176 NEW YORK STATE MUSEUM 



centration of gTaphite as well as of pyrite. In this connection, much 

 interest attaches to Young's recent conclusion^ that the graphitic 

 carbon, relatively abundant in a few of the mines in the Rand con- 

 glomerates, is not of organic origin but is derived directly from 

 magmatic vapors. Young is dealing with conditions which are 

 similar to those that must have prevailed in the Grenville rocks 

 prior to the period of intense metamorphism when a certain amount 

 of concentration of pyrite may have occurred, as already indicated. 

 Any magmatic carbon introduced at that time would evidently be 

 indistinguisliable now from carbon of organic origin, having" been 

 completely recrystallized by metamorphism. Thus, for the graphite, 

 a history somewhat similar to that of the pyrite is indicated, but 

 with the difference that most of the former is thought to be carbon 

 that was original in the sediments, which has undergone some con- 

 centration and may have received minor additions from magmatic 

 sources, while, in the case of the pyrite, the relative importance of 

 these sources is reversed. 



As to the relation of graphite to the precipitation of pyrite there 

 can be no doubt that the original organic matter, from which the 

 graphite v/as formed, was a potent agent both during sedimenta- 

 tion and any subsequent ground water concentration of pyrite. 

 In the postmetamorphic stage, to which the main concentration 

 of pyrite is ascribed, any precipitating effect would be due of course 

 not to organic matter but to graphite. The oft-quoted instance of 

 such precipitating action of graphite at Freiberg described by V^on 

 Cotta- has been supplemented by many other cases, and recently 

 Jenny^ has cited several examples. On theoretical grounds the 

 action can be regarded only as a question of pressures and tempera- 

 tures, and with these properly adjusted there can be no doubt that 

 graphite would act as a precipitating agent. But so little is known, 

 on the one hand, as to what pressures and temperatures would per- 

 mit the necessary reactions and, on the other, as to the pressures 

 and temperatures under which the pyrite was formed that little 

 is to be hoped for in the present case from these general theoretical 

 considerations. But with such a precipitating action possible, the 

 marked association of graphite with the pyrite is, to say the least, 

 very suggestive, and the former being regarded as representing a 



1 Young, C. R. Trans. Geol. Soc. South Africa, XIII, 1911, p. 105-6. 



- Von Cotta. Treatise on Ore Deposits, English Trans., p. 46-47. 



3 Jenny. W. P. The Chemistry of Ore Deposition, Trans. Am. Inst. M. E. 



XXXIII, 1003. p. 455-57- 



