CONVERGENCE OF EVIDENCE 891 



their life cycle the gill-breathing of their ancestors. ^**^ During the Per- 

 mian the reptiles clearly dominate o^'er the amphibians and were differ- 

 entiating into those orders which ruled the later periods. Thus, from the 

 standpoint of vertebrate evolution the Age of Eeptiles began in the Penn- 

 sylvanian, was established in the Permian, and ended with the Cretaceous. 

 This is in fact the classification proposed by Ulrich^*^ on the basis of 

 diastrophism. The diastrophic and organic classification can then be 

 brought into adjustment by placing the periods which mark crescendoes 

 of diastrophism as the beginnings of new eras, not the ends of old ones. 

 This is more logical and'accords with the system of human history in 

 which revolutions and their changed political conditions are placed as 

 the opening events of new eras. It is doubtful, however, if this logical 

 view will prevail in geology, since the older classification is solidly in- 

 trenched in the literature of a century. 



On this new basis the Mesozoic, beginning with the Pennsylvanian and 

 ending with the Cretaceous, would include 205,000,000 years on the 

 minimum estimate of time, 270,000,000 years on the maximum estimate. 

 The N"eopaleozoic, embracing the Silurian-Mississippian periods, would 

 include 140,000,000 or 130,000,000 years, thus falling short of the normal 

 measure of an era. But the Eopaleozoic would include the revolution 

 preceding the Cambrian. The Torridonian, Keweenawan, Beltian, and 

 Grand Canyon systems, constituting the Lipalian era of Walcott, should 

 apparently go into the Eopaleozoic. If the Morogoro granites of Africa, 

 with an age of 660,000,000 years, mark a time in this Lipalian period of 

 revolution characterized by continental uplift and continental deposits, 

 then the Eopaleozoic also is delimited by a duration of 200,000,000 to 

 270,000,000 years, according to which scale of time is used. 



MEAN RATES OF EROSION AND SEDIMENTATION 



This table of geologic time carries several corollaries. The first to be 

 noted is the mean rate of sedimentation for the several eras which would 

 be required if the maximum thicknesses represented continuous sedimen- 

 tation, as postulated by Sollas in his work previous to 1909 and by most 

 geologists writing previous to the present decade. It has been argued in 

 the present paper that this postulate is very far from being true, the very 

 nature of sedimentation implying the presence of breaks of all orders of 

 magnitude, the proportion of lost intervals increasing with the more dis- 

 tant eras. In the axes of maximum thickness, however, the disconti- 



i*« Joseph Barrel! : Origin and significance of tlie Mauch Chunk shale. Bull, Geol. Soc. 

 Am., vol. 18, 1907, pp. 449-476 ; see pp. 469-474. 

 "7 Loc. cit., plate 26. 



