320 SYLVA AMERICANA. 



immediately after the last disruption of the crust ; but in process 

 of time the decay of vegetables and animals form additions to 

 the outer surface of the earth, and constitute what are called 

 soils ; the difference between which and earths is, that the former 

 always contains a portion of vegetable or animal matter. The 

 manner in which rocks are converted into soils, Sir H. Davy 

 observes, may be easily conceived 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 chrystalline form. The feldspar 

 and mica are very compounded substances ; both contain silica, 

 alumina and oxide of iron ; in the feldspar there is usually lime 

 and potassa ; in the mica, lime and magnesia. When a granite 

 rock of this kind has been long exposed to the influence of the 

 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 oxidized state, tends 

 to combine with more oxygen ; 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 vegetables of the kind which 

 are constantly floating in the atmosphere, and which have made 

 it their resting place begin to vegetate ; their death, decomposition 

 and decay afford a certain quantity of organizable 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 nourishment by the agency of water and the 

 atmosphere ; and after perishing, 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 labors of the cultivator. 



