CHAPTER XXIII 

 DISCUSSIONS OF ISOSTASY 



THE STRENGTH OF THE EARTH'S CRUST 



The behavior of the earth's crust under varying loads and stresses was precisely the kind of 

 problem to interest Gilbert's philosophical mind. Its discussion involved the balanced con- 

 sideration of many factors, and he seemed to have a special enjoyment in, as well as an unusual 

 capacity for, such balancing. It involved also the determination of many quantitative relations, 

 and he had always shown a strong leaning toward quantitative work. His relation to the 

 problem was first developed through his own geological studies, particularly in connection with 

 Lake Bonneville, as will now be shown; it was afterwards extended into a discussion of the 

 geodetic investigations carried on by the United States Coast and Geodetic Survey, as will 

 appear below. 



The Bonneville studies had led him to believe that the evaporation of a large lake, nearly 

 1,000 feet deep, had been accompanied by a gentle doming of the lake floor; and he was thus 

 inclined to the opinion that the removal of the lake waters was presumably the cause of the 

 doming. His presidential address on the " Inculcation of scientific method by example, " before 

 the Society of American Naturalists in 1885, was chiefly devoted to an exposition of the trains 

 of thought by which this conclusion was reached. The same problem was discussed in a paper, 

 essentially an abstract of a chapter in the Bonneville monograph not then published, on the 

 "Strength of the earth's crust," his first communication to the newly organized Geological 

 Society of America at its earliest winter meeting in December, 1889. 1 His direct entrance into 

 the subject was characteristic : 



Conceive a large tank of parafine with a level surface. If a hole be dug in this and the material piled in a 

 heap at one side, the permanence of hole or heap will depend on its magnitude. Beyond a certain limit, further 

 excavation and heaping will be completely compensated by the flow of the material. Substitute for parafine 

 the material of the earth's crust, and the same result will follow, but the limiting size of the hole or heap will 

 be different, because the strength of the material is not the same. Assuming the earth to be homogeneous, the 

 greatest possible prominence or depression is a measure of the strength of the material. 



Attention is then directed to the relation of the uplifted Wasatch Range and the depressed 

 Bonneville Basin; the range is unloaded by erosion and the basin is loaded by deposition; and 

 some geologists might regard the progressive displacement of the two areas as a direct consequence 

 of the continual transfer of load from one to the other. But if so, the depression of the basin 

 ought to have been accelerated when an arm of Lake Bonneville, 500 or 600 feet deep, was rather 

 quickly laid upon the piedmont part of the depressed area, and conversely ought to have been 

 retarded when the lake arm was as quickly withdrawn. As a matter of fact, the depression 

 of the basin with respect to the mountain range "continued alike during the presence of the 

 water and after its removal." Hence the transfer of load should not be taken as the primary 

 cause of the displacement. On the other hand, if the doming of the central area of the Bonne- 

 ville Basin floor be due to the removal of the lake waters by evaporation, the effect is not so 

 great as it should be if an isostatic relation had been preserved : 



A stress residuum was left to be taken up by rigidity, and the measure of this residuum is equivalent to the 

 weight of from 400 to 600 cubic miles of rock. 



This leads to the further quantitative conclusion : 



The measure of the strength of the crust is a prominence or a concavity about 600 cubic miles in volume. 

 If this be strictly true, then there should be no single valley due purely to the local addition or subtraction of 

 material, having a greater volume than 600 cubic miles. 



' Bull. Geol. Soc. Amer., i, 1890, 23-25. 



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