492 
Van Eise—Earth Movements. 
result of the erosion of the land, there future mountain ranges 
may be born. 
We may anticipate that in the border of the Gulf of Mexico, 
where the sediments of the Mississippi have accumulated to a 
great thickness, and are still accumulating, at some future 
time a mountain system may exist. That Hall’s law is true is 
shown by the vast thicknesses of sediments which are found in 
the Appalachians, in the Sierra Nevadas, in the Alps, in the 
Himalayas, and in all other great mountain systems of the earth. 
As shown by Willis, 1 an important cause for the location of 
mountain ranges in areas of great sedimentation appears to be 
the initial dips of the strata of the geosynclines of deposition. 
Further, under the stresses at work during deposition the solid 
rocks below the sediments are flexed. When later, great hori¬ 
zontal thrust comes, as a result of contraction, it finds the 
strata in a loaded area already somewhat bent. When a rigid 
mass is once slightly bent at any place it bends farther 
much more easily at that place than elsewhere. Furthermore, 
newly-formed unconsolidated sediments form an outer zone of 
relative weakness. 2 Hence it follows for any given period that 
a large part of the effects of the outer contraction of the earth 
is concentrated along the comparatively narrow zones of moun¬ 
tain-making. 3 
1 The mechanics of Appalachian structure, by Bailey Willis: Thirteenth 
Ann. Kept. U. S. G. S., Part II, 1893, pp. 249-250. 
8 Le Conte, in supposing that the rise of mountains in areas of great ac¬ 
cumulation is due to the weakness and softness of the strata caused by the 
rise of the isogeotherms, altogether overlooks the fact that in regions of lit¬ 
tle or no sedimentation the rocks at corresponding depths are equally and 
probably more highly heated. Otherwise it would have to be supposed 
that complete isothermal equilibrium had been restored in the areas of sedi¬ 
mentation before mountain-making began. (Loc. cit., p. 557.) 
3 As suggestedjby Babbage and Herschel, and advocated by Reade, the 
localization of mountains may possibly also partly be explained by the 
rise of the isogeotherms. (Babbage’s ninth Bridgewater treatise, with 
appendix by Sir John Herschel, 1837, pp. 187-197, 214-217. The origin of 
mountain ranges, by T. Mellard Reade, 1886, pp. 107-116. Manual of 
geology, by J. D. Dana, 4th ed., 1895, pp. 258, 381-383.) It has been held by 
Reade that if this expansion be concentrated it would be sufficient to ex¬ 
plain the rise of mountains. Davison has pointed out (Expansion theory 
