12
AUGITE PORPHYRITE AND ALLIED ROCKS

The intrusive rocks here dealt with form small, stock-like, and more
irregular masses consisting chiefly of augite porphyrite, augite syenite,
and gabbro. Most of the bodies intrude the Bitter Creek argillites near
their contact with the Bear River formation. One elongated body lies
at a contact between the two formations and one lies in rocks of the
Bear River formation. Only the larger bodies are shown on the geological
map accompanying this report. These intrusives have been grouped
because of their general similarity, because all contain a considerable
proportion of augite, and because they are probably of the same age.
They resemble rather closely some phases of lava flows and intrusives
of the Bear River formation and are probably of similar age and origin,
but are considered separately because they have previously been mapped
separately, and because they consist largely of intrusive rock, whereas
the Bear River formation consists largely of volcanic fragmentals and
lava flows. These intrusives are quite different in appearance and mineral
composition from the granodiorite intrusives of the map-area, and are not
believed to be related to them in origin. McConnell has described and
mapped intrusive bodies of augite porphyrite and of gabbro. He believes
that the augite porphyrite is allied to the Bear River formation in origin,
but that the gabbro is associated in origin with the granodiorite intru-
sives.2 In the preparation of the map accompanying the present report,
McConnell’s mapping of the bodies of augite porphyrite was in large part
accepted. Only one body of the gabbro was examined and mapped.

Augite porphyrite occupies a circular area 2 miles in diameter on
Glacier creek, a smaller area on Bear river, a long, narrow band near
Bromley glacier, and several smaller areas in various parts of the map-
area.

The large body on Glacier creek consists of a variety of rocks prob-
ably largely but not solely intrusive. Massive andesite holding large
inclusions of argillite, augite porphyrite, diorite porphyrite, other basic
igneous rocks, and a small amount of volcanic fragmental rock are present.
The main rock type is augite porphyrite, but it also is variable in char-
acter, and in some places, for instance, where it seems to be the only
rock type, the grain is so variable that in a distance of a few feet a rock
with large phenocrysts of augite gives place to a fine-grained rock with
no phenocrysts. The andesite and other igneous rocks may be local phases
of intrusives. The argillite masses are probably inclusions and perhaps
roof pendants. The fragmental rock may be inclusions of breccia and
tuff or may be brecciated phases of intrusives. In some places, the
massive rocks seem to be intrusive, but in other places they are amygda-
loidal and resemble lava flow. The sediments bordering this body on the
north and west dip gently outward. The igneous rocks appear to pass
under these bordering sediments and in part at least seem to form a
sill-like body.

1McConnell, R. G.: Geol. Surv., Canada, Mem. 32 (1913).
*McConnell, R. G.: Geol. Surv., Canada, Mem. 32, pp. 16-20 (1913).