FORMATION OF ARABLE LANDS. 83 



I shall confine myself to these two species of rock, be- 

 cause the others are not so widely extended over the globe, 

 nor do they present themselves in as large masses as these. 

 Neither shall I speak of some substances that are found, 

 more or less, in granite, as hornblende, amphibole, serpen- 

 tine, &c., as these bodies are only secondary there. 



The composition of the various stones which constitute 

 granite, is widely different ; quartz is almost entirely 

 formed of siliceous earth ; feldspar of silex, alumina, lime, 

 potash, and the oxide of iron ; mica contains besides these, 

 magnesia. So that when granite is decomposed, it produ- 

 ces those lands which, upon analysis, afford all these prin- 

 ciples; whilst the washings from the quartz mountains 

 form only beds of siliceous earth ; and the ruins of rocks 

 of micaceous schist contain only the elements of feldspar 

 and mica. 



The calcareous mountains, composed of carbonate of 

 lime, without any appearance of the remains of organized 

 bodies, are ranged by naturalists amongst primitive rocks, 

 and give rise to the formation of calcareous soils. 



All the lands which are produced by the destruction of 

 primitive rocks are of the first formation, and ought to be 

 so designated to distinguish them from those which owe 

 their existence to other causes, of which I am now 

 about to speak. 



Independently of those causes which T have just ex- 

 plained, and which have produced the formation of the 

 greater part of the arable lands, there are others to which 

 some lands owe their origin. The successive destructions 

 which the whole surface of the globe appears to have suf- 

 fered ; the decomposition of pyritous beds, which appear 

 to have covered a part of it ; the numerous lakes which 

 have disappeared by the hand of man, or by the acciden- 

 tal rupture of their natural confines ; the eruption of vol- 

 canoes ; the overflowings of the sea ; the bony remains of 

 animals, and the decay of vegetables buried in the ground, 

 have formed soils of all characters ; and these have after- 

 wards been applied by man to his own use. 



