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Show 104 GEOLOGY OF THE HIGH PLATEAUS. It will be noted that while chemical constitution and mineralogical components are the basis of the larger and broader divisions, the texture may here be employed to distinguish the secondary characters. Group I.â€" RHYOLITES. Sub-groups. Characteristics. Sub-group 1. Nevadite or granitoid rhyo-lite. Having a superficial resemblance to granite; highly crystalline, with conspicuous quartz and feldspar; the crystals rounded, cracked, and irregular in contour. Base resembling some of the coarser varieties of trachyte. Sub-group 2. Liparite or porphyritic rhyo-lite. Having a decided porphyritic texture; compact base; crystals perfect or nearly so, often of large size; not conspicuously vitreous. Sub-group 3. Ehyolite proper or hyaline rbyolite. Having a fluent groundmass, sometimes wholly without crystals, but more frequently with them, but crystals less perfectly developed; vesicular, with vesicles much elongated and drawn out; or not vesicular, but with lines of flow suggesting a vitreous or candy-like mass. Foliated or structureless. Generally fibrolitic or spherolitic. The microscopic characters of the hyaline rhyolites and some of the liparites have been studied and analyzed in a most admirable manner by Professor Zirkel, and described by him in the volume on Microscopic Petrography in the series of Reports of the Survey of the Fortieth Parallel, to which volume the reader is referred. II. TRACHYTES. The trachytic group is characterized chemically by a high degree of acidity, but inferior in that respect to the rhyolites. Its dominant minerals are orthoclase, with a subordinate amount of plagioclase. It is distinguished mineralogically from rhyolite by the absence of free quartz, by the greater abundance of plagioclase, and of the subordinate minerals hornblende, magnetite, augite, and biotite. In its texture and physical characters it is also well separated in most cases, showing a tendency to develop the coarsely granular and porphyritic habitudes rather than the hyaline and vitreous, though the latter are not wanting, nor even extremely uncommon. This group is nearly as varied in character as the |
