CONVERGENCE OF EVIDENCE 889 



great Himalayan-Alpine mountain system from the geosynclines of an- 

 cient Tethys. Great movements took place also in western North and 

 Sonth America. The continents have been warped and the sealevel de- 

 pressed in the Pliocene and Pleistocene to a degree not known to have 

 occiirred before since the earlier eras of earth history. Far-spreading 

 giaciation and aridity have oscillated and rendered large areas of the 

 lands almost or entirely uninhabitable. These conditions have by no 

 means disappeared and the recent retreat of giaciation to high altitudes 

 and latitudes must not be mistaken for the end of the period. In fact 

 the upper Pleistocene appears to have shown a greater persistence of 

 giaciation than was true of the middle or earlier parts of the epoch. 



The Neogene should be used consequently as a term to include the 

 Pleistocene and Eecent, as proposed by Ulrich in 1911/^* but its course 

 should be looked upon as only one-half run. It is seen that the Ap- 

 palachian revolution began with the opening of the Pennsylvanian period 

 and pulsed through this and the Permian. The Laramide revolution 

 began in the closing epochs of the Cretaceous, but it is only a part of a 

 greater cycle of revolution whose convulsive phases have recurred with 

 increasing magnitude through the Cenozoic, consisting of two periods* 

 and whose end lies still hidden in the future, rather than revealed in the 

 past. Comparing the march of diastrophism in the Cenozoic with that 

 through the Carboniferous, beginning with the Pennsylvanian, it is seen 

 that the presenj epoch, which the turning wheel of time will probably 

 reveal to be but an interglacial stage of the Pleistocene, corresponds to 

 about the middle of the Permian. 



The Upper Cenozoic stands so close to us that we see it in detail, and 

 conspicuously subdivided into epochs, emphasized by the rapid evolution 

 of mammals ; but the Permian was similarly subdivided, and^ if the 

 Neogene were as remote from us in geologic time it would doubtless be 

 seen in more accurate perspective in the whole history of the earth, as 

 the period of revolution closing a great era which began with the Triassic. 

 Taking the length of the Mesozoic from the minimum column of geologic 

 time as 135,000,000 years, adding 55,000,000 years for the length of the 

 Cenozoic and 10,000,000 years as yet to be fulfilled, gives to this whole 

 Mesozoic-Cenozoic era a length of 200,000,000 years. 



There is a human tendency, however, to seek for over-much regularity 

 in nature and it is doubtful if much weight should be attached to this 

 cycle of approximately 200,000,000 years. Although extremely suggestive 



"*E. O. Ulrich : Revision of the Paleozoic systems. Bull. Geol. Soc. Am., vol. 23, 1011, 

 pp. 281-680 ; see pi. 26, p. 376. 



