186 



University of California Publications. 



[Geology 



differences in the form of the bonanzas in the two portions of the 

 Comstock lode have been noted by other observers. In Becker's 

 monograph,* von Richthofen is quoted as writing: "The ore is 

 distributed in a different way in the northern and southern parts 

 of the vein. * * * In the northern part the ore is concentrated 

 in elongated lenticular masses of which the greatest axis is not 

 far from vertical. * * * To the south the ore is concentrated in 

 continuous sheets, the principal one of which is very near and 

 parallel to the eastern wall." This difference in the form and 

 occurrence of the ore bodies is a very important matter, both 

 from a scientific and an economic standpoint. This sifijject, and 

 what grows out of it, is the main reason for the writing of this 

 short paper, for a proper view may increase the life and output 

 of the mines very materially. 



One peculiar bonanza occurred in the Gold Hill group, in the 

 Yellow Jacket mine. The explanation of this, as of the others, 

 will be given later. 



HALE AND NORCROSS TUNNEL. ,> 



The structures shown in the Hale and Norcross tunnel will 

 be mentioned together, for the sake of simplicity. This tunnel, 

 running N. 75° W., enters the slope of Mt. Davidson at the Hale 

 and Norcross shaft (see figure 2). At a distance in of 1,080 feet 

 the footwall of the lode is reached, the so-called "black dyke." 

 1. Proceeding in, at a distance of 3,720 to 3,750 feet, appear ap- 

 proximately vertical slips striking N.W.-S.E. parallel and in line 

 with the Silver City lode as shown on the surface. A second 

 well developed slip parallel to these occurs at a distance of 4,550 

 feet in from the mouth. 2. In nearly all portions of the tunnel, 

 but particularly between the lode and 3,500 feet in the tunnel, 

 and from 4,500 feet to the end (5,085 feet on Feb. 12, 1905), 

 are seen slips parallel to the Bullion Ravine fault ; that is, ap- 

 proximately parallel to the course of the tunnel itself. These 

 indicate unmistakably evidence concerning the existence of the 

 Bullion Ravine fault underground. 3. At a distance in the tun- 

 nel of 4,908 feet occurs a very strong vertical north-south fault. 

 The east wall is the diorite of Mt. Davidson, and the west wall 



* G. F. Becker, op. cit., p. 17. 



