GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



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former, dip at 25 to 75° to the south, across an uncertain 

 breadth, but not less than twenty miles in that direction. 

 We are not yet acquainted with the longitudinal extent 

 of this fracture. But we are nevertheless in possession 

 of the interesting fact that two great disrupted lines of 

 elevation range through central Pennsylvania, the first 

 at a distance of ten miles in front of the Alleghany 

 mountain, and the second in the same parallel, at the 

 distance of fifty to fifty-five miles in front of the same 

 mountains. Both these lines range along chains of val- 

 leys and not on hills or ridges. The point where the 

 two planes of lifted or tilted strata rest against each other 

 on their lower sides is not equidistant between the two 

 anticlinal lines, but in the proportion of fifteen to eigh- 

 teen miles breadth from the south line, and twenty-five 

 to twenty-nine miles breadth from the north or first line 

 of elevation, measuring at right angles to those lines. 

 These results are obtained from four sectional lines at 

 the distance of fifty-five, one hundred, and one hundred 

 and thirty miles from each other. From two of these 

 I have prepared the sections, pi. 7, fig. 2 and 3, 

 roughly traced for the present purpose of illustrating the 

 foregoing notice. 



It is also a circumstance of some interest, that this line 

 of depression between the two uplifted planes, passes 

 westward from the foot of the Mahanoy mountain across 

 the Susquehanna, and along the south foot of the Shade 

 mountain, in the bottom of the trough described by me 

 in a former article in this volume, pp. 6 to 15, where this 

 deep valley is filled partly by the Juniata, and partly by 

 numerous beds of fucoides. Pursuing the same line of 

 fracture, as it sweeps round to the south west, we find 

 that for the space of thirty miles it forms the bed of the 

 Juniata, which so far avails itself of the trough produced 

 by the intersection of these vast planes ; and we thus 

 obtain an interesting elucidation of the cause of this 



