448 EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 



The axis of figure of the forming tetrahedroid chanced not to 

 coincide with the axis of rotation and the latter gradually shifted 

 from near Behring Straits to its present position, which is one of 

 stable rotational equilibrium. This happened in pre-cambrian time. 



At this point comes the interesting novelty in the tetrahedral 

 theory. The development of the tetrahedral form from shrinkage 

 would proceed but a little way when rotation would tend to repro- 

 duce the spheroidal form. The tetrahedroid shape would be pushed 

 beyond the strength of the material and collapse would ensue, with 

 reassumption of a more spherical form. In a long period of rest 

 the crust would be recemented and strengthened and the continued 

 escape of heat would then tend to develop the tetrahedroid again 

 and rotation would again restore the spheroid. 



This is brought into connection with the six great geologic cycles 

 as follows : The solidified crust becomes by interior shrinking slightly 

 tetrahedral. This involves elevation with glacial conditions, large 

 continents, inner crustal tensions, foldings, Assuring, mountain- 

 making and outpouring of lava. Through this fissuring the crust 

 becomes weakened, the tangential force of rotation becomes pre- 

 dominant, restoring the spheroid ; great transgressions of the oceans 

 then intervene while mountain-making and volcanic activity approach 

 a minimum. In the relatively long time of submergence and quiet 

 the faults and fissures are sealed up by the circulating waters and 

 the earth becomes again rigid enough to permit the oncoming of a 

 second period of tetrahedral deformation. The oceans are deepened 

 and contracted, the continents elevated and enlarged with mountain- 

 making and this becomes again the cause of a glacial period and vol- 

 canic activity. This cycle is several times repeated. 



We are now in a period of deformation, as is shown by the 

 marked tetrahedral features of the earth, the sinking of the Pacific 

 coral region, the abundant volcanic and earthquake activity and the 

 just passed glacial period. 



The author assumes the nebular hypothesis and Arrhenius's 

 theory of the condensed-gaseous condition of the earth's interior, 

 and noting the unimportance of the present equator for the structure 

 of the earth, and the great importance of the band going through 

 the three Mediterraneans ; that is, the Mediterranean and the East 



