138 AMOUNT OF EARTH CHAP. III. 



and a layer of the marl fragments could be 

 traced at a depth, carefully measured, of 12 

 inches in some parts, and of 14 inches in 

 other parts. This difference in depth de- 

 pended on the layer being horizontal, whilst 

 the surface consisted of ridges and furrows 

 from the field having been ploughed. The 

 tenant assured me that it had never been 

 turned up to a greater depth than from 6 to 8 

 inches ; and as the fragments formed an un- 

 broken horizontal layer from 12 to 14 inches 

 beneath the surface, these must have been 

 buried by the worms whilst the land was 

 in pasture before it was ploughed, for other- 

 wise they would have been indiscriminately 

 scattered by the plough throughout the 

 whole thickness of the soil. Four-and-a-half 

 years afterwards I had three holes dug in 

 this field, in which potatoes had been lately 

 planted, and the layer of marl-fragments was 

 now found 13 inches beneath the bottoms of 

 the furrows, and therefore probably 15 inches 



1809, that is twenty-eight years before the first examination of 

 the field by my friend. The error, as far as the figure 80 is 

 concerned, was corrected in an article by me, in the * Gardeners' 

 Chronicle,' 1844, p. 218. 



