REPORT OF THE DIRECTOR I9II 1/5 



and the assumption of the organic origin of the generally dissemi- 

 nated graphite is therefore well grounded. Even when graphite 

 occurs in the pegmatites it may perhaps have been absorbed from 

 sediments through which the pegmatites have passed. However, 

 where graphite occurs in igneous rocks, the writer feels that 

 the burden of proof rests with those who claim for it an organic, 

 rather than a magmatic; origin. Doubtless most if not all of 

 the carbon of sediments is ultimately of magmatic origin, and 

 while it may become reincorporated in igneous rocks after passing 

 through the atmospheric and organic stages, no reason is apparent 

 why it may not also remain in the magma to appear not only 

 in the compounds liberated by heat in the laboratory but as primary 

 graphite. Many instances^ are on record where no other source 

 seems probable. But where, as in the present case, relatively small 

 intrusions, carrying graphite, cut sediments in which this mineral 

 is quite generally disseminated, absorption of graphite by the intru- 

 sions seems reasonable. Such absorption, however, implies mobility 

 on the part of the graphite that might lead to considerable circula- 

 tion and concentration, and raises the question whether the graphite, 

 so abundant in some of the ores, was actually formed in situ by 

 metamorphism of organic matter, or is in part or wholly a product 

 of secondary concentration. So far as some of the ores are replaced 

 pegmatites, this question does not of course arise, since any 

 graphite present in these rocks must, if of organic origin, be a 

 secondary introduction; but in the ordinary ores formed from 

 gneisses it is of moment. Thin sections give no positive information 

 on the -matter, although in several instances some of the graphite 

 appears to be younger than pyrite, implying the secondary intro- 

 duction of the former. However, the evidence is capable of a dif- 

 ferent interpretation and thus not final. Vein quartz sometimes 

 carries graphite and pyrite so related that their late and practically 

 contemporaneous origin seems necessary. This shows that circu- 

 lation of graphite has occurred in connection with the formation 

 of ores, and it is probable that this circulation has resulted in con- 



1 Weinschenk, E. ]\Iemoire sur I'histoire du graphite. Compt. rend., 

 Vin Congr. Geol. Tnternat., T900, Paris 1901, p. 447. 



Clarke, F. W. Data of Geochemistry, Bui. 330 U. S. Geol. Survey 191 r, 

 p. 311. 



Winchell, A. N. Theory for the Origin of Graphite, etc., Ec. Geol. VI, 

 1911, p. 228. 



Horwood, C. B. The ^lode of Occurrence and Genesis of the Carbon in 

 the Rand-Bankets. Trans. Geol. Soc. South Africa, 13, 1911, p. 65-92. 



