ALLUVIA. 631 



mation of peat; in which latter case they are 

 mixed with powdery peat earth. 



b. Of calcareous sand ; commonly from 

 the decomposition of sea shells, and 

 found under similar circumstances. 



F. Sands of various constitution, found in partial 

 deposits in different places, and commonly, if not 

 always, arising- from the decomposition of rocks. 



a. Quartz and argillaceous schist. 



b. Quartz and felspar. 



c. Quartz and the sand of trap rocks. 



d. Mica: or mica with clay, or felspar, 



or quartz, or hornblende, or all of 

 these. From gneiss and granite. 



G. Mixtures of various kinds, forming the soil 

 of agriculturists. 



Some of these are transported materials, others 

 are the produce of rocks decomposed in situ. 

 Those produced, in the latter case, by the decom- 

 position of many trap rocks, of the argillaceous 

 limestones, and of the argillaceous schists, are 

 the most fertile and valuable. 



H. Vegetable soil, or mould, consisting of a mix- 

 ture of any of the preceding with a hydrocarbonaceous 

 compound, analogous to peat, which results from the 

 decomposition of vegetables. 



Other circumstances being equal, the fertility 



