300 GEOLOGICAL SUEVEY OF THE TEREITOEIES. 



considerable portion of which was driven by hand. The tunnel is wide, 

 well graded, entering the mountain in the course of north 20^ west. At 

 a distance of 797 feet from the mouth of the tunnel, a dike of porphy- 

 ritio granite occurs, 38 feet thick, (Plate VII, a,) striking at right angles 

 to the course of the tunnel, with a southerly dip. Contact between 

 this granite and the following gneiss is a vein striking parallel with 

 the former, containing a body of blende 18 inches in thickness. From 

 that point to the end of the tunnel, »uo further veins have been cut. 

 About 450 feet from the end of the tunnel, a break occurs in the gneiss, 

 parallel in dip to the porphyritic granite ; a small vein of quartzite, 4 

 feet thick, is found about 320 feet farther toward the end. At the east- 

 ern end of Georgetown, the Eclipse tunnel has been driven in a south- 

 erly direction ; it is claimed quite lately, with good success. 



From the few words that have been said about the working of the 

 Georgetown mines, it will have become evident that the plan of opera- 

 tions differs slightly here from that followed at Central. The silver- 

 miners of this section usually drive a tunnel into the side of the mount- 

 ain containing their lode, and, having reached it, sink a shaft. At the 

 level of the tunnel, drifts are made, and the ore extracted by means of 

 overhand stoping. Sometimes a shaft is sunk from the surface, con- 

 necting with the tunnel or one of the levels. 



A number of works have been put lip in Colorado to smelt the lead 

 and silver ores from this and other regions, the greater portion of which 

 are doing Avell. Eventually the concentration of ores will have a regen- 

 erating influence upon mining and smelting enterprise, if the flush pro- 

 duced at present by the extraction of large masses of rich ore should 

 pass away. 



PAET III. 



High up in the mountains of Boulder County, another district of 

 silver-mines remains to be commented upon. Caribou is the mining 

 camp, located in the center of a number of mines that have attracted con- 

 siderable notice. Situated near the timber-line, with snow within three 

 minutes' walk from the hotel-door in the latter part of June, the report 

 of rich lodes has attracted several hundred miners to the spot. As my 

 time was very much limited just then, I can only speak of the two main 

 lodts which I visited, and must again refer to Mr. Mar vine's report for 

 notes on the geognostic formations. 



Predominating is a granite, with accidental admixtures of hornblende, 

 so that at certain localities it may be termed a syenitic granite, and it is 

 therein the Caribou lode is located. The strike of this lode, that a short 

 time ago was sold to a Netherlands company for $3,000,000, is almost 

 due east and west, and it has a dip to the north that on an average is 

 slight — in one or two points increasing however — never varying more 

 than 20° from the vertical. Entering above, the dip to the north is 

 found to be about 20°, continuing for 40 feet ; then follow 70 feet almost 

 vertical, and after that a northerly dip of 15° again. With increasing 

 depth the vein grows wider, so that it is 14 feet thick between walls at 

 the depth of 210 feet, while it is 4 feet at the surface. Both walls are 

 beautifully defined, having an almost polished appearance. On the 

 north wall, the granite seemed to be of a different variety, but it could 

 not be decided with certainty in the mine. A thin clay selvage sepa- 

 rates the vein from either wall. In the distribution of the ore, a sym- 

 metrical arrangement maybe observed, ore being found near either wall, 

 and again in the center. It consists mostly of galenite, sphalerite, argen- 



