CAUSES OF FERTILITY. 225 



solute amount of all the ingredients of an acre of soil, of a 

 tillage depth of six inches. 



In order to show distinctly the influence of alkalies and al- 

 kaline earths, let us first estimate the amount, in a soil com- 

 posed of the same materials as the rocks, allowing the soil to 

 be of the same composition as our ordinary granite, f quartz, 

 f feldspar, and i mica. 



1. Supposing a cubic foot of such soil to weigh 125 lbs., 1 acre 



of tilled surface, 6 in. in depth, would weigh 1361.25 tons. 



Of this there is of silex 74.84 per cent. = 1018.76 " 



Alumina 12.80 « 174.23 " 



Potash 7.43 u 101.82, " 



Magnesia 0.99 « 13.47 u 



Lime 0.37 " 5.03 « 



Oxide of iron 1.93 u 26.37 " 



Oxide of manganese 0.12 " 1.63 u 



Fluoric acid ' ,2i « 2 85 " 



2. In sienite rock, hornblende takes the place of mica, 1 acre 

 of tilled surface 6 in. in depth, would weigh 1361.25 tons. 



Of this there is of silex 74.84 per cent. = 1018.76 " 



Alumina 9.79 « ^3437 „ 



Potash 673 u Q2.29 « 



L 



ime 



2.76 " 37.57 



Magnesia 3.76 « 57.I8 



Protoxide of iron 1.46 « I9 37 



Protoxide of manganese .04 " 54 u 



Fluoric acid .03 u 4q » 



No allowance is made here for vegetable matter, and the spe- 

 cific gravity exceeds that of soils which contain it, and which 

 are in a finely divided state and therefore more bulky, but 

 the amount of potash, lime and magnesia is enormous, com- 

 pared with the same substances in soils which have been 

 cultivated. Here is 100 tons of potash on an acre of soil 

 or of rock six inches in depth, while in the soil of Massachu-' 

 setts, the fine materials, separated from the coarse pebbles 

 accordmg to Prof Hitchcock, contain no potash in a free 

 state, and probably but a small quantity in any state. What 

 19* 



