844 J. BARRELL MEASUREMENTS OE GEOLOGKJ TIME 



their radioactive properties, the electroscope and the fluorescent screen 

 enable the investigator to identify the substances and even to count the 

 atoms which disintegrate. As a result, the half-value period of radium 

 ma}^ be determined by comparison with that of the radium emanation, 

 and uranium by comparison with radium, as well as by other ways. Thus 

 the accuracy with which the rates of decay can be measured, the agree- 

 ment by different methods, and the great duration of uranium unite to 

 fit the series to serve as a measure of geological time. 



The rate of decay is not affected by the nature of chemical combination 

 or physical state. Temperatures up to 2,500° C. and pressures up to 160 

 tons per square inch have been found not to influence the rate of decay 

 of the products of radium. It is highly probable but not as yet actually 

 demonstrated that uranium is similarly unaffected. Thus the atoms of 

 uranium break up with a uniform rate whether they are in elemental 

 form or combined in a salt; whether they are in solid, liquid, or gas. Of 

 course there must be conditions under which an element which is now 

 breaking down was built up, but such a physical environment would ap- 

 pear to be so extreme that it must occur under unknown cosmic conditions, 

 and there is no reason to expect the modification of disintegration rates 

 in the outer crust of the earth, a region in which temperatures and pres- 

 sures are both relatively low. 



The uranium minerals are commonly found associated with pegmatite 

 dikes. Since the time of their growth their temperatures have been well 

 below the crystallization temperature of granite and the pressures have 

 diminished with the progress of erosion from an initial maximum of 

 possibly twenty or thirty thousand pounds per square inch to atmospheric 

 pressures as they are exposed at the surface. From the time radioactivity 

 attained equilibrium in the uranium or thorium mineral, the end prod- 

 ucts have been accumulating within it consequently at a uniform rate. 

 These in a dense .crystalline rock are not removed unless the minerals are 

 subjected to passing solvents, which would then surely record their effects 

 by the alteration of the mineral itself. An atom of uranium which breaks 

 up will ultimately give rise as stable products to eight atoms of helium 

 and one atom of lead. Tf the quantity of these can be measured and 

 compared with the quantity of uranium in the same volume of material, 

 data are obtained for measuring the age of the mineral and with it the 

 age of the rock formation of which it is a part. 



Following this introduction, the methods of measuring the earth's age 

 by radioactive processes and the results which have been attained may be 

 considered in more detail. For this purpose the best presentation will be 



