78 The American Geologist. February, 1900 
surface of whose curved planes is highly polished and displays 
the beautiful oil-green of typical serpentine. Another phase 
of the same action has produced veinlets of chrysotile, which 
are in places quite numerous. 
The serpentine belts, or so-called "dikes" of this region, 
constitute one great formation, about 2,500 feet thick, whose 
normal position is above the coarse crystalline and under the 
mica schists, from both of which it seems to be separated by a 
sharp line, particularly from the latter. It represents the ex- 
tremest stage in regional metamorphism, and was originally a 
sedimentary, probably an argillaceous dolomyte. It is much 
older than the schist series. 
Tlie Mica Schist Formation. — This is composed of thin 
folia of muscovite of dull colors such as gray, light brown, 
yellow and dull red, separated by irregular layers of white 
quartz, representing the original lamin?e. It is often highly 
contorted. In certain belts the quartz predominates to such an 
extent as to cause it to outcrop like great veins of very glassy, 
white and dark blue quartz. The relatively great amount of 
this primary quartz is particularly characteristic of this for- 
mation. There are, also, in places, thin folia of hard blue crys- 
talline limestone, sometimes thickening and clearing to a beau- 
tiful marble. The foliation of the formation is parallel to the 
top and bottom and in general to the original lines of strati- 
fication. It is a highly metamorphosed sedimentary, but yet 
represents a stage of metamorphism much less advanced than 
the serpentine. This dynamo-metamorphism has affected the 
entire formation to about an equal degree as the schist is es- 
sentially alike at all its outcrops. Its thickness in the Sierra 
Costa mountains is about i,oco feet. 
The Hornblende Schist Formation. — The preceding forma- 
tion grades upward through a transition series, several hundred 
feet in thickness, made up of black graphitic and light green 
chloritic schists,' into a great formation of hornblende schist, 
probably 2,500 feet in average thickness. This consists of 
elongated crystals of dark green and black hornblende in folia, 
separated by irregular thin layers of white quartz. Although 
its structure is distinctly schistose, it has often 1)een identified 
as dioryte. It is moderately line-grained, and its general color 
is a dark green. Certain layers are highly calcareous, abound- 
