1879.] OUl [Stevenson. 



One is not fairly justified by the facts in supposing that the system was 

 established after the axes had been elevated enough to affect the topography. 

 Several canoe shaped synclinals lie between the Alleghanies of Pennsyl- 

 vania and Laurel Ridge ; a similar synclinal exists between Laurel and 

 Chestnut Ridges and another between Chestnut and Brush Ridges. A 

 drainage system, like that now existing, could hardly have originated in 

 parallel synclinal troughs separated by distinct ridges such as these moun- 

 tains are. A series of lakes might have been formed, but that they would 

 have been in communication so as to give a uniform drainage system, tend- 

 ing wholly westward and breaking through the axes, seems not altogether 

 probable ; for Laurel and Chestnut Ridges are little, if at all inferior to the 

 Alleghanies of Pennsylvania, either in altitude or in the inclination of the 

 strata. 



It is equally difficult to believe that the streams now flow along lines of 

 original weakness, crossing the several axes, or that their present courses 

 through the mountains mark lines of transverse fracture. A transverse 

 fracture would necessarily be parallel to the direction of the disturbing 

 force, and it could arise only in case the rock on one side of a line has 

 greater power of resistance than it has on the other. Associated with the 

 transverse fracture there would be a greater or less faulting of the axis. 

 So that there would be no difficulty in deciding the presence or absence of 

 such a fracture ; for even were the direct evidence masked by river erosion, 

 the side throw would be distinct. But there are no evidences of such side- 

 throws near the gaps or anywhere else in these mountains, and all the con- 

 ditions go to show that any supposition of their existence is altogether im- 

 probable. For, as I have shown elsewhere,* though the axes are thrown 

 off successively toward the southeast, yet there are no breaks. The two 

 parts of a shifted axis overlap, one becoming gentle toward the north and 

 the other toward the south, until each is over-ridden by the other. 



In addition, such a hypothesis necessarily assumes not only that these 

 fractures must have existed in all of the mountain axes, butalso that much 

 more extensive erosion had taken place in the region west from Chestnut 

 Ridge than any of the more eastern troughs, in order that by deepening 

 the fractures in the several mountain ranges, the waters could be drawn off 

 into the next trough west. 



While neither the hypothesis nor its attendant assumption goes beyond 

 the range of possibility, each oversteps very far the bounds of probability. 

 Erosion must have been going on all the time east from Laurel Ridge, 

 while the valleys were deepening on the west side of Chestnut Ridge; and 

 a system of drainage must have existed in one region just as well as in the 

 other ; unless, indeed, the region east from Laurel Ridge was arid, which 

 is wholly improbable. If these systems were not one from a very early 

 date, it isdifficult to conceive how they could become oue at any later date; 

 at all events it would be impossible by any natural deepening of gaps or 

 removal of divides to divert the drainage from one side of an axis to the 



* Reports on FnyeLte and Westmoreland Dist. of Penn. 



