240 The American Geologist. '^i'''^- ^'*°^- 
The structur.c of the Mesozoic area of eastern Trinity and 
western Shasta counties has been partially described by me in 
several former papers''^ and little new material can be added. 
The area as mapped has a length from north to south of about 
forty miles and a maximum width of about thirty miles with 
a probable average of twenty miles. It occupies a basin-shaped 
depression in the surface of the Devonian rocks, which latter, 
however, do not come to the surface along its western and 
northwestern borders, partly because of faulting and partly 
because of the intrusion of vast masses of peridotite along that 
line. The axis of the basin lies considerably northwest of the 
center, has a course approximately north-northeast to south- 
southeast and occupies the position nearly of the present sum- 
mit of the Trinity mountain. In this central and deeper por- 
tion of the basin, the Bragdon slate outcrops without inter- 
ruption over extensive areas, but nearl}^ all around the borders, 
it has been upturned, eroded and the volcanic series laid bare. 
The northwestern one-third of the basin is characterized 
by the development of a system of beautiful anticlines. They 
strike east-west with a tendency to bend northeastward in en- 
tering Shasta from Trinity county. The Trinity river between 
Trinity Center and Eastman gulch, a distance of eighteen 
miles, traverses about ten of these anticlines. The intensity 
of the folding reaches a maximum a few miles south of Brag- 
don where the elevation of the axis of the anticline may be 
2000 feet above the same strata in the axes of the bordering 
synclines. The strata are closely appressed, so that at and 
near the river-level, the contacts are vertical and in places 
slightly overturned. From' this point both northward and 
southward there is noticeable a progressive decrease in the 
amplitude of the folding vertically, and lengthening of the 
rolls, until the system dies out in comparatively gentle un- 
dulations. 
If we follow the crest of a given anticline, using the con- 
tact between the volcanic series and the overlying black slates as 
our datura plane, we will find it rising and falling in a series of 
gentle undulations. Also, we will find frequent small faults, 
with throws of 10 to 100 feet, with a consequent tilting of the 
* "Origin and Age of Certain Gold 'Pocket' Deposits In Northern Califor- 
nia," Amhk. Geol., vol. xxiv, July, 1899. 
" Metamorphic Formations of Northwestern California," Amer. Geol., 
vol. xxvii, April, 1901. 
