THE ADVENT OF MAN. 



249 



great antiquity. We know tliat in favourable circumstanceJi 

 mud, sand, and gravel may be rapidly deposited in caves by land 

 floods or river inundations, and that debris of various sorts 

 accumulates in such places from decay of rock and vegetable 

 and animal agencies. The deposition of stalagmite is also very 

 variable in its rate ; and the fact that it is being very slowly 

 deposited in any cave now does not prove that more rapid 

 deposition may not have taken place formerly. Dawkins and 

 others have ascertained a rate of a quarter of an inch i)er 

 annum in some caverns ; and this would allow the stalagmite 



Fig. 192. — Sketch of a Mammoth, carved on a portion of a Tusk of the same Animal 



(Lartet). 



crust of Kent's cave, for which an antiquity of half a million of 

 years has been claimed, to have been formed in a thousand 

 years. 



(3) The erosion of river valleys to great depths since the 

 Glacial period fails to establish the great antiquity of the caves 

 left on their sides or the high level gravels of their banks. 

 Throughout the northern hemisphere, the river valleys are of 

 old date, and were merely filled with loose detritus in the Glacial 

 age. The sweeping out of this debris would be a rapid process, 

 more especially when changes of level were occurring, and 

 when the rainfall was greater than at present. Besides, as Croll 



