302 DENUDATION OF THE LAND. CHAP. VL 



have been effected by the carbonic acid in 

 the rain and in the ground, by the humus- 

 acids, and by the corroding power of living 

 roots. Why a thick mass of residue has not 

 been left on the Chalk, wherever the land is 

 nearly level, may perhaps be accounted for 

 by the percolation of the fine particles into 

 the fissures, which are often present in the 

 chalk and are either open or are filled up 

 with impure chalk, or into the solid chalk 

 itself. That such percolation occurs can 

 hardly be doubted. My son collected some 

 powdered and fragmentary chalk beneath the 

 turf near Winchester ; the former was found 

 by Colonel Parsons, R.E., to contain 10 per 

 cent., and the fragments 8 per cent, of earthy 

 matter. On the flanks of the escarpment near 

 Abinger in Surrey, some chalk close beneath 

 a layer of flints, 2 inches in thickness and 

 covered by 8 inches of mould, yielded a re- 

 sidue of 3'7 per cent, of earthy matter. On 

 the other hand the Upper Chalk properly 

 contains, as I was informed by the late David 

 Forbes who had made many analyses, only 

 from 1 to 2 per cent, of earthy matter ; and 

 two samples from pits near my house con- 



