398 University of California. [Vol. i. 



isostasy does not seem to meet the requirements of geological con- 

 tinuity, for it tends rapidly towards stable equilibrium, and the crust 

 ought therefore to reach a state of repose early in geologic time. 

 But there is no evidence that such a state has been attained, and 

 but little if any evidence of diminished activity in crustal move- 

 ments during recent geologic time. Hence we infer that isostasy 

 is competent only on the supposition that it is kept in action by 

 some other cause tending constantly to disturb the equilibrium 

 which would otherwise result. Such a cause is found in secular 

 contraction, and it is not improbable that these two seemingly 

 divergent theories are really supplementary." * 



It is evident that he here has reference to that aspect of the 

 problem which has just been called the geologic side. It is not 

 quite clear, without further statement, why isostasy should rapidly 

 tend to stable equilibrium, unless we assume that there is a limit to 

 the progressive elevation of a region which is being worn down by 

 erosion. The idea of such a limit is not in itself opposed to the 

 theory of isostasy, as Reade and others have thought; for it is 

 quite conceivable that such an elevation would result in bringing 

 deeper and denser portions of the crust closer to the surface, 

 whereby the tendency to rise would finally be accurately counter- 

 balanced. Granting some such explanation as this, it is then evident 

 that the land would all be permanently reduced to sea-level, as the 

 theory of isostasy is inadequate to account for any new rise of land 

 when all has been degraded to that plane. 



Gilbert, f also in 1889, from a study of the up-arching of the cen- 

 tral part of the desiccated Bonneville basin, reaches the conclusion 

 that 600 cubic miles' concavity or protuberance may be taken as 

 the measure of the crust's rigidity. He formulates, as a working 

 hypothesis, the following: "Mountains, mountain ranges, and val- 

 leys of magnitude equivalent to mountains, exist generally in virtue 

 of the rigidity of the earth's crust; continents, continental plateaux, 

 and oceanic basins exist in virtue of isostatic equilibrium in a crust 

 heterogeneous as to density." 



* Loc. cit., p. 352. 



t Strength of the Earth's Crust, Bull. Geol. Soc. Am., Vol. I, p. 23-27; 

 also Monog. I, D. S. Geol. Survey, p. 389 (1890). 



