9

a short distance east of the northerly trending part of Bear river. The
anticline has the shape of an inverted boat plunging at both the north
and south ends. In detail the structure is more complex. Although few
small plications were seen, several folds exist both transverse and parallel,
which complicate the main structure. In the vicinity of the Porter Idaho
mine on Kate Ryan creek, the volcanic rocks dip west. On the eastern
slope of Bear River ridge the prevailing dip is also westward, but trans-
verse folds and possibly faults have modified the main structure in this
place and the detailed structure is not apparent. Along the westerly
trending part of Bear river the rocks lie in a gentle easterly trending anti-
cline, which appears to swing northward to join the main northerly
trending anticline. ‘The transverse fold flattens to the east and plunges
steeply under the Nass formation.

The Bitter Creek argillites are exposed in the central part of the main
anticline. The harder, more competent volcanic rocks have been gently
folded transverse to the main line of folding and the lower, softer, and
less competent argillites have been locally severely plicated.

NASS FORMATION

This formation outcrops in only one or two places and was not
examined. The following description is taken from pages 17 to 18 of
Memoir 32 by McConnell.

“The Nass formation consists of a thick argillite series alternating in places with
coarse clastic beds and bands. These rocks definitely overlie the Bear River volcanics
and in one place appear to alternate with them near their base. The Nass formation
ae ae (is) very similar in most respects to . . . . the Bitter Creek argillites,
but less generally altered. They are seldom strongly cleaved and in some sections
might be classed as shales. Beds and thick bands of tuffaceous sandstone occur with
them in places, but are of subordinate importance. These consist of angular and
subangular grains of quartz and broken feldspar crystals, with occasional small rock
fragments, mostly slate and limestone, enclosed in a dark, fine-grained matrix, usually
considerably altered . . . . . In the mountains north of Bear River summit the
Nass argillites alternate near their base with heavy bands of . .. . . breccia
precisely similar to that occurring in the upper portion of the underlying Bear River
formation.”

Nass argillites occur

“overlying the Bear River greenstones on the summits of the mountains situated
north and south of Bear River pass. Hast of the Coast range they occupy the
ridgy district extending eastward . . . . to the Nass river and probably extend
for some distance beyond.

In the summit ranges the Nass argillites occur in a nearly flat attitude ar &
while along the eastern edge of the Coast range the inclination is steadily east-

ward. . . . . The Bitter Creek argillites . . . . . have prevailing westerly
-. . . dips, thus giving the two formations the appearance of forming opposing
limbs of a wide anticline!. . . . . The principal reason for classing them as two

series is that while the Nass argillites definitely overlie the Bear River volcanics, the
Bitter Creek rocks appear to underlie them to the west, and are intruded by them at
various points. The lithological character . . . . also .. . . exhibits some
differences. Limestones occur in the Bitter Creek formation and the proportion of
coarse fragmentals is much less than in the Nass formation.”

1‘Syncline”’ appears in the report, but obviously ‘‘anticline’ was intended.