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

METAMORPHIC STR,TA-PRASOID ROCKS
The most distinct contortions and highly-inclined positions are, however, shown by a class
of rocks Which have not yet been mentioned. They are, to all appearance, a metamorphosed or
changed portion of the sandstone formation. They are found outcrop ping near the Presidio, south of the Mission, and form the highest ele vations of the north shore of thf Golden Gate. Lime Point is entirely
//l X formed of these rocks, and it is probable that they compose nearly the
euang el:.[ V whole peninsula from Point Cavallos to Point Bonita. At Lime Point they
exhibit regular stratification, with the planes nearly vertical, or inclining
westward. Portions of the strata are very finely stratified, the layers being
not over half an inch thick, and yet they are well defined and apparently
'~ ~ very hard. The whole series is enormously thick, and the principal beds
v are seen to form the crests or culminating points of the principal ridges,
andto outcrop in long lines on the surface. Several of the small islands,
or large rocks under Lime Point, consist entirely of these metamorphic
sediments, and rise above the waves with nearly vertical sides, like
~ steeples. In these islets, and on Lime Point, there are beautiful flexures
~l:H qdusogl ( a and folds of the strata, some of them of considerable extent, and others
are local, showing many bends and short angles within the space of a
I aoo:.~,s u ). square yard, resembling the compressed and crumpled leaves of a book
in the number of the thin layers, and their conformity through all the
E us ~ bends. One of the most interesting displays of these plications is found
in the sides of Needle rock, a high column rising from the waves near the
0 base of the Point.
0 The lithological characters of these strata are very interesting. They
~ are hard, flint-like, and jaspery, and occur of various colors. The most
~~ S common color is a dark reddish-brown, or a chocolate color. but this is
often intermixed with yellow and green. Indeed, some of the fragments
~o.!.o.a V Vare beautifully spotted and banded with different colors, and form good
specimens of ribbonjasper, or prase. Quartz, in thin irregular veins, is
a common accompaniment of the rock, and traverses it in all directions
' without any regularity in the trends of the fissures. It appears, in many
/ cases, to form a complete coating around fragments of the rock, so as to
~u.oJ uod isolate them from the adjoining portions. The fiat surface of one of the
specimens, when viewed at a short distance, appears as if covered by a
a tangled mass of white cord. It is probable that these flinty strata are
!J,~, similar to those seen by Professor Dana, and described in his report as
prasoidrocks.1
/x Similar rocks are found along the road south of San Francisco towards
,,',i~ San Jose, and at New Almaden. At the latter place the color is much
lighter, and the crooked quartz veins are absent. They are, however,
./ very hard, and show the original alternation of shales and sandstone.
'soqoqI ~ulo, \I
1 Report on the Geology of the United States Exploring Expedition under the command of
Lieutenant Charles Wilkes,U. S. N.
"ueooo auorao~:
155