MICROPEGMATITE IN PURCELL SILLS. 
11 
appearance and minute needles of it occur throughout the other 
mineral constituents. The micropegmatite, which forms the 
ground-mass, consists of quartz containing oriented inclusions 
of orthoclase, minutely charged with a great number of micro- 
scopic hornblende needles. Large masses of quartz occur which 
are usually, but not always, free from inclusions. Apatite in 
long needle-like crystals, and magnetite form the accessory 
constituents. This phase is believed to be a variation of the 
acid member of the banded gabbro. 
Associated with the basic sills and sometimes with the banded 
gabbros, are small irregular dykes of aplite, which consist almost 
entirely of plagioelase and quartz in varying proportions, with 
minor amounts of calcite. With a decrease in calcite and 
plagioelase these dykes pass into quartz veins which represent 
the extreme differentiate of the gabbro magma. Associated 
with all these variations occur sulphides of iron and copper. 
Quartz-Diorite or Transition Rock . — With an increase in 
quartz and micropegmatite the gabbro gradually passes into 
quartz-diorite which forms the transition type between the 
gabbro and the granite (micropegmatite). The quartz-diorite 
has a light greyish-green colour and, in the hand specimen, 
shows quartz, feldspar, hornblende, and biotite. Under the 
microscope, the shredded hornblende is seen to have the same 
pleochroism: c, bluish-green, b, dark green, a, yellowish green, 
as the secondary hornblende described as occurring in the hypers- 
thene gabbro, and is embedded in a ground-mass of quartz and 
micropegmatite. In the latter the quartz holds the feldspar, 
which is clouded with a great number of dust-like inclusions. 
Plagioelase — andesine to labradorite — is rather plentiful, while 
biotite, in small plates, is present sporadically throughout the 
rock. Chlorite and zoisite occur as secondary minerals. The 
following analysis is given by Daly: — 1 
1 Daly, R. A., Festschrift zum siebzigsten Geburtstage von H. Rosenbush, 
1906, p. 217. 
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