442 STEVENSON— THE FORMATION OF COAL BEDS, t^'ov. i. 



Observer," has discussed such occurrences in detail. Green-^' de- 

 scribed several instances seen in the Yorkshire area. The Hands- 

 worth Rock, 40 feet of sandstone, disappearing towards north and 

 south, fills a trough eroded in shale. The great deposit, known as 

 the Rotherham and Harthill red rock, was deposited in a valley 

 eroded in upturned Middle Coal Measures ; and the deposition was 

 earlier than the Permian, since the red rock underlies the magnesian 

 limestone. The little coal basin of Commentry in France shows a 

 striking example of contemporaneous erosion, which has been ex- 

 posed in cross section by two of the great cpiarries. Toward the 

 close of the period of deposit, a valley was eroded in the Coal 

 Measures. Afterwards, this was filled by successive deposition with 

 light-colored sands and gravels. At a somewhat later date, an 

 eruption of igneous rock in the immediate vicinity pushed these 

 horizontal beds into a compressed, somewhat complex synclinal and 

 folded the older beds beyond into overturned folds, while faulting 

 the Grande Couche, to whose roof the erosion had reached.-" The 

 same process of erosion was repeated in later times and a newer 

 filled-valley has been exposed during deepening of the eastern 

 tranchee. 



Contemporaneous erosion provides evidence that the rocks 

 underwent folding during deposition of the Coal Measures. A few 

 notes from southwestern Pennsylvania will suffice for illustration 

 and, no doubt, they will recall to the reader instances in other locali- 

 ties.-" The Washington-Brady Bend anticline is crossed by the 

 Monongahela River at Pittsburgh and the Claysville anticline is 

 crossed by the Ohio River at a short distance farther west, near 

 Woods run. These folds existed in the later part of the Cone- 



^A. H. Green, "The Geology of the Yorkshire Coalfield," London, 

 1878, pp. 397, 456, 481-484. 



^"J. J. Stevenson, "The Coal Basin of Commentry in Central France," 

 Ann. N. Y. Acad: Sci.. Vol. XIX., 1910, pp. 198, 199. H. Fayol has pre- 

 sented a very different explanation of the phenomena in " Reunion extra- 

 ordinaire dans I'AUier," Bull. Soc. Gcol. France, III., Vol. XVI.. separate, 



pp. 35-37- 



'^ Second Geol. Survey Pennsylvania, Rep. K, 1876, pp. 310, 311, 324-326; 



Rep. KK, 1877, p. 303. 



