GOLD MINING m COLORADO. 495 



It crops out on the northern slope not far below the crest and several hundred 

 feet above the base of Bobtail Hill. The hill itself is on the south side of 

 Gregory Gulch, about half-way between Central City and Black Hawk. 

 West of it is Gregory Hill, from which it is separated by a shallow ravine. 

 The western limit of what is commonly known as the Bobtail lode is at or 

 near this ravine, and is traced thence easterly about 800 feet along the steep 

 side of the hill referred to. 



This vein is one of a number closely related to each other, of which 

 the relative positions are approximately indicated in the accompanying sketch 

 on Plate XXIX. The prominent members of this system, if such it may be 

 termed, are the Bobtail, Fiske, Gregory, and Bates, all of which, so far as yet 

 appears, have convergent courses, so that, if continuous, they probably inter- 

 sect or unite with the Mammoth lode, a large vein opened further west and 

 traced for a distance of several thousand feet, coinciding closely in course, dip, 

 and alignment with the Bobtail. This relation, though not clearly established 

 by actual developments, seems very probable. The Bobtail is by many per- 

 sons considered as a continuation of tbe Mammoth, faulted or somewhat dis- 

 placed, perhaps a hundred feet, in the neighborhood of the ravine that sepa- 

 rates Bobtail Hill from Gregory Hill. However this may be, there is a lack 

 of visible continuity between the western portion of the Bobtail on the east 

 side of the ravine and the distinctly recognized openings on the Mammoth on 

 the western side of the ravine ; so that, practically, the name of Bobtail only 

 applies definitely to the lode east of the ravine. 



The average course of the lode, for the length of 800 feet east of the 

 ravine, is 75° east of magnetic north; or, allowing 15° for variation of the 

 compass, is east and west, true. Its dip is almost vertical, varying a little in 

 places, and sometimes inclined to the northward, sometimes to the southward. 

 The width of the vein varies from a few inches, or a mere seam, to 10 or even 

 15 feet, but does not exceed 3 or 4 feet on an average. 



The inclosing rock may be generally described as gneiss, though having 

 sometimes a distinctly granitic character. It possesses the common character- 

 istics of the, probably metamorphic, rock that prevails throughout the more 

 elevated portion of the mountain range. It frequently shows a thinly-banded 

 structure, the lines of banding or bedding dipping easterly. The walls of the 



