16 
MUSEUM BULLETIN NO. 4. 
A reddish zeolite, which is almost certainly heulandite, 
forms druses on some of the joint planes, and occurs also as 
veinlets in some of the tuffs. 
Trachytes — Mgirite-augite trachyte. 
This specimen was taken from a boulder about three feet 
in diameter, which was not found in place. Other rocks of 
similar appearance are of frequent occurrence in the volcanics, 
so it may be that the trachyte occurs as a flow in this part of the 
district. However, microscopic study shows the other 
specimens that were collected to be pyroclastics, but of such a 
strikingly similar appearance to this primary type that they were 
not distinguished as secondary in the field. There is little 
doubt that a magma of this nature furnished material to form 
a considerable portion of the volcanics. 
The hand specimen (field number 35), shows deep pink, 
rectangular, and quadratic phenocrysts of feldspar up to an inch 
long, averaging about half an inch, forming about 20 per cent of 
the rock, somewhat irregularly distributed in a green, very 
finely crystalline groundmass which is speckled with shiny, 
black phenocrysts of pyroxene up to 2 or 3 mm. long. 
In thin section the feldspar phenocrysts resemble ortho- 
clase, except that inclined extinctions on 010 up to 14 degrees have 
been observed. This, in connexion with the absence of twinning 
and the low index of refraction, indicates that the mineral is 
soda orthoclase. They are slightly altered, and show kaolini- 
zation in cloudy reticulate bands, often obscuring most of the 
section. 
The phenocrysts of pyroxene are idiomorphic or tabular 
pointed individuals from 0*3 mm. to 1 *0 mm. long, showing com- 
binations of the prism and pinacoids in cross section. The 
extinction angles of C A c = 74 degrees, the grass green colour, 
and the green to yellow pleochroism determine the mineral as 
aegirite-augite. Zonal banding is occasionally seen, the centres 
of some crystals giving a higher extinction than the sides. 
Twinning parallel to the elongation has been noted. The 
mineral is remarkably fresh, and the outlines are sharp. 
