186 CHEMICAL ANALYSES. 



examined by the lens. The different degrees in fineness of the granite debris constitute the 

 chief difference in the soils, regarding them mechanically. The soil of the Pimas valley is 

 finest in texture ; next, and differing hut little from it, is that of the lower Gila, while that of 

 the desert is much coarser. In chemical composition they also approach each other. They are 

 essentially light, sandy soils, in which the quantity of useful mineral matters vary from 15.80 

 to 9.5 per cent. 



They are poor, granite soils, of which the desert specimen is the type ; to this has been 

 added some vegetable matter, and some carbonate of lime in impalpable powder. The soil 

 of the lower Gila has vegetable matter also, and a lesser amount of the calcareous element ; this, 

 in both instances, has been derived from the river alluvium. There is, however, lime as silicate 

 in the desert soil, which, could it be cultivated, would soon yield carbonate. 



The desert soil so nearly approaches the others in chemical constitution, that we may con- 

 clude that the sterility is not due to the soil, per se, for transplant this soil to the margin of 

 the Gila, and in a few years it will resemble the others. It is to the presence of organic matter 

 (vegetable) that the fertility is due, and the vegetable matter is due to the moisture of the 

 soil, so that, ultimately, it is to the presence of water that fertility is owing, and it is to its 

 absence that sterility is imparted. While, therefore, from this we learn that all the bare and 

 apparently desert low country may be cultivated with success, where it can be irrigated, we 

 also learn that on the upland, where irrigation cannot be practised, and where a few showers 

 per year are all the water received, the brand of perpetual sterility is inalterably fixed. 



