BEARINGS OF RADIOACTIVITY ON GEOLOGY 677 



The special distribution of the radioactive substances among 

 the different kinds of igneous rocks is no doubt full of meaning, 

 but as yet the determinations have not been sufficient to justify 

 more than a few broad generalizations, and these must be held 

 subject to revision. 1 It may be said safely that the igneous rocks 

 carry a higher ratio of radioactive substance than the average 

 sediments. The reason for this is simple. The sediments are 

 derived from the igneous rocks, and in the process of derivation 

 some of the radioactive matter inevitably goes into the waters 

 and into the atmosphere, and this diversion leaves the content in 

 derivative rocks lower than that of the original rocks. If all the 

 radioactive matter that is lost into the waters and the air were 

 gathered into the derivative rocks, their content should equal 

 that of the igneous rocks from which they came, if no account be 

 taken of the loss by dissolution. 



The earlier determinations of the amounts of radium in the 

 igneous rocks by Strutt seemed to show that the acidic class hold 

 more radioactive matter, on the average, than the basic class, and 

 a portion of the later determinations seem to support this generali- 

 zation, but the determinations of Eve and Toly, which have been 

 important, seem to bring the richness of the basic class into some- 

 what near equality with that of the acidic, and even to make the 

 preponderance of the one class over the other doubtful. The 

 point of special interest here lies in the inference that, if the lique- 

 faction and eruption of the igneous rocks is dependent on the heat 

 derived from radioactivity, the distribution of radioactive sub- 

 stances in the erupted rocks should be inversely proportional to 



1 The larger number of determinations of radioactivity in rock have been made 

 by Strutt: Proc. Roy. Soc, LXXVI A (1905), 88 and 312; LXXXVII A (1906), 472; 

 LXXVIII (1906-7), 150; LXXXA (1907-8), 572; Eve: Phil. Mag., September, 

 1906, p. 189; February, 1907, p. 248; August, 1907, p. 231; October, 1908, p. 622; 

 Am. Jour. Sci., XXII, (December, 1906), 477; Bull. Roy. Soc. Con., June, 1907, pp. 3 

 and 9; July, 1907, p. 196; Joly: Nature, January 24, 1907, p. 294; Phil. M ag., March 

 1908, p. 385; Radioactivity and Geology (1909), general treatment with references; 

 Elster and Geitel: Phys. Zeit., II (1900-1901), 590; III (1901), 76. 



For the physics of radioactivity see J. J. Thomson: The Conduction of Electricity 

 through Gases; E. Rutherford: Radioactivity; (1904); Radioactive Transformations 

 (1906); F. Soddy: Radioactivity (1904); The Interpretation of Radium (1909); R.J. 

 Strutt: The Becquerel Rays and the Properties of Radium (1904); and the papers of 

 Boltwood, McCoy, and many others. 



