.136 AMOUNT OF EARTH CHAP. III. 



inch in length ; and other fragments of coal- 

 cinders together with a few white quartz 

 pebbles. Beneath this layer and at a depth of 

 4^ inches from the surface, the original black, 

 peaty, sandy soil with a few quartz pebbles 

 was encountered. Here therefore the frag- 

 ments of burnt marl and cinders had been 

 covered in the course of 15 years by a layer 

 of fine vegetable mould, only 2J inches in 

 thickness, excluding the turf. Six and a half 

 years subsequently this field was re-examined, 

 and the fragments were now found at from 

 4 to 5 inches beneath the surface. So that 

 in this interval of 6 years, about 1^ inch of 

 mould had been added to the superficial layer. 

 I am surprised that a greater quantity had 

 not been brought up during the whole 21 J 

 years, for in the closely underlying black, 

 peaty soil there were many worms. It is, 

 however, probable that formerly, whilst the 

 land remained poor, worms were scanty ; and 

 the mould would then have accumulated 

 slowly. The average annual increase of thick- 

 ness for the whole period is *19 of an inch. 



Two other cases are worth recording. In 

 the spring of 1835, a field, which had 



