42

common facies exhibits numerous, blocky, white, $-inch plagioclase pheno-
crysts in a finer grained, light grey groundmass, with readily visible biotite
flakes. Other facies of about the same grain size contain a few quartz
eyes, which, locally, are as abundant as the feldspar phenocrysts. Black
amphibole needles are occasionally seen. Finer grained facies are com-
monly so thoroughly weathered that only scattered tiny quartz grains or
biotite needles are discernible in the chalk-like kaolinized groundmass. The
most coarsely crystalline rock, such as is found in the Nep Peak-Comb
Peak stock, contains scattered 4-inch to 2-inch orthoclase crystals in a
groundmass of blocky 4-inch plagioclase grains interspersed with a few
cloudy quartz grains and biotite flakes. In rare instances rounded quartz
eyes are as much as 4 inch in diameter and weather out like pebbles from
the kaolinized groundmass. Be

Specimens examined under the microscope varied in composition from
granodiorite to quartz diorite. About 80 per cent of the phenocrysts are
of strongly zoned plagioclase ranging from oligoclase to andesine; others
include rounded quartz grains, minor orthoclase, biotite, and rarely
amphibole. The groundmass is an equigranular interlocking aggregate of
quartz (as much as 20 per cent), twinned and clear feldspar, and minor
biotite, chlorite, sphene, apatite, and carbonate.

Structure

The Kastberg intrusions form one principal stock from which has been
carved the isolated mountain mass extending more than 4 miles from
Nep Peak to Comb Peak. Elsewhere they form tabular bodies ranging
to more than 300 feet in thickness: a great many of these are gently
dipping sills essentially parallel with the layering of the enclosing strata,
but others are steeply dipping dykes cutting abruptly across the enclosing
formations. One westerly dipping sill outcrops over much of the western
slope of Scallop Mountain. A few sills display rude columnar jointing.
The very fine-grained, indistinctly porphyritic intrusions are generally
closely fractured and form talus slopes of small, sharply angular or platy
fragments.

Sills or sill-like exposures of Kastberg porphyritic intrusions are
abundant within parts of a U-shaped zone extending southeasterly from the
Comb Peak-Nep Peak stock to Scallop Mountain and thence curving from
southwest to northwest along the northeast side of Bear Lake Valley.
They dip southwesterly on Scallop Mountain, are nearly horizontal on
Kastberg Creek near latitude 56 degrees, and dip northeasterly near Bear
Lake. It is probable that many of them emanate from the stock and
comprise a synclinal zone that conforms approximately with the enclosing
upper formations of the Takla group and the lower strata of the Sustut
group, this zone being essentially continuous beneath the northwesterly
trending mountain range that lies between Bear Lake and Comb Peak.
The parent stock occupies the crest and flanks of the contiguous anticline
involving Sustut formations.

Contacts between the sills and Sustut formations are generally sharp
and unsheared, suggesting that the Kastbere rocks were injected when
folding had nearly or entirely ceased. It is indeed not improbable that
their emplacement was a late phase of an early Tertiary period of mountain
building that deformed the Sustut formations.