CHALK MOUNTAIN NEVADITE. 347 



quartz or feldspar, the larger portion of the latter being inonoclinic. The groundmass 

 of the more compact varieties of the rock is cryptocrystalliue. Gas pores of irregu- 

 liir shape are present between the granules in all modifications. 



While there is no niicrofelsitic substance and no persistent glassy base, properly 

 speaking, there are irregular, disconnected patches or particles of a clear, structure- 

 less, isotropic matter, with branching arms filling spaces between grains of the ground- 

 mass. This substance is most clearly developed in the coarser parts of the rock mass, 

 and it is apparently identical in character with the glass observed in the rhyolite from 

 the Hohenburg near Berknm, on the Bhine, Germany, first described by Zirkel. 1 In 

 manner of occurrence of this glass residue the two rocks are very similar, though it is 

 more abundant in the German rock. The latter contains plagioclase abundantly in 

 the groundmass and its basic silicate is hornblende. 



Drusy cavities In the coarser -grained parts of theNevadite body are numerous 

 small cavities lined by minute crystals. At the northwestern point of Chalk Mount- 

 ain they reach the maximum in size, some observed being several centimeters in great- 

 est diameter. In the larger ones the crystals reach a determinable size and are found 

 to be chiefly sanidine, in delicate glassy tablets that are always Carlsbad twins, 

 with some quartz, biotite, and topaz. A few stout crystals seem likely to be triclinic 

 feldspar, but they could not be definitely determined. A coating of manganese binox- 

 ide is often upon the crystals and dark spots in ihe mass of the rock seem due to 

 the same substance. Both sanidine and topaz from these druses are worthy of special 

 notice and are described below. No minerals which can be considered alteration prod- 

 ucts are found in these druses and a natural explanation for the occurrence is to 

 regard all the crystals as sublimation products. 



Topaz Usually but a single topaz is present in one of the druses, and that is 

 larger and more perfect in development than any other crystal. The topaz is attached 

 directly to the walls of the cavity and often bears small tablets of sanidine upon it. 

 The crystals which can be recognized vary from 0.5 mm to 3 mm in length, but it seems 

 quite probable that there are some smaller ones, indistinguishable from quartz. 



The determination rests upon the crystalline form, which is very distinct and is 

 that of common topaz. One crystal, measuring S"" 11 in length and l mm in thickness, 

 was removed from the rock, and its angles were measured with a Fuess reflection 

 goniometer. This crystal presents oo I 3 , o= ?2 and 2Poo as the dominant forms; OP is 

 a narrow face and 4? 00 , 2P co , 2P, and P are minute, but very distinct. The angles 

 measured are as follows: 



ooP A*P 124 16' 



ooPSA^P aoveroofoo 93 7' 



OP A 2 foe 136 30' 



OP A P 1* 11' 



OP A 2P 115 55' 



2P oo appears as a very narrow face in the zone of 2P to 2P. This is the usual habit, 

 with the occasional addition of oo ? oo , and a more prominent development of OP. 

 This crystal is also imperfectly terminated at the attached end, showing 2p oo most 

 prominently, with 4f' oo and 2P also recognizable, and there are no signs of henii- 

 morphism. 



1 Mik. Beschaf. der Mill, uud Gesteiue, p. 343. 



