TECH ET TEEN 
METAMORPHIC STRATA—PRASOID ROCKS 155 
| The most distinct contortions and highly-inclined positions are, however, shown by a class 
euang q1 A 
Bay of San Francisco. 
“ITH 17 
"IHH uerssng 
"ompisa1q 
*iutog yoy 
és 
IAEA 
"Soqo'T 3uroq 
۰۷۲900 9yroeq 
a 
a, a, a, San Francisco sandstone ; b, Metamorphosed sandstone ; s, Serpentine of Fort Point. 
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 the Golden Gate. Lime Point is entirely 
formed of these rocks, and it is probable that they compose nearly the 
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 
are seen to form the crests or culminating points of the principal ridges, 
and to 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 
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 
square yard, resembling the compressed and crumpled leaves of a book 
in the number of the thin layers, and their conformity through all the 
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 
base of the Point. 
The lithological characters of these strata are very interesting. They 
are hard, flint-like, and jaspery, and occur of various colors. The most 
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 
are beautifully spotted and banded with different colors, and form good 
specimens of ribbon-jasper, 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 às to 
isolate them from the adjoining portions. The flat surface of one of the 
specimens, when viewed at a short distance, appears as if covered by a 
tangled mass of white cord. It is probable that these flinty strata are 
similar to those seen by Professor Dana, and described in his report as 
prasoid rocks.! 
Similar rocks are found along the road south of San Francisco towards 
San José, 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. 
1 Report on the Geology of the United States Exploring Expedition under the command of 
Lieutenant Charles Wilkes,U. S. N. 
