204 _ TRANSACTIONS OF THE [Apr. 24 
Under the microscope the gneiss shows a mineral composi- 
tion seldom very different from that of the granites, though, as 
a rule, containing more plagioclase. Slides from different 
localities show no great variation in the character of the 
minerals present, though different proportions prevail. Even 
the dark bands are usually composed of the same minerals as 
the normal gneiss, but with a larger content of mica or horn- 
blende. There are, however, exceptions to this rule. Quartz, 
orthoclase, plagioclase, biotite and hornblende make up the 
bulk of the gneiss ; with zircon, apatite, ilmenite, magnetite, 
rutile, garnet and sillimanite as minor constituents. 
Quartz is never absent, though varying somewhat in quantity. 
It often occurs in rather large patches, which under crossed 
nicols break up into several] distinct parts. Fluid inclusions 
are very abundant, usually in bands which extend through 
adjacent individuals. Zircon is also a common inclusion, while 
the hair-like bodies supposed to be rutile are rarely present. 
The quartz nearly always shows undulatory extinction, which, 
together with the shattering often apparent, bears witness to 
the strains to which the rock has been subjected. 
The feldspars show much variation in the relative propor- 
tions of different species present. On the whole, plagioclase 
seems rather more abundant than orthoclase, but the reverse is 
so often true that the examination of a larger number of 
sections might show it to be the rule. In a few instances 
microcline is an abundant constituent, but it is more generally 
absent. A very marked feature in a majority of the sections 
examined is a great abundance of the microperthitic inter- 
growth of orthoclase and plagioclase. The peculiar banded, 
dotted and striated sections produced by it are generally 
apparent in ordinary light, but are brought out more clearly 
with crossed nicols. Sections of orthoclase show a varying 
proportion of their area occupied by slender spindle-shaped 
masses of plagioclase. These spindles extinguish simultan- 
eously, and have decidedly stronger double refraction than the 
surrounding orthoclase. In most instances the microperthite 
has the appearance of a contemporaneous crystallization of two 
feldspars ; but enough sections contain absolute proof of its 
secondary nature to render it extremely probable that in this 
gneiss it is never an original intergrowth, Evidence of this 
secondary origin is seen in plagioclase spindles passing 
unbroken across cracks in the orthoclase, and in the evident 
optical continuity of the material of the spindles with secondary 
feldspar filling cavities and cracks adjacent to the microperthite. 
Figure 1 shows a case of the latter kind. A crack between 
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