154 SCIENTIFIC AGRICULTURE. 



are swept along by the impetuous stream, tlieir corners worn 

 off, and they, together with the finer particles, are deposited 

 on the plains or in the valleys, in the form of sand, gravel and 

 "boulders." 



The action of the air contributes powerfully to the decompo- 

 sition and crumbling of rocks. Water, also, which falls into 

 the cleavage or crevices of rocks, and becomes frozen, often 

 cleaves large masses asunder by its expansion in passing into 

 ice : these masses are again subdivided in the same manner* 

 until entire hills of marble, slate, and other rocks, 'are com- 

 pletely pulverized. The affinity of the gases of the air and 

 water for these elements of the various rocks produce the same 

 effect. The combined action of ice and water in transporting 

 masses of rock is another powerful agency in the formation 

 and distribution of soils. Immense blocks of rocks are fre- 

 quently frozen into ice, which is subsequently broken up and 

 floated by streams and freshets to great distances from their 

 original locality : these, in process of time, become pulverized, 

 and add their elements to the soil. 



The action of fire in volcanic districts produces immense 

 effects in chaging the character of rocks, leveling hills, and 

 filling up valleys with ashes and lava: so vast is the quantity 

 sometimes thrown out at a single eruption, that the whole 

 country for many miles around is covered with the melted 

 rocks, scoria, ashes and cinders. 



In this way Pompeii, Herculaneum and Stabia were inhumed 

 A. D. 79, by a single eruption of Vesuvius. Whole strata of 

 rocks are sometimes broken up by earthquakes, and are after- 

 wards disintegrated and mingled with the soil. The agency 

 of winds in wafting fine particles of alluvium and sand has in 

 many places entirely changed the character of the soil; in 

 some places vast barren plains have keen formed, " dunes " or 

 sand hills raised, fertile fields stript of alluvium, and others 

 coveted by sand containing no other elements of a fertile soil. 



