Febkuaet is, 1910] 



SCIENCE 



24tl 



wth cycle. 



(«+ 1) cycle. 



> («. + 2) cycle. 



more difficult to define the respective 

 periods. 



It is possible to recognize at least three 

 grand cycles from late pre-Cambrian time 

 to the close of the Mesozoie, each grand 

 cycle consisting of a long period of activ- 

 ity and a still longer time of relative quiet, 

 thus : 



Late pre-Cambriau activity (ap-"| 

 parently somewhat general ) , ( 

 Cambrian and Ordovioian inac- 

 tivity ( general ) , J 

 Silurian and Devonian activity 1 

 (qualified by circumpolar quiet 

 in the north), r 

 Late Devonian and early Carbon- 

 iferous inactivity (general), J 

 Later Carboniferous and early ^ 

 Mesozoie activity (non-contem- 

 poraneous in different dynamic 

 provinces), 

 Later Mesozoie and early Tertiary 

 inactivity ( general ) , 



Later Tertiary and Quaternary then con- 

 stitute the initial, ,°"tive period of the 

 («. -j- 3) cycle. 



While these grand cycles may be recog- 

 nized for the whole world as far as we 

 know the facts, it is found that each one 

 may be divided into epicycles consisting of 

 shorter periods of emergence and sub- 

 mergence especially if attention lie fixed 

 upon a single ocean iasin and the conti- 

 nents adjacent to it. The North Atlantic, 

 for instance, is bounded on the east and 

 west by lands, which have been disturbed 

 or have been at rest during the same 

 epochs, and the several cycles have been of 

 much shorter duration than those enumer- 

 ated above for the whole world. These 

 cycles are indeed those on which the time- 

 scale of geologic history is based, and each 

 one corresponds in general with a standard 

 period. Carboniferous, for example. 



Lands about the Arctic Ocean did not 

 share in the Atlantic movements of Silur- 

 ian, Devonian or late Paleozoic epochs. 



On the contrary, the great epicontinental 

 seas of those periods were circumpolar. 

 Nor do lands about the North Pacific, from 

 California to China, record a history par- 

 allel with that of eastern North America 

 and northwestern Europe, with the At- 

 lantic history. In the Atlantic provinces, 

 the Paleozoic era closed with marked dias- 

 trophism, while comparative tranquillity 

 reigned around the Pacific ; but the Pacific 

 provinces were greatly disturbed in the 

 middle Mesozoie when quiet had super- 

 vened about the Atlantic. Again, a dis- 

 tinct series of movements is recorded in the 

 great geosyncline of Eurasia, that which 

 stretches from India to Spain and is now 

 marked by the system of mountain chains 

 of which the Himalaya and the Pyrenees 

 are the extremities. Similar movements 

 appear to characterize the West Indies 

 and northern South America. If, as I 

 believe, these parallel movements in Eu- 

 rasia, South America and the Indies origi- 

 nated in a common dynamic region, then 

 that region is the great ocean of the south- 

 ern hemisphere, including the South At- 

 lantic, the South Pacific and the Indian 

 oceans. 



The principle of periodicity is necessar- 

 ily qualified by these facts and the general 

 law should be supplemented by one which 

 recognizes unlike dynamic histories of dif- 

 ferent oceanic regions. It may be stated 

 thus : 



Tke phenomena of diastrophism are 

 grouped according to several distinct dy- 

 namic regions. Each region has experi- 

 enced an individual history of diastro- 

 phism, in which the law of periodicity is 

 expressed in cycles of m,ovement and 

 quiescence peculiar to the region. The 

 cycles of one region have been, however, 

 to some extent parallel, though not conter- 

 minous, with the cycles of other regions, 

 and thus major cycles of ivorld-tvide condi- 



