160 The American Geologist. March, 1892 
The line of contact between the Cretaceous and the older rocks 
has been particularly favorable for the intrusion of the periditi- 
tic rock from which the serpentine has been derived, and this to- 
gether with a general covering of the rocks with soil makes it 
hard to find good exposures. The best contact observed was on 
Klk creek in Colusa county; here the soft black shales rest di- 
rectly against the green silicified schists. A few hundred feet 
distant the shales havea dip of 40° to the east; as the contact is 
approached they dip more and more, finally becoming somewhat 
broken and reversed, while for several feet adjoining the schists 
they are crushed to a clayey mass. The change to the vertical 
green schists is abrupt. Toward the crest of the mountain five 
miles away they become more silicified. Black slate and horn- 
blende schists are also to be observed in places. The clay at the 
contact has been formed by an upward movement of the meta- 
morphic ridge, a condition noticed at several points farther south, 
and which, to a certain degree obscures the non-conformity. 
This is undoubtedly the reason for the apparent conformity be- 
tween the Aucella bearing strata and the metamorphics of Mount 
Diablo, mentioned by Mr. Becker as a proof of the unity of the 
two formations. * 
About the headwaters of Stony creek the Cretaceous is separ- 
ated from the metamorphics by large areas of serpentine, and 
ancient porphyritic rocks in part amygdaloidal. Snow Mt. 
consists of jasper, silicious schists and hard sandstones. Along 
the ridge southward between Lake and Colusa counties glanco- 
phane schists appear. Black slate becomes more abundant and 
is particularly well exposed on the North Fork of Cache creek, 
where it splits into large slabs when not too much crushed. 
The eastern boundary of the metamorphic rocks follows a di- 
rection a little east of south, passing about three miles east of 
Lower lake, through the western edge of Pope valley, and then 
becomes covered with volcanic material so that it can be traced no 
farther. Two outlying areas are to be found to the east of this 
line. One forms a ridge near the head of Sulphur creek, Colusa 
county and consists of sandstone, schists and jasper, with dikes 
of old eruptives. From this ridge the aucella bearing strata dip 
in opposite directions. The other is of much greater extent, 
forming a high range west of Beyressa valley, with a length of 
*(reology of the Quicksilver Deposits, p. 184. 
