CLIMATE AND TERRESTRIAL DEPOSITS 287 



beds exposed during the summer months to temperatures of from 110° 

 to 120° Fahr. in the shade. ^ As indicating the influence of heat 

 may be mentioned the uniform restriction of red soils, neglecting those 

 derived directly from red formations, to the warm temperate and 

 especially the torrid zone. That the contrast is not due to the younger 

 age or glacial origin of the soils of the cold temperate zones is found 

 upon examination of driftless areas in such regions. As indicating 

 the influence of prolonged exposure to the air, Crosby notes that it is 

 the older soils and especially the surface portions in the warm regions 

 which show red to a striking degree.^ 



Wliere reds in surficial formations are noted in arid regions they 

 are frequently the accompaniments of aeolian action. Dune sands 

 may be white, yellow, or red. Red deserts have been noted in both 

 Africa and Asia and Phillips has shown that the red sands of the Ara- 

 bian desert owe their color to a coating of ferric oxide deposited after 

 the grains of sand had become round. The source of the iron oxide, 

 which comprised about 1-500 of the total weight, he was unable to 

 determine. 3 All these facts emphasize the influence of lapse of time 

 and the presence of moderate heat, such as that of the torrid regions, 

 as causes sufficient partially to eft'ect the dehydration of ferric oxide, 

 even without the agency of pressure ; thereby transforming the yellow 

 and light-brown colors into the brilliant reds which characterize the 

 tropical regions of rainy climate. 



In view of these facts, one of the chief true causes of the red color 

 of the older ferruginous formations seems to have been reached by 

 Crosby, whose statements are as follows : 



The dehydration of the ferric oxide is not wholly dependent upon heat or 

 pressure or any obvious extraneous agency, but it is in a large degree, apparently, 

 a spontaneous process. Of this we have abundant evidence in nature and in the 

 laboratory. When the iron, which exists in the various silicate minerals chiefly 

 in the ferrous state, is liberated and peroxidized during the decay of these species, 

 it combines naturally with a very large and indefinite proportion of water, form- 

 ing the yellow hydrate, which is seen as a flocculent or a gelatinous colloid in 

 the waters of springs, bogs, and marshes, and when the hydrate is obtained as 



I Op. cit., p. 42. 



3 On the contrast in color of the soils of high and low latitudes, American Geologist, 

 Vol. VIII, 1891, p. 77. 



3 "The Red Sands of the Arabian Desert," Quarterly Journal 0} the Geological 

 Society, Vol. XXXVIII, 1882, pp. 1 10-13. 



