322 NdTICES OF AUTHOKS. 



the excavated earth thrown up to form a dike on 

 one side. On removal of the dike, the original sur- 

 face, which no doubt, at the time the dike was 

 formed, consisted of vegetable clay mould similar to 

 the surface around, is always found to be close, heavy, 

 poor clay, containing little or no carbonaceous or 

 vegetable matter. In this case, from the draining 

 effect of the ditch, the original surface under the 

 dike must have been drier than the subsoil of the 

 crowns of the ridges. 



The difference of depth and richness of vegetable 

 moidd, may nearly always be referred to existing 

 causes, such as the original surface (diluvium, or de- 

 cayed rock), being a combination of earths favourable 

 to vegetation ; occupying a genial situation ; being 

 favourably placed with regard to moisture, that is, less 

 or more moist, according as the original surface has 

 been clayey or sandy, or open or close bottomed ; and 

 is in no way connected with those flood torrents to 

 which we owe the diluvium deposits themselves — 

 tills, sand and gravel, in which we have never found 

 any vegetable matter, excepting in the coaly or mi- 

 neralized state. 



Unless in the case of alluvium, or of drift sand, 

 or where surface earth has been rolled down from 



