Appalachia?i Geosyncline. 109 



whole northern half of Pennsylvania. East of the Allegheny 

 plateau it would outcrop only in the steep limbs of the anthra- 

 cite synclines, to the northeast the whole of the Pocono plateau 

 becoming removed. Following this principle of "Willis, our 

 distant descendants would argue therefore from the limits of 

 the outcrops that the Pocono sandstone in eastern Pennsyl- 

 vania was a local deposit confined to the anthracite coal basins. 



As another illustration, — between the New Jersey Coastal 

 plain and the bold Silurian outcrop known as the Kittatinny 

 or Shawangunk mountain is an intensely folded and faulted 

 region 50 to 60 miles in width, now occupied by considerable 

 areas of Cambro-Ordovician limestones and slates. Erosion of 

 the Appalachian province to another mile in depth would 

 destroy most of these rocks and perhaps even the most deeply 

 infolded remnants. Their easternmost outcrops would then 

 appear on the steep walls of the synclinorium of the Appa- 

 lachian valley. The inhabitants of the earth in such a period 

 would, if they 7 followed the principle which is here under criti- 

 cism, declare that the Ordovician seas never encroached east- 

 ward beyond this margin. 



To come to the Upper Devonian formations in particular, — 

 their gradations in thickness across the geosyncline are best 

 revealed in southern Pennsylvania. Proceeding from the west 

 to the east, — at the Allegheny Mountain front they measure 

 6000 feet ; around the isolated Broad Top coal basin between 

 7000 and 8000 feet, in the southern anthracite basin over 9000 

 feet. Consequently the suggestion both from the changes in 

 thickness as well as the nature of the sediments is that they 

 formerly extended an unknown distance to the eastward. The 

 original basin has been uplifted on the east and the Upper 

 Devonian formations eroded back to the region of their maxi- 

 mum thickness. How far eastward and southeastward these 

 formations once extended must be estimated from other lines 

 of evidence : — the internal evidence given by the sediments 

 now remaining, and the external evidence as to adequate 

 sources of supply for the original volume of the Upper 

 Devonian waste. 



[To be continued.] 



