232 J. Barrett — Upper Devonian Delta of the 



ancient land they measure but 1100 feet.* The Catskill rocks 

 are given by Rogers as 6000 feet thick at the Susquehanna Gap 

 above Harrisburg and Claypole gives this figure for the whole 

 of Perry County, f A large part of the thinning of the Port- 

 age-Chemung to the southeastward is doubtless due to the 

 passage of beds into the Catskill facies in that direction as 

 observed both to the northeast and southwest. 



The sura of the Upper Devonian rocks in the northern part 

 of Perry County as derived from these measurements amounts 

 to 9400 feet. Sixteen miles south at the Susquehanna Gap 

 it is 7200 feet. The amount of shortening due to folding on 

 this line was measured by the writer from the sections by R. T. 

 Chamberlin4 The base of the Catskill was used for this pur- 

 pose and it was determined that 21*5 miles had been shortened 

 to 16 miles, 75 per cent of the original length. Assuming 

 that the thinning is accurately determined and not in part due 

 to a greater pressure on the east during folding, it gives a rate 

 of 2200 feet in 21*5 miles, approximately 100 feet per mile. 

 This is a higher rate of thinning than is observed anywhere 

 toward the west, on the western side of the basin. There the 

 highest rate, as seen between the anthracite basins and the 

 Allegheny front, is not more than 60 or 70 feet per mile. A 

 higher rate on the eastern side of the basin is however quite 

 possible, and since this is near the center of the basin the rate 

 may have increased to a still higher figure farther east. A 

 higher rate of thinning than 100 feet per mile is not demanded, 

 however, by the other lines of evidence. This would bring 

 the original margin approximately to a line passing through 

 New York, Philadelphia, and Baltimore, the margin of the 

 present Coastal Plain. If the crust was shortened 25 per cent 

 by the later folding of this zone, it would bring the original 

 limit to a present line 15 to 20 miles northwest. This is the 

 position shown on the map, fig. 1 (p. 225). In a later part of this 

 article it is shown that the sources of erosion were southeast of 

 the margin of the present Coastal Plain from rocks whose 

 foundations are now concealed. Consequently the sediments 

 either approached to the region of the present Coastal Plain 

 or there was in the Upper Devonian a more or less broad neu- 

 tral zone between the regions of erosion and deposition. 



The bearing of lithologic evidence from the Green Pond 

 Mountain outlier is considered in the next section, but such 

 discussion as deals with the thinning of the strata may be given 

 here. The marginal Upper Devonian outcrop in New York 



* Second Geol. Surv. Pa., vol. F-2, pp. 71, 164, 1885. 

 fLoc. cit., pp. 34, 36, 244, 316. 



JThe Appalachian Folds of Central Pennsylvania, Jour. Geol., xviii, 233, 

 1910. 



