346 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



On the southern edge of tbe mountain and on the northwestern slope the rock 

 has an even more striking development than that just described. Both quartz and 

 sanidine, but specially the latter, occur in large crystals, and, while the quartz is dark, 

 as before, the sauidine possesses a most beautiful, brilliant, satiny luster upon a sur- 

 face nearly parallel to the orthopinacoid, which is particularly marked in fractured 

 crystals. At the same time biotite and ore specks appear in sufficient quantity in the 

 subordinate groundmass to give it a tinge of gray and cause it to stand out plainly 

 from the feldspars. The dark, smoky tinge of the quartzes, the delicate but brilliant 

 luster of the sauidiues, together with the general freshness of all constituents, give 

 to the rock an extraordinarily beautiful appearance. On Plate VIII, page 88, is a 

 heliotype representation of this Nevadite, which but feebly expresses the strong con- 

 trast between various constituents. 



Macroscopic constituents Of the feldspars in this rock only the sauidine is at all 

 prominent, although plagioclase appears in small crystals and sparingly in the ground- 

 mass. The plagioclase must be an oligoclase poor in lime, as is shown by the rock analy- 

 sis later. Sauidiue, much more glassy and fresh in appearance than the plagioclase, is 

 by far the most interesting component of the rock. Many of its crystals are Carlsbad 

 twins, sometimes polysynthetic, and exhibit the faces oc P, co P &, OP, and 2P ob. The 

 luster which has been mentioned is highly characteristic and is described in detail 

 below. 



Some of the large, lustrous sanidiues exhibit a peculiar internal structure. On 

 breaking open several crystals there appeared a kernel partially detached from an 

 outer zone or shell about l mm in thickness. All free surfaces of the kernel are glis- 

 tening crystal faces, and the inner surfaces of the shell are likewise regular crystal 

 planes, upon which miuuto projections are found to be like attached crystals, with the 

 same orientation as the larger individual. The shell usually exhibits the satiny luster 

 more markedly than the kernel, but no other difference was noticed between the 

 substance of the two parts. 



From a clear crystal in which the luster was not pronounced a section was pre- 

 pared nearly at right angles to the edge between OP and P do, and the optical axes 

 were found to lie near together in a plane normal to so P <x. 



The quartz crystals and grains of this rock are quite free from mineral inclu- 

 sions; glass has never been observed in them and arms or inclusions of the ground- 

 mass are alike nire. Gas pores are, on the other hand, quite abundant, being in part 

 negative crystalline in form. Many pores, seeming at a low power to be merely filled 

 with gas, are really fluid-inclusions with a relatively small amount of fluid. This is 

 very plain if the cavity is irregular, the fluid being pressed into the angles or pro- 

 jecting arms, while the main part of the cavity is occupied by the bubble. 



Biotite is very sparingly developed in small hexagonal leaves. Magnetite is the 

 only ore mineral and is present in very small quantity. Apatite and zircon are the 

 remaining accessories, and both are much less abundant than is usual in the rocks of 

 the district. 



The groundmass Quartz and feldspar in a very even-giained mixture are almost 

 the sole constituents of the grouudmass. In the coarser variety of the Nevadite the 

 average size of the grains is 0.02 mm to 0.05 mm , and the greater part can be identified as 



