EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 461 



heavier than iron, and from its way of conducting earthquake waves, 

 more rigid than steel, and with rigidity increasing centerward. 



We may accept it as highly probable that a condition of ap- 

 proximate isostacy exists over the area of the United States, with 

 compensation of the lighter land and deeper adjacent sea areas 

 within perhaps one hundred miles of the surface. ^° It, however, 

 remains to be proved whether this is true of other continents or a 

 constant condition of any continent. This must be reconciled with 

 the existence of long periods of peneplanation when the base-leveled 

 surface is not raised as the load is removed but often submerged 

 beneath the waters of a transgressing sea. 



The theory of isostacy must also meet the fact that the lavas of 

 midoceanic regions are nowhere ultrabasic, but rather intermediate 

 between basic and subacid. They range from rare nepheline basalts 

 (SiOo 39, sp. gr. 2.9) to rhyolite (SiOo 76, sp. gr, 2.4). The average 

 is basalt and andesite (SiO. 53, FeO = 20, su. gr. 27-2.95). While 

 all the masses of terrestrial metallic iron, the diamontiferous olivine 

 rocks (sp. gr. 3.2-3.5), the greatest accumulations of magnetite, the 

 greatest areas of heavy " norites with titanic iron borders " are 

 found in the old highlands. 



The diamond-bearing rocks would seem to have come from great 

 depths which could furnish great pressures, unless the Ovifak irons 

 and the diamond-bearing Vaalite are planetesimals. 



The postulates of the planetesimal hypothesis are distinctly 

 favorable to the tetrahedral hypothesis. The possible considerable 

 irregularity in the accumulation of the matter would supply a needed 

 condition for any such deformation and especially for a deformation 

 into a somewhat irregular and one-sided tetrahedroid. 



The storage of outgoing heat in an outer shell which should pro- 

 mote the formation of a plastic stratum along which flow could take 

 place would be an additional favorable condition." 



It is quite possible that the planetesimal hypothesis may be found 

 to supplement rather than supplant the nebular hypothesis. 



The impact hypothesis, suggested by the great multitude of 



10 J. F. Heyford, " Figure of Earth and Isostacy," Coast Survey, 1909. 



11 Chamberlin, " Geology," p. 539. 



