196 GEOLOGY. 



The next deformative movement was one of the greatest in post-Cambrian 

 history, and appears to have involved some movement of nearly every mountain 

 range whose history is known, as well as a very marked withdrawal of the sea, 

 as indicated by buried or submerged erosion channels traversing the continental 

 shelves. These are not however regarded as indicating an elevation of the con- 

 tinent equal to their depth below the present sea-level, but chiefly as indicating flexures 

 of the continental border attending the deformative movement; even thus 

 interpreted they imply sea-withdrawal. This great deformation is held to give 

 a better definition to the Pliocene period than any assigned percentage of living 

 and extinct shells in the sediments of the time. The extinction of species during 

 this period seems to have been greatly lessened by reason of the extinctions 

 and adaptations which had already been brought about by the Oligocene move- 

 ments and the Miocene cold currents; hence the importance of these changes 

 is not fully revealed in the immediate faunal change. Their biological influence 

 can only be fully measured when the secondary effects, through climatic and 

 other means, have worked themselves out, and this will require a long period, 

 a part of which is still in the future. 



An immediate secondary effect is probably found in the glacial invasions 

 which, because of their great influence in the history of the land, have been 

 regarded as constituting a period by themselves, the Pleistocene. In a strict 

 deformative classification, however, this should be united with the Pliocene, 

 for important movements seem to have been in progress during the glacial period, 

 and perhaps the same may be said of the present. 



It should perhaps be repeated that this deformative, dynamic classification 

 is not in accord, in all its details, with the classification by species percentage, 

 even in its modified form; but there is no serious discrepancy between them, 

 and if the dynamical considerations shall be supported by future extensions 

 of knowledge in the less known regions of the earth, the existing rather arbi- 

 trary classification may easily merge into the dynamical one. 



FORMATIONS AND PHYSICAL HISTORY OF THE EOCENES 



The formations of the Eocene system are found in widely sepa- 

 rated parts of the North American continent (Fig: 418), but they do 

 not appear at the surface over extensive areas. Within the conti- 

 nental area, their extent was not great at the outset, and in many 

 places they are concealed by younger beds. They include (1) beds 

 laid down in the sea or below sea-level, and (2) beds deposited on 

 the land. The former include formations of (a) marine, and (b) brack- 



1 For review of all the literature of the Eocene of the continent up to 1891, see 

 Clark. Bull. 83, U. S. Geol. Surv. For later publications, see Bulls. 130, 135, 146, 

 153, 156, 162, 172 and 177. See also article by Dall in 18th Ann., U. S. Geol. Surv., 

 Pt. II, where bibliography up to 1898, is given. 



