BATE OF WEATHEEING 257 



of the rocks of the granitic group. It is possible also that these 

 dark colors cause them to become more highly heated, where 

 exposed to direct sunlight, and hence subject to mechanical dis- 

 integration. The fact that many of our trappean rocks, as seen 

 in dikes cutting other rocks, do not in all cases succumb with 

 greater comparative rapidity is due to their very compact struc- 

 ture, whereby percolating waters are so largely excluded. 



(3) Rate of Weathering influenced by Humidity. — The ra- 

 pidity of rock weathering and soil formation is, even among 

 rocks of the same nature, widely variable, being dependent 

 upon climatic conditions of any particular locality. In the arid 

 regions north of Flagstaff, Arizona, are wide areas of country 

 covered with coal-black lapilli ejected from volcanoes whose 

 craters are now occupied by growing pines upwards of two 

 feet in diameter Yet these fields are, with the exception of the 

 pines, as bare of vegetation as though but yesterday scorched 

 by fire. The fine lapilli, resembling nothing more than crushed 

 coke, cover everywhere the undulating plains, greedily absorb- 

 ing the moisture from melting snows and scanty rainfalls, but 

 undergoing no appreciable decomposition and affording foot- 

 hold for only a few desert shrubs and grasses. Yet in a 

 moister clime, and one more adapted for luxuriant vegetation, 

 we might expect that these lapilli should long ago have suc- 

 cumbed and given fairly fertile soils. (See further, p. 263.) 



(4) Rate of Weathering influenced by Position. — Among the 

 siliceous crystalline rocks superficial disintegration is undoubt- 

 edly greatly aided by temperature variations, which, by render- 

 ing the rocks porous, facilitate chemical decomposition. Such 

 action must, however, be merely superficial, and at considerable 

 depths below the surface the change must be purely chemical. 

 The chief conditions favoring chemical action are those of con- 

 tinual percolation by waters carrying carbonic acid, as already 

 described. It naturally follows, therefore, that a purely chem- 

 ical decay will progress more rapidly where the rock mass is 

 covered by such a layer of vegetable soil as shall keep the surface 

 moist and give rise to the decomposing solutions. Hence, that 

 such an accumulation having begun, decomposition will keep on 

 at an ever-increasing rate to a depth concerning which we have 

 at present no data for calculation. It must not be too hastily 

 assumed from this that rocks thus protected do in reality break 

 down more rapidly than those on bare hillsides, since, in the 



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