250 KEMP'S ORE DEPOSITS. 



ledges, or reefs. Some discussion has arisen over the condition of 

 the gold in the pyrite, but in most cases it is the native metal 

 mechanically mixed, and not as an isomorphous sulphide. It has 

 been detected in the metallic state, in a thin section of a pyrite 

 crystal from Douglass Island, Alaska, as later set forth (2.13.04), 

 and the fact that it remains as the metal when the pyrite is dis- 

 solved in nitric acid makes this undoubtedly the general condition. 

 The association of gold with bismuth, which has been shown by 

 R. Pearce to occur in the sulph'urets of Gilpin County, Colorado 

 (referred to on p. 212), and the difficulty experienced in amalga- 

 mating some ores, indicate the possibility of other combinations. 

 When crystallized, gold has shown, in one specimen and another, 

 nearly all the holohedral forms of the isometric system, but the 

 octahedron and rhombic dodecahedron are commonest. The veins 

 are younger than any of the igneous dikes with them. They may 

 have been filled, as thought by Whitney, during the metamor- 

 phism of the rocks attendant upon their upheaval in post-Jurassic 

 time. Certain it is that a very extensive circulation of siliceous 

 solutions was in progress. For the gold in the similar veins of 

 Australia a precipitation by organic matter has been urged. (See 

 William Nicholas, "The Origin of Gold in Certain Victorian 

 Reefs," Engineering and Mining Journal, Dec. 15, 1883.) In 

 the development of explanations of origin, however, a wide field 

 for study yet remains. 1 



1 M. Attwood, " On the Wall Rocks of California Gold Quartz and the 

 Source of the Gold," Rep. Cal. Mineralogist, 1888, p. 771 (thought to be 

 due to igneous injection in diabase). W. P. Blake, "On the Parallelism 

 between the Deposits of Auriferous Drift of the Appalachian Gold Field 

 and those of California," Amer. Jour. Set., ii., XXVI. 128. "Remarks 

 on the Extent of the Gold Region of California and Oregon," etc., Ibid. 

 ii., XX. 72. "The Carboniferous Age of a Portion of the Gold-bearing 

 Rocks of California," Ibid., ii., XLV. 264. W. H. Brewer, reply to above, 

 Ibid., ii., XLV. 397. A. Bowman, " Geology of the Sierra Nevada in Re- 

 la^ion to Vein Mining," Min. Resources West Rocky Mountains, 1875, p. 

 441. W. H. Brewer, " On the Age of the Gold bearing Rocks of the Pacific 

 Coast," Amer. Jour. Sci., ii., XLII. 114. F. G. Corning, "The Gold Quartz 

 Mines of Grass Valley, California," Engineering and Mining Journal, 

 D. c. 11, 1886, p. 418. W. M. Courtis, Gold Quartz," M. E., 1889, Ottawa 

 meeting. H. W. Fairbanks, " Geology of the Mother Lode," Tenth Ann. 

 Rep. Cal. Mineralogist; also in briefer form in Amer. GeoL, April, 1891, 

 p. 201. Rec. "On the Pre-Cretaceous Rocks of the California Coast 

 Ranges," Amer. GeoL, March, 1892, February, 1893. J. H. Hammond, 



