General Geology

a red, fine- to medium-grained rock that in hand specimen shows phenocrysts
of pink orthoclase, minor oligoclase, rare quartz, and fine needles of horn-
blende. Under the microscope the rock is seen to consist of about 65 per cent
cloudy orthoclase, 25 per cent oligoclase, 5 per cent quartz, and 5 per cent
hornblende and magnetite. Graphic intergrowths of quartz and orthoclase
are common and the general character of the rock is similar to that of the red
granite described above except that it contains much less quartz.

Mount Bolom Stock (10). A large part of Mount Bolom is underlain
by a light coloured, medium- to fine-grained, porphyritic granite that cuts the
late Lower Cretaceous rocks on Laventie Mountain and Swing Peak. It is com-
posed of phenocrysts of cloudy orthoclase in a finer groundmass of plagioclase
and quartz with a little biotite and hornblende. Under the microscope, the
quartz, which comprises about 15 per cent of the rock, is clear. The orthoclase
is cloudy and commonly in slight graphic intergrowths with the quartz.

Swing Peak Stock (9). A light coloured, porphyritic, plutonic rock
that outcrops on Swing Peak may be associated with the mass on Mount
Bolom. It weathers light brown to rusty, and is composed of phenocrysts
of feldspar, biotite, and hornblende, in a light grey groundmass.

Structural Relations and Age

Contacts of the Coast Intrusions with older rocks may be either clear-
cut intrusive, gradational through a metamorphic zone, or faulted. Adjacent
to the main mass there is commonly a metamorphic zone containing rocks of
a ‘hybrid’ character. This metamorphic zone is irregular in size, shape, and
continuity. It may be interrupted by a tongue of batholithic rock, or be
present as an embayment in the edge of the batholith. Such zones are mixtures
of batholithic material, older volcanic and sedimentary rocks, and intermediate
or hybrid types that have some of the characteristics of each of the main
components. Small plutons, cupolas, and dykes of batholithic material may
cut these hybrid rocks, but commonly the beds are not greatly disturbed and
the contact is a zone rather than a sharply defined line.

The above contact relations differ from those associated with the small
plutons occurring some distance away from the main mass. The contacts of
these bodies are sharper and better defined, metamorphism is not so great;
‘hybrid’ types are present in minor amounts. Commonly the older rocks are
more disturbed than is the case near the main mass. These relations hold
true on Sibola Range and Chikamin Range and on other similar stocks. On
the ridge north of Fenton Lake the granitic rock—Hazelton group contact

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