MOUNTAINS AND PLATEAUS 365 



The cause of the great deformations, such as those recorded in the 

 Alps, the Appalachians, and other ranges, is believed to be found 

 in the distribution of heat beneath the surface. It is thought that 

 the heat of the interior " would be conducted from the deep interior 

 to the outer zone 800 to 1200 miles thick, faster than from the latter 

 outward, with the result of raising the temperature of the outer 

 zone while that of the deep interior falls. The result of this should 

 be a severe crowding of the outer zone upon itself, in shrinking to 

 fit the deep interior as it loses heat and shrinks. " (Chamberlin and 

 Salisbury.) The folding of great areas therefore results, according 

 to this theory, from the crowding of the thick outer zone on itself. 



The extrusion of lava from the deeper zones of the earth cooperates 

 with the cooling of the heated interior in causing a shrinkage. The 

 outpouring of the hundreds of thousands of square miles of lava in 

 Oregon and neighboring states, and in the Deccan peninsula of Asia, 

 undoubtedly contributed to the shrinkage of the interior, although 

 the total effect was slight. 



The Elevation of Plateaus and Mountains. — The statement is 

 often made that great mountain ranges are formed solely as a result 

 of lateral pressure and also, when a region only a few hundred feet 

 above sea level is found to be underlain by much folded and meta- 

 morphosed rocks, that "erosion has laid bare the mountains to their 

 roots and that the ancient heights may at one time have rivaled 

 the Alps in majesty." Such assumptions must, however, be accepted 

 with caution. It seems safe, at least, to assume that, if great areas 

 of the earth's surface can be raised by vertical movements to form 

 plateaus, the elevation of a folded region may be largely due to similar 

 vertical movements. This brings us to the modern theory of isostasy 

 (Greek, isos, equal, and stasis, standing still). 



The Theory of Isostasy. 1 — If oil and water are balanced in a 

 U-tube, it is evident that, since water is the heavier, its surface will 

 be lower than that of the lighter oil. It is upon this principle that 

 the theory of isostasy is based. The ocean basins are believed to 

 be underlain by heavier materials than the continents and are conse- 

 quently lower, since they are drawn more strongly toward the center 



1 Hayf ord, J. F., — The Figure of the Earth and Isostasy from Measurements in the 

 United States: U. S. Coast and Geodetic Surv., 1909. 



Hayford, J. F., — The Effect of Topography and Isostatic Compensation upon the 

 Intensity of Gravity: Special Publication 10, U. S. Coast and Geodetic Surv., 191 2. 



Reid, H. F., — Isostasy and Mountain Ranges, Bull. Am. Geog. Soc, Vol. 44, 191 2. 

 p. 354 et seq. 



