GEOLOGICAL SOCIETY OF PENNSYLVANIA, 



201 



variety has been found on Jack's mountain, near the 

 Juniata narrows. The two former varieties are manu- 

 factured at furnaces situated within this valley. 



Having disposed of these details;, it only remains to 

 revert to the supposed lake, and to consider the circum- 

 stances attending the emptying of its ancient basin. 



From our previous description, and by consultation 

 with our accompanying illustrations, may be ascertained, 

 that the waters occupied this trough to the depth of 

 three hundred feet in the centre, and gradually shoaled 

 off towards the extremities, as the bottom ascended in 

 those directions. The deepest part is immediately in 

 front of the gorge, where the beach line appears to be 

 four hundred and fifty feet above the present water 

 level of Kishacoquillas creek. Supposing the basin to 

 be refilled, it is obvious that the greatest pressure would 

 be at the precise point at which the creek now dis- 

 charges itself. 



I cannot however view this as a case of sudden burst- 

 ing or breaking down of a barrier by overpowering 

 pressure, as in some instances is said to have occurred. 

 Here the outlet and point of fracture is on one side of 

 the trough, and the barrier is no less than a ridge eight 

 hundred feet high and nearly two miles broad ; whose 

 highly inclined and compact silicious beds have been 

 cut down to the very base, and that at right angles to 

 their direction. We have illustrated this in the section 

 by the line C D. 



I see difficulty also in admitting this to result from the 

 slow and gradual action of discharging waters. The 

 transverse slopes of the ravine are probably, in its 

 deepest part, at as high an angle as 40 or even 45°, 

 notwithstanding the immense quantity of fallen debris. 

 It is quite obvious to an observer that this transverse 

 section is of a very dilferent age ; that is, of much later 

 origin, than the ridge itself, whose sides are for the most 

 I. — 2 A 



