292 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



the case if the planes of foliation led inward and upward beneath the 



Thus it seems we may reasonably conclude that suboceahic foliation 

 is oriented in an approximately horizontal attitude, but rises in the broad 

 marginal regions to join the curve that extends up into the eroded conti- 

 nental areas. 



The preceding statement, which applies to ocean deeps ^ in general, 

 should not be understood as excluding the occurrence of overdeepened 

 hollows. On the contrary, overdeepening would be likely to result under 

 certain conditions as a local effect. 



Thus, suppose that the support of a suboceanic body be melted and 

 pressed outward toward the surface. If the foliation be horizontal or 

 gently inclined, the melt would escape laterally and the body would sink. 

 Overdeepening must result, if isostatic equilibrium existed previously. 

 However, a comparison of the volume of extruded igneous rocks with the 

 areas of ocean deeps indicates that overdeepening due to their extrusion 

 can be of notable amount only if narrowly localized. The Tonga Deep 

 would seem to offer, a case in point, but the Pacific, Indian, and Atlantic 

 deeps in general are too extensive. 



The horizontal diameters of an area which might thus assume a disk- 

 like structure under an ocean basin are not indicated by the preceding 

 analysis. It is clear, however, that one possible limit is a continental 

 margin along which the foliation would rise from under the loaded zone 

 to the coast. Another limit is the divide, so to speak, between two disk- 

 like masses, both of which, together with the intermediate zone, are sub- 

 merged. Such a divide would correspond, for instance, with the mid- 

 Atlantic ridge. 



DISKS AND INTERDISKS 



In a preceding section, in discussing the departures from average 

 densities, it was suggested that we may recognize an average density of 

 the oceanic underbody and also an average density of the continental 

 underbody, which is lighter; and that within each of these there are 

 bodies which are in the former case heavier, in the latter case even 

 lighter. It is not supposed that the distinctions as to density are in any 

 way precise. They are merely relative, but they exist if the postulate of 

 isostatic equilibrium be true. 



Curved or discoidal schistosity has now been deduced as a structure 

 rising from beneath a heavier into a lighter body. The underbodies con- 

 sidered in the discussion have been those which give the more striking 

 illustrations, those of continents as contrasted with those of ocean basins ; 



