DETAILED DESCRIPTION OF SLIDES. HI 



and, on being- mounted in balsam, jiroved to be groundmass and feldspar. 

 Hornblende and mica, most of the quartz, some feldspars and decomposition- 

 products sank. 



Slide Kil. West end of railroad tunnel above Red Jacket. 



Feisitic variety. — Tliis is a greenisli gray, fine-grained, rhyolitic-looking rock. 

 Under the microscope, too, it differs in general appearance from the ordi- 

 nary quartz-porphyries of the District. In detail, however, it is found to 

 correspond with them. The quai'tzes, of which there are but few, and those 

 minute, carry nvimerous fluid inclusions, many of them with active bubbles. 

 One of the quartzes also carries a comparatively large glass inclusion with 

 a cut bubble, the hemispherical space being of course filled with balsam. 

 The groundmass shows traces of fluidal structure, and is pseudo-spherolitic 

 in places. The feldspars are badly clouded, but a few are plagioclase, and 

 the remainder appear to be orthoclase. Hornblende, mica, titanite, and 

 ilmenite are present. In short, the rock appears to be merely a feisitic 

 variety of the ordinary quartz-porphyry. 



Collection of the Exploration of tbe Fortieth Parallel. Slides 265 and 266. 



40th Parallel slides. — Profcssor Zirkcl's description of these slides excellently 

 represents the phenomena, with one or two exceptions. While many of the 

 feldspars are clouded with decomposition products, others are nearly free 

 from exti'aneous matter. Most of these are unstriated and appear to give 

 the angles of extinction of orthoclase. The quartzes of both slides contain 

 fluid inclusions with moving bubbles, though they are neither very frequent 

 nor of large size. In slide 266 there are good glass inclusions in quartz, 

 penetrating the section and remaining dark between crossed Nicols. The 

 thin sections and specimens correspond entirely with those described in 

 this 2>Hper as quartz -porphyry. 



Collection of the Exploration of tbe Fortieth Parallel. Slide 333. 



40th Parallel slide. — This slide is Very graphically described by Professor 

 Zirkel. It happens to be a very small one, and shows only two or three 

 minute quartzes, in which I have detected no inclusions. The specimen 

 from which it was taken, however, presents quartzes in abundance. The 



