EECITAL OF CERTAIN OUTSTANDING FACTS 291' 



tend to spread the mountain at its base, raise the surface just beyond 

 the base, and at the same time force the spreading mass under the raised 

 part on the mountainward side, simulating the familiar phenomena of 

 overthrust. Such sagging may or may not be adequate to explain large 

 lateral thrusts, but I know of no folds of post-Cretaceous age in the 

 Southern Rockies which could not be as readily explained in this way as 

 by lateral thrust. 



In brief, I can see no essential difference, except size, between the 

 Southern Eockies, the Black Hills, and certain still smaller groups of 

 mountains which are subcircular in outline and which may be compared, 

 to raised blisters rather than to wrinkles on the face of the earth. 



The Southern Rockies as a quantitative Test 



If it be conceded that the Southern Rockies were formed by vertical 

 uplift rather than by lateral pressure, it remains to find adequate causes^ 

 of the uplift. The processes resulting from the existence and main- 

 tenance of isostatic equilibrium, discussed at the Amherst Meeting of 

 this Society a year ago and in recent papers by William Bowie and 

 others, considered qualitatively, seem to offer an explanation of the rise 

 of the mountains, but a quantitative test is needed. It seems to me that 

 the Southern Rockies offer as good an opportunity for making this test 

 as is likely to be found. 



Considering the great quantity of detrital material derived from the 

 Ancestral Rockies, it seems reasonable to assume that the ancient range 

 was comparable to the present mountains. It was formed at the close 

 of the Pennsylvanian epoch, let us say thirty million years ago. It was' 

 eroded through succeeding ages and became baseleveled about the middle 

 of the Cretaceous period. In the meantime, presumably at the close of 

 the Triassic period, the western landmass, which may be called the 

 N"evada continent, was raised, and sediments from it spread eastward in 

 Jurassic and Lower Cretaceous time. At the beginning of the Upper 

 Cretaceous epoch the baseleveled site of the Ancestral Rockies began to 

 settle beneath sealevel and was covered by the water of the Interior 

 Cretaceous sea. From this time to the end of the Cretaceous period the 

 Nevada continent supplied rock debris which lodged in the Cretaceous 

 basin, to thicknesses in the vicinity of the present mountains approxi- 

 mating 10,000 feet. At the end of the Cretaceous period a reversal of 

 movement set in, and from that time to the present the movement has^ 

 been haltingly upward. 



