MUSEUM OF COMPARATIVE ZOOLOGY. 281 
The blackened parts of this brown hornblende are magnetic, and it is 
probable that much of the magnetite scattered through the groundmass 
came from the destroyed hornblende. Often these hornblendes, occur- 
ring in the andesites that have a reddish color, have been torn into 
ferritic fibres, which have been heaped up or else scattered throughout 
the groundmass. These changes have apparently been produced by 
the remelting of pre-existing rocks of the same kind. The augite varies 
inversely with the hornblende, and in its relations to that mineral leaves 
no doubt that it comes from a recrystallization of the hornblende mate- 
rial dissolved in the magma. 
The feldspar and augite are here products of crystallization of the 
magma. These rocks are subject to alteration, but in a less degree than 
the basalts, the changes being somewhat similar, and giving rise to 
several varieties now classed by lithologists as distinct species, some of 
the most important of which are propylite, porphyrite, hornblende- 
porphyry, black porphyry, part of the felsite or felsite porphyry, diorite, 
etc. No distinction can be legitimately drawn between the hornblende 
and augite andesites, on account of their mutual relations, as given 
above; as well might we have olivine and augite basalts. 
We next pass into that most obscurely defined of the volcanic rocks, 
trachyte, which has been made to include the rhyolites on one side and 
the andesites on the other. The range of silica here is perhaps from 
65 to 70 per cent. The base is a light-gray, almost colorless glass, 
somewhat micro-felsitic ; sanidin begins to predominate, the plagioclase 
diminishes, biotite appears, and the hornblende is partly now a min- 
eral of the first class and partly of the second. Augite rarely occurs, 
while quartz, which has been occasionally seen as a mineral of the first 
class in the basalts and andesites, begins to be more abundant. This, 
like the others, has its old and altered forms. 
Next we pass on to the rhyolites, which contain from about 70 to 
80 per cent of silica, possessing a clear, glassy base, which is, how- 
ever, often colored by foreign materials, holding crystals and fragments 
of quartz belonging to the minerals of the first class, and more abun- 
dant sanidin, biotite, and hornblende, crystallizing out of the magma. 
The base is most prominent in the compact rhyolites, and its devitrifi- 
cation gives rise to the various structures that characterize them. 
This devitrification gives rise in the older and more altered rhyolites 
to the feldspar, quartz, and micro-felsitic (so-called) base, that has so 
puzzled lithologists in the study of the felsites. The rhyolites of all 
voleanic rocks pre-eminently show lamination produced by flowing, 
