ESTIMATES OF TIME 815 



inheritance from a time before a proper geological perspective had been 

 attained. 



MAGNITUDE OF THE PALEOZOIC ERA ON EVIDENCE CHIEFLY FROM 

 SEDIMENTATION 



Estimates of the duration of the Paleozoic era rest especiall}^ npon the 

 rate of accumulation of the strata and the valuation of the intercalated 

 disconformities and unconformities. In order to perceive if the data can 

 be equally well or even better interpreted to give greatly longer estimates 

 of time than has been customary, the lines of reasoning of two leadino- 

 geologists will be examined, Walcott and Sollas, since their work has had 

 great influence on geologic thought in these matters. 



Walcott, in his vice-presidential address before Section E of the Amer- 

 ican Association/® gave the results of a careful study of the deposits oi 

 the Cordilleran sea as a basis for estimating the length of Paleozoic time. 

 Giving ratios to the Paleozoic, Mesozoic, and Cenozoic as 12, 5, and 2, he 

 obtained the following results : 



Period Time duration 



Years 



Cenozoic, including Pleistocene 2,900,000 



Mesozoic 7,240,000 



Paleozoic 17,500,000 



Algonkian 17,500,000 



Archeau 10,000,000 ( V) 



Walcott then states : 



"It is easy to vary these results by assuming different values for area and 

 rate of denudation, the rate of deposition of carbonate of lime, etcetera ; but 

 there remains, after each attempt I have made that was based on any reliable 

 facts of thickness, extent, and character of strata, a result that does no^pass 

 below 25,000,000 to 30,000,000 years as a minimum and 60,000,000 to 70,000,000 

 years as a maximum for post-Archean geologic time. I have not referred to 

 the rate of development of life, as that is virtually controlled by conditions of 

 environment. 



"In conclusion, geologic time is of great but not of indefinite duration. I 

 believe that it can be measured by tens of millions, but not by single millions 

 or hundreds of millions of years." ^° 



As a basis for these figures Walcott takes the area of the Cordilleran 

 sea as 400,000 square miles, the area of Cambrian drainage into it as 

 1,600,000 square miles, a minimum thickness of Cambrian mechanical 

 sediments below the Tipper Cambrian as 10,000 feet, a rate of erosion of 



*» C. D. Walcott : Geologic time, as indicated by the sedimentary rocks of North 

 America. Smithsonian Report for 1893, pp. 301-334, 1894 ; Jour. Geology, vol. iii, 1893 

 pp. 639-676. 



50 Jour. Geology, vol. 1, pp. 675, 676. 



LXI — Bull. Geol. Soc. Am., Vol. 28, 1916 



