OF VOLCANIC ROCKS. 1] 
most advanced stage of the science a few years ago, because it was guided by a definite 
principle ; though when compared with contemporaneous works on petrology, it gives, 
on the other hand, a remarkable illustration of the great difference in the way in which 
the same names have been applied. Since then, the name ‘rhyolite ” (Richthofen, 
Studien aus den ungarisch-siebenbiirgischen Trachytgebirgen, in Jahrbuch der K. K. 
geologischen Reichsanstalt in Wien, Vol. XI [1860], pp. 153-277) has been introduced 
for a very distinct class of volcanic rocks. Adding this to the previous list, there 
results a number of names which, in geological treatises, are either grouped com- 
pletely at random,? or in an arbitrary order, or arranged by artificial principles, when 
the whole classification ordinarily comprises volcanic and ancient eruptive rocks pro- 
miscuously. In order to establish a more natural system, we have, not to make groups, 
but to find them. Dropping all of those @ prior? principles which may be conceived 
having an artificial basis, we must endeavor to discover whether any great divisions 
are established by nature herself, and if so, of what character they are. We may 
then apply, as second in the order of their importance, those results which are obtained 
in the laboratory or geological cabinet, for defining and subdividing those groups. 
Most of the natural divisions which may be derived from geological observation, coin- 
cide essentially with those based on artificial principles, but are more naturally 
limited as regards each other. Hach of them has its own more or less independent 
part in the architecture of mountain ranges, and a distinct geological age in reference 
to the other groups. Hach of them comprehends a series of rocks, which, besides, are 
closely connected by the relations of their petrographical characters, chemical compo- 
sition, texture, specific gravity, and other properties. The test of the natural founda- 
tion and general validity of these groups will be their recurrence, with mutual rela- 
tions unchanged, in different parts of the globe, of which test we are never to lose 
sight. 
The following is the classification, the approach of which to a natural system 
of volcanic rocks, I will endeavor to set forth in the course of this paper : 
Orver First: Rhyolite. 
Family 1. Nevadite, or granitic rhyolite. 
‘2. Liparite, or porphyritic rhyolite. 
3. Rhyolite proper, or lithoidic and hyaline rhyolite. 
ce 
Orper Seconp: Trachyte. 
Family 1. Sanidin-trachyte. 
‘2. Oligoclase-trachyte. 
2 That this is even done in books of the highest standard, may be seen by reference to one so prominent as Lyell’s 
Elements of Geology. The following is the order of names of which, under the head of “volcanic rocks,” definitions are 
given (p. 592, pp. of 6th Am. ed., 1866) : Basalt, augite rock, trachyte, trachytie porphyry (in connection with which the 
name “andesite” is mentioned), clinkstone, greenstone, porphyry, amygdaloid, lava, scoris or pumice, yolcanic tuff or trap- 
tuff, agglomerate, laterite. 
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