The Age of the Earth 25 



It is in this way that scientists build upon observed facts 

 inferences as to past happenings. It is by the careful 

 measurement of present-day happenings that they build up 

 inferences as to the rate at which events of various kinds 

 took place in the past. From such studies and calculations 

 various estimates have been made regarding the age of the 

 earth. These estimates of the earth's age, made by various 

 methods, do not, it is true, agree very closely with one another, 

 since it is necessary to speak in terms of millions of years. 



The Age of the Earth 



A consideration of salt in the sea has served to furnish 

 one series of estimates. From the careful analyses of waters 

 that empty into the ocean we may form an estimate of how 

 long it took for the salt in the ocean to accumulate. Un- 

 fortunately, estimates of the total amount of water in the 

 ocean must leave a rather broad margin of error. 



Similarly, we may make estimates of the earth's age from 

 the thicknesses of various layers of deposits of sandstone or 

 other sedimentation, from the rate at which exposed rocks 

 are worn away by weathering and rain, from the rates at 

 which glaciers move, and so on. In every case there is neces- 

 sarily a considerable element of inaccuracy. The greatest 

 objection to each of these methods, and the probable reason 

 for their lack of agreement, is that we cannot be sure that the 

 processes measured went forward at a uniform rate. Salts 

 may have been carried to the sea much more rapidly, or much 

 more slowly, in past ages. Sedimentation in the ocean might 

 have been faster or slower, depending perhaps upon the den- 

 sity of the waters due to the amount of salt in solution, or 

 upon varying depths. 



The most reliable methods for estimating the age of the 

 earth have been developed in very recent times from a study 

 of the curious fact that the element uranium steadily 

 throws off certain particles or emanations and becomes con- 

 verted into lead. These discoveries have been made only since 

 the discovery of radium by Pierre and Marie Curie in the 



