494 THE GLACIAL LAKE AGASSIZ. 



Reade^ believe it to be solid, while Whitney," Button,^ Powell,* Wads- 

 worth,^ Crosby," Claypole,' Phillips,* Airy," Fisher,'" and Jamieson^' 

 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 isostasy'' or gravitational 

 equilibrium upon the heavier liquid or viscous mobile interior or layer 

 enveloping the interior, subject, however, to stresses and resulting defor- 

 mation because of the earth's contraction. The thickness of the crust, 

 according to this hypothesis, is variously estimated to be from 20 to 50 

 miles, or possibly 100 miles or more. 



Another statement of the probable relationship of the earth's crust to 

 the interior, which seems to come between these diverse opinions and in 

 some measure to express the important features of each, is given as follows 

 by Gilbert, in his discussion of faults and displacements of the Wasatch 

 range and the area of Lake Bonneville : 



We are forced to conclude that the mountain ranges of the Bonneville basin 

 and the valleys between them do not, with reference to each other, obey the law of 

 flotation. 



It follows with equal cogency that the faults do not penetrate to a layer charac- 

 terized by fluidity or semi-fluidity, implying by these terms the power to flow under 

 small shearing strain, but terminate in a region of rigidity, implying by that term 

 the ability to withstand relatively large shearing strain. I conceive them to termi- 

 nate at the upiier limit of the region of plasticity by pressure, implying by that 

 phrase that at and below a certain depth the rocks of the crust, however rigid, are 



' The Origin of Mountaiu Ranges, 1886, pp. 6, 7, 256, 267, 270, etc. Philosopliical Magazine, June, 

 1891 (also in Am. Geologist, Nov., 1891). 



'^Earthquakes, Volcauoes, and Mountain Building, 1871, pp. 77-87. 



■■> Penu Monthly, Vol. VII, pp. 364-378, and 417-431, May and June, 1876. U. S. Geol. Survey, Fourth 

 Annual Report, pp. 183-198; Sixth Annual Report, pp. 195-198. 



' Science, Vol. Ill, pp. 480-482, April 18, 1884. The Forum, Vol. II, pp. 370-391, Dec, 1886. 



""Am. Naturalist, Vol. XVIII, June, July, and August, 1884. 



eProc, Boston Soc. Nat. Hist., 1883, Vol. XXII, pp. 443-485. Geol. Magazine (2), Vol. X, 1883, 

 pp. 241-252. 



' Am. Naturalist, Vol. XIX, pp. 257-268, March, 1885. Am. Geologist, Vol. I, pp. 382-386, and Vol. II, 

 pp. 28-35, June and July, 1888. 



Vesuvius, 1869, pp. 324, 329. 



3 Nature, Vol. XVIII, pp. 41-44, May 9, 1878. 

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

 " Geol. Magazine (3), Vol. IV, 1887, pp. 344-348. 



•°A term proposed by Capt. C. E. Duttou in a paper "On some of the greater problems of physical 

 geology," Bulletin of the Philosophical Society of Washington, Vol. XI, pp. 51-64, April 27, 1889. See 

 also au important discussion of this condition of the earth's crust, ''The Gulf of Mexico as a measure 

 of isostasy," by W J McGee, in Am. Jour. Sci. (3), Vol. XLIV, pp, 177-192, Sept., 1892. 



