242 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



are fairly safe in assuming that the change in temperature for the 

 first mile of the earth's crust is about 50° C. If in any particular 

 area we have sediments laid down to a depth of 6 miles, and this is 

 not beyond the probable depth, and if the temperature gradient is 

 the same for the whole depth of the block, then there would be 

 a change in the temperature of the material of approximately 300° 

 C. It is reasonably certain that as the block is lowered, as sediments 

 are placed on it, the material will not increase its temperature as 

 rapidly as it subsides. There undoubtedly will be a very great lag 

 in the temperature and it will be a long time after the deposition 

 of sediments has ceased that the temperature of the column will 

 become normal — that is, that the geoisotherms will rise within the 

 lowered block to their normal depths below sea level. As a result 

 of the increase in the temperature there will be a thermal expansion 



- V 



J$ *»* t0 lower end ° f ° ,ock t0 •"*** b a/aoc 



Fig. 7. — Displacement of geoisotherms due to isostatic adjustment. 



As sediments are placed on a block of the earth's crust the block is depressed and 

 each of its elements goes to a zone which is hotter than the one it originally occupied. 

 As erosion takes place the block of the crust under the erosion area is lightened, thus 

 causing material to enter the lower end of the block. Each element of the block will be 

 forced up into a zone that is colder than the one that it previously occupied. The geo- 

 isotherms are depressed in the block under the sedimentary area and are elevated in the 

 block under the erosion area. 



of the block which may amount to as much as 3,000 feet. The 

 surface of the block will be raised approximately that amount, but 

 this is an elevation far less than we find existing in most mountain 

 systems and, therefore, we have to find some additional process to 

 cause the uplift. It can not be proved, of course, that the other 

 cause is a physical or chemical action taking place in the column as 

 a result of the increase in temperature, but this seems to be the 

 only thing left which could assist the thermal expansion in decreas- 

 ing the densities of the block and extending its length from about 

 (50 to 62 miles. An expansion of about 3 per cent is necessary to 

 raise a sedimentary area to an elevation of 2 miles. 



After a mountain area has been base-leveled, that is, reduced in 

 elevation to such a point that erosion ceases, a downwarping is the 



