OF VOLCANIC ROCKS. 15 



Difference of Rhyolite from other Rocks nearly related to it. — Several varieties of 

 rhyolite bear so close a resemblance to other rocks, that some mention must be made 

 of their distinguishing features. Rhyolite may be easily distinguished from granite. The 

 varieties by which it mostly approaches the same are those which contain crystals of 

 quartz, feldspar and mica in unusually large proportion and size. But in the case of rhy- 

 olite, the paste in which they are imbedded is never wanting. Moreover, the orthoclase 

 and oligoclase are of the vitreous varieties, and quartz is present either in crystals or in 

 rounded crystalline grains, while in granite it usually permeates the interstices between 

 the other component minerals. Much closer is the affinity which certain other varie- 

 ties of rhyolite bear to quartzose porphyry, especially those which have a paste of 

 homogeneous appearance containing no crystals but those of quartz inclosed. Geo- 

 logical observation will never fail, in such instances, to establish the nature of the 

 questionable rock, as it will show its association either with true rhyolite or with 

 true porphyry. The same test has to be applied occasionally with respect to some 

 other varieties which contain the silica equally diffused through the paste, and bear a 

 close resemblance to trachyte. In this instance, however, even geological observations 

 will sometimes fail to determine the exact position. There is a gradual passage in 

 character between every two nearly related rocks, such as rhyolite and trachyte, or 

 granite and syenite, and it frequently happens that either name may be used with 

 equal right. 



Subdivisions. — In establishing the subdivisions of most orders of eruptive rocks, 

 mineral composition affords a principle, not only the most convenient for practical 

 application, but one that answers well the requirements of the natural system, when 

 made subordinate in value to those higher principles which determine the limits of 

 classes and orders. In the case of rhyolitic rocks, however, it is not as applicable as 

 in that of other orders. They should, from this point of view, be subdivided into 

 those which contain quartz and those which are devoid of it, or into such as carry 

 sanidin and such as contain both sanidin and oligoclase. But, since rhyolite of any 

 certain chemical composition may contain its surplus of silica, either visibly segregated in 

 crystals of quartz, or dissolved in the mass of the rock, and as the case may be similar 

 in regard to the occurrence of either species of feldspar, the application of this princi- 

 ple would lead us to -combine into one group quartzose rocks differing considerably 

 among themselves as to the proportion of silica they contain, while another group 

 might comprehend rocks of virtually the same nature as those of the first, and differing 

 from them only accidentally in external character. More natural subdivisions of rhy- 

 olitic rocks are obtained by taking as a basis of classification their difference in texture, 

 which either approaches, to a certain degree, that of granite or is porphyritic or hya- 

 line. It is a singular fact, and one difficult of explanation, that rhyolite, at every place 

 where it has been hitherto observed, presents, either solely or chiefly, one of those 

 .three modes of texture. Lassen's Peak, for instance, presents the granitic variety 

 almost exclusively. Sonoma, in California, and the Tokay Mountains in Hungaiy, 

 only the hyaline, and other places exclusively the porphyritic varieties. This circum- 

 stance, which is peculiar to none but rhyolite among eruptive rocks, indicates the 



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