302 AV. T. LEE BUILDIXG OF SOUTHERN ROCKY MOUXTAIXS 



It is obvious that without surface changes capable of initiating move- 

 ment, such as the changes in load by erosion and deposition, internal 

 forces tending toward deformation of the earth's surface must ultimately 

 reach equilibrium. Such a condition of affairs could be reached only 

 when erosion and deposition are absent, as on the moon. The height to 

 which mountains can rise on a globe like the moon, on which there is no 

 atmosphere, is. dependent on initial temperature, force of gravity, and 

 plastic properties of the materials constituting the isostatic shell : they 

 should, therefore, be capable of determination b}' the methods of mathe- 

 matical analj^sis. 



Prof. R. S. Woodward^ has solved a problem which gives a measure 

 of the effect of change in surface temperature on the earth. He assumes 

 that the mean annual temperature over the Bonneville Basin was raised 

 10 degrees Fahrenheit. The change in elevation of the l3asin due to the 

 change of only 10 degrees Fahrenheit in the mean annual temperature 

 for different time intervals is as follows : 



Time interval. Changes in elevation. 



Years Feet 



10.000 0.41 



100.000 1.28 



1,000.000 4.06 



50,000,000 28.7 



100,000,000 41. 



These figures show that even moderate fluctuations in temperature on 

 the surface of the earth, covering long intervals of time, produce forces 

 which tend to check subsidence and initiate elevation, or, conversely, they 

 may check elevation and initiate subsidence. 



A rise of 25,000 feet in 5,000,000 years, as shown by Lee for the Rocky 

 Mountains, is equivalent to a uniform rise of 0.005 foot per year, or an 

 extension, 



f = 1,578 X 10-^S 



in a column of rock 60 miles in height. That is, 



GO X 5280 X 1578 X 10-^^ = 0.005 foot, 



or a little less than 1/16 of an inch in one year. 



The rocks in the crust of the earth are always in a state of strain; 

 consequently we are not concerned with initial strain, but a subsequent 

 strain or flow in the rocks due to carrying loads for long intervals of 



* R. S. Woodward : On tbe elevation of the Bonneville Basin by expansion due to 

 change of climate. U. S. Geol. Survey. Mon. I (Lake Bonneville, Gilbert). 1890, pp. 

 425-6. 



