382 OS GALE 5 BALLERS Lava 
having disappeared by decay, but deep in the formation the cob- 
bles are comparatively fresh. 
The age of this deposit is considered to be the same as that 
of the Hay Fork, Indian Creek and Hyampour, because it is of 
similar lithologic character, occupies a similar valley, and has 
suffered great erosion since its abandonment by Trinity River. 
No coal is known from this area, and no tuffs have with certainty 
been discovered in it. No bones or fossil plants have yet been 
reported from it. However, it has not been closely examined 
for them, and even the tuff may occur and have escaped notice. 
Conditions of accumulation. — Several interesting problems are 
presented by the old Trinity River deposit. What were the con- 
ditions which caused the accumulation of 1,600 feet of alluvial 
gravel andsand? Why did the river abandon its old course and 
cut a new valley on a longer course? 
Let us first inquire into what fixed the site of the old valley. 
The abrupt southeastern face of the Sierra Costa Mountains on 
the west side of the old Trinity valiey has the appearance of a 
degraded fault scarp. After long consideration of the matter, I 
have concluded that while there may be an old fault on this line, 
it has had very little influence on the present topography. It is 
along this line that the resistant serpentine and granodiorite of 
the higher mountains meet the rather soft and easily eroded 
Bragdon slate and the Clear Creek volcanic series, the Mesozoic 
representatives in this region. The mountains on the west are 
high because the rocks are very resistant, and those on the east 
are low because the rocks are softer. Along the junction between 
the two areas was a line of unusual weakness, perhaps because of 
an old fault, and that controlled the Trinity River between Trin- 
ity Center and Weaverville. In the course of time the river 
varied somewhat from the fault(?) line, and at one place cut its 
deep cafion entirely on the serpentine side of the line. This 
strengthens its interpretation as a valley of erosion. 
The accumulation of 1,600 feet of gravel and sand by river 
action. could have occurred under one of only two conditions ; 
either a depression of the land to a lower level relatively to sea 
