24:4 Watson — Association of Native Gold with Sillimanite. 



quently fills them, it is clear that it has been introduced 

 subsequent to the formation of both the original and meta- 

 morphic minerals, such as garnet and sillimanite. Some pyrite 

 is present in the rock, and while the gold is closely associated 

 with it in some instances there is no evidence suggesting the 

 derivation of the gold from pyrite by alteration. In the 



Fig. 2. 



& " 







tfvjy^ 





K f 



^-, H^ 





^■•x! 





- 



: Iq 



•j% ^^ijli 





* v<»*\v 



V 



Fig. 2. Photomicograph showing relation of native gold (in black) to 

 sillimanite, the fine fibrous mineral which occupies most of the field. Gold 

 is shown as irregular black areas in the sillimanite partly lying between 

 fibers and partly filling fractures. * White mineral occupying marginal posi- 

 tions is muscovite showing some thin foils of gold (black) interleaved along 

 cleavage directions. Intermediate dark mineral is biotite with partial rims 

 of free gold (black). Magnified 60 diameters. 



absence of accurate knowledge of the field relations, conclu- 

 sions based wholly on the single hand specimen and thin sec- 

 tion are of doubtful value ; but so far as these can be relied on 

 the evidence suggests that the gold was probably introduced 

 into the wall rock with the pegmatite-making solutions and 

 formed in the relations to the rock minerals as described above 

 and as indicated in the two figures photographed from the thin 

 section. 



Brooks Museum, University of Virginia. 



