474 
Van Else—Earth Movements. 
nearly sufficient and greatly deficient amounts of material. As 
just seen where as a result of erosion an area becomes deficient 
in material, upward movement will not begin until the deficiency 
is so great that this and the other forces combined, sur¬ 
pass the elastic limit of the rocks. The movement, once set up, 
would continue even if the deforming forces were less than at 
first, for when rigidity is once overcome less force is required 
to continue deformation. Relative uplift might continue faster 
than denudation, until the differential stresses became too small 
to produce further effect. Denudation would then continue its 
work until the area was so deficient in gravity that the differ¬ 
ential stresses would again overcome the rigidity of the rocks, 
and a second uplift would occur. This is offered merely as a 
partial explanation for some local uplifts; for general uplifts, 
as explained (pp. 483-4), probably result from differential sub¬ 
sidence of sea beds and of continental areas or from changing 
rotation. 
At all times and places no sooner is the land above the water 
than the epigene forces begin their work of denudation. At 
various times in many regions a land area remained so long 
above the ocean that the epigene forces were able to hew it 
nearly or quite to the surface of the water before the vertical 
movement came which relatively raised the area or lowered 
it, and in the latter case caused the sea to override the land. 
If there are no differential movements of subsidence which re¬ 
sult in relative elevation of the land, the epigene forces always 
conquer. The part of the continent above the sea is slowly, al¬ 
though surely, cut away. To illustrate, we may direct our at¬ 
tention to North America. It is certain that before the rocks 
were deposited which bear evidence of the earliest life, three 
times were the land masses overridden by the sea and sub¬ 
sequently emerged from it. Since the time of the earliest 
life at least three times more have the seas overridden the 
North American continent, and three times have the continen¬ 
tal masses again emerged from the water. Also there have been 
a number of other every great differential movements which have 
resulted in the submergences and emergences of considerable 
