328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



upon the lithosphere thus appear to have been the more effective 

 factors. 



That the marginal areas of the continents were at times elevated 

 and folded is, of course, accepted by all — even by Suess and his fol- 

 lowers, who speak of the continents as having the character of 

 "horsts" and of the ocean basins as being permanently "sunken 

 areas." Suess, however, believed that the median areas of the conti- 

 nents are essentially stable, a view adopted by Schuchert, who holds 

 " that the continent (North America) is a horst, that the great 

 medial region remained unmoved, while the margins were often 

 folded and elevated. The seas periodically flowed over this medial 

 land — in fact, were elevated over it — owing to the detrital materials 

 unloaded into the oceanic areas, thus filling them and causing them 

 to spill over on to the lands." 



I can not subscribe to this opinion. On the contrary, though ac- 

 cepting the idea of permanent oceans and continents, it seems to me 

 that the crust of the lithosphere was subject to periodic movement 

 away from the poles ; that the surface of the lands was exceedingly 

 unstable in the median areas as well as along the borders of the 

 continents. Schuchert's paleogeographic maps, indeed, offer con- 

 vincing proof of such instability; and the more detailed maps made 

 since his appeared further substantiate my claim. 



In reaching these conclusions I am mainly influenced by a life- 

 time study of Paleozoic formations and their faunas. The criteria 

 and principles used in the course of these stratigraphic investiga- 

 tions are defined and discussed in my Revision of the Paleozoic Sys- 

 tems, published in 1911. In this work more than 100 previously un- 

 described instances of differential vertical movements of lands and 

 consequent shifting of seas are discussed in varying detail. Since 

 1910 much additional information has been gathered concerning such 

 oscillations in North America. 



On this occasion I shall mention briefly some of the more con- 

 vincing of the published cases and in greater detail a few of the 

 more recently determined instances — enough of both to show that 

 from the beginning of Cambrian time the surface of the continents 

 was exceedingly unstable and subject to frequent oscillation, and 

 that the epicontinental seas were correspondingly inconstant, shal- 

 low, relatively small, and frequently withdrawn in part or entirely. 

 Even in the same geological provinces the outlines of the new sea 

 may agree essentially and often very closely, in parts, with the next 

 preceding or some earlier sea, but in other parts the new shore line 

 departs radically from the older. 



These movements occurred in Paleozoic ages which, unlike the 

 Pleistocene, have left no record of great ice accumulations. Doubt- 

 less even in the Paleozoic there were times of relative frigidity, 



