THE 



AMERICAN JOURNALOFSCIENCE 



[FOURTH SERIES.] 



Art. XIX. — Some Peculiarities of Rock -Weathering and 

 Soil Formation in the Arid and Humid Regions j by 



E. W. HlLGARD.* 



Some of the differences in the processes and the results of 

 weathering in the arid and humid regions respectively have 

 been long commented upon ; but we do not find anywhere a 

 measurably full discussion of the subject. I have discussed 

 the various phases as observed by myself, in various publica- 

 tions ; but I desire to give in this place a summary of the 

 points noted, especially as regards soil-formation, and the con- 

 clusions to be drawn therefrom. 



The potent effects of water upon both the mechanical and 

 chemical processes of rock decomposition being well under- 

 stood, the relative scantiness of rainfall in the arid regions leads 

 us at once to expect a slower rate of decomposition of rocks 

 and of their component minerals under the arid regime. The 

 old observation of the freshness of the surfaces of half-finished 

 obelisks in the quarries of Syene, and the good conservation of 

 the same in the obelisks of Lower Egypt under slightly more 

 humid conditions, as compared with the fate of Cleopatra's 

 Needle at New York, are familiar to all. It is not, of course, 

 moisture alone, but very essentially the temperature conditions 

 accompanying both its abundance and scarcity, that are con- 

 cerned in the effects produced. 



Aridity being intimately correlated with the existence of 

 deserts, we are at once led to associate sand and dust with 

 arid conditions. The dust storms of the arid regions are as 

 proverbial as are the sands of the desert ; and the latter are in 

 the public mind the symbol of sterility. It is only of late that 



* Eead at the December meeting of the Cordilleran Section of the Ameri- 

 can Geological Society, at Berkeley, Cal. 



Am. Jour. Sci.— Fourth Series, Vol. XXI, No. 124. — April, 1906. 

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