THE STRENGTH OF THE EARTH'S CRUST 291 



VARIABLE OR CONSTANT DEPTH OF COMPENSATION 



The Cordilleran region awoke to an era of great orogenic and 

 igneous activity near the beginning of the Tertiary, and, especially 

 in the Neocene, has become broadly elevated into one of the great 

 plateau regions of the world. Large areas like the Colorado 

 plateaus, which since the beginning of the Paleozoic had rested near 

 sea-level, at times beneath and again slightly above, have been lifted 

 many thousands of feet. Block-faulted structures indicate the 

 dominance of vertical forces rather than surficial compression as 

 the cause of these movements. The uplift has not been of the 

 nature of a broad even upwarp, and adjacent regions show great 

 contrasts in elevation. These diflferent surface results of the 

 interior forces suggest differences in elevatory forces at compara- 

 tively shallow depths. The region is known to be in a fair degree 

 of isostatic equilibrium notwithstanding the high relief. Davis has 

 shown why these movements cannot be regarded as differential 

 sinkings toward the center of the earth.^ These features suggest, 

 then, subcrustal decreases in density during the Tertiary as a cause 

 of the broad movements of elevation. 



The rising of great bodies of magma to high levels in the zone of 

 isostatic compensation, their irregular distribution, the great 

 quantities of heat and gases which would invade the ^oofs are 

 suggested by the observed evidences of regional igneous activity at 

 the surface as the probable causes of the changes in density and 

 regional vertical movements. A consequence of such a cause 

 would be a lessened strength of the crust to resist strain, a lessened 

 depth to the zone of isostatic compensation, and a decreased size of 

 the unit areas departing from equilibrium. 



The history of the Cenozoic in the Cordillera has repeated the 

 history of other regions at other times, either in the Archean 

 igneous activity or later. The slow conduction of this excess heat 

 from the outer crust, the solidification of the reservoirs of magma, 

 would, in the course of ages, bring about a new rigidity. Upon 

 disturbances of the equilibrium by erosion or compressive forces 

 there would be found a new and greater depth to the zone of 

 compensation. 



^"Bearing of Physiography upon Suess's Theories Abstract," Intern. Geog. 

 Cong., 8th Report, 1905, p. 164; Amer. Jour. Science (4), XIX (1905), 265-73. 



