404 



University of California. 



[Vol. i. 



geologists, notwithstanding the power and brilliancyof its supporters, 

 occupies a very unassured position, and that this uncertainty has not 

 been diminished by the latest light thrown upon the subject of isos- 

 tasy. It is, moreover, impossible to review the literature without being 

 impressed by the contrast presented between the general vigor and 

 dash of the supporters of isostasy and the apparent apathy shown 

 by the advocates of rigidity. However assured the latter may feel 

 in the strength of their position, attacks directed against it with 

 such ability, and by men of such acknowledged prestige as Dutton 

 and Gilbert, may be considered as calling for more searching criti- 

 cism than they seem to have experienced. 



It will be the purpose of the following section to see how far 

 this isostatic hypothesis is applicable to explain the facts met with 

 in the Great Valley of California, more particularly the subsidence 

 of its floor during late geological times. 



THE THEORY OF ISOSTASY AS APPLIED TO THE GREAT VALLEY. 



The description of the Great Valley given in the preceding 

 pages has shown that it is an area of closely associated sedimenta- 

 tion and subsidence. Such an area would probably be classed as 

 a typical example of isostatic subsidence due to loading, by such 

 geologists as consider the sinking of great river deltas and 

 other restricted areas of deposition as due to the same cause; and, 

 as we shall see, it was so regarded by Becker. But it has been 

 shown that the hypothesis thus appealed to can not be looked upon 

 as occupying an unassailable position at the present time, and it is 

 accordingly unwise to shut our eyes to this fact and proceed to use 

 the "doctrine of isostasy " as if it were a generally accepted theory 

 in geological science. However beneficial the formulation of hy- 

 pothesis may be as an exercise of the scientific imagination, it is a 

 process not profitably resorted to when a given set of phenomena 

 can be accounted for by the application of known laws. 



With these thoughts in mind, it appears that the question of 

 the fitness of applying the hypothesis of isostasy to the Great 

 Valley may be approached by two distinct steps, which may be 

 conveniently indicated by the two questions — (i) Is it necessary? 

 and (2) Are there any facts which invalidate it? Having answered 



