234 , FORMATION AND GROWTH OP 



as the old subsoil surface still remains ; therefore the 

 mould must have increased in thickness upwards, and 

 necessarily by the decay of the vegetable substances. 

 It may be said that this is an exceptional case ; for 

 this road-like surface would not be formed naturally 

 over the subsoil. However, if the thickness of the 

 mould increases downward, as suggested by Darwin, 

 is it not remarkable that over each different kind of 

 subsoil the thickness of the vegetable mould should 

 be so uniform? Naturally the mould is only a few 

 inches thick over gravel, a little thicker over clay, a 

 good depth over a subsoil formed of a combination of 

 clay and sand, more especially if it is limey, while 

 over an alluvial subsoil it may be deeper still ; unless, 

 indeed, that meteoric abrasion in each several case 

 has removed from the surface an exact equivalent 

 to the increase ; but this is highly improbable. 



Mr John Edward Lee, F.S.A., F.GLS., the transla- 

 tor of Dr Keller's " Lake Dwellings of Switzerland," 

 in a note on the depth of soil covering the " Main- 

 land Settlement of Ebersberg," records a remarkable 

 growth of soil at Caerleon, South "Wales. Mr Lee 

 thus writes : " In a field which forms the south-west 

 portion of the ancient city of Isca Silurum, I have 

 frequently excavated for the sake of archaeology, and 

 in one instance, when the summer was dry, the grass 

 showed where walls were probably to be found, indi- 

 cating the ancient Roman houses forming the corner 



