EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 459 



changes which decrease bulk and increase density. Thus equiHbrium 

 will be destroyed without producing a common level, and a slow 

 surface creep of the lighter and higher land areas toward the sea 

 will ensue, and as a result beneath this surface creep a great slow 

 undertow from the ocean areas toward the continents. The under- 

 tow being attached continuously to the surface strata, and the two 

 moving in opposite directions, there must be shearing between them 

 or crumpling of the surface layers, which are free to relieve part of 

 the tension by folding. Therefore the mountain chains are a short 

 distance inside the continental borders and parallel to them. 



Wilhs accepts essentially the conclusions of Heyford, but utilizes 

 exclusively the lower layer underthrust from the oceanic areas. He 

 speaks of a " suboceanic spread," i. e., "the expansion of suboceanic 

 masses within the upper hundred miles of the crust in consequence 

 of the efficiency of stresses due to greater density to direct move- 

 ments occasioned primarily by molecular or mass changes under 

 varying temperature and pressure." 



Much is made of the idea of great areas of habitual elevation 

 and depression. These must be subordinate to the great persistent 

 continental elevations and oceanic depressions. 



The rhythmicality is explained by the unproved consensus in the 

 rhythm of several causes none of which are shown to be rhythmical.* 



The special tendency to collapse when the centers of the coigns 

 rise too high would explain the central seas on the three shields, as 

 the Baltic and Hudson's Bay. It is interesting in this connection that 

 Heyford declares" the earth to be a failing body. He reconciles this 

 inward thrust with Suess's idea that the Asian chains flowed sea- 

 ward by saying that the thrust of the ocean bed beneath the coastal 

 parts of the continents would produce the same effect as an outward 

 superficial motion of the land. 



" Gondwana land," he says, " has been carried north with the 

 deep underflow "® which passed beneath and wrinkled up the Hima- 

 laya. But Gondwana land is a rising and thus a lighter area against 

 which the flow should have impinged and formed mountains on its 



6 " Asia," II., 130. 



■^Heyford, "Geodetic Evidence of Isostacy," Proc. Wash. Acad., VIII., 

 36-39, 1906. 



s Loc. cit, p. 133. 



