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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 rapidly. The feldspar, 

 which is as it were the cement of the stone, forms a 

 fine clay: the mica partially decomposed mixes with 

 it as sand; and the undecomposed quartz appears as 

 gravel, or sand of different degrees of fineness. 



As soon as the smallest layer of earth is formed on 

 the surface of a rock, the seeds of lichens, mosses, and 

 other imperfect vegetables which are constantly float- 

 ing in the atmosphere, and which have made it their 

 resting place, begin to vegetate; their death, decompo- 

 sition, and decay afford a certain quantity of organi- 

 zable matter, which mixes with the earthy materials of 

 the rock; in this improved soil more perfect plants are 

 capable of subsisting; these in their turn absorb nour- 

 ishment from water and the atmosphere; and after per- 

 ishing afford new materials to those already provided: 

 the decomposition of the rock still continues; and at 

 length by such slow and gradual processes, a soil is 

 formed in which even forest trees can fix 'their roots, 

 and which is fitted to reward the labours of the culti- 

 vator. 



In instances where successive generations of vege- 

 tables have grown upon a soil, unless part of their pro- 

 duce has been carried off by man, or consumed by 



