514 THE WE AGE IN NORTH AMERICA. 



ain plants identical with those of the Arctic zone ; * but also we 

 may now more satisfactorily bridge over the tropics and equa- 

 tor, by uplifts and subsidences of mountain-ranges, so that 

 species incapable of enduring a torrid climate could sometimes 

 become dispersed even to such distances as from north tem- 

 perate latitudes to Tierra del Fuego and the Cape of Good 

 Hope, f 



It seems probable that the rate of the earth's contraction 

 has been somewhat uniform throughout the vast ages known 

 to us by the researches of geology ; but the corrugation of the 

 earth's surface in mountain-building has been much more rapid 

 in some epochs than in others, and between the times of for- 

 mation of great mountain-ranges there have been long intervals 

 of quietude. J The slowly progressing contraction of the globe 

 has been uninterrupted, and in some way the cooled outer part 

 of the crust which has not shared in this diminution of volume 

 has been able to accommodate itself to the shrinking inner, 

 mass. As stated on a previous page, this has probably resulted 

 in distortion of the earth's form, both of the whole thickness 

 of the crust and of the probably molten interior, within mod^ 

 erate limits during the periods of quiet, until so much lateral 

 pressure has been accumulated as to compress, fold, and uplift 

 the strata of a mountain-range. In attributing the severe 

 climate of glacial epochs to great uplifts of the areas glaciated 

 through such deformation preparatory to the process of mount-* 

 ain-building, it is distinctly implied that the Quaternary period 

 has been at first exceptionally marked by such broad crustal 

 movements, and has since gained comparative rest from the 

 lateral stress to which they were due by equally exceptional 



* " Sequoia and its History," " Proceedings of the American Association for 

 the Advancement of Science," Dubuque, vol. xxi, 1872, and "American Jour- 

 nal of Science,'" III, vol iv ; " Forest Geography and Archaeology," " American 

 Journal of Science," III, vol. xvi, 1878 ; " Characteristics of the North Ameri T 

 can Flora," " Report of the British Association for the Advancement of Science," 

 Montreal, 1884, and " American Journal of Science," III, vol. xxviii. 



f Darwin's " Origin of Species," chapter xi. Wallace's " Geographical Dis- 

 tribution of Animals," chapter iii, and his " Island Life," chapter vii. 



% Dana's " Manual of Geology," third edition, p. 795 ; Prestwich's " Geology," 

 vol. i, chap. xvii. 



