26 The Ma\e-Up and the Wordings of the Earth 



nineties of the last century. The transformation of the heavy 

 element uranium is known to proceed at a very definite rate. 

 Moreover, this rate appears to be uninfluenced by tempera- 

 ture or by any of the other conditions to which the uranium 

 can be exposed in the laboratory. It is possible for the skill- 

 ful worker in the laboratory to determine the proportions 

 of uranium and lead in certain minerals, and from this pro- 

 portion to calculate the age of the minerals, assuming that 

 all of the lead present is derived from uranium. Thus, pitch- 

 blende obtained from the Black Hills was estimated to be 

 1,500 million years old. A mineral from very old rocks 

 in Norway appears to be 900 million years old. Studies of 

 the atomic weight of lead enable the chemist to tell whether 

 a given sample was lead to begin with, so to say, or derived 

 from the breaking down of uranium. Determination of the 

 ratio of uranium and lead in minerals of corresponding pe- 

 riods obtained from different parts of the world show re- 

 markably close agreements. 



We must remember that in dealing with such enormous 

 figures, where a few million years more or less are of relatively 

 slight consequence, there is necessarily a considerable range 

 of disagreement among the various calculations. On the 

 other hand, there is a tendency toward substantial agreement 

 regarding the age of the earth as the various methods of 

 calculation become more refined. 



Relative Ages of Different Formations 



After all this speculation regarding the age of the earth, 

 it must be admitted that for our purpose it does not really 

 matter how old the earth may be. It is important for us 

 only (i) that we be clear as to the facts and assumptions 

 upon which the age of the earth is calculated; and (2) that 

 we be clear as to the reliability of our judgments regarding 

 the relative ages of the various layers. 



Wherever the layers or strata are perfectly horizontal 

 it is readily agreed that they resulted from sedimentation of 



