6 WM. H. HOBBS 
texture in which the occurrence of two or more generations of 
the same constituent mineral indicates that the process of con- 
solidation was not a continuous one but consisted of two or more 
stages. 
The time honored but now obsolescent classification of igne- 
ous rocks on the basis of geological age has left us as alegacy a 
double nomenclature for the rocks of porphyritic texture, and 
this may be well illustrated by the terms “quartz porphyry” and 
‘rhyolite’? applied to rocks of porphyritic texture having a 
chemical composition similar to the granites. The former in its 
traditional, and also in its present German signification, refers to 
rocks of pre-Tertiary age, the latter to Tertiary or later rocks. 
The tendency of American petrographers seems to be to aban- 
don entirely terms of the class of ‘‘quartz porphyry’’ and to 
extend the terms correlated with ‘‘rhyolite”’ to cover the rocks 
which were previously included in both groups. This tendency. 
seems to me to be an unfortunate one since it results in classing 
together rocks which are essentially unlike. There may be no 
important difference between a particular ‘‘quartz porphyry ’”’ and 
a particular ‘“‘rhyolite,”’ but compare a drawer of hand specimens 
of the former with one of the latter and argument is unnecessary 
to show that as classes they are essentially different. The 
‘‘quartz porphyries”’ are, as a class, devoid of -vesicular and 
fluxion structures—they are in their mode of occurrence hypa- 
byssal—and they more generally show the effects of devitrifica- 
tion, weathering, etc. 
The ‘‘rhyolite” class of rocks may be conveniently distin- 
guished from the ‘“‘quartz porphyry” class by the possession 
of either vesicular or rhyolitic (fluxion) textures. Correspond- 
ence with some representative American petrographers has indi- 
cated to me that a restriction of the terms, rhyolite, trachyte, 
andesite, basalt, etc., to describe porphyritic types possessed 
of rhyolite or vesicular textures, would meet with considerable 
favor. Though of these terms rhyolite alone in its derivation 
calls attention to a fluxion texture, the others by their usage 
(trachytic structure, andesitic structure, etc.) have been given 
