890 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



of a new perspective, there are not enough terms, nor are they sharply 

 enough defined, above those of lesser magnitude to give this indication 

 more than such suggestive value. In regard to the lesser rhythms, con- 

 sisting of the geologic periods and leading up to crescendoes of two or 

 three geologic periods in span, the evidence is more clear. It is made 

 apparent in this new time-table through the time-spacing which is neces- 

 sary in order to bring adjustment between the radioactive and strati- 

 graphic evidences. In such crescendoes, marking the' liberation of forces 

 pent up in the rigid body of the earth, we should not expect too much 

 regularity of recurrence, nor similarity of expression, and yet such 

 rhythms, the heart-beats of the earth-mother, running through her life- 

 time, must serve as the great division points determining the crises in 

 the evolution of the protoplasmic life which she has born. 



A graphic representation of these composite rhythms in diastrophism. 

 showing their rise into crescendoes, is to be published by Professor LuU, 

 to which he adds the graph of the pulse of life.^*^ 



There are seen to be grounds for maintaining that, from the stand- 

 point of diastrophism as a basis of correlation, the Eecent is a part of 

 the Pleistocene, and the Pleistocene an epoch of the Neogene period. 

 The older Tertiary, or Paleogene, is a separate period, and both are divi- 

 sions of the same order of iriagiiitude as the four periods of the Mesozoic. 

 They stand in -somewhat the same relation to these earlier periods as the 

 Permian and Pennsylvanian do to the preceding periods of the Neo- 

 paleozoic. 



This brings out the difference between a classification founded on a 

 diastrophic basis which begins an era with the close of a period of revolu- 

 tion and that founded on an organic basis. From the latter standpoint 

 the Cenozoic, the Age of Mammals, is a separate era from the Mesozoic, 

 the Age of Eeptiles. But, for that matter, the reptiles date back into 

 the Pennsylvanian. They seem there to occupy a place subordinate to 

 the amphibians, but this is largely due to the fact that the coal swamps 

 were the natural habitat of the amphibians; the drier uplands that of 

 the reptiles. The sedimentation in the habitat of the amphibians ac- 

 counts largely for the more abundant record of these lower forms. The 

 aridity of the upper Mississippian must have been a stimulating cause, 

 acting on ancestral amphibians, dependent on a life in water in embryonic 

 and adolescent stages, which urged their transition into a class of verte- 

 brates whose eggs were incubated in air and whose young dropped from 



"■"• R. S. Lull : The pulse of life. Chapter IV of the evolution of the earth and its 

 inhabitants. Yale Sigma Xi lectures for 1916-1917. 



