CRYSTALLINE ROCKS OF OAK HILL AREA 165 
greenish variety weathers to this same product, would tend to confirm 
this view. 
These patches disintegrate on exposure more readily than the 
massive green variety, and in some instances accentuate the very 
irregular and fantastic forms that result from surface weathering. 
This change of the mineral serpentine to magnesium silicate, which 
is common in serpentine areas, seems to be an intermediate one, and 
frequently precedes the release and subsequent deposition of silica 
and the taking-up of carbon dioxide, thus forming a rock made up of 
carbonates and various forms of silica, together with compounds of 
iron, etc. Such a rock occurs quite abundantly in the vicinity of the 
Oak Hill area, and will now be described. 
‘“SILICA CARBONATE SINTER’’ 
Its occurrence in the serpentine appears to be somewhat sporadic, 
and there is no fixed evidence to indicate that it is either an inclusion, 
a dike or a vein. On the other hand, it seems to bear relationships 
with the serpentine with which it is found and into which it grades. 
At times it occurs in the form of a dike or vein, but may be irregular 
in outline. ‘The alteration in some cases is so complete that the rock 
has many characteristics of a vein. 
Petrographic characters.—Professor Lawson described wnat is 
evidently this same rock under the tentative name ‘“‘Silica-Carbonate 
Sinter,”’? and his description applies so well that I quote it here: 
Petrographically the rock is an exceedingly irregular and intricate mixture 
of silica in the form of opal and chalcedony and carbonates of lime, magnesia, 
and iron. The silica is present usually in the form of a mesh-work of veinules, 
which, however, do not seem to fill fissures in the carbonate, but to be of con- 
temporaneous formation with it. These veinules anastomose, but do not com- 
monly intersect, and they vary greatly in their thickness. In weathering, the 
carbonate of iron yields an abundant ochre, the other carbonates are leached 
out, and the silica remains as a honeycombed mass, giving rise to exceedingly 
irregular and fantastic, pitted and cavernous forms, which project ruggedly 
above the general surface. It is difficult to get a fresh mass of the rock quite 
free from the yellow ochre. In the least decomposed specimens the carbonates 
are seen to have the crystalline texture of marble with a yellowish color. Occa- 
sionly there is a bright green strain apparent in the rock which may be a silicate 
of iron. 
t “Geology of the San Francisco Peninsula,” Fifteenth Annual Report of the U. S, 
Geological Survey, p. 435.- 
