192 GEOLOGY. 



ral origin. Certain formations in the London and Paris basins were taken as 

 the type of the Eocene, and they contained 3^ per cent of living species, as then 

 determined. Certain other formal ions in southern France, containing 17 per 

 cent of recent species, as then determined, were taken as the type oi the Miocene, 

 and others in Italy of much larger and varying percentages, as the type of the 

 Pliocene. Dana 1 generalizes the criteria as follows: Eocene, no species, or 

 less than 5 per cent living; Miocene, 20 to 40 per cent living; Pliocene, more 

 than half the species living. 



It is not surprising that it was soon found that this scheme did not 

 fit the facts in Germany, and an additional division, Oligocene (few recent), 

 was introduced between Eocene and Miocene, taking something from 

 each. In practice, the criteria have not been closely adhered to, and 

 movement toward a natural system has been in progress; but common con- 

 sensus of opinion as to what constitutes the true basis of a natural system has 

 not yet been reached, and the movement is not very definitely directed. There 

 are geologists who do not believe that there are natural divisions of general appli- 

 cability, the divisions that are natural for one region being unnatural for other 

 regions. With the qualification that all views must yet be put to the test when 

 the whole world shall have been carefully worked over, and that views now 

 expressed must not be held as authoritative, or even necessarily representative, 

 it is proper that we state our convictions, and their application to the unsettled 

 questions of Cenozoic classification and nomenclature. 



We believe that there is a natural basis of time-division, that it is recorded 

 dynamically in the profounder changes of the earth's history, and that its 

 basis is world-wide in its applicability. It is expressed in interruptions 

 of the course of the earth's history. It can hardly take account of all local details, 

 and cannot be applied with minuteness to all localities, since geological history 

 is necessarily continuous. But even a continuous history has its times and 

 seasons, and the pulsations of history are the natural basis for its divisions. 



In our view, the fundamental basis for geologic time divisions has its seat 

 in the heart of the earth. Whenever the accumulated stresses within the body 

 of the earth over-match its effective rigidity, a readjustment takes place. 

 The deformative movements begin, for reasons previously set forth, with a 

 depression of the bottoms of the oceanic basins, by which their capacity 

 is increased. The epicontinental waters are correspondingly withdrawn into 

 them. The effect of this is practically universal, and all continents are 

 affected in a similar way and simultaneously. This is the reason why the 

 classification of one continent is also applicable, in its larger features, to 

 another, though the configuration of each individual continent modifies the 

 result of the change, so far as that continent is concerned. The far-reaching 

 effects of such a withdrawal of the sea have been indicated repeatedly in the 

 preceding pages. Foremost among these effects is the profound influence exerted 

 on the evolution of the shallow- water marine life, the most constant and reliable 



1 Manual, 4th ed., p. 880. 



