460 EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 



south, or if Gondwana land is carried north with the deep underflow 

 Angara land should be carried forward also by the larger Pacific 

 flow. 



This underthrust would hardly produce the glacier-like lobing of 

 the Asian chains so characteristic of the outflow of ice, and would 

 not explain the northward overthrust of the mountains across 

 Europe from the Pamir to the Pyrenees, where the oceanic area is 

 wanting and the thrust must have come from Arabia and Africa. 

 It does not explain the contrast between the festooned Asian chains 

 and the -Straight American coasts, nor all the complexity of the zone 

 of the intercontinental seas. 



Such a band thrust far under the continental mass must have had 

 behind it an enormous force to overcome the resistance to shear 

 (which may have approached the breaking strength of the rocks) 

 over all its broad upper and under surfaces and have surplus force 

 to upfold the many festooned mountain chains of Asia. Indeed this 

 suboceanic spread occupying the greater portion of the hundred 

 miles in depth would have caused vertical elevation of the sea bottom, 

 instead of being transmitted so far inland beneath so small a load. 

 We may contrast with this the superficial movement down a slope 

 having shear only on an under surface softened by an internal heat. 

 This sliding might be carried dowai a very low slope, solicited as it 

 were, by the constant stresses of the earth tides and occasional earth- 

 quake vibrations, especially in soft and water-soaked strata recently 

 emerged from the sea. 



The hypothesis as presented by Heyford can, however, coexist 

 with the tetrahedral hypothesis, since an elevation of the central con- 

 tinental mass would favor the superficial flow and hinder the deep- 

 seated one. 



It would seem, however, that for the formation of the earth's 

 largest features much deeper portions of the earth would be con- 

 cerned than are involved in the compensations of isostacy.'' 



Heyward bases his theory upon the observed fact of isostacy but 

 this fact itself is still sub judice. 



Because of the heatgradient we may assume the centrosphere 

 to now consist of gas above the critical point, by compression made 



» See Cliamberlin and Salisbury, " Geology," p. 556, 1904. 



