498 THE ICE AGE IN NORTH AMERICA. 



with not less rigidity than that of glass or steel. In def- 

 erence to their researches, this conclusion is accepted and 

 taught in recent text-books of geology by Le Conte and A. 

 Geikie ; but in similarly recent text-books Dana and Prestwich 

 teach that the earth probably consists of a comparatively thin 

 crust underlaid by a molten interior, which may change within 

 a moderate depth to a great nucleal solid mass. Among other 

 geologists and physicists who have discussed the condition of 

 the earth's interior, King* and Shalerf believe it to be solid ; 

 while Whitney, X Dutton,* Powell, || Wadsworth, A Crosby, Q 

 Gilbert,J Claypole, % Airy, % Fisher,** and Jamiesou,f f believe 

 that it is molten, or, at least, is surrounded by a molten layer, 

 and that the earth's crust floats in a condition of hydrostatic 

 equilibrium upon the heavier liquid or viscous mobile interior 

 or layer enveloping the interior, subject, however, to strains 

 and resulting deformation because of the earth's contraction. 

 The thickness of the crust, according to this hypothesis, is 

 variously estimated to be from twenty to fifty miles, or possibly 

 a hundred miles or more. 



It must be confessed that we have only a very inadequate 

 knowledge of the conditions which would result from the enor- 



*" United States Geological Exploration of the Fortieth Parallel," vol. i, 

 "Systematic Geology," 1878, pp. 117,696-725. 



f " Proceedings of the Boston Society of Natural History," 1866, vol. xi, pp. 

 8-15; 1868, vol. xii, pp. 128-136 ; 1874, vol. xvii, pp. 288-292. "Memoirs of 

 the Boston Society of Natural History," 1874, vol. ii, pp. 320-340. " Scribner's 

 Magazine," vol. iii, pp. 201-226, February, 1888. 



\ "Earthquakes, Volcanoes, and Mountain Building," 1871, pp. 77-87. 



* "Penn Monthly," vol. vii, pp. 364-378, and 417-431, May and June, 1876. 

 " United States Geological Survey, Fourth Annual Report," pp. 183-198 ; "Sixth 

 Annual Report," pp. 195-198. 



|| "Science," vol. iii, pp. 480-482, April 18, 1884. 



A " American Naturalist," vol. xviii, June, July, and August, 1884. 



"Proceedings of the Boston Society of Natural History," 1883, vol. xxii, 

 pp. 443-485. "Geological Magazine," II, vol. x, 1883, pp. 241-252. 



% "American Journal of Science," III, vol. xxxi, pp. 284-299, April, 1886. 



$ "American Naturalist," vol. xix, pp. 257-268, March, 1885. "American 

 Geologist," vol. i, pp. 382-386, and vol. ii, pp. 28-35, June and July, 1888. 



% " Nature," vol. xviii, pp. 41-44, May 9, 1878. 



** "Physics of the Earth's Crust," 1881, pp. 223, 270, etc. 



ft " Geological Magazine," III, vol. iv, 1887, pp. 344-348. 



