CHAPTER X. 



THE PENNSYLVANIAN (COAL MEASURES, CARBONIFEROUS PROPER) 



PERIOD. 



Formations and Physical History. 



The system of rocks lying upon the Mississippian was formerly 

 known as the Carboniferous proper, and this name is still in common 

 use. The name has its explanation in the fact that this system con- 

 tains the great coal deposits of the eastern part of North America. 

 The need of a name to distinguish this system of rocks from those 

 which have been described under the name Mississippian has long been 

 felt, and the name Pennsylvanian, which has recently come into wide 

 use in this country, was adopted because the system is well developed 

 and well known in Pennsylvania. The Pennsylvanian system includes 

 the Pottsville conglomerate (Millstone grit) below and the Coal Meas- 

 ures above. 



The Pottsville Conglomerate (Millstone Grit). 



The lowermost formation of the period in the Appalachian region 

 is generally sandstone or conglomerate (Pottsville conglomerate at 

 the north; Lee conglomerate, Lookout conglomerate, Millstone grit, 

 etc., farther south [see p. 557 et seq.]), which betoken shallow water. 

 Over wide areas it is unconformable on the Mississippian system, as 

 already noted. In some places the Pottsville conglomerate appears to 

 be a true basal conglomerate, representing the deposits made by the 

 sea as it transgressed the surface of the recently exposed Mississippian 

 formations. In such places it is sometimes made up partly of the 

 cherts derived from the Mississippian beds, showing that the latter 

 had undergone prolonged decay before the deposition of the conglom- 

 erate. In other places the pebbles of which the conglomerate is com- 

 posed seem not to have been derived from local sources. This like- 



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