DEVELOPMENT OF THE DISCOIDAL HYPOTHESIS 279 



sealevel surface as a reference datum, and this has been questioned by 

 MacMillan, who argues that "a true isostasy must be based on a level 

 9,000 feet below the sea." ^^ These doubts, however, touch only the degree 

 of perfection of isostatic adjustment at the present time. They do not 

 invalidate the general hypothesis that the relief of the earth's surface has 

 always approximated that which the larger elements of the lithosphere 

 tend to assume because of differences of density. That hypothesis re- 

 mains the one which most nearly satisfies geodetic and geologic evidence 

 and is therefore the safest guide. Accepting, then, the depths and shal- 

 lows of the ocean basins as an approximate expression of the distribution 

 of heterogeneous masses, we may analyze the underbodies of the oceans 

 accordingly. 



Groll's bathymetric maps constitute the latest presentation of tlie avail- 

 able data as to the distribution of ocean depths, whicli, by hypothesis, 

 express the distribution of denser and lighter suboceanic materials. It 

 is not necessary here to describe what may better be seen in the maps 

 themselves. Plate 12 gives a reproduction of the north and south central 

 Atlantic region, adapted from Groll, and brings out the four major 

 basins, together with many minor ones, which constitute the deeps of the 

 oc€an. The map also illustrates clearly the winding central ridge and its 

 branches, which divide the basins and occasionally widen out into broader 

 shallows. 



Our knowledge of the depths of the Pacific and Indian oceans is much 

 less complete than that of the Atlantic, but where soundings are numer- 

 ous differences of depths are characteristic, and it is evident that the 

 deeps of these two oceans are of the same general order of magnitude as 

 those of the Atlantic. 



Thus it seems safe to generalize that the underbodies of the oceans 

 consist of elements of suboceanic horizontal dimensions which, according 

 to the hypothesis of isostasy, indicate differences of density by the greater 

 or less depth which their surfaces have assumed. 



The general postulate with which we proceed is, then, that the under- 

 bodies of continents and of oceans consist of masses of subcontinental or 

 suboceanic dimensions which differ in density; that these masses are 

 recognizable within continental areas by their tendencies to stand rela- 

 tively high or relatively low, as recorded by geologic history; that by 

 analogy the masses composing the underbodies of the ocean have shown 

 similar tendencies during the same prolonged history. 



«• W. D, McMillan: On the hypothpsis of Isostasy, Chicago .Tour, of Geol., vol. xxv, 

 ini7, pp. 105-11]. 



XIX — Bull. Gkol. Sor. Am., Vol. 31, 1919 



