96 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



rather of an eruptive origin, and contains relatively more mica and quartz 

 than that found in Buckskin gulch. The gneiss is of the normal gray type, 

 generally rich in quartz and biotite. Its feldspar occurs often in large 

 Carlsbad twins. The microscope detects plagioclase, microcline, and mus- 

 covite ; also, abundant fluid inclusions in the quartz, sometimes double and 

 with salt cubes and moving bubbles. A schist found locally on the northern 

 face of Mount Lincoln is of dark-green color and contains only biotite, 

 muscovite, and tourmaline, with a little feldspar, which is scarcely visible, 

 even under the microscope, and then appears in a stage of alteration into 

 muscovite. The white pegmatite masses are specially prominent, as al- 

 ready mentioned, on the faces of the spurs on either side of the gorge at 

 Montgomery. Their color is due to the large proportion of white ortho- 

 clase feldspar, which in the mass gives its tone to the quartz also, while the 

 mica, generally muscovite, occurs in bunches of subordinate importance, 

 growing between the crystals of the other constituents. 



The more prominent eruptive masses observed and which are indicated 

 on the map are : 



1. Half a mile above Montgomery a dike of porphyry crosses the 

 valley at right angles and can be traced for a considerable distance up either 

 wall. It is a light-colored, felsitic-looking rock, in which only very small 

 quartz grains and biotite leaves can be detected by the naked eye. It most 

 nearly approaches the Mount Zion, or fresh variety of White Porphyry, 

 and has a holocrystalliue structure as seen under the microscope. 



2. A mile above Montgomery is a wider dike of light-gray quartz- 

 porphyry, whose distinguishing peculiarity lies in brilliant-green grains 

 of epidote, which are scattered uniformly through the rock and which are, 

 in part certainly, the result of the decomposition of biotite. Its ground- 

 mass is also microcrystalline. 



3. A third dike is particularly noticeable for its peculiar form, changing 

 half way up the cliff from a vertical to a horizontal sheet. This change of 

 form is not unusual in dikes which extend up into the Paleozoic or regularly 

 bedded rocks ; but this is the only instance in which it has been observed in 

 the Archean. The rock belongs to the Mosquito Porphyry type, and is 

 identical with that found (type No. 2) on the south face of Mount Lin- 



