Branner.] *>*" [ Feb . 19, 



Mr. Chamberlin lias happily represented the glaciers of the second epoch 

 as moving in this manner. I would suggest that the scratches referred to 

 on pp. xxi and xxii of Report Z may have been varied by such means, if 

 the topography itself of the region cannot account for the change. 



The glacial striae, wherever observed in this valley or along its borders, 

 seem, in every instance, to prove : — 



1st. That the glacier, when at its greatest thickness, was influenced 

 only by the great average topographical features of the glaciated region, 

 and, consequently, that what appears to have been the upward movement 

 of the ice is upward only in a local sense. 



2d. That as the ice-sheet began to grow thinner and to retreat, its 

 southern margin came more and more under the influence of local topo- 

 graphy, and ended in local glaciers. 



3d. That when more than one set of striae are found in the same place, 

 they are clue to the direction of the thinning ice having been changed by 

 topography. 



Wearing Power. 



The variation in the direction of striation in the case above the West 

 End breaker at Mocanaqua amounts to 120°, without the original (?) striae 

 being obliterated. Other instances of double striation have also been re- 

 ferred to. I was at first inclined to think that such cases might be taken 

 as conclusive evidence of the small wearing power of ice. But such a con- 

 clusion would evidently be unwarranted, for, whatever the original wear- 

 ing power of the ice may have been, that power certainly diminished as 

 the ice grew thinner and the glacier retreated. The later striae cannot 

 fairly, therefore, be taken to represent the wearing power of the ice when 

 it was thickest. Indeed it is quite evident, from almost any one of the 

 cases found, that the furrows made by the ice when thickest, were very 

 deep, while later ones were so shallow as to fail to entirely obliterate the 

 former ones. Furthermore, we have no means of knowing the compara- 

 tive length of time the ice was moving in the different directions recorded. 

 It may have moved for a long period in the direction indicated by the 

 oldest of the preserved striae, and, so moving, may have worn what the 

 most extravagant claim for it (at least as far as any evidence to the con- 

 trary, found in this region, is concerned) ; while motion in the other direc- 

 tions may have been only of long enough duration to leave the markings 

 we now see upon the rocks. 



The Drift, its Character, Origin, Distribution and Arrange- 

 ment. 



The material composing the drift found through this region appears to be 

 almost entirely local. In no instance did I find a single boulder of granite, 

 or of any other archsean rock, though I watched carefully for such speci- 

 mens. Only along the Susquehanna river did I find a few pehhles of 

 archsean origin, but these were so small and water-worn that lam obliged 



