General Geology

Previous workers included all the islands in this vicinity with the
Tertiary rocks. The writer, after careful examination, concluded that the
rocks in this general area, with perhaps the exception of the two islands
mentioned above, are more characteristic of the Hazelton group than of the
younger rocks,

The gabbro dyke in the northeast corner of the map-area is a light
grey porphyritic rock with phenocrysts of labradorite An;, in a fine-grained
groundmass of plagioclase, biotite, and subordinate augite.

Internal Structure

Rocks of this group are relatively undisturbed and thus are commonly
flat lying. On Mosquito Hills the flows have a gentle southerly dip of 25
degrees at a contact between two flows. The white ash bed between the flows
on Whitesail Range dips 35 degrees southeast. Angles of dip, measured on
columnar structures on Chef Ridge, are as high as 40 degrees at one place,
though most flows in this region are flat lying. Many of the dips may be due
to initial inclination for it seems most probable that these rocks were ex-
truded over a terrain of at least moderate relief as they are found undisturbed
both on plateau regions above timber-line and in valley bottoms. On the
north shore of Eutsuk Lake east of Sand Cabin Bay the four buttes or hills
mentioned above rise to as much as 1,000 feet above the general level of
the valley bottom and are aligned in a direction south 45 degrees east.
These four structures resemble volcanic plugs and were most probably
sources for some of the basaltic material.

External Relations

Flow rocks of Oligocene or later age overlie unconformably all other
consolidated rocks in the map-area. On Mosquito Hills and the eastern end
of Whitesail Range they overlie the Hazelton group rocks with an angular
unconformity. On Mount Wells and Chef Ridge they overlie batholithic
rocks. They have themselves been deeply eroded during Pleistocene time and
now occur merely as remnants of a much wider and thicker cover.