I9I3.] STEVENSON— FORMATION OF COAL BEDS. 127 



sandstones ; in some places it shattered the coal and rubbed the frag- 

 ments to polished surfaces, at times reducing the coal and shale to 

 a flaky structure like that of pastry. The conditions are those so 

 familiar in the Logan and Pottsville coal areas of southwest Vir- 

 ginia and southeastern Kentucky, as well as in the Allegheny area 

 of Broad Top in Pennsylvania. They are commonplace in some 

 Cretaceous areas at the west. The extent of disturbance increases 

 toward the place of greatest outburst, where one finds faults and 

 slips in abundance. The remarkable " Glissement de I'Esperance " 

 is in no sense due to a slide on the slope of a submerged delta. 

 This glissement marks the course of a valley, eroded after the Coal 

 Measures deposition had been completed. It was filled with 

 materials dift"erent from those of the adjacent rocks and extended 

 for a considerable distance toward the southeast. Similar material 

 was seen in a fragmentary exposure along the railroad at about 3 

 miles northwest. When the eruption took place, these new, light- 

 colored rocks of the valley were folded into a close irregular syn- 

 cline, the finer dark shales of the valley wall were pushed over into 

 recumbent folds and a sharp horizontal fault was made underneath 

 the syncline ; other valleys of similar type were observed in the 

 basin. The structure, in its striking features, is in no wise original 

 and has no bearing whatever on the mode of deposition. 



Practically no coal in economic quantity was formed in the Com- 

 mentry basin until after not less than 500 meters of rock had been 

 deposited. Suddenly one comes to the Grande Couche in les 

 Pegauds, with maximum thickness of not less than 12 meters, and 

 to a similar bed with greater maximum in les Ferrieres — the other 

 and smaller sub-basin. This abrupt appearance of the great coal 

 deposits is a phenomenon for which the delta theory offers no ade- 

 quate explanation. The streams had brought down a marvelous 

 quantity of inorganic material, converting much of the lake area 

 into dry land and, of necessity, making much of the still water- 

 covered area very shallow. But during this period, no vegetable 

 materials had been brought down, aside from those composing the 

 insignificant streaks of anthracite along the northern border. The 

 new land on the northern side of les Pegauds, barely half a mile 



