THE STRENGTH OF THE EARTH'S CRUST 35 



lie after erosion in close isostatic equilibrium; that whatever stress 

 this implies can be carried by the earth for long periods of time. 



The ancient peneplains are now broadly warped and uplifted. 

 The rivers, as a rule, are intrenched in youthful valleys; or their 

 seaward courses are drowned and not yet reclaimed by delta build- 

 ing. These features testify to the recency of world-wide crustal 

 unrest, marked chiefly by movements of a vertical nature; move- 

 ments which presumably diminished the vertical stresses in the 

 outer portions of the earth and has produced at the present time, as 

 Wilhs has argued, a higher degree of isostatic compensation than 

 has been customary through the long periods of quiet which sepa- 

 rate the epochs of movement. 



There are difficulties, however, in using ancient base-leveled 

 surfaces now upwarped as measures of the previous stress. It is 

 known that a region like the Colorado plateaus which now stand 

 markedly high tended to lie near sea-level from the beginning of 

 the Paleozoic to the end of the Mesozoic. Presumably a decrease 

 of density within the zone of isostatic compensation has taken place 

 here during the Cenozoic and the uplift has accompanied or 

 followed the internal change. 



Furthermore, if there are stages in the uplift, a considerable 

 volume of rock is removed during each stage, so that at no one time 

 has the average elevation of the region been as high as the residual 

 masses might be thought to imply. Allowing for these qualifi- 

 cations, however, there seems no doubt that the study of erosion 

 cycles will throw light upon the limits of stress due to unloading 

 which the crust can resist, and also upon progressive changes in 

 subcrustal densities through geologic times. This evidence of 

 considerable crusted rigidity, shown by freedom from compen- 

 sating movements during a cycle of erosion, or by warpings not in 

 sympathy with isostatic stresses during cycles of crust movements, 

 has been pointed out before. Hayford has sought to explain it 

 away by invoking, first, the slight crustal cooHng which would 

 occur in regions of erosion because of removal of the upper rock, 

 heating in regions of deposition. Second, he assumes as probable 

 the existence of a high coefficient of compressibility sufficient to 

 make eroded regions rise in appreciable ratio to the thickness of 



