270 
GEOLOGY. 
about five hundred feet in length. The roek is compact greenstone, and is traversed in 
several directions by veins. The main lode dips at an angle of twenty-two degrees, and is 
stoped from the level of the tunnel, or adit, to the surface. There are several other veins 
which are not worked out to such an extent. One of the veins is only three or four inches 
wide, and is pen along the centre, the quartz being deposited on each wall, so that the crystals 
on the oppo - ng surfaces interlock. It is a good example of a true fissure vein. The color 
of the quart , which is peculiarly stained by oxide of iron, and the character of the crystals, 
conforms exactly with specimens which I obtained in San Francisco, which were said to be 
from Grass Valley. These consisted chiefly of broad plates partly imbedded in quartz, or 
supported between the ends of the crystals. This, I was assured, is the character of the 
specimens obtained from the vein. The quartz veins of this locality do not appear to be very 
highly charged with pyrites, and yet they are stained by iron as if from its decomposition. 
Pyrites appears to exist in the wall-rock in considerable quantities. 
The machinery for crushing and washing the quartz is on a great scale, and has been erected 
at great expense. The crushers and stamps are driven by a powerful condensing engine from 
the establishment of James Watt & Co. There are three large tubular boilers set very beauti¬ 
fully, without brick-work. The crushing apparatus consists of two sets of Cornish crushers, 
twenty-six inches in diameter, and twelve stamps. It is the intention to increase the number 
of stamps to twenty-four. 
The mine of the Rocky Bar Mining Company is upon Massachusetts Hill. Several shafts 
have been sunk upon the vein, from twenty to fifty feet deep, until water was reached. It is 
not now worked, but preparations are making to sink an engine shaft to intersect the vein at 
a depth of 110 feet. The vein is said to dip towards the northeast at an angle of thirty-five or 
forty degrees. The quartz which has been removed is reported to have yielded about sixty 
dollars to the ton. 
There are many other interesting mines and crushing mills at Grass Valley, but they could 
not be visited during the short stay made in the place. 
GRASS VALLEY TO COLOMA. 
Grass Valley to Auburn .—We travelled over a rolling, uneven country, .and crossed many 
small creeks. The rocks are covered by drift and soil, which sustains a growth of oaks, pines, 
and the peculiar shrub called mancinita. A few miles before reaching Auburn we found the 
outcropping edges of argillaceous and talcose slates, and they form the bed-rock at Auburn. 
The surface of this foundation of rock is very irregular and rough, but is made smooth in places 
by a thin layer of auriferous drift, from six inches to two or three feet in thickness. These 
elevated flats are of considerable extent, and have been completely dug over by miners. The 
drift is much rounded, and is not very coarse, but large masses of the subjacent slates are found 
with it. A considerable part of the gold is in coarse lumps, and masses weighing from ten to 
eighty ounces are common. 
Auburn to Coloma .—The prevailing formations from Auburn to the crossing of the North 
Fork of the American are clay slates and talcose slates, standing nearly on edge, and traversed 
by intruded dykes of basaltic or trappean rock. The rock, at the crossing, is a fine-grained 
syenite, cleaving like trap-rock. It appears to be a dyke of but slight thickness, and the slates 
are seen again a short distance beyond. We passed upon a fine-grained syenitic granite about 
