DEVELOP^rEXT OF THE DISCOIDAL HYPOTHESIS 293 



but there is nothing in the controlling conditions to limit the argument 

 or conclusion to those contrasts. Differences of density within a conti- 

 nental underbody or within an oceanic underbody may have similar 

 effects, provided the initiative activity of internal heat shall reach the 

 area. 



In the analysis of continental structure we have distinguished positive 

 and negative elements, and we would anticipate that the former would 

 exhibit steeply dipping schistosity, which by hypothesis should curve 

 outward beneath the negative areas. The latter constitute the conti- 

 nental matrix, as it were. They are commonly confluent. A section 

 across a negative area from one positive element to another should show 

 a structure which would be convex downward — that is, would exhibit a 

 s}Ticlinal foliated structure; but a section along the axis, which winds 

 between positive elements, would transect nearly horizontal foliation. 



The oceanic underbody contrasts with that of the continent in that the 

 average density is less than the density of the surrounded masses. The 

 contrast may be stated thus : Within continents the heights are isolated ; 

 beneath oceans the deeps represent isolated bodies. The foliation in the 

 oceanic underbody should, then, be flat under the deeps and should curve 

 upward around them in the surrounding matrix. 



Let us call any mass which is characterized by foliation that is flat or 

 convex downward a dish, and let us designate any mass in which the 

 foliation rises to a steeply dipping attitude by the term interdish. We 

 may then say: In both continents and oceanic underbodies disks repre- 

 sent the heavier, negative elements ; interdisks represent the lighter, posi- 

 tive elements. Thus, within either a continental or a suboceanic mass as 

 a whole the terms disk and interdisk have the same significance as to 

 relative density of the parts. But if we contrast the continental with the 

 oceanic the disks of the former are lighter even than the interdisks of 

 the latter, and extremes of unlike densities exist between suboceanic disks 

 and continental interdisks. 



In the continents the disks in general are confluent and constitute the 

 matrix in which the interdisks are isolated, whereas in the oceanic under- 

 body the interdisks in general are continuous and the disks are separated. 



There is obviously no sharp boundary between disk and interdisk, 

 either in fact or by definition.' There is in each of them a characteristic 

 attitude of foliation, but between them the change in attitude is gradual. 



