Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 5, Pt. 2

GRANITE-METAMORPHOSED SLATES.
up the ravine, several open cuttings have been made across the vein, and many fine "leads"
are exposed. Gold can be detected in the fragments thrown out, but the best indications are
the presence of considerable cellular quartz and o)xide of iron. These veins are not now worked,
and no machinery has been erected. The veins have furnished some very rich specimens.
Bowlders and fragments of iron ore are abundant along the bed of the creek.
MORMON ISLAND.
Mormon Island is on the bank of the American River at the point where the foot-hills of the
Sierra Nevada become merged into the plains of Sacramento. Mining is conducted in the bed
of the stream and on the bars; but at a higher level, sixty feet above the river, the operations
extend over a much greater area, being in the older drift and alluvium. The place is also interesting, geologically, for the extensive outcrop of a very superior granite which forms bluffs on
each bank of the stream. This granite is highly structural; the minerals are arranged in
parallel planes or layers, appearing as lines on the surface. They determine a direction of easy
rift, or cleavage, which is favorable to the operation of quarrying and splitting the stone for
building purposes. The color of the rock is a pleasing gray, not unlike that of Quincy syenite.
Its trend is N. 26~ E. Lenticular masses, or agglomerations of hornblende and mica, of a dark
color, are found in parts of the rock, and are frequently drawn out into sheets or lines. They
are like the masses seen in the granite of the summit of the Sierra Nevada at the Tejon Pass.
CONCLUDING OBSERVATIONS ON THE GEOLOGY AND THE AURIFEROUS DRIFT.
The principal geological features of the central portion of the Gold Region which was
traversed. as detailed in the foregoing notes, may be briefly enumerated.
Talcose and clay slates are the prevailing rocks, and in general present a low degree of
metamorphism. The color is generally lighlt, and apparently little changed from the original
tint of the sediment. More highly metamorphosed portions appear to occur in narrow belts,
and to be near the lines of intrusive rocks.
The strata are, in general, upon edge, or inclined at high angles, and trend in a northwesterly direction with great uniformity, and without abrupt and local plications or disturbances
of the beds. The plications into which they have been thrown are upon a magnificent scale
and very regular. This formation of slates occupies the wide space between the intrusive rocks
which form the axis of the Sierra Nevada and a belt of granitic rocks at the base of the slope,
and at the margin of the great California -plains. - The slates are also traversed by erupted rocks
at many intermediate points, and are thus cut up into a series of parallel bands or belts of
metamorphic and intruded rocks, the former predominating. These intrusions are chiefly trap
or greenstone, and a serpentinoid rock which resembles that at Fort Point, and may be considered by some as a metamorphosed sediment; but the evidences favor the conclusion that it is
of igneous origin. Granite is found at Nevada and Grass Valley, and again near Coloma, and
appears to be succeeded on both sides by slates; but the relations of these outcrops-whether
they form one belt, or two or more-I am unable to decide without a map on which the places
are laid down with accuracy.
The lo~ver or most western outcrop, to which reference has been made, appears to form a welldefined belt of considerable length and breadth. It was crossed on the road between Sacramento
and Auburn, and further south at Mormon Island, in the range of the trend. This belt is one
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