Page 722 - calmining1890

672 REPORT OF THE STATE MINERALOGIST.

SONOMA COUNTY.

By W. A. Goodyear, Geologist, and Assistant in the Field.
i
Between Knight's Valley and the Geysers nothing was seen but meta
morphic rocks; that is, sandstones, serpentine, etc. The quantities of th(
latter rock are large. A considerable body of it occurs on the southerr
slope of the Pine Mountain Ridge before reaching Pine Flat, and then
are numerous masses of it between the latter point and the summit o
the ridge. On the northern slope, going down toward the Geysers, then
was less of it seen. The strike of the rocks on the southern slope of this
ridge seems generally northwesterly and the dip northeasterly. Bui
they seem to have been much disturbed, and the stratification is oftenei'
obscure than otherwise.
From the Geyser Springs, we climbed the Geyser Peak. The road
which crosses the mountains here from the Geysers to Healdsburg passes
within a few hundred yards of the crest of the peak. We estimated the' '
distance by the road from the Geyser Hotel to its summit to be about'
seven miles. It is an easy pleasure ride, and the view from here is the'
finest that can be obtained in the immediate vicinity of the Geysers;
though, as before stated, it bears no comparison with that from Mount
St. Helena. The sharp and narrow chaparral-covered ridge, running"
northeasterly from the peak, and connecting with the larger mass of the 1
ridge that lies between it and Pluton Creek, is called the Hog's Back,
and at one point on its crest I noticed the rocks striking north 65 degrees
to 70 degrees west, and dipping 75 degrees to 80 degrees northeast. All
the rocks in this region are metamorphic, and nothing volcanic was seen
southwest of the Cobb Mountain and northwest of Knight's Valley.'
There is a large body of serpentine in the Hog's Back, and also other
masses irregularly distributed in the ridge between it and Pluton Creek,
and probably all through this section of the country. In the metamor-
phic sandstones, the bedding is generally either very heavy or else almost
entirely obliterated, and their stratification usually difficult and fre-
quently impossible to make out without more time than we could give
them. There also occur occasionally large bodies of jaspery rocks, and
these are frequently in the form of thin-bedded shales, whose strike and
dip, however, vary largely, showing the rocks to have been much dis-
turbed.
At the distance of about two miles to the west of Geyser Peak, how-
ever, there is in the low er hills a fine exposure of a large mass of
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apparently heavy-bedded sandstones, which seem to have been not so
irregularly disturbed, and perhaps not so highly metamorphosed as
most of the rocks in this region. A line running from the peak south
35 degrees west magnetic would nearly touch the southeast extremity,
and a line north 80 degrees west would touch the northwest edge of this
exposure. As nearly as could be judged from such a distance, these
rocks strike about north 20 degrees east, and dip about 20 degrees north-