166 E. P. CAREY AND W. J. MILLER 
In a recent letter to Mr. Carey referring to the genesis of his rock, 
Professor Lawson says: 
I have seen a good deal of that so-called “‘Silica-Carbonate Sinter,” since 
I wrote my paper on the geology of the San Francisco Peninsula, and have estab- 
lished to my own satisfaction that the rock in question is an alteration product 
of serpentine, and that the cases where it appeared to be interbedded must be 
cases of intrusive sills. It is thus a chemical rock, but due to alteration, and 
I think the term ‘‘Sinter” therefore a misnomer and will not myself use it. 
This rock, then, is the final or end product of an interesting chemi- 
cal transformation in connection with serpentine. It possesses the 
composition and properties that would be predicted for it apart from 
any evidence. Remnants of serpentine have been found in this rock, 
and all stages in the transformation observed. 
THE GLAUCOPHANE SCHISTS 
The glaucophane schists of the Oak Hill area and vicinity occur 
as distinct outcrops on the southern and eastern portion of the hills, 
in general near the periphery of the serpentine. They present 
nearly as many facies as there are occurrences. It is difficult to 
observe contact phenomena, but in one instance at least it appears 
that the glaucophane schist is completely surrounded by serpentine; 
two other outcrops are closely associated with, if not actually in con- 
tact with, jasper; while still in another, glaucophane schist is in the 
immediate vicinity of sandstone and appears to grade into it. 
The glaucophane schists of the Oak Hill area may be described 
under three heads. 
t. The ordinary typical glauco phane schist.—In this facies the schis- 
tosity is eminently developed, and the schist can be readily split into 
large thin pieces. It is uniformly bluish in color, and contains flakes 
of mica which are plainly visible to the naked eye. 
The microscope shows this rock to be made up essentially of 
glaucophane, with some mica, lawsonite, and titanite. The glauco- 
phane prisms show the well-established glaucophane orientation. 
In the glaucophane the @ ray is pale greenish-yellow; the ) ray is 
purplish-violet parallel to the long diagonal of the basal sections; the 
ray is sky-blue, and the inclination of ¢ to C’ is about 8°. The 
absorption is ) << ¢ > @. In addition to the prism, clinopinacoidal 
