350 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



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

 prominent in tlie grounrtmass. Quartz occurs in abundant, slightly smoky crystals. 

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

 netite is also insignificaut as a constituent. 



Fluid inclusions appear in 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 groundm<iss is granular and shows no glass 

 substance like that in the Nevadite. 



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

 sauidine, 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 cruptives 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]. 



M'^NULTY GULCH RHYOLITE. 



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 McNulty gulch (not indicated on the map), 

 which runs north and entei's the Ten-Mile Eiver 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 occurs in the synclinal fold at this point. All these rocks 

 extend northward into the Ten-Mile district, and they will be more fully treated in 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 biotite leaves, with a subor- 

 dinate ashen-gray groundinass between them. Few crystals exceed 0.5'=™ in diameter, 

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

 in alternating bauds 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 tliese 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 in a similar manner, and 

 also on the contact with wall rock. 



