20 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



had found almost one hundred coal beds in the South Wales coal- 

 field and. with rare exceptions, each overlies a clay bed from six 

 inches to ten feet thick. The clay varies much in composition but 

 it is a persistent deposit, so that coal beds which have thinned out in 

 the workings have been found again by following the clay. Ordi- 

 narily. Stigmaria occurs abundantly in the clay and Logan thinks 

 that plant was the source of most of the coal. 



Soon after the field work of the Virginia and Pennsylvania sur- 

 veys was completed, H. D. Rogers''" gathered the salient facts bear- 

 ing upon the origin of coal beds and presented them in a paper 

 which has become classical. It bears the impress of the time, but 

 it was based on broad observations by the author and his equally 

 celebrated brother, William B. Rogers, aided by a corps of able 

 assistants ; the studies, lasting six years, were in detail for an area 

 of somewhat more than 20,000 square miles, but in addition less 

 detailed studies had been made in Ohio and Kentucky, so that the 

 region under consideration was not far from 40,000 square miles. 

 The discussion was the first serious attempt to account for the origin 

 of the Coal Measures, which was based on actual study of a vast 

 area. 



At the outset, Rogers pronounced against any theory of delta 

 formation, as according to his belief the Appalachian ocean deep- 

 ened toward the west and northwest. ^^ The deposits are traceable 

 coastwise for 900 miles, so that it seems improbable that fluviatile 

 currents could have assembled them. 



The sandstones decrease in thickness and coarseness as they re- 

 cede from the ancient shoreline at the east ; the shales increase in 

 that direction for a time and then decrease, while the limestones, 

 wholly wanting near the shore line, increase in thickness and purity 

 so as to become imposing before the Ohio River has been reached. 

 The animal remains found in the limestones are marine. There 



'° H. D. Rogers, "An Inquiry into the Origin of the Appalachian Coal 

 Strata, Bituminous and Anthracitic," Reps, of Amer. Assoc, of Geologists and 

 Naturalists, Boston. 1843, pp. 434, 459, 463-467. 



^' It should be noted here that wlicn Rogers wrote the conditions on the 

 west side of the Appalacliian l)asin were not known; but does not affect the 

 general argument. 



20 



