RANSOMli.] 



The Great Valley. 



395 



down approximately to a plane, and that it does not, after a certain 

 period, continue to rise higher and higher above the sea-level, as the 

 theory of elevation and subsidence by loading and lightening would 

 seem to demand."* 



In the same year Professor Le Conte, f speaking of the causes of 

 elevations and subsidences of portions of the earth's crust, presented 

 the most incisive protest against the adoption of the unmodified 

 theory of equilibrium that had yet appeared, and the passage is here 

 quoted in full. He says that "in these later times there has been 

 a tendency to regard elevation and subsidence, when unattended by 

 plication, as a simple matter of equilibrium of a floating crust. 

 According to this view, wherever abundant sedimentation is going 

 on, there the crust, weighted down by the increasing" mass, subsides 

 pari passu, and wherever erosion is exceptionally active, as in great 

 mountains and high plateaus, there the ever-lightening crust rises 

 pari passu. Thus subsidence and elevation are caused by weighting 

 and lightening. Doubtless this is a real cause which must not be 

 neglected, but it can not be the principal cause. Doubtless the 

 proposition is true, but the converse proposition is much more true, 

 viz.: that subsidence is the cause and necessary condition of sedi- 

 mentation, and elevation the cause of exceptional erosion. The 

 plateau region, for example, during the Carboniferous, Permian, 

 and whole Mesozoic times, was a region of subsidence to the extent 

 of 15,000 feet (for such is the thickness of the strata there). Since 

 that time it has been a region of elevation, and has risen, probably, 

 at least 20,000 feet. But the extreme general erosion (2. e., leaving 

 out the canon-cutting) has been only about 12,000 feet, leaving the 

 region still 8,000 feet high in its highest parts. Now, first, why did 

 the rise commence at all? and, second, how can a lightening by 

 removal of 12,000 feet cause an elevation of 20,000 feet? In fact, 

 at every step the erosion has lagged behind the elevation, as it 

 ought, if it be effect. The fundamental cause of subsidence and 

 elevation over large areas must therefore be sought elsewhere, 

 although, doubtless, weighting and lightening, by adding to the 



* Loc. cit., p. 297. 



fThe Elevation of the Sierra Nevada, Am. Jour. Sci., Vol CXXXII, pp. 

 167-181. 



