MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 581 



miles, a little over a mile on each side, a trivial amount compared with 

 the foldings on the adjacent continental borders. 



The influence of the domed form of the surface. — Because of the 

 spheroidal form of the earth, each portion of the crust is ideally an arch 

 or dome. When broad areas like the continents are considered, it is 

 the dome rather than the arch that is involved, and in this the thrust 

 is ideally toward all parts of the periphery. It is probably for this 

 reason that mountain ranges so often follow curved or angulated lines, 

 or outline rude triangles or polygons. The sigmoidal courses of the 

 ranges of southern Europe, the looped chains of the eastern border 

 of Asia, and the curved ranges of the Antillean region, are notable 

 examples. The border ranges of the Americas, of the Thibetan plateau, 

 and of other great segments, illustrate the polygonal tendency. The 

 general distribution of the great ranges is such that a nearly equal 

 portion of crustal crumpling is thrown across each great circle, as theory 

 demands. The common generalization that mountain ranges run 

 chiefly in oblique directions, as northeast-southwest, northwest-south- 

 east, is but a partial view of the more general fact that the hues of 

 distortion must lie in all directions to accommodate the old crust to 

 the new geoid, if there be equable contraction in all parts. 



Theoretical strength of domes of earth-dimensions. — As the domed 

 form of the crust has played an important part in theories of deforma- 

 tion, it is important to form quantitative conceptions of the strength 

 of ideal domes having the figure and dimensions of segments of the 

 earth's crust. According to Hoskins,^ a dome corresponding perfectly 

 to the sphericity of the earth, formed of firm crystalline rock of the 

 high crushing strength of 25,000 pounds to the square inch, and having 

 a weight of 180 pounds to the cubic foot, would, if unsupported below, 

 sustain only g-^^ of its own weight.^ This result is essentially independ- 

 ent of the extent of the dome, and also of its thickness, provided the 

 former is continental and the latter does not exceed a small fraction 

 of the earth's radius. If this ideal case be modified by supposing the 

 central part of the spherical dome to rise above the average surface, 



^ Computations made at the request of the authors. See also Fisher, Physics of the 

 Earth's Crust, p. 36. 



2 Of Uke import is the statement of Woodward — "If the crust of the earth were self- 

 supporting, its crushing strength would have to be about thirty times that of the best 

 cast steel, or five hundred to one thousand times that of granite." Mathematical 

 Theories of the Earth, Proc. Am. Assoc, for Adv. Sci., 1889, p. 49. 



