292 W. T. LEE BUILDIXG OF SOUTHERX EOCKY MOUXTAIXS 



EXPLAXATIOX OF THE MOVEMENT 



Thus far the geologist ma}' feel secure in offering the final word; but 

 for explanation of the movements he must seek advice. From the 

 numerous unsolved problems, I venture to select three and inquire if 

 geophysicists can offer solutions. 



First. Was the time — let us say 20,000,000 years — between the close 

 of the Pennsylvanian epoch and the opening of the Upper Cretaceous 

 epoch, during which the Ancestral Eockies were reduced to baselevel, 

 sufficient for the establishment of perfect adjustment of physical con- 

 ditions ? Had the movement of isogeotherms ceased ? Had such re- 

 cr3'stallization or metamorphism as was appropriate for the changed tem- 

 perature and pressure been accomplished ? Had perfect balance between 

 those forces which make for rise and fall of land surface been attained? 



Second. Observation establishes the fact that downward movement 

 ceased at the close of the Cretaceous period and the deeply buried rocks 

 of the mountains began to rise. Are changes in temperature, mineral 

 constitution, specific gravity, etcetera, within the rocks of the mountain 

 region competent to halt and reverse such a movement, or must some 

 external force be called in? 



Third. Observations establish the fact that the rise of the Southern 

 Eocky Mountains inaugurated at the close of the Cretaceous period 

 continued until not only was the old surface, which had been depressed 

 10,000 feet, restored to its former position, but was raised in some 

 places to more than 15,000 feet above sealevel. This rise took place 

 periodically during the time, perhaps 5,000,000 years, since the close of 

 the Cretaceous period. Are the changes in the physical character of 

 the rocks of the mountain region competent to produce a rise of 25,000 

 feet in post-Cretaceous time, let us say 5,000,000 years, or must we look 

 to some other cause for an adequate explanation? 



The rise of the Eocky Mountains, because of superficial transfer of 

 .material or lateral thrust of the crust of the earth, seems to be barred 

 from consideration by the observed structure. Their rise because of 

 injection of large masses of matter or transfer of deep-seated material 

 from a distance seems barred by the isostatic equilibrium of the crust 

 l)eneath the mountains, for pendulum observations show no excess of 

 mass. Highlands and lowlands alike are found to be in essential iso- 

 static balance — a condition which could scarcely exist if the mountains 

 had been forced up by injection of material. In other words, the high- 

 lands are highlands because the material of the mountain block is less 

 'dense than the material of neighboring blocks. The mountains rise 



