354 HISTORICAL PALAEONTOLOGY. 



logically the lowest, since they are obviously much older than 

 the low-level gravels, as these are than the recent gravels. 

 How much older the high-level gravels may be than the low- 

 level ones, it is impossible to say. They occur at heights 

 varying from 10 to 100 feet above the present river-chan- 

 nels, and they are therefore older than the recent gravels 

 by the time required by the river to dig out its own bed to 

 this depth. How long this period may be, our data do not 

 enable us to determine accurately ; but if we are to calculate 

 from the observed rate of erosion of the actually existing 

 rivers, the period between the different valley-gravels must 

 be a very long one. 



The lowest or recent fluviatile deposits which occur beside 

 the bed of the present river, are referable to the Recent period, 

 as they contain the remains of none but living Mammals. The 

 two other sets of gravels are Post-Pliocene, as they contain 

 the bones of extinct Mammals, mixed with land and fresh- 

 water shells of existing species. Among the more important 

 extinct Mammals of the low-level and high-level valley-gravels 

 may be mentioned the Elephas aiitiquus, the Mammoth (Ele- 

 phas primigenius}, the Woolly Rhinoceros (R. ticJwrhinus), the 

 Hippopotamus, the Cave-lion, and the Cave-bear. Along 

 with these are found unquestionable traces of the existence 

 of Man, in the form of rude flint implements of undoubted 

 human workmanship. 



The so-called " Cave-deposits," again, though exhibiting 

 peculiarities due to the fact of their occurrence in caverns or 

 fissures in the rocks, are in many respects essentially similar 

 to the olde,r valley-gravels. Caves, in the great majority of 

 instances, occur in limestone. When this is not the case, it 

 will generally be found that they occur along lines of sea-coast, 

 or along lines which can be shown to have anciently formed 

 the coast-line. There are many caves, however, in the making 

 of which it can be shown that the sea has had no hand; and 

 these are most of the caves of limestone districts. These owe 

 their origin to the solvent action upon lime of water holding 

 carbonic acid in solution. The rain which falls upon a lime- 

 stone district absorbs a certain amount of carbonic acid from 

 the air, or from the soil. It then percolates through the rock, 

 generally along the lines of jointing so characteristic of lime- 

 stones, and in its progress it dissolves and carries off a certain 

 quantity of carbonate of lime. In this way, the natural joints 

 and fissures in the rock are widened, as can be seen at the 



