702 C. R. KEYES MID-COWTINENTAL EOLATION 



More recently I have treated®^ this phenomenon as it bears upon ore 

 deposition in arid regions, and I have also described it in its relations to 

 general deflation.^^ Eecent writings of McGee on the Sonoran district 

 of northwestern Mexico/^ and of Passarge on the South African re- 

 gions/^ contain valuable data in corroboration of this statement. 



Since redness of color is not an indication of the subaerial oxidation 

 of desert soils, but of soils of moist climates, it can hardly be pointed to 

 as proving desert conditions when a formation so colored was being 

 deposited. Red rocks occurring in arid regions should, upon disintegra- 

 tion, quickly lose their bright coloration. Red sandstones in which the 

 component grains are coated with films of red oxide of iron soon part 

 with the latter through constant trituration of the grains. The fine 

 iron oxide and red clay blow off in the Justs. The "red dust storms" 

 of the southwestern plains come directly off the "Red Beds" tracts. 

 "Red fogs," as described by Milne, '^^ and "blood rains," which from time 

 to time are reported in the Mediterranean region, offer strong support to 

 this suggestion. 



RELATIONS OF AREAS OF DEFLATION AND AeROPOSITION 



Until lately the terranal characteristics of continental deposits have 

 received little discriminating attention. The controlling condition ena- 

 bling such formations to be preserved is that they be deposited upon a 

 slowly sinking area, such as might be expected to exist in front of a 

 growing mountain range. Whether the deposits be eolic or fiuviatile 

 in character, the necessary conditions are the same. Eolic depositions 

 are likely to be extensive and remarkably homogeneous. The Great 

 Plains region seems to afford ideal conditions. What modification the 

 element of streams issuing from the mountain belt on the border intro- 

 duces may not be measurable at the present time. Their influence, as 

 will be seen later on, is probably small. Whatever stream materials are 

 brought into the Plains area and deposited must so suffer at once from 

 the effects of deflative action that they are soon converted into eolic ma- 

 terials in the same way that river sediments entering the sea are thence- 

 forward marine deposits. As already stated, eolic terranes are not as a 

 rule formed in desert tracts, but in the semi-arid and moist belts beyond. 



68 Trans. American Inst. Mining Eng., vol. xli, 1911, p. 543. 



69 Bull. Geol. Soc. America, vol. 19, 1910, p. 569. 

 7« Bull. Geol. Soc. America, vol. 8, 1897, p. 991. 



"^1 Zeitsch. d. deut. geol. Gesellsch., Ivi Bd., 1904, Protokol, p. 196. 

 "Nature, vol. xlvi, 1892, p. 128. 



