THE BIRTH-TIME OF THE WORLD 39 



in the early crust of the earth. Sediments are scarce among 

 these materials. 1 



There can be little doubt that in this underlying floor of 

 igneous and metamorphic rocks we have reached those surface 

 materials of the earth which existed before the long epoch of 

 sedimentation began, and before the seas came into being. 

 They formed the floor of a vapourised ocean upon which the 

 waters condensed here and there from the hot and heavy 

 atmosphere. Such were the probable conditions which pre- 

 ceded the birth-time of the ocean and of our era of life and 

 its evolution. 



It is from this epoch we date our geological age. Our next 

 purpose is to consider how long ago, measured in years, that 

 birth-time was. 



That the geological age of the earth is very great appears 

 from what we have already reviewed. The sediments of the 

 past are many miles in collective thickness : yet the feeble silt 

 of the rivers built them all from base to summit. They have 

 been lifted from the seas and piled into mountains by move- 

 ments so slow that during all the time man has been upon 

 the earth but little change would have been visible. The 

 mountains have again been worn down into the ocean by 

 denudation and again younger mountains built out of their 

 redeposited materials. The contemplation of such vast events 

 prepares our minds to accept many scores of millions of years 

 or hundreds of millions of years, if such be yielded by our 

 calculations. 



The Age by the Thickness of the Sediments 



The earliest recognised method of arriving at an estimate 

 of the earth's geological age is based upon the measurement 

 of the collective sediments of geological periods. The method 

 has undergone much revision from time to time. Let us 

 briefly review it on the latest data. 



The method consists in measuring the depths of all the 

 successive sedimentary deposits where these are best developed. 

 We go all over the explored world, recognising the successive 

 deposits by their fossils and by their stratigraphical relations; 



1 For a description of these early rocks, see especially the monograph of Van 

 Hise and Leith on the Pre-Cambrian Geology of North America (Bulletin 360, 

 U.S. Geol. Survey). 



