Apeh, 20, 1906.] 



SCIENCE. 



631 



rock that follows a comparatively ancient 

 fault. In all these cases, it is true enough that 

 erosion has been guided by a fault; but it is 

 certainly desirable to distinguish between 

 scarps of that kind and scarps directly pro- 

 duced by faulting. 



It is not easy to determine, on reading Pro- 

 fessor Kemp's article, whether the scarps that 

 he describes are of one kind or of the other. 

 Certain supposable conditions, not excluded by 

 anything in his article, would allow an ancient 

 date for the faulting, and would leave the 

 freshness of the scarps to relatively modern 

 erosion, especially to glacial erosion. It is of 

 course possible that the full knowledge which 

 Professor Kemp possesses of the rock struc- 

 tures in the Adirondacks may enable him to 

 exclude these supposable conditions. 



There is one feature of the Adirondacks 

 which seems to me to favor the idea of a re- 

 mote date for at least those faults by which 

 the graben-like valleys are determined. This 

 may be made clear by the following alternative 

 considerations. If the graben-like valleys 

 were the result of faulting of so modern a 

 date that their scarps are still steep as a direct 

 result of displacement, it is evident that the 

 date of such faults must be of later date than 

 the general erosion of the region. In this 

 case there should not be expected any partic- 

 ular relation between the course of the older 

 valleys and the course of the new grabeu. On 

 the other hand, if the faults are of ancient 

 date, the general valleys of the region might 

 have been eroded along them, and might have 

 gained a wide-open expression appropriate to 

 their advanced age; while the graben-lUie val- 

 leys might then be due to more modern erosion 

 ■ — especially to glacial erosion — along the zones 

 of fault-shattered rock. In this case there 

 should be a close association between the direc- 

 tion of the general valleys and the course of 

 the graben. As far as I know the region from 

 its maps the latter relation seems to prevail. 



W. M. Davis. 

 Haevabd Univeesity. 



To THE Editor op Science : Professor Davis 

 has kindly submitted his letter to me before 

 publication and I gladly embrace the oppor- 



tunity to add a word regarding the points 

 raised. I assume that the relief is admitted 

 to be primarily due to faulting and that the 

 question of faults as against joints or other 

 influences I need not take up. While faults 

 in the central region of crystalline rocks are 

 less easy to demonstrate than in the outer and 

 contrasted sediments, yet in the areas of the 

 latter they are frequent and they can be fol- 

 lowed in instances into the mountains them- 

 selves. 



Generally speaking, the region of the Colo- 

 rado River can not with justice be compared 

 with the Adirondacks, because in the former 

 we have canyons alone ; whereas in the Adiron- 

 dacks we have both canyons and broad, open 

 valleys of equal depth and believed with rea- 

 son to be older topographic forms. Rivers, 

 even if following faults, could imder the cir- 

 cumstances scarcely yield canyons, since they 

 would be tapped off by the older valleys long 

 before they could accomplish much; but two 

 opposing faults with a dropped strip or a 

 ' Grabensenkung ' could. If we had one scarp 

 produced because a stream worked down a dip, 

 on a hard stratum and against a soft one, 

 erosion without faulting could be urged, but, 

 in these old crystallines, we usually have no 

 contrasts in hardness, which would lead to 

 sapping, and in the valleys we very often have 

 precipitous fronts on each side. 



The production of the scarps by other proc- 

 esses than faulting chiefly centers around the 

 work of the ice sheet. The question essen- 

 tially boils down, as soon as we know the coun- 

 try, to the decision whether or not the ice 

 alone was able to pluck away the fronts of 

 single escarpments and to carry oil the inter- 

 vening rock between two so as to leave the 

 ' Grabensenkung.' 



Evidences of an old plateau or peneplain 

 are found in certain flat-topped mountains. 

 Dr. I. II. Ogilvie has remarked a striking 

 one on Treadway Mountain in the Para- 

 dox Lake quadrangle (Bulletin New York 

 State Museum, No. 96, p. 468) and an- 

 other appears in Coot Hill in the north- 

 eastern portion of the town of Crown Point, 

 Ticonderoga quadrangle. We have imagined 

 these and others like them to be remnants of 



