CHAPTER VI 

 UPPER CARBONIFEROUS OR PENNSYLVANIAN^ 



GEORGE H. GIRTY 



The Upper Carboniferous, rather in contrast with the Lower, 

 was a period of emergence of shores and of shallowed waters, and it 

 presents the variety that appertains to such conditions. In con- 

 sidering the stratigraphic relations of the Pennsylvanian and Permian 

 one cannot fail to be struck by the local character of the phenomena, 

 and the vast amount of detail, from which it is difficult to disengage 

 facts of broader significance. One of the facts of larger moment 

 is the general unconformity which occurs at the base of the Pennsyl- 

 vanian rocks. The extent of the phenomenon may be gauged by this : 

 that an unconformity probably occurs at this horizon all the way from 

 Pennsylvania to the Mexican boundary, except possibly in the deeper 

 troughs. The underlying strata range in age from pre-Cambrian 

 to Upper Mississippian. This is evidently, therefore, an uncon- 

 formity by overlap, but the overlap is sometimes not appreciable 

 unless extensive areas be kept in view. Very rarely, I believe, is 

 any angular unconformity to be observed, but there are basal con- 

 glomerates and in many places unmistakable evidence of erosion 

 in the subjacent strata. Some of the most noteworthy instances of 

 erosion are to be found in Missouri where shales of Pennsylvania 

 age were deposited in sink holes and subterranean channels in the 

 Lower Mississippian limestones. On the other hand, evidences of 

 erosion are often wanting and sometimes any physical suggestion of 

 an interruption in sedimentation. A striking instance of this sort 

 occurs in southern Arizona. In the Bisbee area limestones of Pennsyl- 

 vanian age rest upon limestones of Lower Mississippian age, the two 

 series being extremely similar in physical characters, though carrying 

 different faunas. The same condition probably exists in the Redwall 

 limestone of the Grand Canyon region, whose lower part is of lower 



I Published by permission of the Director of the U. S. Geological Survey. 



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