Colorado Geological Survey

METAMORPHIC ROCKS -1
east of Pando. The gneiss varies from coarse to fine grain in different
localities. Essential component minerals are orthoclase, microcline, quartz,
and biotite.
QUARTZ-DIORITE GNEISS
A small exposure of quartz-diorite gneiss is found west of the mouth
oz Homestake Creek. The rock has a coarse texture and rough gneissic
structure. Probably half the rock mass is composed of hornblende and
biotite in nearly equal quantities. The feldspar, insofar as determinable,
is plagioclase. Quartz forms about 15 per cent of the rock.
GRANITIC GNEISS
Gneiss that resembles granite in mineral composition, and of uncertain
or mixed origin, may be called granitic gneiss. One variety is pinkishgray,
and composed chiefly of quartz and feldspar. It also carries an appreciable
quantity of muscovite and very little biotite. The rock may be
a metamorphosed arkose or feldspathic sandstone. The proportion of
feldspar is a little low for typical granite. Another vlariety bears a resemblance
to the biotite gneiss described below, but contains a higher
proportion of injected igneous material. A phase of this rock carries much
soda-lime feldspar, and approaches granodiorite in mineral composition.
MICA GNEISS
The mica gneiss differs from the granitio gneiss chiefly in the much
higher proportion of mjica or quartz or both. The feldspar content is low.
Part or all of the rock here called mica gneiss is of sedimentary origin.
The rock is dark gray or reddish-gray, and has pronounced gneissic structure.
Quartz in most specimens makes up 50 to 80 per cent of the rock.
The highly quartzose variety might be called a quartzite, but the pronounced
banding is more characteristic of gneiss. Biotite composes about
15 to 30 per cent of the rock mass. Muscovite is present in some specimens.
PYROXENE GNEISS
North of Yoder Gulch, about 200 feet above the railroad, is a small
exposure of fine-grained pyroxene gneiss. The rock is dark in color, and
contains thin veinlike streaks and lenslike patches of quartz. The most
abundant mineral is pyroxene determinable only in thin section. It is
grass-green to greenish-gray, monoclinic, and has a birefringence of about
.028 or .029. It is probably ferriferous diopside or common pyroxene.
Quartz, orthoclase, microcline, soda-lime feldspar, and a little titanite and
zircon are present.
SILLIMANITE GNEISS
South of Eagle River, west of the mouth of Homestake Creek, is a
small exposure of peculiar rock whose weathered surface shows many bluish
patches of a mineral with a splintery fracture. This mineral is shown by
the microscope and chemical tests to be sillimanite. Other minerals present
are quartz, andesine, and biotite. Any one of the minerals may be bunched,
and associated with smaller amounts of the others. That part of the rock
rich in sillimanite is about one-half sillimanite in grains up to three-