144 AMOUNT OF EARTH Chap. III. 



certainly more than forty years before, a 

 large hole had been filled up with coarse red 

 clay, flints, fragments of chalk, and gravel ; 

 and here the fine vegetable mould was only 

 from 4|- to 4f inches in thickness. In 

 another and undisturbed place, the mould 

 varied much in thickness, namely, from 6^ 

 to 8£ inches ; beneath which a few small 

 fragments of brick were found in one 

 place. From these several cases, it would 

 appear that during the last 29 years mould 

 has been heaped on the surface at an 

 average annual rate of from *2 to *22 of an 

 inch. But in this district when a ploughed 

 field is first laid down in grass, the mould 

 accumulates at a much slower rate. The 

 rate, also, must become very much slower 

 after a bed of mould, several inches in thick- 

 ness, has been formed ; for the worms then 

 live chiefly near the surface, and burrow 

 down to a greater depth so as to bring up 

 fresh earth from below, only during the 

 winter when the weather is very cold (at 

 which time worms were found in this field at 

 a depth of 26 inches) and during summer, 

 when the weather is very dry. 



