346 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Ou the southeru edge of the mouutaiu and ou the uorthwesteru slope the rock 

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

 sanidiue, 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 sufScieut 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 au extraordinarily beautiful appearance. On Plate VIII, page 88, is a 

 beliotype 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 sanidiue is at all 



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

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

 sis later. Sauidine, much more glassy and fresh iu appearance than the plagioclase, is- 

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

 twins, .sometimes polysynthetic, and exhibit the faces cc P, co P c», OP, aud 2P *. The 

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

 below. 



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

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

 outer zone or shell about 1™™ 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 miuuts? 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 uot prououuced a section was pi'e- 

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

 were found to lie near together iu a plane normal to » P a. 



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 iuclusious of the grouud- 

 mass are alike rsire. Gas pores are, on the other hand, quite abundant, being iu i)art 

 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 iuto the angles or pro- 

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



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

 only ore mineral aud is present iu very small quantity. Apatite and zircou are the 

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

 the district. 



The groundmass — Quartz and feldspar iu a very eveugiained mixture are almost 

 the sole constituents of the g;-ouudmass. In the coai-ser variety of the Nevadite the 

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



