ESTIMATES OF TIME 813 



has discussed these features for the mountains of central Wyoming.*^ 

 The magnitude of the work of erosion and the number of events since 

 the opening of the Miocene are also illustrated in the Coast ranges of 

 California. The newer interpretations of the structure and erosion his- 

 tory of the Alps testify to the great length and complexity of the geolog- 

 ical history of the Miocene and Pliocene as compared to the Pleistocene. 

 The maximum thicknesses of sediments deposited in the several periods 

 have been made the basis by Sollas of estimates of age. The increase in 

 knowledge concerning the divisions of the Tertiary is represented by the 

 increase in his figures from the year 1900 to 1909. These are as fol- 

 lows :*^ 



A. D. 1900 A. D. 1909 

 Feet Feet 



Recent and Pleistocene 4,000 4,000 



Pliocene 5,000 13,000 



Miocene 9,000 14,000 



Oligocene 12,000 12,000 



Eocene 12,000 20,000 



42,000 63,000 



Turning to a reinterpretation of the data, it is to be noted that discon- 

 formities and unconformities occur between many of the formations, and 

 reasons have been discussed in connection with the significance of valley 

 forms for believing that the mean rate of denudation through the Tertiary 

 probably has not equaled more than half of the rate for the Pleistocene 

 and the Recent. From these varied lines of evidence it seems reasonable 

 to regard the total Pleistocene as at least 1,500,000 years in length, and 

 the younger Tertiary, including the Miocene and Pliocene, as perhaps ten 

 times as long as the Pleistocene. But back of the younger Tertiary lies 

 the earlier Tertiary. This was formerly all embraced in the Eocene and 

 lower Miocene, but is now subdivided into Oligocene and Eocene, and 

 the basal Eocene in turn is now set apart by many as the Paleocene. 

 The older Tertiary was probably considerably longer than the younger 

 Tertiary, perhaps twice as long, and the whole of the Cenozoic is as much 

 as twenty and more probably thirty times the length of the Pleistocene. 



Another line of attack is found in estimates of the amount of organic 

 evolution accomplished in successive epochs, not forgetting, however, that, 

 as evolution depends largely upon changes in environment, the times of 

 rapid change will also be times of rapid evolution. ISTo one is better quali- 



*8 Eliot Blackwelder : Post-Cretaceous history of the mountains of central western 

 Wyoming. Jour. Geol., vol. xxiii, 1915, pp. 97-117, 193-217, 307-340. 



*' W. J. Sollas : Presidential address. Quart. Jour. Geol. Soc, vol. 65, 1909, p. cxii. 



