16 RICHTHOFEN NATURAL SYSTEM 



dependency of the mode of texture upon deep-seated influences which acted at the very 

 source of the eruptive matter, and produced a certain molecular condition of the latter, 

 varying at each locality. Some light on this subject may be expected from minute 

 geological observation, accompanied by exact chemical and microscopical researches. 



The following are the subdivisions which may be distinguished in regard to the 

 texture : 



Fam. \st. Nevaditeor Granitic Rhyolite. The name " Nevadite " is derived from 

 that of the State of Nevada, where these rocks have been first met with in larger 

 accumulations. The local derivation may answer in this instance, as granitic rhyolite 

 is little known from other countries, excepting the neighboring California. In the 

 Carpathians, it occurs isolated in Transylvania, but by no means as characteristic as in 

 Nevada. The name " granitic rhyolite " is designed to indicate the general resem- 

 blance of these rocks to granite, which is conspicuous in boulders, or on large exposed 

 faces, but disappears on closer examination. It is chiefly produced by the similarity 

 in color, which is of light shades of gray and red in Nevadite, and by some affinity 

 in mineral composition. Nevadite contains crystals of quartz in large proportion ; the 

 corners are usually rounded, and the quartz itself cracked, like glass when rapidly 

 cooled. Sanidin occurs in crystals of larger size than oligoclase, sometimes of an inch 

 in diameter. The crystals of both are often cracked throughout their mass, and 

 rounded at the corners. Black mica and hornblende are ingredients of frequent occur- 

 rence. These minerals are, in more or less quantity, enclosed in a paste which is 

 probably a partially microcrystalline, and partially amorphic aggregation of the same 

 ingredients, and has a highly vesicular texture, rendering it rough to the touch, even 

 more so than is the case with trachyte. Geologically, Nevadite appears to have been 

 produced as frequently by volcanic activity as by massive eruptions. An interesting 

 occurrence is that at Lassen's Peak, in California, where it was discovered by Prof. W. 

 H. Brewer and Mr. Clarence King. 



Fam. 2d. Liparite, or Porphyritic Rhyolite. The name "Liparite," which was pro- 

 posed by J. Roth for this whole class, may be conveniently retained for those varieties 

 of rhyolite which approach quartzose porphyry in character, as they appear to occur 

 on the Liparic Islands, either solely or at least in larger proportion than other varieties. 

 They consist of a paste which has a similar texture to that of quartzose porphyry, and 

 incloses crystals either of quartz only, or of quartz and sanidin, or of quartz, sanidin, 

 oligoclase, and black mica, or of one or both kinds of feldspar, without quartz being 

 present. The crystals have sharp corners and are seldom cracked ; oligoclase is rarely 

 of the vitreous variety. Typical varieties of the rocks of this family occur largely in 

 the hills of Bereghszasz in Hungary. 



Fam. 3d. Rhyolite proper, or Hyaline Rhyolite. The extensive range of varieties 

 afforded by all manner of modifications of hyaline texture are a peculiar feature of 

 rhyolite, distinguishing it from any other eruptive rock. In outward appearance they 

 remind one of artificial glasses cooled under the most varied conditions. Obsidian, 

 pumice-stone, and pearlite, constitute but a small portion of the varieties occurring ; 



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