General Geology

weather away from the basalt and cover the outcrop as loose particles. Most
of the amygdules are hollow but many are solid. One of the local inhabitants
had several of the solid variety polished and buffed into small ornamental
stones.

A thin section of a basalt flow, containing the large phenocrysts of plagio-
clase, showed little else than the euhedral forms of labradorite Ans, in a
microcrystalline groundmass of feldspar laths, magnetite, and interstitial glass.
Olivine and apatite are present as accessory minerals. Calcite and limonite
have formed as secondary minerals. The feldspar laths of the groundmass
show distinct flowage around the large porphyritic feldspars.

Andesites are about as common as basalts and are generally light grey,
purple, or black. The porphyritic feldspars are not as large as in the basalt
flows. The islands in Ootsa Lake opposite Streatham were found to be
composed almost entirely of andesitic flows that exhibited a crude columnar
structure. These islands have been completely covered by the flooding of
the lake.

A thin section of a purple andesite flow from the south shore of Ootsa
Lake near these islands showed phenocrysts of plagioclase Anz; and a lesser
number composed of altered hornblende in a groundmass of feldspar laths and
magnetite. In this also the feldspars of the groundmass showed distinct flow
structure around the phenocrysts.

Rhyolite flows are perhaps the most distinctive rock types of the group.
They outcrop along the hills bordering Ootsa Lake and commonly form
distinctive ridges and knolls. Much of the road metal used on the old Ootsa
landing-Wistaria road was broken rhyolite from nearby outcrops. The rocks
are white to creamy white, light brown and, rarely, light pink. They are hard,
dense, fine-grained rocks, in part porphyritic containing small phenocrysts of
quartz and feldspar. The phenocrysts are commonly orthoclase or oligoclase
and the groundmass consists of quartz, feldspar, and partly devitrified glass.
Ferromagnesian minerals are rare, but a few crystals of hornblende do occur.
Intergrowths of quartz and orthoclase are common. Spherulitic structures are
common in the flows along Ootsa Lake, some of the spherulites being up to
4 inch in diameter. Perlites and perlitic rhyolites occur in similar rocks near
Francois Lake north and east of the map-area, but were not seen within its
boundaries.

It is believed that the perlitic flows associated with the group outside the
map-area are stratigraphically higher in the group and are therefore slightly
younger than the flows along Ootsa Lake.

The individual rhyolite flows vary greatly in thickness. They may be as
little as 2 feet or as much as 100 feet, but most are 35 to 50 feet thick.

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