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Even the exhausting powers of crops will be 
influenced by like circumstances. In cases where 
plants cannot absorb sufficient moisture, they 
must take up more manure. And in Ireland, 
Cornwall, and the western Highlands of Scot- 
land, corn will exhaust less than in dry inland 
situations. Oats, particularly in dry climates, are 
impoverishing in a much higher degree than in 
moist ones. 
Soils appear to have been originally produced 
in consequence of the decomposition of rocks and 
strata. It often happens, that soils are found in 
an unaltered state upon the rocks from which they 
were derived. It is easy to form an idea of the 
manner in which rocks are converted into soils, 
by referring to the instance of soft granite , or 
porcelain granite . This substance consists of three 
ingredients, quartz, feldspar, and mica. The 
quartz is almost pure siliceous earth, in a crys- 
talline form. The feldspar and mica are very 
compounded substances ; both contain silica, alu- 
mina, and oxide of iron : in the feldspar there is 
usually lime and potassa ; in the mica, lime and 
magnesia. 
When a granitic rock of this kind has been long 
exposed to the influence of air and water, the lime 
and the potassa contained in its constituent parts 
are acted upon by water or carbonic acid ; and the 
oxide of iron, which is almost always in its least 
oxided state, tends to combine with more oxygene ; 
the consequence is, that the feldspar decomposes, 
and likewise the mica ; but the first the most ra- 
pidly. The feldspar, which is at it were th e 
