranean hollows previously formed ; and thirdly, there were such 

 changes in the configuration of the region as caused the engulfed 

 rivers to be turned into new channels, and springs to be dried up, 

 after which the cave-mud, breccia, gravel, and fossil bones would 

 bear the same kind of relation to the existing drainage of the 

 country as the older valley drifts with their extinct mammalian 

 remains and works of art bear to the present rivers and alluvial 

 plains. 



In the first of the periods above supposed the operations are en- 

 tirely subterranean. We know that in every limestone district the 

 rain water is soft or free from earthy ingredients when it falls upon 

 the soil, and when it enters the rocks below, whereas it is hard, or 

 charged with carbonate of lime, when it issues again to the sur- 

 face in springs, which, by failing after long droughts, and by in- 



* Ly ell's Antiquity of Man, p. 111. 



