Formation of Mould. 509 



a layer three-fourths of an inch below the surface, that thickness consisting- 

 of peaty soil. 



The second case is more interesting. It has been ascertained that a field, 

 which has since been ploughed^ was covered about eighty years ago with 

 marl ; an imperfect layer of it, but sufficiently distinct to be traced, is now 

 found at a depth, very carefully measured from the surface, of twelve inches 

 in some parts and fourteen in others : the difference corresponding to the top 

 and hollow of the ridges produced by ploughing. It is certain, the marl must 

 have sunk or been buried before the field was ploughed, for otherwise the 

 fragments would have been scattered in the soil : this conclusion, moreover, 

 explains the circumstance of the layer being horizontal, whilst the surface is 

 undulating. At the present time no plough could possibly touch the marl, as 

 the land in this country is never turned up to a greater depth than eight 

 inches. In the preceding communication, I have shown, that in a field lately 

 reclaimed from being waste land, three inches of mould had been prepared by 

 the worms in the course of fifteen years. We now find, that within a period of 

 less than eighty years, (but how much less cannot be told, unless the date 

 when the field was first ploughed were known) the earth-worms have covered 

 the marl, which was originally strewed on the surface, with a bed of earth of 



November 14, 1837. 



3u 2 



