191^-] STEVENSON— THE FORMATION OF COAL BEDS. 487 



the oxidation was by free oxygen and not by that derived from ferric 

 oxide. The discussion in the later paper covers the whole question 

 and the author fortifies his position with the wealth of illustration 

 that seems to leave little room for disputing his conclusions ; but 

 the discussion is so elaborate that only the final statement can be 

 given here and the reader must be referred to the memoir itself for 

 the detailed argument. The red color of ferruginous rocks as con- 

 trasted with the predominating yellows of alluvium is due to three 

 cooperating causes : Spontaneous dehydration, operating to some 

 extent at the surface in warmer regions ; dehydration under great 

 pressure and moderate temperature, nearly universal in sediments 

 which become buried and consolidated ; diffusion, operating under 

 conditions of warmth and moisture, whether these be at the surface 

 as in warm and humid regions or beneath the surface as may occur 

 in any portion of the earth. By these three means, the light-colored 

 yellow or brown muds and sands may become red shales and sand- 

 stones. The chief condition for formation of red shales and sand- 

 stones is merely the alternations of seasons of warmth and dryness 

 with seasons of floods. 



Gruner and Dannenberg lay stress upon the fact that reds 

 occur in the sterile measures. Certainly the reds seem to have 

 been deposited under conditions which were unfavorable to the 

 accumulation of coal, for that is almost wanting in the central 

 area of West Virginia ; great coal beds, traceable for hundreds or 

 even thousands of square miles, thin out to disappearance as they 

 approach that area. At the same time one must not forget that the 

 reds mark local, not general conditions ; that they abound in the 

 productive as well as in the less productive portions of the column. 

 Within the Central area they are as important in the upper Alle- 

 gheny and in the Monongahela as in the Conemaugh and Wash- 

 ington. Even in the limited Loire basin the same is true, for the 

 reds there are practically confined to that portion of the Saint- 

 Chamond area, which is micaceous. Whether or not a similar rela- 

 tion exists in the Saar'bruck area, the writer has not been able to 

 ascertain. 



The distribution of Pennsylvanian red shales forbids the sup- 



PROC. AMER. PHIL. SOC. LI. 207 E, PRINTED DEC. l6, I9I2. 



