CONVERGENCE OF EVIDENCE 883 



that the relative lengths and the order of magnitude were satisfied by the 

 following : 



Era Time duration 



Years 



Cenozoic, inclufling Pleistocene 2,900,000 



Mesozoie 7,240,000 



Paleozoic 17,500,000 



Algoiikiaii 17,500,000 



Archean 10,000,000 ( ?) 



Schuc'liert, following Walcott's results for the length of the eras, pub- 

 lished in 11)10 an estimate of the proportionate time to be given to each 

 period from the beginning of the Paleozoic.^*^ These figures are listed in 

 the following table, page 884. Lately, however, in a still unpublished 

 manuscript,^*^ he has revised his ratios and concedes a far longer duration 

 for geologic time. 



Sollas, in 1909, revised upward his previous estimates, giving 80,000,000 

 years as the most probable value of the age of the earth, as determined by 

 the salt of the sea. He still estimated the duration of each period by 

 means of the maximum thicknesses of sediments known in each, taking 

 one foot as equivalent to a century and making up the balance of the 

 80,000,000 years by means of the time value assumed for unconformities. 

 Following his method of measuring time by means of the maximum 

 known thicknesses of sediments accumulated in each period, Sollas would 

 with present data presumably raise the Jurassic to 2,000,000 years, since 

 in northwestern Alaska the Cape Lisburne section shows about 20,000 

 feet of sediments, of which 15,000 bears plant fossils showing Jurassic 

 g^gg 143 j|- -^Yould appear on this basis that the estimate for the Triassic 

 might also be raised, since the very thick Newark deposits are restricted 

 to Upper Triassic time. ^ 



The estimates by these geologists may be greatly magnified, but the 

 ratios are nevertheless of value as expressing their judgment as to the 

 relative lengths of the periods by those who have made special studies of 

 stratigraphy. They have been made on a uniformitarian basis, with a 

 knowledge of the limitations set by Lord Kelvin, and without reckoning 

 in the significance of rhythms, but they must form a starting point for 

 any revision of the duration of the several periods. These authors place 

 more reliance on the relative length of the periods as expressed by ratios. 



1*1 C. Schuchert : Paleogeography of Nt)rtli America. Bull. Geol. Soc. Am., vol. 20, 

 1910, plate 101. 



1*2 C. Schuchert : The earth's changing surface and climate. Chapter II of The evolu- 

 tion of the earth and its inhabitants. Yale Sigma Xi lectures for 1916-1917 ; to he pub- 

 lished. 



"3 A. J. Collier: Bull. TI. S. Geol. Survey, Nos. 218, 259, 278, 1903, 1906. 



