Appalachian Geosyncline. 233 



and Pennsylvania opposite the outlier has a thickness between 

 5000 and 6000 feet. The Upper Devonian in the outlier is 

 estimated as still 2500 feet thick and an unknown portion has 

 been eroded at the top. If the original thickness of the Upper 

 Devonian be assumed here as only 2500 feet, and the forma- 

 tion regarded as part of a general mantle of sediments, it gives 

 a thinning of about 3500 feet in 28 miles after folding. 

 Assume this to be 35 miles before folding and the thinning 

 comes to 100 feet per mile ; the same figure as from Perry 

 County, Pennsylvania, 150 miles southwest. This, it will be 

 noted, is a maximum estimate. More probable figures for the 

 rate of thinning near the present outcrops would be 80 feet 

 for each mile of original distance. This may have steepened 

 to a higher figure near the margin. 



The rate of thinning at the northern margin, where the rocks 

 are not much folded, may be approached from another stand- 

 point : that of the dips in the successive formations. In south- 

 ern New York the upper Devonian beds are known to thin 

 from east to west, the Portage beds, for example, thinning 

 westward from 1500 feet to 1200 feet in fifty miles, from long. 

 77° to 78°. This, therefore, is on the farther side of the geo- 

 syncline. The rate of thinning to the north is, however, more 

 difficult to determine since the same bed is not exposed over 

 great distances in that direction and gentle folds slightly warp 

 the strata. In the higher strata, however (the basal Mississip- 

 pian), the regional dips are to the southwest at an average of 

 about 20 feet to the mile. In the Middle Devonian the dips 

 average in direction 20 to 30 degrees nearer south and at aver- 

 age inclinations of from 30 to 50 feet per mile. These figures 

 give between long. 76° and 78° a thickening of the Upper 

 Devonian to the south-southeast, at a rate of from 20 to 30 feet 

 per mile. In western New York south of Lake Erie, G. D. 

 Harris gives data* from which it may be computed that the 

 rate of thickening of the Middle and Upper Devonian together 

 is 36 feet per mile to the southeast. Data given in the Warren, 

 Pa., folio, however, make this seem somewhat high. These 

 results serve as checks to the contour lines of the structure map 

 accompanying the present paper, which are primarily based on 

 measured or estimated sections and indicate a marginal rate of 

 thinning of 22 to 23 feet per mile. It is possible that the thin- 

 ning may have been accentuated beyond the limits of the pres- 

 ent outcrops. It was not, however, the direction from which 

 the sediments were dominantly derived, and from the absence 

 of diastrophism in that direction a smaller rather than a larger 

 rate of thinning in the portion now eroded might as readily 



* Notes on the Geology of Southwestern New York, Amer. Geol., vii, 

 164-178, 1891. 



