34 JOSEPH BARRELL 



The movement of the crust could not keep pace with the climatic 

 change but it shows by means of these fossil water planes its incom- 

 petency to bear without at least partial yielding a burden as broad 

 and as heavy as the Pleistocene climates placed upon it. 



Gilbert, in 1889, was led by reflection upon the changes of load 

 imposed by the waters of extinct Lake Bonneville to use them as a 

 measure of the strength of the earth's crust to resist isostatic 

 adjustments,^ and as previously stated, tested the conclusions 

 drawn therefrom by comparisons with the volumes of mountains 

 made by extravasation, or circumdenudation, or their combination, 

 and of valleys of erosion. Of Lake Bonneville he states: 



Considering the main body of Lake Bonneville, it appears from a study of 

 the shorelines that the removal of the water was accompanied, or accompanied 

 and followed, by the uprising of the central part of the basin. The coinci- 

 dence of the phenomena may have been fortuitous, or the unloading may have 

 been the cause of the uprising. Postulating the causal relation, and assuming 

 that isostatic equilibrium, disturbed by the removal of the water, was restored 

 by viscous flow of crust matter, then it appears (from observational data) 

 that the flow was not quantitatively sufficient to satisfy the stresses created by 

 the unloading. A stress residium was left to be taken up by rigidity, and the 

 measure of this residuum is equivalent to the weight of from 400 to 600 cubic 

 miles of rock. 



From these phenomena and theoretic considerations arises the working 

 hypothesis that the measure of the strength of the crust is a prominence or a 

 concavity about 600 cubic miles in volume. 



THE EVIDENCE FROM EROSION CYCLES 



Erosion base-levels folded and upHfted tracts, leaving for a time 

 during the process mountains of circumdenudation whose local 

 stresses have previously been discussed. The development of 

 peneplains implies a rigidity of the crust sufficient to prevent 

 responsive vertical movement until after the completion of the 

 cycle of denudation. It may be difficult to determine the original 

 average elevation and the degree of progressive uplift pari passu 

 with erosion which preceded the peneplanation, but the fact that 

 broad areas become flat and are controlled until the next deforma- 

 tive movement by the level of the sea suggests that they cannot 



' Bull. Geo!. Soc. Am., I (1889), 23-27. 



