264 EARLY MAN IN BRITAIN. [CHAP, vin 



river-valley, where both are close together; the Pleis- 

 tocene fluviatile strata occurring in various levels, either 

 above or below the present level of the stream, while 

 the Prehistoric deposits consist of alluvia close to the 

 present level of the stream, or of subaerial accumulations 

 of loam and the like, the result of the rain -wash, 

 covering the lower grounds like a mantle. In the 

 former the severity of the winters is marked by the 

 confused manner in which the pebbles have been 

 accumulated, owing to the floating ice in the streams ; 

 while in the latter the sediments are sorted by the 

 ordinary action of running water, without the interven- 

 tion of ice. The line of demarcation is equally clear in 

 the caverns, 1 in which the late Pleistocene accumulations 

 are generally mapped off from those of the Prehistoric 

 age by a layer of stalagmite, sometimes of considerable 

 thickness. This, however, offers no measure of the 

 interval between the two periods, because the rate of 

 accumulation depends upon the currents of air in the 

 caves, and the amount of water passing through the 

 limestone, both of which are variables. In the Irigle- 

 borough cave, in Yorkshire, it has been so swift that 

 between 1845 and 1873 a stalagmitic boss known as 

 the Jockey Cap has grown at the rate of '2941 inch 

 per annum. In Kent's Hole it has been so slow that 

 an inscription bearing the date of 1688 on a similar boss 

 is only covered by a film not more than one-twentieth 

 of an inch in thickness. It therefore follows that very 

 great thicknesses may be formed in a short time ; while 

 on the other hand it may take a long series of centuries 

 to form a thin layer of a few inches. 2 



1 Cave-hunting, c. viii 



2 Cave-hunting, p. 439 ; Pengelly, Kent's Cavern, Science Lectures for 



