36 



KEMP'S ORE DEPOSITS. 



fissure perhaps at the time small has become once filled, subse- 

 quent movements take place, it may strip the vein from one wall and 

 cause a new series of minerals to be deposited, with the previously 

 formed vein on one side and the wall rock on the other. This oc- 

 casions unsymmetrical fillings. But it may also happen that, with 

 otherwise symmetrical fillings, one layer may be lacking on one 

 side or the other. Where portions of the wall rock have been torn 

 off by the vein matter in these secondary movements, they may be 

 buried in the later deposited vein filling, and form great masses of 

 barren rock called horses. The vein then forks around them. If 

 the ore and the gangue have partly replaced the wall in deposition, 



East 



FIG. 6. Banded vein at Newman Hill, near Rico, Colo. After J. B. Far- 



ish, Proc. Colo. Sci. Soc., April 4, 1892; Engineering and Mining 



Journal, Aug. 20, 1892. 



unchanged masses of wall may also become inclosed and afford 

 horses of a different origin. An originally forked fissure gives an 

 analogous result. 



It is a curious fact that veins are often most productive just at 

 the split. If the masses are small, or if the vein fills a shattered 

 strip and not a clean fissure, or if it occupies an old volcanic con- 

 duit, deposition and replacement may surround unchanged cores of 

 wall rock with concentric layers of ores and minerals. Thus the 

 Bassick, at Rosita, Colo., referred to above, consists of rounded 

 cores of andesite, inclosed in five concentric layers of metallic sul- 

 phides. The Bull Domingo, in the same region, exhibits shells of 

 galena and quartz mantling nodules of gneiss. Such cores strongly 



