Orogenic Movements . 
487 
general principle of gravitative adjustment already explained, 
for an equal or greater mass at the equatorial areas subsides a 
corresponding amount. 
Another factor disturbing the clear-cut effects of differential 
subsidence is the lateral attraction between the land and sea. 
Where adjacent to the sea the land is high, the water is raised 
above the normal level. The same effect would be produced to 
some extent by a continental ice sheet. Woodward 1 calculates 
that if there were a North American ice sheet 10,000 feet thick 
sloping to the sea, the water would be raised several hundred 
feet as a result of lateral attraction. 
Other important factors affecting the submergence and emer. 
gence of the land areas are orogenic movements and vulcanism. 
As shown later, more or less extensive elevations and depressions 
may result from these processes, and the sea shore thereby be 
greatly shifted. 
Finally, it has been seen that epigene transfer of material 
may also produce a wide shifting of the beach lines. 
But however complicated are the causes of movements and the 
resultant movements, the center of gravity of the entire mass 
moved is lower than before the movements. When the land 
areas subside with relative rapidity, they fall below the sea; 
when the sea beds sink more rapidly the water follows, leaving 
the land areas at an apparently higher level. Thus we have 
great vertical movements 2 of the surface of the earth, the effect 
of which is now to place large parts of the continents above the 
water, now below the water, the greater phenomena being the 
result of differential subsidence. Furthermore, there is every 
reason to believe that these differential vertical movements 
affect the continents all the time, and there is no reason to be¬ 
lieve that any extensive area is ever quiescent. However, the 
movements are exceedingly slow. 
Orogenic Movements . —Accompanying the vertical continen¬ 
tal movements of the first order of magnitude, there are im- 
1 On the form and position of the sea level, by R. S. Woodward: Bull. 
U. S. Geol. Surv., No. 48, 1888, p. 70. 
2 The extension of uniformitarianism to deformation, by W. J. McGee: 
Bull. Geol. Soc. Am., Vol. 6, 1894, pp. 55-70. 
