SYMPOSIUM OX R.\DIOACTIVITY. 61 



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.* 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 

 the 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 Joly, 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 

 liquefaction and eruption of the igneous rocks is dependent on 

 the heat derived from radioactivity, the distribution of radio- 

 active substances in the erupted rocks should be inversely propor- 

 tional to their temperatures of mutual solution or of fusion. But 

 it must be observed that even if such a causal distribution pre- 

 vailed in the rock-matter when first it took the liquid form, this 

 distribution might not persist indefinitely, for selective segrega- 

 tion has apparently taken place during the later processes. It is 

 quite clear that the radioactivity is concentrated in some constitu- 

 ents rather than others, as for example in zircon, pyromorphitc, 

 apatite and some other minerals, and in pegmatite and some other 



*The lareer number of determinations of radioactivity in rock have been made by 



STRUTT: Proc. Rov. Soc. "6A, 1905, pp. 88 and 312; 77A, 1906, p. 472; 78, 

 1906-7, p. 150; 80A, 1907-8, p. 572. 



EVE: Phil. Mag., Sept. 1906, p. 189; Feb. 1907, p. 248; Aug. 1907, p. 231; 

 Oct. 1908, p. 622; Am. Jour. Sci. 22, Dec. 1906, p. 477; Bull. Roy. Soc. Con., June 

 1907, pp. 3 and 9; Julv 1907, p. 196. 



JOLY: Nature, Tan. 24, 1907, p. 294; Phil. Mag., March 1908, p. 385. Radio- 

 activitv and Geology, 1909, general treatment with references. 



ELSTER and' GEITEL: Phys. Zeit. 2, 1900-1, p. 590; 3, 1901, p. 76. 



For the physics of radioactivity see T. J. Thomson : The Conduction of Elec- 

 tricity through Gases: E. Rutherford: Radioactivity, 1904, Radioactive Transforma- 

 tions, 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. 



