350 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



crystals. Both feldspars are present in numerous crystals, but orthoclase alone is 

 prominent in the groundmass. Quartz occurs in abundant, slightly smoky crystals. 

 Biotite, in small hexagonal leaves, is sparingly scattered through the whole, and mag- 

 netite is also insignificant as a constituent. 



Fluid inclusions appear iu both orthoclase and quartz, particularly in the latter, 

 and sometimes carry white cubes, apparently of salt. There are glass inclusions also 

 in the quartz, but not plentifully. The groundmass is granular and shows no glass 

 substance like that iu the Nevadite. 



The orthoclase, though fresh looking, has none of the glassy appearance of 

 sanidine, and it< must be confessed that there is little evidence in the observed charac- 

 teristics of the rock demanding that it be separated from the quartz-porphyries. There 

 is no direct evidence of its age, and its classification as a younger rock rests chiefly 

 upon the following facts. In mode of occurrence and in composition it is more nearly 

 related to the Chalk Mountain Nevadite than, to any other rock of the region described. 

 It lies separated by a considerable space from all other eruptives of the map, but is 

 adjoined at no great distance on the south and southeast by a large series of rhyolites 

 and andesites. It is regarded as most probably related to these in its origin. A silica 

 determination in fresh rock gave 69.54 per cent. [140]. 



MNTTLTY GULCH EHYOLITE. 



Occurrence On the northern boundary of the area mapped, at the western base 

 of Bartlett Mountain, occurs a rhyolite of peculiar character. It appears in one large 

 and several small bodies at the head of McNnlty gulch (not indicated on the map), 

 which runs north and enters the Ten-Mile River at Carbonateville. White Ridge, 

 between Chalk ranch and Chalk Mountain, is also formed of this rock, as are one or 

 two minor bodies west of Chalk Mountain, which are not shown upon the map. 



At the head of McNulty gulch this rock cuts porphyrite and the fresh-looking 

 quartz-porphyry which < ccurs in the synclinal fold at this point. All these rocks 

 extend northward into the Ten-Mile district, and they will be more fully treated iu the 

 forthcoming report upon that region. 



Description in the largest body of this rhyolite, indicated upon the map, the 

 prevailing habit is that of a light-colored rock, showing numerous slightly pinkish 

 quartz crystals, white glassy feldspars, and bright brown uiotite leaves, with a subor- 

 dinate ashen-gray groundmass between them. Few crystals exceed 0.5 cm in diameter, 

 and the average is much less. Intimately associated with the above variety, usually 

 in alternating bands or streams, with rapid though gradual transitions, is a darker 

 modification, in which the development of the quartz in particular has been hindered, 

 while feldspar and biotite are abundant in smaller individuals than before. The ground- 

 mass becomes at once more prominent and darker brown in color, determining the 

 general hue of the rock. The thicker these dark portions are the more completely the 

 quartz disappears. In the most compact parts of the rock a fluidal structure is macro- 

 scopically visible and small glistening prisms of hornblende appear. About included 

 fragments of sandstones, etc., this rhyolite grows compact iu a similar manner, and 

 also on the contact with wall rock. 



