892 PRINCIPLES OF STRATIGRAPHY 



also seems to point to the phenomenon of polar migration. (Sim- 

 roth-37.) The signs further point to the existence, in the border 

 regions of the present Atlantic, of large continental masses which 

 became submerged with the close of the PaLneozoic. Thus Appa- 

 lachia, the source of clastic sediments in eastern North America 

 during the whole of Palaeozoic time, disappeared at the end of that 

 period ; the disappearance being coincident with, or closely related 

 to, the formation of the Appalachian folds. Similar conditions have 

 been ascertained for western Europe. As demonstrated in an ear- 

 lier chapter, the clastic sediments of North America indicate that 

 in Palaeozoic time this continent came under the influence of the 

 trade winds, at least so far as the eastern portion is concerned, 

 while in ]\Iesozoic and later time it had passed into the region of 

 the westerlies. This suggests that the end of Palaeozoic sedimenta- 

 tion was brought about by a profound change in physical geography 

 which affected not only North America, but also the earth as a 

 whole. A comparatively rapid shifting of the position of the earth's 

 axis would be sufficient explanation of the profound changes which 

 brought about the all but universal extinction of life, so that only 

 the pelagic and deep-sea types of the oceans (especially of the 

 Pacific), and the terrestrial types in the interior of the continents, 

 remained unaffected. These probably formed a source from which 

 the earth was repopulated after the widespread extermination 

 of life on the margins of the continents and in shallow water. 

 That migrations of the poles occurred before and after this great 

 cataclismic shifting is indicated by the nature of the sediments, 

 the changes in climate and life during the successive periods, and, 

 especially, by the occurrence of glacial conditions in Pleistocenic 

 times in regions not now subject to such conditions. (See ante, 

 Chapters II and VIII.) The map (Fig. 245), copied from Walther, 

 shows the location of the Pleistocenic ice sheets, and the suggested 

 position of the pole, to account for its occurrence. All such wan- 

 derings of the poles, however, if they occurred, are insignificant 

 in comparison with the catastrophic change which brought to an 

 end the period of Palaeozoic sedimentation and biologic develop- 

 ment. 



The Pendulation Theory. Though frequently suggested as 

 a possible cause of changes in the geologic development of the 

 earth, the theory of polar migration seems to have been first def- 

 initely formulated by Paul Reibisch in 1901. Reibisch developed 

 the theory of polar pendulation, or swinging, i. e., a back and forth 

 migration of the poles along certain well-defined paths. Dr. Hein- 

 rich Simroth, of the University of Leipzig, developed this theory 



