301 Status of the Theory of Isostasy. 



1915 under the general title, "The strength of the earth's 

 crust.' ' The results confirmed Gilbert's views, expressed 

 in 1889, which he stated as follows : 12 



"It is believed that the following theorem or working hypothe- 

 sis is worthy of consideration and of comparison with additional 

 facts : mountains, mountain ranges, and valleys of magnitude 

 equivalent to mountains, exist generally in virtue of the rigidity 

 of the earth 's crust ; continents, continental plateaus, and oceanic 

 basins exist in virtue of isostatic equilibrium in a crust heteroge- 

 neous as to density. ' ' 



It was in recognition of the agreement with Gilbert's 

 brief statement that this longer series of papers, pub- 

 lished a quarter of a century later, was given the same 

 title ; also to offset the title, of the more recent paper by 

 Hayford entitled "The earth a failing structure." 13 



A review of the principal lines of argument given in 

 the set of papers published in 1914 and 1915 on "The 

 strength of the earth's crust" will be given here in order 

 to clear the ground for such further discussion as is 

 needed. 



In Part I, on " Geologic tests of the limits of strength, ' ' 

 it is noted that the broad and thick continental ice sheets 

 of the Pleistocene seem to have depressed the crust, since 

 an upwarp followed their melting, an upwarp so closely 

 related in time and locality as to constitute strong evi- 

 dence that the cause was an isostatic response to the 

 relief of load. On the other hand, the deltas of the 

 Nile, and especially the Niger, are constructional forms 

 built out into the ocean basins. The maintenance of their 

 forms shows that the crust is strong enough to bear the 

 load, or at least that the yielding is much slower than 

 the accumulation, very different from the geological 

 rapidity with which the earth responded to the ablation 

 of the greater mass of the ice sheets. Yet a conserva- 

 tive estimate of the mass of the Niger delta is as follows : 



Area within assumed limits 195,000 sq. km. (75,300 sq. mi.) 



Kadius of equivalent circle 250 km. (155 mi.) 



Average thickness within assumed limits. . 1.1 km. (3,600 ft.) 



Equivalence in rock upon land 0.6 km. (1,980 ft.) 



Maximum thickness 3.0 km. (9,900 ft.) 



Equivalence in rock upon land 1.65 km. (5,450 ft.) 



12 G. K. Gilbert, The strength of the earth 's crust, (Abstract) Bull. Geol. 

 Soc. Amer., 1, 23-25, 1890. 



13 J. F. Hayford, Bull. Philos. Soc. Wash., 15, 57, 1907. 



