Ransomk.] 



The Great Valley. 



407 



tion for every isolated case of the subsidence or elevation of a 

 restricted area of the earth's surface. We know that such move- 

 ments take place, on a large and small scale, through the operation 

 of a hidden mechanism to which we do not possess the key, and 

 which, as far as we can see, is unaffected in its workings by super- 

 ficial loading or unloading. Having seen that the subsidence of 

 the Great Valley is not necessarily delicately adjusted to the 

 amount of sediment available for deposit upon its floor, this is in 

 itself enough to show the unnecessary character of the isostatic 

 hypothesis, while the sinking itself can be looked upon as merely 

 an example of one of those mysterious movements just referred to, 

 which can not be comprehensively explained through the shift- 

 ing of material on the earth's surface. 



But the Great Valley does not seem to demand an immediate 

 retreat to this refuge. There appears to be a possibility in this 

 case of pushing the inquiry yet a step farther back, and arriving at 

 at least a proximate cause for the movement which its floor has 

 undergone. The forces which initiated the elevation of the Coast 

 Ranges at the close of the Miocene were antecedent in their activity 

 to the formation of the valley, and were consequently independent 

 of any loading and subsidence of its floor. The movements then 

 initiated have continued to the present time, and are, broadly speak- 

 ing, orogenic in character. What effect can such orogenic move- 

 ments have had upon the bottom of the adjacent valley? Accord- 

 ing to the ideas of mountain formation held by nearly all geol- 

 ogists, the effective force by which these ranges were folded and 

 elevated, whatever its source may have been, was a lateral pressure 

 acting along lines approximately perpendicular to the trend of the 

 Sierra Nevada. This statement will apply to Reade's theory as 

 well as to the ordinary contractional hypothesis. During this pro- 

 cess of elevation and compression, the floor of the Great Valley 

 must have acted as a strut, against which the thrust was exerted, 

 and which perhaps transmitted it in part to the Sierra Nevada 

 mass. We know that a little later, during Pliocene times, the 

 Sierra Nevada also began to be elevated, a movement which 

 partially culminated at the end of the Pliocene, but which seems to 

 be still in progress. It is improbable that the crustal block form- 



