bascom.] SUMMARY. 67 
The first are typical noncrystalline porphyries, characterized by a 
soda-feldspar and by the presence in many cases of accessory piedmont- 
ite. The second group are the prototypes of the modern rhyolites, 
differing from them only in the loss of a vitreous base through devitri- 
fication. They are without phenocrysts, with inconspicuous pheno- 
crysts, and with abundant and conspicuous phenocrysts. Like the 
porphyries, they are characterized by a soda-feldspar — that is, they are 
of the pantellerite type. The evidence for devitrification lies in the 
abundant presence of structures peculiar to glassy lavas, in the present 
holocrystalline character of the rocks, and in the empirical knowledge 
that g-lass may become crystalline through lapse of time. 
The sericite schists are a metamorphic product of the first two 
classes by means of dynamic action. 
The alteration which the original types have undergone subsequent 
to consolidation is, in the case of the aporhyolites, devitrification 
(statical metamorphism); in the case of the schists, sericitization 
(dynamical metamorphism); and in the case of all three groups, includ- 
ing the quartz-porphyries, an epidotization (weathering). 
