166 QUICKSILVER DEPOSITS OF THE TACIFIC SLOPE. 



liottoni of which would ha\e been in general a sediniented area. Tn certain 

 localitie.s one may .suppose that oceanic currents might have cut through 

 thi.s stratum of sediment and eroded the underlying primeval rocks to some 

 extent, but it is certain that action of this description would soon find a 

 limit, and that thereafter no sensible mechanical action would be exerted 

 on the primeval rocks. Consequently, the quantity of sediment could 

 never increase perceptibly beyond a certain fixed and ver}- moderate limit. 



Effect of upheavr.;s. — If uplieaval Averc now sujoposed to be introduced into 

 terrestrial economy, portions of the universal sedimented area would be 

 raised into continents and would undergo erosion. The stratum of sedi- 

 ment having been removed, the primeval rock would again be exposed and 

 its degradation would increase the total amount of sedimentary material. 



If upheaval were confined to certain areas and were a continuous 

 process, while corresponding subsidence took place in other and distinct 

 areas, primeval rocks Avould continue to be exposed in continental regions, 

 at least for a very long tiuie. If upheaval and subsidence were to alter- 

 nate in the same areas, but if in certain regions the upheavals were on the 

 whole somewhat in excess of the subsidences, primeval rocks Avould appear 

 at the surface of these areas from time to time and the total quantity of 

 sediment on the globe would at these times receive accessions. 



Upheavals and subsidences could alternate and balance one another 

 on each portion of the globe only if the influences tending to produce these 

 movements were everywhere exactly balanced. The mere fact that the 

 poles receive less heat than the equatorjal regions establishes a difference 

 of phj'sical conditions on various portions of the earth, which certainly in- 

 fluences erosion and cannot but affect changes of level. However complex 

 and remote the connection may be between upheaval and evaporation, some 

 I'elation certainly subsists between them, and it is not possible that on a 

 globe like ours there should not be a tendency to a greater prevalence of 

 uplifts in some regions than in others 



Bearing of Dana's continental theory — It is clear tliat, if Professor Daua's theory 

 of the permanence of continental areas is correct, it substantiates the con- 

 clusion drawn above, that there are areas in which the tendency to upheaval 

 on the whole exceeds the tendency to subsidence. There is much evidence 



