150 SOILS, THEIR ORIGIN AND NATURE. 



and metallic constituents of the soil. Now we find that 

 nearly all the rocks which exist in large quantity are 

 composed chiefly of silica,* alumina,! lime, and oxide 

 of iron ; and these are. the ingredients that are found 

 almost invariably in soils. Magnesia is also usually 

 present in small quantity ; as is also manganese in the 

 soils of New England. Silica is in the largest quantity, 

 both in the rocks and the soils ; alumina next ; while 

 the other ingredients are in much smaller proportion. 

 I ought also to add potassa and soda, Avhich are very 

 widely diffused, though not usually in large quantity. 

 To give a numerical statement, derived from numerous 

 analyses, such rocks as most of those in New England 

 contain sixty-six per cent, of silica, sixteen per cent, of 

 alumina, six or seven percent, of potassa, five per cent, 

 of oxide of iron, and of lime and magnesia a much less 

 quanty ; and the composition of our soils will probably 

 be found to correspond very nearly with these numbers, 

 with the exception, perhaps, of the potassa, which may 

 have in a good measure disappeared by the operation of 

 vegetation. 



Classification of Soils. The above ingredients 

 are combined in different proportions in the different 

 rocks, so as to constitute several sorts. Hence we 

 should expect, and in fact fiud, a corresponding differ- 

 ence in the soils resulting from their decomposition. 

 Indeed, with some exceptions, the geologist is able to 

 ascertain the nature of the rock from the character of 

 the soil that covers it. And I apprehend that it will 

 not be dilficult to point out the characteristics of the 

 soils derived from the different rock formations of 

 Massachusetts, so that they can be distinguished by 

 those not familiar with practical geology. This 

 geological classification is the only one which I shall 

 attempt to give of our soils ; and this seems to me all 

 that is necessary or useful in addition to the common 



* Sandy. t Clayey. 



