Metamorphic Rocks.—kuirbanks. 163 
granite and have been. metamorphosed by it.* Crystalline lime- 
stones probably Carboniferous are found on both sides of the 
Mariposa beds, and must have been concerned in the same up- 
heaval. Northward toward Pence’s ranch no physical break has 
been detected, At this place are Carboniferous limestones and 
slates in part of the same age. Carboniferous limestone is also 
found in the eastern part of Shasta county interbedded with 
slates. From all the evidence at hand I believe it is impossible 
to separate the Mariposa beds from the other metamorphic rocks 
of the Sierras, and consequently their period of upheaval, ad- 
mitted to be the same as the metamorphic rocks of the Coast 
Ranges, is coeval with that of the Carboniferous of Shasta 
county and other parts of the Sierras. 
With regard to the peculiar metamorphism of the Coast Ranges 
it was certainly pre-Cretaceous. The formation of a network of 
minute quartz veins, so wide spread, seems to be confined to the 
harder rocks which were broken and crushed in the violent oro- 
graphic movements, while the more argillaceous rocks were re- 
duced, often to a clayey mass impenetrable by the  silicifying 
agencies. 
The metamorphic series is also distinguished by the presence of 
much intrusive rock which, as far as my observation goes, does 
not appear in the Knoxville. These intrusions are so much de- 
composed that it is difficult to get at their real character. When 
the amygdaloidal structure is absent they closely resemble decom- 
posed sandstone. Good exposures of them appear north and 
west of Clear lake, in Pope valley, and Capel creek. In the 
Geology of the Quicksilver Deposits it is stated that no pre-Ter- 
tiary eruptives are to be met with between Clear lake and New 
Idria. 
Serpentine is the most common pre-Tertiary eruptive in the 
Coast Ranges. That it is younger than those just described is 
shown by the fact that it cuts both the Knoxville and metamor- 
phie series. It was held by Prof. Whitney and his assistants to 
be an altered silicious or silico-argillaceous rock, the magnesia 
having been introduced by some process of substitution. Dr. 
Becker follows in nearly the same line and endeavors to prove the 
origin of serpentine through metasomatic processes, either directly 
*Tenth Annual Report of the State Mineralogist, p. 30. 
+Auriferous Gravels, p. 19. 
