Bragdon Formation in N. W. California. — Hcrshey. 251 
before the geologic public. It is warranted by the importance 
and interesting character of the formation, for in the Bragdon 
series lies the record of a peculiar event in California geology. 
THE EASTERN OR TYPE AREA. 
The principal and least altered area of the Bragdon form- 
ation lies in the eastern portion of Trinity county and the west- 
ern portion of Shasta county, mainly in what may be called the 
Trinity Mountain country. It is roughly oval in outline, its 
major axis trending north-northeast to south-southwest. It 
has a length of about thirty miles, a width of ten to twenty 
miles with a probable average of about fifteen miles, and in 
addition there is a spur twelve miles in length and two to four 
miles wide extending southeastward to Ono in the Sacramento 
valley. The northeastern portion is trenched by the Sacra- 
mento river both above and below Delta, the Trinity river tra- 
verses it near the Avestern border, and Clear Creek emerges from 
it about a mile below the Tower House. In the central portion, 
the formation outcrops without interruption over broad areas, 
but around the borders, especially on the western side, small 
masses have been isolated, by erosion, from the main era. 
The formation is a succession of black, laminated shales and 
blue-gray quartzytes, with conglomerates locally developed. 
The bedding is regular and very prominent at nearly every out- 
crop. Layers of black, thin-bedded, hard shales (or slates as I 
have formerly denominated them because of their alteration) 
from ten to fifty feet thick alternate with heavy-bedded layers 
of coarse sandstone two to five feet thick, although layers ten 
• to twenty-five feet thick are not uncommon. Where the form- 
ation lies nearly horizontal, extensive outcrops as seen from a 
distance have a banded appearance, as in the high bluff at the 
bridge near Trinity Center. 
Except in a contact metamorphic zone near Lewiston. the 
shales have not been metamorphosed to the extent of the de- 
velopment of mica, but they have been silicified, so that they 
resist weathering and fragments commonly lie about on the 
surface in flat pieces. The black color I suppose to be due to 
carbon. The shales in many places weather to a light yellow 
color, as along the stage road crossing Trinity mountain be- 
tween French Gulch and Trinity Center. The distinct lamin- 
ation has generally escaped destruction, but the formation is 
