EXPLAXATIOX OF THE MOVEMEXT 297- 



material )3eneatli it has two important effects : First, the isogeothermal 

 planes are raised farther than they would be by the ex2:>ansion alone of 

 the material of the ])lock under increasing temperature, for hot rock is 

 raised bodily into a position where its temperature is above normal and 

 where its density tends to increase because of lowering temperature; 

 second, the material added to the column from below to take the place of 

 the material of less density of the rising block may 1)e dense and there- ' 

 fore tend to check the rise. It will be pointed out (page 298) that change 

 in density may reverse crustal movement and cause a rise of the surface 

 about 1,200 feet under the conditions postulated. The tendency of the 

 earth column to "tloaf when its density is decreased makes possible a 

 much greater rise as the toj) is eroded, and Lambert shows in the accom- 

 |)anying paj^er that the removal of 10,000 feet of sediments will allow of 

 209 to 376 feet more. Probably many times that amount has been eroded, 

 and it is probably safe to place the rise of the surface due to relief of 

 pressure and lessening density as about 2,000 feet. 



From the ajopended computation by Yan Orstrand it appears that the 

 effect caused by changes of temperature due to elevation or depression of 

 parts of the earth's crust lags so far behind these changes that fixity in 

 j)osition of isogeotherms is not to be expected at any time in a region of 

 repeated crustal oscillation, such as that of the Southern Rocky Moun- 

 tain Province. As the rocks of the subsiding Cretaceous Basin, once at 

 the surface and therefore cool and correspondingly dense, were buried 

 their temperature increased, their density decreased, and in time the 

 downward movement was halted; but, on account of slow transfer of 

 heat, this halting occurred only after a subsidence of about 10,000 feet. 

 Thereafter the increasing heat would tend to still further decrease the 

 density of the material in the earth block and cause the surface to rise, 

 as shown by Bowie. ° 



The effect on volume due to change in temperature may be computed. 

 Accepting Bowie's figure^ for cubical expansion of rock as 0.000038 for 

 1 degree centigrade, and the principle that the increased volume is ex- 

 pressed in linear expansion because the rocks can expand only in an 

 upward direction, and accepting further an increment of temperature of 

 1 degree centigrade for 100 feet, by a depression of 10,000 feet the earth 

 block is lowered to a position where it is 100 degrees centigrade cooler 

 than is normal. The return of the temperature to normal produces an 

 expansion of about 1,200 feet in the 60-mile block — that is, 0.000038 X 



5 William Bowie : The earth's crust and isostasy. Cieog. Review, vol. xii. 1922. p. 618. 



6 William Bowie : Theory of isostasy. Bull. Geol. Soc. Am., vol. 33. 1022. p. 285. 



