o 



00 AV. T. LEE BUILDIXG OF SOUTHERN ROCKY MOUXTAIXS 



as the top is eroded away, in the manner illustrated by ice, which rises in 

 water as the upper part is melted. Because of the tendency of the cohinin 

 of rock to behave as a floating mass, the rise is not restricted to the mere 

 expansion of the mass. A search for cause of the building of the South- 

 ern Eockies fails to reveal evidence of lateral thrust of sufficient magni- 

 tude to account for them l)v crustal shortenino-. Field observations indi- 

 cate that they were formed chiefly by vertical uplift. Considering avail- 

 able sources of information, local forces acting within the crust beneatli 

 these mountains seem competent to build them without calling on forces 

 originating at a great distance. 



The following "Xotes on isostasy,'' by C. E. Van Orstrand, of the 

 United States Geological Survey, and "The elastic yielding of the earth's 

 crust under a load of sedimentary deposits," by Walter D. Lambert, of 

 the Coast and Geodetic Surve}', were pre^^ared lor this paper and should 

 be considered parts of it. 



XoTEs ox Isostasy; BY C. E. Tax Orsteaxd 



The question as to the movement of isogeotherms can be answered hj 

 reference to the diagram, which gives the distribution of temperatures 

 in a dike and the adjacent rocks for different time intervals. The dike 

 is assumed to be of very great depth and length; the thickness is taken 

 to be 160 kilometers, or about 100 miles. The entire dike is assumed to 

 be at some constant temperature and the whole is suddenly injected be- 

 tween rocks whose temperature for convenience is assumed to be degree. 

 Points on the curves for given time intervals, say 5,000,000 years, repre- 

 sent the temperatures at that point, expressed in percentages of the 

 initial temperature of the dike. For example, if the initial temperature 

 of the dike was 1,000 degrees, the temperature at a distance of 90 kilo- 

 meters from the center of the dike was 0.30 X 1,000 = 300 degrees. 

 Twenty million years later the temperature at this point had risen to 

 about 410 degrees, as shown on the 25,000,000-year curve. The tem- 

 perature at the center of the dike had been lowered only about 60 degrees 

 in this same time interval. At the end of 100,000,000 years the tem- 

 perature at the center was about 650 degrees, a reduction of 350 degrees. 



The calculations do not apply rigorously to the elevation and subsidence 

 of a mountain range, for the reason that heat flow from the bottom of 

 the block has been omitted, and, furthermore, the block is not put in- 

 stantly in place, as is here supposed. However, with these corrections, 

 there will still l)e a very considerable lag of the isogeothermal surfaces^ 

 during elevation and subsidence. 



