32 JOSEPH BARRELL 



give to it an added resistance to stress differences as great as indi- 

 cated by the experiments of Adams, then the strains imposed by 

 the deltas may be permanently borne. 



This confrontation of the conclusions drawn from various paths 

 of approach raises the problems which are treated in the second 

 part. 



MOUNTAINS BUILT BY COMPRESSION OR IGNEOUS ACTIVITY 



Mountain ranges made by folding or extravasation must be 

 independent to some degree from vertical forces, but these are not 

 suitable geologic tests of the rigidity of the crust, since it is known, 

 as noted in the introduction, that they are secondarily connected 

 with diminutions of density in the zone of isostatic compensation 

 and in many cases are rejuvenated after partial erosion by later 

 up warping. 



The individual mountains or plateau remnants left standing by 

 circumdenudation, or piled up as volcanic cones are clearly burdens 

 upon the earth. The volume which rises above the average level 

 is a measure of the stress. Gilbert has so used them and obtained 

 values ranging from. 40 to 700 cubic miles.^ These volumes, how- 

 ever, might be called minimum estimates, as may be seen upon 

 examination of tljeir nature. 



If a certain broad upwarping reduces the vertical stresses to a 

 minimum and erosion follows without further adjustment, it is the 

 volume of the valleys rather than the mountains which soon comes 

 to measure the larger possible departures from equilibrium. The 

 remaining mountains by their weight produce local downward 

 stresses, but the more regional stresses are upward and are due to 

 the breadth of the field of erosion. These regional stresses will 

 become larger ultimately than the local stresses due to the residual 

 masses. 



Volcanic cones do not continue to be built up until their base 

 begins to sink into the crust as fast as the upward growth takes 

 place. On the contrary, their growth ceases when the hydrostatic 

 pressure of the high column of lava or a decadence of pressure in 

 the reservoir below leads finally to a shifting of the vents. 



' "The Strength of the Earth's Crust," Bull. Gcol. Soc. Am. (1889), I, 25. 



