214 GROVE KARL GILBERT— DAVIS [MEMores [vSxi; 



Various examples of volcanic mountains and eroded valleys are next cited, the volume of 

 which is much less than this limit, and which are therefore regarded as being sustained by the 

 rigidity of the crust. A general conclusion that is held to be worthy of consideration is as follows : 



Mountains, mountain ranges and valleys of magnitude equal to mountains, exist generally in virtue of the 

 rigidity of the earth's crust. Continental plateaus and oceanic basins exist in virtue of isostatie equilibrium 

 in a crust heterogenous as to density. 



It would be interesting to know what opinion Gilbert would have had as to the stability 

 of the huge volcanic cones that rise from the bottom of the Pacific, with a total weight greater 

 than the limit he regarded as supportable by crustal rigidity, even after allowing for the weight 

 of the water replaced by the submerged part of the cones. 



A chapter in the Bonneville monograph continued the discussion just outlined. It intro- 

 duced a curious analogy in comparing "the earth when loaded by the water of Lake Bonneville 

 with a bowl of jelly upon which a coin has been laid"; thus likening the apparently solid earth 

 to the pliable jelly and the mobile lake waters to the rigid coin; nevertheless the analogy served 

 its purpose remarkably well by very reason of its seeming contradictions; it was precisely the 

 pliability of the earth's crust under the definite weight of the coinlike lake that was to be empha- 

 sized. After a more detailed statement on the same lines as those of the paper on the "Strength 

 of the earth's crust," it is thought probable that, all things considered, "the Wasatch range and 

 the parallel ranges lying west of it are not sustained at their existing heights above the adjacent 

 plains or valleys by reason of the inferior specific density of their masses and of the underlying 

 portions of the crust, but chiefly and perhaps entirely in virtue of the rigidity or strength of the 

 crust." It is further said that "a mountain of the first class is the greatest load that can be 

 held up by the earth"; but it is intimated at the end of the chapter that all of its conclusions 

 are only tentative; even the doming of the Bonneville Basin floor may be due to the accidental 

 coincidence of a compelling force of upheaval with the time and place of the vanishing lake waters. 



It thus appears that while Gilbert's work in the Cordilleran region led him to regard the 

 earth's crust as possessing a great measure of rigidity, he believed also that the inequalities of 

 pressure due to continental masses would be greater than the rigidity of the crust could bear, 

 and that the continental parts of the crust must therefore owe their height to being less dense 

 than the suboceanic parts of the crust, so that, at a certain depth, both the continental and the 

 suboceanic parts would exert a uniform pressure on the next underlying earth shell; in other 

 words, that the density of the earth's crust varies inversely with the height of its surface above 

 the shell of uniform pressure; or in still other words, that the crust is in equilibrium: such 

 being the essence of the doctrine of isostasy. 



Brief allusion to this doctrine is made in Gilbert's presidential address on " Continental 

 problems " before the Geological Society of America at Ottawa in the winter of 1892-93 : "The 

 weight of opinion and, in my judgment, the weight of evidence lie with the doctrine of isostasy, " 

 so far as the larger reliefs of the earth's surface are concerned. It is here pointed out that 

 if a spheroid be imagined, the oceanic parts of which are coincident with the average 2-mile 

 depth of the oceans, the rock pressure on its subcontinental parts would exceed the water 

 pressure on its suboceanic parts by about 12,000 pounds to the square inch and that such an 

 excess of pressure urging the continent down and the ocean floors up could not be withstood. 

 It thus appears again that in Gilbert's view the continental masses and their larger features 

 are in isostatie equilibrium, but not individual mountains or mountain ranges of the second 

 order. 



THE GEODETIC TREATMENT OF ISOSTASY 



Before taking up Gilbert 's later papers on certain geodetic aspects of the isostatie problem, 

 it is desirable to outline briefly the manner in which the geological doctrine of isostasy came 

 to be involved in geodetic calculations. If the earth were a smooth, nonrotating sphere, com- 

 posed of concentric shells, each of uniform composition and temperature and all systematically 

 increasing in density from crust to center, gravity would be the same at all points of the surface; 

 and each shell receiving a uniform pressure from the shell exterior to it, the whole earth would 



