470 EMERSON— RECURRENT TETRAHEDRAL DEFORMATIONS. 



Salisbury-^ and that of Schuchert-- averaging the results of his ex- 

 tensive and valuable work on the paleogeography and paleometeor- 

 ology of North America. The two tables are in substantial agree- 

 ment with the table of Arldt (see p. 447). The larger disturbances 

 given by Schuchert agree with Arldt's cycles except that the Grand 

 Canyon revolution is local and the Caledonian cycle is less marked 

 in North America than the others. He brings out very clearly the 

 brevity of the elevation and the great length of the intervening times 

 of depression. 



" Granting all this," says Schuchert-^ (after reviewing all the 

 theories to explain the " climates of geological time " except the 

 tetrahedral hypothesis), "there still seems to be back of all these 

 theories a greater question connected with the major changes in 

 paleometeorology. This is : What is it that forces the earth's topog- 

 raphy to change with varying intensity at irregularly rhythmic in- 

 tervals ? . . . Are we not forced to conclude that the earth's shape 

 changes periodically in response to gravitative forces that alter the 

 body form." The tetrahedral hypothesis is certainly trying to force 

 this same conclusion. 



The idea of a spherical recovery and extensive transgression and 

 exceptionally equable climate far poleward would take away largely 

 the need from the biological side of many assumed continental con- 

 nections across the deep oceans as bridges for migrations. Their 

 migrations could take place during equable climates by long cir- 

 cuitous land connections extending far poleward, and would remove 

 many apparent conflicts with the supposed tetrahedral configuration 

 of the earth, which appear in many restorations of early geological 

 periods. This was written in 191 3 and the important and authorita- 

 tive article by Mathew on " Climate and Evolution,"-^ brought so 

 full confirmation of this suggestion and so strong condemnation of 

 the indiscriminating bridge building which has been customary for 



-1 H. W. Shimer, " Broader Features of the Geological History of North 

 America," Technology Quarterly, Vol. XX., p. 287, 1907. 



-~ " Textbook of Geology," Pt. 2, p. 980, 191 5. 



23 " Climates of Geologic Time," Pub. Carnegie Inst, No. 192, p. 289. 



24 W. D. Mathews, "Climate and Evolution," An. N. Y. Acad. Sc, Vol. 

 24, pp. 171-318. 



