CHAPTER IV 



Geological Time and Methods of Reckoning 



There are a great many ways of estimating the duration of geo- 

 logical time or the age of the earth. This is a fascinating subject 

 that has always invited speculation by physicists and biologists 

 as well as geologists. Most of these methods are dependent on 

 the data furnished by the comparison of past processes with present 

 processes, such as the rate of the formation of peat in bogs, or of 

 stalactites in caverns, or the rates of accumulation of different 

 kinds of sedimentary materials. Other methods take as their 

 basis the rate of cooHng of lavas, the amount of salts in solution 

 in the oceans compared with the rate at which rivers are bringing 

 them down to the seas, the effects of tidal stresses in retarding the 

 earth's rotation, the rate at which the sun is supposed to be losing 

 its heat, and by other less obvious methods, such as the state and 

 rate of change of radium minerals in rocks. 



These various estimates vary through very wide limits since 

 no method has been discovered that does not involve a variety of 

 unknown factors. There is, however, a certain amount of con- 

 cordance in them all and it is possible by carefully estimating the 

 total thickness of the sedimentary rocks and comparing the rate 

 of accumulation at the present time of the different kinds of rocks 

 such as limestones, sands and clays to get a rough approximation 

 of the time necessary to account for the deposition of similar 

 materials in past times. That the results are not accurate is 

 obvious when it is recalled that conditions of erosion, amount of 

 material and all of the other factors of the environment vary greatly 

 so that one limestone may accumulate rapidly and another slowly. 

 Furthermore very Httle account can be taken of the time that is 

 represented by the numerous intervals in geological history when 

 the rocks were being worn away by erosion and the resulting de- 

 posits are beyond our ken, or when the surface of the land was so 



14 



