484 STEVENSON— THE FORMATION OF COAL BEDS. L^'ov. i, 



for isolated deposits, which are numerous and often thick. These, 

 in many cases, were far away from any marine invasion, in regions 

 of coal beds and freshwater limestones. Within the Central area, 

 there is almost total absence of limestone, the coal beds are indefinite 

 and most of those which are recognized on the borders are wholly 

 wanting. 



The origin of the red color in shales and sandstones has been 

 sought by many students. Crosby*'* has shown that in the northern 

 United States and Canada the soils are rarely red except when de- 

 rived from a red rock ; but southward from latitude 39° redness 

 increases until, in the West Indies and South America, it becomes 

 intense. The -color is not due to the rocks, for the red deposit at 

 the south rests on primary rocks not differing from those at the 

 north where no red is found. He thought that the contrast is due, 

 very probably, to climate, the dehydration of ferric hydrate and con- 

 secjuent change of color being caused by solar heat. In a later paper, 

 he says that six years of additional study had led him to assign 

 less importance to solar heat as the converting agent, but he still 

 recognizes it as one of the important agents. Many illustrations 

 are given, which certainly appear to go far toward fortifying his 

 position. 



Russell*'^ took up the subject at a somewhat later date and reached 

 conclusions differing from those of Crosby. The depth to which 

 decay of rocks extends in the Appalachian region increases south- 

 wardly, becoming 100 feet in large areas, and he cites Belt as report- 

 ing that, in Nicaragua, the depth at times is 200 feet. In the great 

 limestone valley, south from James River in Virginia, the clay re- 

 maining after solution of the limestone is red and sometimes 50 feet 

 thick. He objects to the conclusion that climate or solar heat caused 

 the red color, because in summer the soil is heated as strongly in 

 the northern as in the southern states. The red beds of the Rocky 



•^W. O. Crosby, "Colors of Soils," Proc. Bost. Soc. Nat. Hist.. Vol. 

 XXIII., 1885, pp. 219-222; "On the Contrast in Color of Soils of High and 

 Low Latitudes," Ainer. GeoL, Vol. VIII., 1891, pp. 78-82. 



•^I. C. Russell, "Sub-aerial Decay of Rocks," U. S. Geol. Surv. Bull. 

 No. 52, 1889. 



