Appalachtan Geosyncline. 91 



Over the western part of Pennsylvania the bottom of the 

 Upper Devonian is nowhere exposed. In southern New York, 

 on the other hand, the upper beds have been removed and 

 detailed geologic mapping has not crossed that wide belt 

 between the middle Devonian and the Mississippian outcrops. 

 In northern New Jersey also the isolated Green Pond Moun- 

 tain axis preserves merely a remnant whose original thickness 

 is not known. In the neighborhood of Lake Erie, however, 

 more exact figures are determinable. But although these 

 difficulties prevent high accuracy, the errors are small in com- 

 parison with the total thicknesses of thousands of feet. 



A striking fact in regard to the distribution of thicknesses 

 is that they are greatest near the southeastern margin. It is 

 to be noticed further that the boundary from Harrisburg to 

 Lake Erie is everywhere oblique to the lines of equal thick- 

 ness, decreasing from about 9000 feet in the southeast to 2000 

 feet in the northwest. In the region of the folded Appala- 

 chians the thicknesses range from 6000 to a little over 9000 feet. 

 It appears therefore as if there were preserved to our geologic 

 period but a portion of the original basin. The eastern side 

 is almost wholly gone and on the north erosion has etched 

 back the margin for an unknown distance. The restoration 

 of this basin showing its original extent and the thickness and 

 character of the strata is the task which we have set. 



Comparison of this Restoration with Other Maps. 



Before entering upon a detailed analysis of the problem it 

 is desirable to see how the present restoration of the Upper 

 Devonian as shown in fig. 1 compares with others which have 

 been made. 



J. D. Dana in 1895 published a map of North America at 

 the commencement of the Carboniferous era.* On it the limits 

 of the sea, and of the sediments, are shown as approximately 

 following the north boundary of Pennsylvania, Lat. 42°, and 

 lying close to the limiting outcrops on the southeast. 



The most detailed map w T as published by Willis in 1899,+ 

 and is the map with which the present one should be most 

 closely compared. The two differ materially. In Willis' map 

 the whole area of sedimentation is indicated as a shallow sea and 

 the north limit of the basin is drawn at Lat. 43° 15' where the 

 present map indicates a former thickness of between 2000 and 

 3000 feet of Upper Devonian strata. On the east the shore 

 line as shown by Willis follows closely the marginal outcrops. 

 The thicknesses taken by Willis include the Hamilton, which 



* Manual of Geology, p. 633. 



f Maryland Geol. Surv., vol. iv, pi. vii. Geography of the Eastern 

 United States During and at the Close of the Devonian Period. 



