168 GEOLOGY AND MINING INDUSTRY OF LEADVILLE 



describe somewhat in detail the rock as found here. That from the west 

 face of Lamb Mountain (46), which is comparatively fresh, is a compact 

 rock, of a light pinkish-brown color, whose only visible crystals are a few 

 small and well-defined orthoclase individuals. No quartz is to be seen. 

 Minute cavities lined with yellow ocher indicate a former constituent, but 

 the forms of the cavity are not sufficiently well preserved to indicate it* 

 character. It may have been pyrite. Under the microscope the rock 

 appears granular, with easily determinable quartz, orthoclase, plagioclase,. 

 and muscovite. There is no trace of a microscopic groundmass between 

 the grains. Both orthoclase and plagioclase are abundant, but muscovite 

 is less developed than is usually the case in White Porphyry; contrary 

 to the usual rule, it is as often found forming in plagioclase as in ortho- 

 clase. With a low power, the feldspars seem full of fine dust or specks, 

 which in many cases are evidently arranged on the cleavage plane. These 

 specks are also seen, though less frequently, in the quartz. By the use of 

 a higher power it is seen that some of these specks are fluid inclusions, 

 with rapidly moving bubbles, and it is therefore probable that a sufficiently 

 high power would prove that all are similar inclusions. No glass inclu- 

 sions were found. The main rock on the northwest slope of Sheep 

 Mountain (47) is of porphyritic appearance, owing to the large devel- 

 opment of muscovite; otherwise it does not differ microscopically from the 

 Lamb Mountain rock. A contact specimen of this body is so fine grained 

 that its exact composition cannot be made out, yet it does not seem to differ 

 essentially from the average rock. Portions of the body are perfectly white 

 and homogeneous, and when breathed on have a strong earthy smell The 

 specimens examined contained little, if any, plagioclase and almost no mus- 

 covite. The body included in the Weber Grits formation (49) is exactly 

 the same as the ordinary rock. That from the saddle between Sheep and 

 Lamb Mountains contains even more plagioclase than the Lamb Mount- 

 ain type. That from White Ridge (44), in the Four-Mile amphitheater, is 

 extremely white and very compact, so that the constituents are mostly 

 indistinguishable. The process of alteration from feldspar to muscovite 

 can readily be distinguished by the naked eye. 



