HHYOLITE. 87 



eruption of these rocks had apparently no influence on the ore deposition 

 of the region, since that, as well as can be determined, was pre-Tertiary, 

 and no ore bodies have been found in connection with these rocks. Their 

 interest is therefore chiefly lithological. 



EHYOLITE. 



The most important body, both in mass and in lithological interest, is that 

 of Chalk Mountain, on the northern edge of the map, which, as the name 

 of the mountain indicates, is prominent on account of its dazzling white color. 

 It is a very crystalline rock, in which the groundmass is so subordinate as 

 to appear in the hand specimen entirely wanting ; it corresponds, therefore, 

 to the generally accepted definition of Nevadite. Its prominent constitu- 

 ents are sanidine, generally in large crystals and having a peculiar satiny 

 luster, and smoky quartz. The microscope also detects some plagioclase, a 

 little biotite, with magnetite, apatite, and zircon in relatively small propor- 

 tion as compared with the quartz porphyries. The quartzes contain fluid 

 inclusions. A careful study of this rock by Mr. Cross has developed the 

 fact that the peculiar luster of these feldspars is due to an actual parting, 

 analogous to cleavage, which has already been determined as that which 

 gives the blue color observed in the feldspar of many rocks, notably labra- 

 dorite and some rhyolites. He also found crystals of topaz in some of the 

 druses of this rock, the first instance, so far as known, in which this mineral 

 has been found in Tertiary rocks. On Plate VIII is the reproduction of a 

 photograph of a hand specimen of this rock, in which the smoky quartz 

 grains appear black; above this are two microsections which show the sim- 

 ilar granular structure of this rock and of White Porphyry. 1 



The next important body of rhyolite is that at the west base of Bart- 

 lett Mountain, at the head of McNulty gulch, a tributary of the Ten-Mile 

 Creek; it here cuts across porphyrite and quartz porphyry. This rock, 

 though generally light colored, is not as white as the Chalk Mountain rock, 

 nor is it so decidedly of the Nevadite type, the groundmass being often quite 

 prominent. It contains glassy feldspars, quartz, and biotite. In darker 



1 In some of the plates, by an error iu proof-reading, the title White Porphyry, which belongs to 

 the left-hand section, has been placed below the right-hand section and vice versa. The reader will 

 bear in mind that the section containing the large crystal is Nevadite. 



