EMPIRE GULCH RHYOLITE. 351 



The smaller masses, tbongb sometimes light-colored, seldom cowtnin mncb macro- 

 scopically visible quartz, and horublende is usually more or less abuudant with the 

 biotite. 



Microscopical — The quartz-beariug variety shows under the microscope a decided 

 preponderance of sanidine over plagioclase. The former is in most cases in fragments 

 of crystals, while the plagioclase is often in well-detiued individuals. A few glass 

 inclusions were seen in both quartz and feldspar, while no fluid inclusions were noticed. 

 Apatite and magnetite are rather sparingly present. No hornblende could be found 

 accompanying the biotite in this form. The grouudmass is cryptocrystalline and is 

 made up of colorless grains and ferritic s[)ecks which are undeterminable. It seems 

 probable that there is some microfelsitic matter present, but it could not be definitely 

 recognized and there is certainly no glass base. 



The microscope shows almost as much hornblende as biotite in the compact rock 

 and there is also a larger determinable amount of plagioclase than in the preceding 

 variety, with the same distinction noticed before in contrast with the sanidine, viz, 

 that the latter mineral is more frequently in a fragmentary state, while the former is 

 well crystallized. Quartz is present in clusters of small irregular grains and rarely 

 in crystals. The groundmass is, as before, cryptocrystalline, but its component i>ar- 

 tides are often minute prisms or flakes and there are more jellowishor opaque grains. 



In the darkest modifications a small amount of quartz can always be recognized, 

 but by no means enough to represent the crystals of the light-i'olored variety. Still 

 everything seems to indicate that the various forms are modifications of one magma 

 and do not differ greatly in chemical composition. So far as the silica is concerned, the 

 truth of this idea was fully established by three determinations made, respectively, in 

 the quartz-bearing variety, the compact form associated directly with it, and, thirdly, 

 in a very light-colored rock from a small isolated occurrence not visibly connected 

 with any other. These yielded, in the order named, 65.75 per cent., 65.21 per cent., 

 and 65.03 per cent, of silica. 



EMPIRE GULCH RHYOLITE. 



About opposite the Long and Derry mine, on the south side of Empire gulch, 

 there is a small body of rhyolite occurring as a bed below the Silurian Limestone [208]. 

 This is uidike any other of the rocks examined and deserves a short description. It 

 is white, barring the specks of biotite, and very fine-grained, although the lens shows 

 many clenr and sharp quartz crystals. The feldspar is distinguishable from the ground- 

 mass through its superior whiteness and is apparently no longer-fresh. The average 

 size of the visible crystals is about 1™". 



Under the microscope the minute quartz crystals are seen to be well-shaped and 

 to contain very characteristic clear glass inclusions, with none of fluid or grouudmass, 

 and a very few apatite needles. The feldspars are chiefly orthoclase, though accom- 

 panied by plagioclase, and both seem to be much altered, calcite being the most 

 Iirominent decomposition product. They contain some inclusions of glass, now much 

 devitrified. The biotite is fresh and characteristic. Magnetite is very sparingly 

 j)resent. 



