EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 463 



up to heights notably above the plane of isostatic equilibrium, and 

 that these portions gradually settle back toward equilibrium."^* 

 That " the conditions prerequisite to baselevelling involve a high 

 degree of stability through a long period of time." The great base- 

 levellings and the great sea transgressions which are little more than 

 alternative expressions for the same thing have as their fundamental 

 assumption a sufficient stability of the surface to permit baselevelling 

 to accomplish its ends. 



Chamberlin states these stages as (i) That of climacteric base- 

 levelling and sea transgression favoring the expansional evolution 

 of shallow water life and Avide migrations and comminglings leading 

 to cosmopolitan faunas. 



(2) The stages of retreat which are the first stages of diastrophic 

 movement after the quiescent period marked by abundant erosion 

 and deposition of deep soil mantles, limited life area, and lessened 

 migration. 



(3) The stages of climacteric diastrophism and greatest sea re- 

 treat marked by restrictional evolution of shallow water faunas, in- 

 creased land deposits, broadest continents, diversity of land surfaces 

 and climatic extremes. 



(4) The stages of progressive degradation and sea advance, 

 marked by the reexpansion of the narrowly provincial shallow water 

 faunas formed in isolated areas in the previous period. 



The tetrahedral hypothesis thus presents itself as a welcome in- 

 troduction or preliminary to Chamberlin's suggestion of diastrophism 

 as the foundation of correlation, since it gives a cause for a rhyth- 

 mical recurrence of short periods of diastrophism with long inter- 

 vening periods of quiescence. In harmony with this hypothesis is 

 the remarkable generalization of White and Knowlton,^^ that a uni- 

 form warm humid climate extending beyond the polar circles has 

 been the rule from early paleozoic, interrupted by relatively short 

 periods of climatic extremes when great glacial areas coexist with a 

 torrid zone.^° 



14 Putnam and Gilbert's pendulum studies indicate that the part of our 

 continent uplifted in late Tertiary is still above the level of equilibrium. 



15 Scietice, XXXI., 760. 



16 Variations of the sun's heat have been adduced as cause of varying 

 climate and even the passage of the solar system through cold areas in space. 



