<S14 J. 15AKKEJ.L MEASUREMENTS OF GEOLOGIC TIME 



lied than W. D. Mattliew to weigh this evidence for the evolution of tlie 

 mammals through the Cenozoic. He has discussed this especially for the 

 horses, but finds the results supported by a comparison with the evolu- 

 tion in other mammalian phyla. *^ On this basis Matthew estimates that 

 the entire Tertiary is about one hundred times as long as the Pleistocene, 

 dating the latter from the first great glacial advance. His impressiou 

 from the evolution of vertebrates during the Mesozoic is that each of it< 

 four periods were comparable in length to the Tertiai-y. Matthew tlic i 

 raises the question as to what length of time should be adopted for the 

 Pleistocene, the unit of measurement. He takes 100,000 years as repre- 

 sentative of the more moderate figures between the 25,500 years of Wriglil 

 and the 1,500,000 of Penck and other authorities. The resulting estimate 

 of 10,000,000 years for the Tertiary and 40,000,000 for the Mesozoic doe- 

 not seem to Matthew unreasonable. 



Matthew's results show a leaning toAvard higher estimates than nearly 

 all stratigraphic geologists have admitted, but he has kept them down only 

 by adopting what will seem to most students of the Pleistocene an ultra- 

 conservative estimate of its shortness. With the recognition of the com- 

 plexity of the Pleistocene and the amount of diastropliism and erosion 

 accomplished in it, 500,000 years has come to be set as a minimum esti- 

 mate of its length, rather than 100,000 years. As an upper limit, on the 

 other iiand, 1,000,000 to 1,500,000 years are not unreasonable figures, and 

 the higher of these is adopted by Penck, one of the foremost authorities 

 on the Pleistocene. Using only 500,000 years as the unit, Matthew's 

 ratios give a length of 50,000,000 years for the Cenozoic and 200,000,000 

 years for the Mesozoic. These figures for the length of the Cenozoic and 

 Mesozoic the writer believes to be of the right order of magnitude and 

 supported by strong evidence, to be brought out in the later part of this 

 paper. 



The foregoing discussion suggests that the early estimate of 3,000,000 

 years, which has entered so largely into geologic literature, may well be 

 multiplied by twenty. The evidence from radioactivity, to be presented 

 later, suggests that 60,000,000 years may readily be granted to the Ceno- 

 zoic era, and the geologic evidence as interpreted in this article is in har- 

 mony with this magnification, though, being qualitative rather than quan- 

 titative, it could, if necessary, be interpreted in terms of half this tinvj. 

 The geologic evidence can not be adjusted, however, to an age as short 

 as 10,000,000 years without ignoring the factors discussed in the earlier 

 parts of this article, and the estimate of 3,000,000 years is clearly hu 



*^ W^ D. Matthew: Time ratios in the evolution of the mammalian phyla. A contr 

 bution to the problem of the age of the earth. Science, vol. xl, 1914, pp. 232-235. 



