286 , STUDIES FOR STUDENTS 



that the red is due to igneous agencies, oxidizing iron pyrites, or to 

 volcanic dust or the heat of igneous intrusions. All such hypotheses 

 fail, owing to the widespread occurrence of red rocks, their uniform 

 color, and the absence of metamorphism. Another group of hypothe- 

 ses ascribes the oxidation to weathering at the time of origin of the 

 sediments and this is undoubtedly correct, the iron peroxide being a 

 component part of the accumulating sediments. Nearly all writers, 

 however, assume that the oxide which is now more or less completely 

 dehydrated and consequently red was in this dehydrated state at the 

 time the sediments were deposited. This assumption runs through 

 the work of Russell and is the basis of many such statements as that 

 of Darton that "red shales and sandstones, such as make up the red- 

 beds, usually result directly from the revival of erosion on a land sur- 

 face long exposed to rock decay and oxidation and hence covered by 



a deep residual soil There is such uniformity of the deep-red 



tint that this is undoubtedly the original color. "^ Russell considers 

 that the incrustation of the sandstone grains of the Newark formation 

 by ferric oxide, resembling that surrounding the grains of quartz in 

 the residual soil of the southern states, indicates that the grains were 

 transported without sufficient wear to remove the original incrustation. 

 From the distribution of present red earths, he further concludes that 

 their formation requires the presence of heat and moisture. All such 

 special modes of origin, however, are difficult of acceptance in view 

 of the great predominance of red, or red-brown in ancient ferruginous 

 formations and the comparative absence of yellow tones such as domi- 

 nate modern alluvium. 



In view of such a conflict of opinions and the special nature of the 

 hypotheses used to explain the general nature of the phenomenon, 

 the need of discussion is seen. Certain essential facts which have 

 bearings on the conclusions may be stated as follows: As indicating 

 the influence of a moist climate, Russell notes the brilliant reds and 

 yellows of the soils of the South Atlantic states as compared with 

 the ashen tints of the Mojave desert,^ and he elsewhere speaks of the 

 creamy- white color of the playa-lake deposits of Nevada and Arizona, 



1 "Geology of the Bighorn Mountains," Professional Paper No. §i, U. S. Geo- 

 logical Survey, 1906, pp. 105-7. 



2 T. C. Russell, op. cit., p. 27. 



