748 GEORGE DAVIS LOUDERBACK 



0.613 X io~" to 9.56X io~^^ grams of radium per gram of rock. All 

 samples showing above 3.37X10"'^ grams of radium per gram of 

 rock are granites or syenites ; the basalts and peridotites are all below 

 1.86X10-". 



Strutt shows that if the interior of the earth be considered at thermal 

 equilibrium, and if the whole amount of heat is due only to radium 

 (part must be due to uranium and other active elements), and "always 

 supposing that the heat production of radium is not materially dimin- 

 ished, under conditions prevailing inside the earth;" if, further, the 

 radium be considered evenly distributed throughout the earth mass, it 

 cannot much exceed i. 75X10"'^ grams per cubic centimeter. Ruther- 

 ford, ^ using somewhat different values for temperature gradient, etc. , 

 calculates the equivalent in radium bromide as 2.6X10^^3 grams per 

 cubic centimeter or 1.52 Xio"'^ grams radium. But the weakest 

 rock examined (Disco Island basalt) gives tests over ten times this 

 amount, and the granites 240 or 250 times! The question becomes, 

 then, why is the interior so cool ? Strutt offers the explanation that 

 only the external or crustal portions of the earth are radioactive. 

 Taking the moderate value of 5 X io~^^ grams radium per cubic centi- 

 meter as representing the average rocks of the crust, radioactive rocks 

 need extend only 45 miles below the surface to establish the present 

 temperature gradient. If we assume that the granites represent the 

 constitution of the outer crust, 5 or 6 miles are sufficient ; if basalts of 

 the weakest activity so far examined, only 96 miles are necessary. ^ 



As supporting his assumption of a crustal layer of 45 miles or less 

 in thickness and of different constitution from the interior, Strutt refers 

 to the conclusions of Professor Milne that a study of the propagation 

 of earthquake waves through the earth's interior indicates a rather 

 abrupt change at about 30 miles ' depth, the material below that being 

 rather uniform throughout the globe. 



1 Rutherford, Radioactivity, 2d ed. (Cambridge, 1905), p. 494. 



2 To account for the greater density of the interior of the earth, and from the hint 

 given by the constitution of meteorites, we may suspect that the inner parts of the earth 

 are of "basic" materials. The hmit of basic materials tested is found in meteoric iron, 

 which gives no measurable radioactivity. We may well suspect, then, that the radio- 

 active crust is rather thin, the amount of activity not being constant to a certain definite 

 point, but gradually decreasing toward a zero value at somewhere between 30 and 90 

 miles in depth. 



