246 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Pine Tree having reached it after crossing 145 feet of White Porphyry. It 

 is a coarse-grained, gray rock, containing white and rather glassy feldspars, 

 quartz in smoky, rounded grains, and biotite in distinct crystals. Cavities 

 filled with white opaque calcite are frequently found. The gray color of 

 the groundmass is due to numerous black specks, many of which are ore 

 grains and others minute biotites. The feldspars under the microscope are 

 seen to be partly triclinic, although nionoclinic feldspar predominates. Both 

 quartz and feldspars contain inclusions of the groundmass and glass inclu- 

 sions. In the quartz, in one case, fluid inclusions with a moving bubble 

 are also observed. Calcite is present in considerable quantity, both in the 

 groundmass and in the feldspars. In general, from the microscopical exam- 

 ination alone, Mr. Cross would have been inclined to class this rock among 

 the Tertiary eruptive rocks. If it be so, it is probably not an intrusive sheet, 

 as has been assumed, but an irregular dike. These indications do not, 

 however, seem sufficiently decisive to outweigh those of its field habit and 

 mode of occurrence, which ally it to the later intrusions of porphyry of pre- 

 Tertiary age. 



Lake beds. Lake beds were found in a prospect hole near the shaft 

 M-41, were passed through by the Pine Tree shaft, and penetrated to a 

 depth of 175 feet in the Continental shaft (M-50), which was sunk in the 

 Iowa south moraine. Several shafts and tunnels have been run in this mo- 

 raine and have very probably penetrated the underlying Lake beds, but, as 

 far as known, have not reached rock in place on the south of Iowa gulch. 



Iowa gulch. On the north bank a number of shafts and tunnels have 

 proved the existence of outcrops of Blue Limestone in the vicinity of the Nisi 

 Prius workings, one of whose tunnels has followed the contact for a distance 

 of 700 feet, disclosing a considerable body of contact vein material. The 

 Little Birdie (N-18) tunnel was driven 200 feet in the moraine without 

 reaching rock in place. 



Dome Ridge. On Dome Ridge the principal developments have been 

 made near the outcrops of the Blue Limestone, the few shafts in porphyry 

 at considerable distance east of this not having been sunk to any great 

 depth. No definite data are therefore obtainable as to the aggregate thick- 

 ness of the White Porphyry sheet. The principal workings are those of the 



