S812 THE JOURNAL OF GHOLOGY. 
with here and there an oval area representing amygdaloidal 
cavities, which are now filled with secondary quartz. The pheno- 
crysts are for the most part corroded dihexahedral crystals of 
quartz, with less commonly phenocrysts of plagioclase and ortho- 
clase. These lie in a fine grained groundmass, which is holocrys- 
talline and composed of feldspar and quartz, with some zircon in 
small crystals, and here and there magnetite. 
The quartzes average perhaps the size of a small pea, and 
hence are macroscopically quite plain. They frequently stand 
out on the fractures and show their crystal form, and in other 
cases we see the angular cavities out of which they have fallen. 
In the thin sections the crystal contours are seen to be more 
or less rounded, with here and there embayments of the ground- 
mass projecting into them. The crystal form is, however, clearly 
marked. Insome cases the individuals have been broken before 
the cooling of the magma, the fragments being seen to conform 
to one another. That some of them have been subjected to 
pressure is shown by the slight undulatory extinction and by the 
separation of the black cross of uniaxial minerals into hyperbole. 
The quartzes are quite clear though they contain some inclusions 
of groundmass and numerous liquid inclusions in which there are 
dancing bubbles. The liquid inclusions have very commonly an 
hexagonal form, corresponding to the contours of the enclosing 
quartz. The possession of an imperfect rhombahedral cleavage 
is very noticeable in a number of the quartzes, and especially 
those which, being on the edge of a section, are very thin. 
(Fig. 1.) The quartz phenocrysts in all the porphyries, with the 
exception of those from two localities, are surrounded by zones 
of varying widths, considerably lighter than the remainder of 
the groundmass. They have the same optical orientation as the 
phenocrysts, and therefore extinguish with them. In those sec- 
tions in which the zones occur they are found around every 
quartz individual. 
The feldspars present are orthoclase and plagioclase, the 
latter apparently predominating. They occur usually in rounded 
crystals, very rarely in grains with irregular more or less angular 
contours. They are always altered, and have associated with 
Pee — 
