Ransome.] 



The Great Valley. 



427 



ment upon which the .sediments were deposited, often elevated 

 above its original position; in other cases it has been replaced by a 

 great batholitic intrusion of plutonic rock ; but no case is recalled 

 in which the sediments themselves can be shown to have been 

 forced down to serve as a buoyant float for the superincumbent 

 mass. 



If we adopt the hypothesis of Fisher,* that the earth's crust is a 

 sharply differentiated, solid shell, floating upon a heavier interior 

 magma, and assume that the entire thickness of this crust is 

 involved in the orogenic corrugations, then the question may be 

 answered. For by the local thickening-up of this crust under 

 lateral compression, and its downward protrusion into the fluid 

 magma, the mountains might be buoyed up, and the sediments 

 themselves elevated instead of depressed. But there are many 

 objections to this view, and the modern tendency is rather to 

 regard the earth as substantially solid, with no hard and fast line 

 between crust and interior. Moreover, the hypothesis of a greatly 

 thickened solid crust directly beneath a mountain range is not 

 easily brought into accord with the frequent occurrence of plutonic 

 intrusions along the axis of the range, which seems to indicate a 

 thinning rather than a thickening of the crust.t 



Reversal of the Movements of Subsidence and Elevation. — As is 

 well known, areas receiving sediment neither subside indefinitely, 

 nor yet come to a state of final rest, but are ultimately elevated 

 above sea-level and exposed to erosion. Likewise, denuded areas 

 at length cease to rise, and then sink down, and become covered 

 by new sediments. This reversal of movement has been frequently 

 cited against the theory of isostasy, and appears still to form an 

 unanswered objection. Probably the most striking actual case of 

 such movements is that described by King J eighteen years ago in 

 western United States, where, after the accumulation of 40,000 feet 

 of Palaeozoic sediments, the relative positions of land and sea were 

 reversed by a great fault, and 25,000 feet of Mesozoic strata depos- 

 ited upon the old denuded Palaeozoic surface then changed into 



* Physics of the Earth's Crust, 2d ed., pp. 183, 184. 

 t Le Conte, Elements of Geology, 2d ed., p. 261. 

 \ Explor. 40th Parallel, Vol. I, p. 731. 



