400 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Willis's views on periodicity of diastrophism. — Willis^^ recently pub- 

 lished a brief statement of his views on the periodicity of geologic events 

 and processes. He recognizes the principle, but qualifies it by grouping 

 the phenomena of diastrophism according to several distinct dynamic re- 

 gions, each of which he supposes to have had its individual history, "ex- 

 pressed in cycles of movement and quiescence peculiar to itself." "The 

 cycles of one region have been, however, to some extent parallel, though 

 not coterminous, with the cycles of other regions, and thus major cycles 

 of world-wide conditions are constituted by coincidence of regional condi- 

 tions." This qualification is based on the belief that the diastrophic his- 

 tory of the lands adjacent to the north Atlantic differs in essential par- 

 ticulars from that of the lands bordering the Arctic and Pacific oceans. 

 He says that the "lands about the Arctic Ocean did not share in the Atlan- 

 tic movements of Silurian, Devonian, and late Paleozoic epochs;" that 

 "in the Atlantic provinces the Paleozoic era closed with marked diastro- 

 phism, while comparative tranquillity reigned around the Pacific," and 

 that "the Pacific provinces were greatly disturbed in the middle Mesozoic 

 when quiet had supervened about the Atlantic." 



On the face of the accessible sedimentary record in the respective re- 

 gions these allegations may seem well founded. But even the visible 

 part of the evidence is not altogether in favor of the quoted statements. 

 Neither are we justified in claiming, because abundant clastic deposits 

 and other indications of active diastrophism are preserved in one area 

 and little or nothing of the kind is found in another at approximately 

 the same age of the geologic time scale, that the two areas did not share 

 in the same deformative movements. The share in the one case may 

 have been small, in the other large. In fact, no two areas, however near, 

 participated in exactly the same way or to exactly the same degree in the 

 movements of any period. The more conspicuous phenomena induced 

 by body deformation of the earth, such as mountain making, are merely 

 local eft'ects of the movement as a whole. While the positive areas were 

 elevated, immediately adjacent negative areas were relatively depressed; 

 and more distant regions may have shared in the movement only to the 

 extent of either gentle subsidence or perhaps slight emergence. Whether 

 it built mountains in one place and caused but gentle warpings in an- 

 other, or whether the mountains bordering one oceanic basin are contem- 

 poraneous with a similarly located slight elevation in another, or even a 

 subsidence in a third, they are all only different manifestations of the 

 same period of diastrophism. 



85 Bailey Willis : "Principles of Paleogeography." Science, vol. xxxi, No. 790, Feb. 18, 

 1910, pp. 246-249. 



