70 J. W. SPENCEB — HIGH CONTINENTAL ELEVATION. 



Lawrence and Maine, which have, however, been more or less affected by 

 terrestrial movements. 



The length of time required to excavate the channels of these gnat rivers 

 commenced as for back as the Paleozoic days. However, the culmination 



of that of tin- Mississippi was not until in the later Tertiary, before the 

 Pleistocene period. As the St. Lawrence, now submerged to a depth of 

 over l'_'i»<» feel for a distance of SiiO miles, is mostly cut out of rocks of the 

 Palm/Mir group, except a belt of the Triassic across the lower portion, 

 more or less involved in mountain uplifts), its antiquity must be very great. 

 The culmination was also probably in the later Tertiary era. like that of 

 the Mississippi, and the channels on the California coast, for then' are sub- 

 merged Tertiary rocks off the coasts of Massachusetts and Newfoundland, at 

 elevations much higher than the beds of the old channels. 



Although the excavating forces took so many periods to form the valleys, 

 and required a high continental elevation, yet the extreme altitude of over 

 two thousand feet appears to have been of comparatively short duration, 

 for otherwise the deep chasms in which the submerged channels terminate 

 would have extended farther inland than we find them, and would have 

 been headed by more gentle slopes, in place of precipitous cliffs, over which 

 the waters of the former rivers were precipitated in great cascades. In the 

 fjords of Norway, merging into rapidly contracting valley.-, or headed by 

 great vertical walls, hundreds of feet in height, having the structure named 

 cirques, may be seen to-day the counterpart of the coast of the American 

 continent, when it- marginal plateaus stood 3,000 feet higher than at present; 

 yet Noi-wav stood once much higher than now, but was afterwards submer- 

 :. from which depression it has only recently been re-elevated bo that its 

 plateaus, close upon the sea, rise to three or four thousand feet, and its 

 mountain- -till higher. The old hydrography is more or less distorted by 

 warpings of the earth's crust, which, however, do Dot obscure the valleys, 

 although rendering the features somewhat more complex. The amount of 

 distortion has yet to be determined. 



University of Georgia, August, 1889. 



