Ransome.] 



The Great Valley. 



421 



the "great earthquakes" of history are said to have originated in 

 such alluvial plains and deltas. 



But even if it were a true generalization, it is not at all clear 

 why earthquakes should accompany a subsidence taking place 

 because of loading, any more than that they should be associated 

 with a subsidence, under a load it is true, but due to some other- 

 cause. 



Tivo Classes of Earth Movements. — Both Dutton and McGee, 

 perceiving that the greater vertical, or epeirogenic, oscillations of 

 the earth's crust do not admit of explanation through superficial 

 loading or unloading, were led to distinguish between the two 

 classes of movements. According to McGee, the greater movements 

 of debatable cause are "antecedent movements," while the more 

 restricted oscillations due to loading and unloading are "consequent 

 movements." McGee would limit the term isostatic to these conse- 

 quent movements. But to this limitation the physicist can take 

 immediate exception. For, taking the simple definition of isostasy 

 as stated by Dutton, it is obviously impossible that the earth's crust 

 should be in such delicate equilibrium as to respond to superficial 

 transfer of sedimentary material, and yet over larger areas be sub- 

 ject to vertical movements many thousand feet in extent, which are 

 not isostatic. In other words, if the greater movements by which 

 continents are upheaved or depressed are not in the nature of true 

 isostatic adjustment, then they constitute a defiance of isostasy, so 

 great and so glaring that it becomes absurd to consider the possi- 

 bility of sedimentary deposits having any effect upon the solid 

 crust beneath them. If, then, we can speak of isostasy at all as 

 having any bearing upon geological science, then we must look 

 upon these great epeirogenic movements as isostatic, and as they 

 are obviously not caused by loading and unloading, then we are 

 driven to the final conclusion that changes calling for isostatic 

 adjustment of the outer crust of the lithosphere must at times take 

 place within the latter, quite independently of the superficial results 

 of erosion and deposition. The attempt to make two distinct 

 classes of the vertical movements of the earth's crust is an unphilo- 

 sophical one, and it is at least misleading to call the more restricted 

 movements isostatic to the exclusion of the greater. It is these 



