264 ROCHESTER ACADEMY OF SCIENCE. [April 2^^ 



as we see it, is the result of steady action of the same geologic forces 

 and agencies that are working to-day, it was naturally believed that 

 the age of the earth must be of indefinite duration. For merely the 

 sedimentary rocks a minimum time of hundreds of millions of years 

 was claimed. For the pre-Silurian and crystalline rocks, and the 

 preceding molten stage of the earth no limit could be given. This 

 was the inevitable swing of the intellectual pendulum away from the 

 catastrophic or cataclysmic theory and the l>iblical chronology. 



To these ex'treme views a check was given by the physicists. In 

 1862 Sir William Thompson challenged the geologists by announcing 

 that from the laws of heat radiation not over loo millions of years 

 could be allowed for the cooling of the earth to its present condition 

 from a fluid state. Other physicists later gave much less range of 

 time. The geologists were led to moderate their claims, and to make 

 closer estimates, until now there is substantial agreement between the 

 two classes of scientific men. 



By a comparison of the character and amount of sediments the 

 relative lengths of the great geologic time divisions are not diffi- 

 cult to approximate. But a determination in years is difficult because 

 of the lack of any constant quantity with time value. As a time unit 

 various phenomena have been taken ; the rate of degradation of the 

 continents ; the growth of river deltas ; the formation of river 

 canyons ; and the arhount of rock disintegration and stream erosion 

 since the ice invasion in our northern lands. The results are confess- 

 edly inexact, but have a fair agreement. 



In the past year three important essays upon the subject have 

 appeared, one from the physical standpoint, and two from the geolog- 

 ical. In the Januar}', 1893, issue of the American Journal of Science, 

 Mr. Clarence King revises the physical conclusions in the light of 

 new data upon the behavior of diabase rock under experimental 

 conditions of heat and pressure. His conclusion is that the age of 

 the earth since its molten state cannot be over 24 million years. An 

 article by Mr. Warren Upham in the March, 1893, number of the 

 same journal, reviews the arguments and estimates of earlier writers, 

 and favors 48 million years for our stratified rocks (since beginning 

 of Cambrian time), or 100 millions for geologic time (since the ocean 

 existed). The Vice-Presidential Address of Mr. C. D. Walcott 

 before Section E of the American Association for the Advancement 

 of Science, at Madison, Wisconsin, in August last, was printed in the 

 American Geologist in December. By a careful and detailed study of 



