SUMMARY OF CONCLUSIONS 315 



elides, most of which obviously are every-day knowledge in physics and 

 geology. To summarize: Combination of (1) the greater competence of 

 rock, down to a certain depth, in the horizontal direction than in the 

 vertical direction, (2) the spherical form of the earth assuring the con- 

 tinuity of the competent part of the outer shell and the confined status 

 of the more freely yielding subjacent material, and (3) the weakening 

 and strengthening of adjacent elements of the outer shell by readjust- 

 ments brought about by erosion and deposition under effective isostatic 

 control, may permit tangential elastic stresses to be generated, sustained, 

 and transmitted to weak regions, so as to build mountains under effective 

 isostatic control, so long as any effective compacting of the interior of the 

 earth continues from any cause. Still, just as formerly, it is to a shrink- 

 ing interior of the earth that geology looks for the prime mover by which 

 its grander dynamical phenomena are brought about. 



The moral of this discussion is that it appears possible to bring con- 

 ceptions of the extreme isostasists and the most conservative students of 

 earth dynamics into much closer harmony without doing violence to the 

 fundamental views of either group. Nevertheless it is clear that certain 

 of the dynamical concepts appealed to in geology are in need of fresh and 

 critical scrutiny in the light of physics and the properties of materials. 



Eeferences 



1. The strength of the earth's crust. Journal of Geology, volume XXII, num- 



bers 1-8, 1914; Journal of Geology, volume XXIII, numbers 1, 5, and 6, 

 1915. 



2. The structural failure of the lithosphere. Science, new series, volume 



LIII, March 4, 1921, page 1366. 



3. Discoidal structure of the lithosphere. Bulletin of the Geological Society 



of America, volume 31, 1920, pages 247-302. 



4. On a possible causal mechanism for heave-fault slipping in the California 



Coast Range region. Bulletin of the Seismological Society of America, 

 volume 4, December, 1915. 



5. The effect of topography and isostatic compensation upon the intensity of 



gravity. United States Coast and Geodetic Survey Special Publication 

 number 10, 1912. 



6. Investigations of gravity and isostasy. United States Coast and Geodetic 



Survey Special Publication number 40, 1917. Our present knowledge of 

 isostasy from geodetic evidence. Journal of Geology, volume 25, 1917, 

 page 422. 



7. Investigations of isostasy in Himalayan and neighboring regions. Trigo- 



nometrical Survey of India, Professional Paper number 17, 1918. A 

 brief review of the evidence on which the theory of isostasy has been 

 based. Geographical Journal (London), volume 56, 1920, page 47. 



