80 GENERA AND SPECIES. 



the generic name should he taken from that, as 

 clayey soils; where two prevail, to all appearance 

 equally, then their names must be conjoined in 

 naming their genus, as clay and sand, or lime and 

 clay, etc. The great thing to be observed is pre- 

 cision in applying the terms. Sir Humphrey Davy 

 observed : * The term sandj/ soil should never be 

 applied to any that does not consist of at least seven- 

 eighths sand ; sandy soils which effervesce with acids 

 should be distinguished by the name of calcareous 

 sandy soils. The term clayey soil should not be 

 applied to any land which contains less than one- 

 sixth of impalpable earth matter not considerably 

 effervescing with acids; the word loam should be 

 limited in its application to soils containing at least 

 one-third of the same, copiously effervescing with 

 acids. A soil to be considered as peaty should 

 consist of at least one-half of vegetable matter. In 

 cases where the earthy part of a soil is evidently 

 the decomposed matter of one particular rock, a 

 name derived from it may with propriety be applied. 

 Alluvial soils may be designated as silicious, calca- 

 reous, or argillaceous ; and in some cases the term 

 saline may be added as a specific distinction appli- 

 cable, for example, at the embouchure of rivers, 

 when the alluvial remains are overflown by the sea. 

 " In naming the species, greater nicety is required 

 than in naming the genera. The species are always 

 determined by the mixture of matters, and never by 



