286 AV. T. LEE BUILDING OF SOUTHERN EOCKY MOUNTAINS 



question is raised, if changes in temperature, density, mineral constitu- 

 tion, etcetera, can explain such elevations and depressions as are known 

 to have occurred. The building of the Southern Rockies might be dis- 

 cussed from the standpoint of probability and possibility, but when re- 

 quested to speak on this subject it seemed to me that the only way in 

 which I could contribute to the much-discussed question of mountain- 

 building would be to outline a definite problem and to seek for such 

 quantitative data as might help in solving it. 



EeCITAL of CERTAIN OUTSTANDING FaCTS 



In this brief jDaper minor details must be omitted, as must also many 

 references to publications on which statements are based. Some of the 

 large outstanding facts which I shall make use of, and which have not 

 been seriously challenged, are as follows : In late Carboniferous time 

 mountains which have been called the Ancestral Rockies, comparable in 

 size to the present Rocky Mountains, occupied the site of the Southern 

 Rockies and sediments were shed from them in all directions. East of 

 these mountains the sediments accumulated chiefly as upland and 

 brackish water deposits to form the gypsiferous red beds of the West. 

 AVest of the mountains the material found its way toward the Pacific 

 through shallow seas in Permian and Lower Triassic time and accumu- 

 lated as lowland deposits in late Triassic time. Through the succeeding 

 ages the Ancestral Rockies were eroded to such an extent that at the 



LEGEND FOR FIGURE I 



A, curvature of the earth's surface along the 39th parallel from the high Sierra in 

 California eastward for 1.000 miles, showing the relative position of the Great Basin, the 

 high plateaus, the Southern Rocky Mountains, and the Great Plains. At this latitude 

 the earth's radius is 3,968.6 miles. The chord of the 1,000-mile arc is about 997 miles 

 and the maximum distance between chord and arc is nearly 34 miles. The part of the 

 arc between Wasatch Mountains and central Kansas, about 700 miles, represents the 

 Interior Cretaceous Basin. The maximum distance between this arc and its chord is 

 about 12 miles. The sedimentary rocks which filled this "basin" are less than 2 miles 

 thick ; therefore they form an onion-skin shell laid down on a surface which was always 

 convex, although slightly less so than the normal convexity of the earth's surface. 



The illustrative profiles B-G, representing the same arc in successive ages, are drawn 

 for convenience with straight line rather than arc representing sealevel. and with exag- 

 gerated vertical scale. B represents early Triassic time, when the Ancestral Rockies were 

 high and the sea occupied areas west of them ; C = late Triassic time, when the moun- 

 tains were eroded, the sea filled, and products of erosion accumulating above sealevel to 

 form the non-marine Triassic ; D = early Jurassic time, after the rise of the Nevada 

 continent and the reduction of the Ancestral Rockies to a peneplained condition ; E = 

 the beginning of the Upper Cretaceous epoch, when the Ancestral Rockies were base- 

 leveled ; F = the close of the Cretaceous period, after the Cretaceous Basin Avas filled 

 with sedimentary rock ; G ^ the present time, showing the Great Basin lower than the 

 plateaus whose material was derived from it and the still higher Southern Rockies 

 standing in the middle of the area formerly covered by the Cretaceous Sea. 



