ENDLICH.] 



GEOLOGY BOBTAIL LODE. " 285 



the quartz ; galenite, in little cubical crystals, contained in the gangue-rock 

 as a matrix ; quartz, as vein-matter and in small crystals ; orthoclase- 

 feldspar, partly decomposed, forming caolinite, found in the gangiie, 

 and delicate flakes of a white talc also contained in the gangue. 



It is stated that during the month of May, 1873, 745 cords of ore were 

 hoists fKom this minfi at S ounces per coidl A cord is between 7 and 

 8 tons. 



With the Bobtail lode we commence the second series of veins, 

 striking approximately east and west. This lode is located on the north- 

 ern slope of Bobtail Hill, which was named after it^ and has a course of 

 east 8^ north. A number of claims' have been taken up on the same 

 lode, so that it is necessary to mention -that the mine on this lode that 

 will be treated of in the following pages is that owned by the Bobtail 

 Gold-Mining Company. At the western end of Black Hawk, a tunnel 

 has been driven into Bobtail Hill in a southerly direction, intending to 

 strike the vein, which was accomplished after driving something over 

 1^00 feet. At the point of intersection, the shaft had reached a depth 

 of 480 feet, and was then sunk farther. The vein runs between granite 

 on the south side and gneiss on the north, and is, therefore, a contact vein. 

 Possibly the gneiss on the northern side may not extend down to any 

 very great depth, because its outcrop is isolated, in which case granite 

 would supply its place. A dip of 15° to 20° to the south was observed 

 at the lowest level, while higher up it seemed to become almost vertical. 

 The distance be+^^ween walls may be given from 1 foot, where the vein 

 sends out shoots or ramifies, to 5 feet. Very clean and well-defined 

 walls are found separated from immediate contact by the characteristic 

 narrow clay selvage. To the west, the vein continues beyond the shaft 

 of this mine in a regular course, while it splits toward the east. 

 Whether this split, that had reached a width of about 18 feet in one of 

 the levels during my visit, will continue, or whether merely a " horse " 

 conies iUj could not then be determined, but the latter seemed more 

 probable. With the exception of this case, great regularity in every 

 feature characterizes the Bobtail vein. Small spurs, from the thick- 

 ness of a sheet of paper to several inches, separate from the main vein 

 and enter either wall. 



The ore is, as in the preceding instance, pyrite and chalcopyrite, 

 massive quantities of the former having been found up to 9 inches in 

 thickness ; of the latter, of several inches. A partly symmetrical 

 arrangement can be noticed by observing the diagram taken at a depth 

 of 520 feet across the vein. (Plate HI.) Pyrite occurs on either side of 

 the vein, next to the wall, separated from it by a narrow selvage, then 

 scattering quantities of chalcopyrite and isolated threads of sphalerite 

 and galenite are found, until the middle again is formed by a thicker 

 band of pyrite. 



Of minerals, the following were found: Pyrite and chalcopyrite, mass- 

 ive, and the former in crystals — rarely, however, in cubes ; sphalerites, 

 in crystalline threads ; galenite, dispersed either in threads or small 

 crystals j mispickel, massive, with pyrite, or in small crystals on quartz ; 

 gold, showing crystallized faces, on quartz ; composing the gangue-rock, 

 gray and white quartz, a pink, yellow, and white orthoclase, i^artially 

 decomposed : flakes of talc and clay in the selvage. 



As the Bobtail mine is one of the deepest in the central mining- 

 region, a trouble that is not yet experienced to any extent in mines 

 of less depth occurs here. It is the accumulation of water, which has 

 been very efficiently overcome by means of a pump supplied with a 

 movable suction-hose, which has been placed in the second lowest level. 



