CHAPTER I. 



THE PHYSICAL PROCESSES OF SOIL FORMATION. 



SINCE the physical and mechanical effects of the agencies 

 mentioned above usually precede, in time, the chemical changes, 

 which are materially facilitated by the previous pulverization of 

 the rocks, the former should be first considered. 



Effects of heat and cold on rocks. Most rocks are aggre- 

 gates of several simple minerals; a few only (limestone, 

 quartzite and a few others) expand or contract alike in all their 

 parts. Of the minerals composing the compound rocks, 

 scarcely any expand to exactly the same extent under the in- 

 fluence of the sun's heat, especially when their colors differ; 

 nor, in the great majority of cases, does one and the same 

 mineral expand alike in all three directions. It follows that at 

 each change of temperature there is a tendency to the forma- 

 tion of minute fissures between adjacent crystals or masses of 

 different simple minerals; and especially in the case of large 

 crystals of certain kinds, this action alone will gradually result 

 in the disruption of the rock surface, so that individual crystals 

 may be detached with little difficulty. In any case, the cracks so 

 formed are gradually widened by a frequent repetition of the 

 changes of temperature, coupled wih access of air, water, dust, 

 and the rootlets of plants ; all of which brings about a gradu- 

 ally increasing rate of surface crumbling. This is especially 

 conspicuous at the higher elevations of mountains, where the 

 temperature changes are very great and abrupt ; and also in the 

 clear atmosphere of deserts, where owing to the extent and 

 suddenness of temperature- changes between day and night, 

 caused by the free radiation of heat into the clear sky, even 

 homogeneous pebbles are known to be almost explosively dis- 

 rupted in the mornings and evenings of clear days. 



I 



