191^] STEVENSON— THE FORMATION OF COAL BEDS. 455 



• 



they seem to be in channels within sandstone, whereas those of 

 Pennsylvania are channels within coal beds. The coal pebbles in 

 the latter localities could not have come from the enclosing coal ; 

 all of them, coal, sandstone, shale, limestone, are rounded as by 

 stream transport and the rounding is such that they must have been 

 brought from a considerable distance. Everything about these 

 pebbles of coal indicates that, during much of Coal Measures time, 

 a considerable part of the area of deposition was land, showing out- 

 crops of the various types of Carboniferous deposits, whence flow- 

 ing streams gathered their loads of detritus. With varying condi- 

 tions, streams shifted their courses or washed down the materials 

 accumulated on their beds to fill the lower reaches of their channel- 

 ways. 



The phenomena of non-conformity and of contemporaneous 

 erosion compose a body of evidence that throughout the Pennsyl- 

 vanian the progress of events was like that in previous and in suc- 

 ceeding periods of the world's history; there were foldings of the 

 crust, there were differential elevations and subsidences and at all 

 times much of the region was exposed to subaerial erosion. 



The Distribution of Sedimentary Rocks. 



The Coal Measures rocks are sandstones, shales, limestones and 

 coals, the terms being employed in the broad sense. All occur in 

 each of the great eras and limestone seems to be wanting in only the 

 Pocahontas or earliest stage of the Pottsville. An understanding of 

 the geographical distribution and structural variations of these rocks 

 should give some insight into the conditions prevailing at the time of 

 their deposition ; but determination cannot be complete, as erosion has 

 removed the beds from a broad strip on the eastern border between 

 the anthracite fields of Pennsylvania at the north and the Alabama 

 line at the south. Beyond the latter, one finds on the eastern side 

 only Pottsville beds ; later formations are unrepresented ; it is more 

 than possible that they never were represented. 



Pocahontas deposits occupied an area on the eastern side, very 

 narrow at first but widening gradually toward the west until the close 

 of the stage, attaining the greatest width near the Tennessee-Virginia 



PROC. AMER. PHIL. S< )C. LI. 207 C, PRINTED DEC. I4, I912. 



