8)

lar replacements of white calcite are frequently seen, and most
of the dacites are impregnated by finely divided pyrite.

Under the microscope the essential minerals are seen to be
andesine and quartz, with subordinate augite and rarely magne-
tite, biotite, and apatite. Secondary minerals are, chlorite and
calcite in large amounts, kaolin, sericite, and an irregular scaly
alteration product which may be zeolitic. The andesine is,
without exception, greatly altered to kaolin and sericite, so that
a close determination of its composition is not practicable; it
is lath shaped with irregular ends, and averages less than 1 mm.
long. Quartz is not very abundant, usually from 10 to 15 per
cent, and is in anhedral grains, interstitial to the andesine.
A matrix of fibrous, radiated, and scaly, pale green, faintly pleo-
chroic chlorite surrounds the andesine and quartz. This is
an alteration product from the original mafic mineral, not a
trace of which now exists. As augite occurs in the closely related
andesites, it is probable that it was the mafic mineral in the da-
cites also. The change to chlorite and the recrystallization have
been so complete that the original relations of the augite and
andesine are obscured. The dacites are completely crystalline,
and the lath shaped, occasionally tabular, andesine is divergently
arranged in the matted chlorite matrix. Alteration is extreme
in every rock examined and the great development of chlorite,
the alteration of the feldspars and impregnation by pyrite,
afford evidence of strong hydrothermal action on these dykes.

Quartz Bearing Andesite. These rocks are very similar to
the dacites, differing from them only in containing a smaller
amount of quartz. They are clearly transitional between the
andesites and the dacites.

Andesite. Andesite is the most abundant type represented
among the injected rocks. The andesites are closely related to
the dacites and resemble them in texture and colour, the same
varying shades of grey being seen. Some of the andesites are
porphyritic, and amygdaloids are occasionally found, both in
dykes and sills.

In thin section the essential minerals are andesine, forming
60 to 80 per cent of the rock, and augite. Subordinate minerals
are magnetite and apatite; and secondary chlorite, calcite,