ORIGIN OF EOLIC DEPOSITS 



701 



stance was especially emphasized by some of the Eussian geologists, nota- 

 bly by Sokolow. The same fact is noted by Walther** and by TJdden.®^ 



The calcareous nature of desert deposits is perhaps one of their most 

 distinguishing properties. The ashen hue of adobe soils appears to be 

 largely due to the high content of lime. In the case of the loess, the 

 large amount of calcareous material present has always remained inex- 

 plicable. It may be that this feature may prove to be the strongest evi- 

 dence of its eolic origin. That there should be such a high lime content 

 in desert soils is not surprising. The lime is everywhere brought to the 

 surface of the ground through capillarity, forming what in the Spanish- 

 American States is called tepetate or caliche. In New Mexico, for ex- 

 ample, favorable situations show beneath a few inches of pulverulent 

 ashen soil a soft, snow-white lime-rock which often attains a thickness 

 of 10 to 20 feet. It is frequently well exposed in railway and highway 

 cuttings, and is encountered in other excavations. It is formed solely 

 by tlie deposition of lime salts in adobe clays. As the uppermost layers 

 of soil are removed by the winds, a large proportion of the lime materials 

 must also be finally carried away through the air. 



Eed coloration, as indicating subaerial oxidation, and inferentially 

 desert conditions under which formations possessing it were deposited, 

 does not appear to be a yerj trustworthy criterion. Eeds and browns 

 are surely not characteristics of desert-formed soils. Ashen or gray 

 everywhere prevail. My personal observations have been rather wide in 

 the western arid country, extending from British Columbia to Guate- 

 mala; they have extended through northern Africa and around the Cas- 

 pian region, but nowhere do I recall a single instance of desert deposits 

 with red coloration due to exposure to the atmosphere. The red soils of 

 arid regions appear to be derived entirely from old red rocks. In the 

 dry tracts of the Great Basin the red and brown belts are invariably erup- 

 tive or very old red elastics perfectly unaffected by recent surface oxida- 

 tion. 



The idea of red coloration of soil as a proof of subaerial oxidation 

 seems to apply only in moist climates. Its extension to arid areas ap- 

 pears unwarranted. Von Eichthofen^^ was the first, I think, to call 

 attention to the fact that in cold and in dry climates chemical decompo- 

 sition of rocks is almost unknown. EusselP^ also emphasizes this point. 



"Abahnd. d. K. Sachischen Gesellsch. d. Wissensch., xvl Bd., 1801. p. 149. 



«» Popular Science Monthly, vol. xlix, 1896, p. 655. 



^ Fiihrer fiir Forschungsreisende, Berlin, 1886, p. 100. 



" Bull. Geol. Soc. America, vol. 1, 1890, p. 134. 



