Orogenic Movements. 
489 “ 
alone. When the time shall come, for any reason, that in any 
part of the earth a regional movement becomes less forceful, or 
altogether ceases, the excess of weight of the plateaus and 
mountain masses may dominate, and local subsidence, tending 
toward isostatic equilibrium, may slowly begin. From the fore¬ 
going we see that mountain-making movements are local dis¬ 
turbers of isostacy. 
It seems reasonable to suppose, as above explained, that the 
rigidity of the rocks is sufficient to explain the uplift and to 
sustain the comparatively narrow mountain masses. But the 
question arises as to the cause of the elevation of the broad 
mountain systems and extensive plateaus of the earth such as 
those of Thibet and western America, and how they are sus¬ 
tained. Some of the mountain systems are tens or scores of 
miles across. Thibet is said to have an area of several hundred 
thousands of square miles, and an average elevation of 14,000 
or 15,000 feet. The plateaus of western America above 6,000 
feet in height are extensive. The larger plateaus are traversed 
and bounded by mountain ranges or systems. Many plateaus 
are partly composed of marine rocks of comparatively late age. 
Elsewhere rocks of the same age are at low levels. If the ele¬ 
vation of the plateaus were explained by differential subsidence 
alone, it would be necessary to believe that other portions of the- 
continental areas bearing similar rocks and the bed of the sea 
had subsided to a far greater extent than the plateau districts. 
This would involve a differential subsidence amounting to sev¬ 
eral thousands of feet since Eocene time. It seems rather im¬ 
probable that the surface of the sea could have been so far from 
the center of the earth at so recent a date, and if this were not 
the case, there must have been absolute elevation of the plateaus. 
In order to have produced absolute elevation there must have 
been thickening of the outer crust by plications along the mount¬ 
ainous areas, and deep-seated flowage of material under the non- 
plicated areas from other places. 
In order that the crust shall be thickened by plications over 
an extensive area, it is necessary that the effects of the contrac¬ 
tion of the outer part of the earth shall be largely concentrated 
along certain zones. We may suppose that the plications first 
