OF VOLCANIC ROCKS. 15 
Difference of Rhyolite from other Rocks nearly related to t.—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 erystalline grains, while in granite it usually permeates the interstices between 
the other component mine rals. 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 rotks 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. lLassen’s Peak, for instance, presents the granitic variety 
almost exclusively. Sonoma, in California, and the Tokay Mountains in Hungary, 
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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