MECHANICAL CAUSES. 55 



mica; the first the most rapidly. The felspar, which 

 is 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 

 sa^d 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 floating 

 in the atmosphere, and which have made it their 

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

 composition, and decay afford a certain quantity of 

 organizable matter, which mixes with the earthy 

 material of the rocks. In this improved soil those 

 perfect plants are capable of subsisting. These in 

 their turn absorb nourishment from water and the 

 atmosphere, and, after perishing, afford new ma- 

 terial to that already provided. The decomposition 

 of the rocks still continues ; and at length, by such 

 a slow and gradual process, a soil is formed in which 

 even forest trees can fix their roots, and which is 

 fitted to reward the labors of the cultivator." 



Boussingault thus gives the mechanical causes 

 effecting segregation. " Water, by reason of its 

 fluidity, penetrates the masses of rocks that are at 

 all porous, and filters into their fissures. If the 

 temperature now falls, and the water congeals, it 

 separates by its dilatation the molecules of the 

 minerals from one another, destroys their cohesion, 

 produces clefts, and slowly reduces the hardest 



