150 
GUIDEBOOK OF THE WESTERN UNITED STATES. 
From Fenner to Danby the railway descends the valley near its 
center. 
A short distance to the west rises a high ridge known as 
Clipper Mountain,' which consists mainly of a thick succession of 
Tertiary voleanic rocks, which lie on gneiss that crops out extensively 
at the north end of the range. 
This ridge is about 10 miles long and 
some of its peaks rise more than 1,500 feet above the surrounding 
plain. The rugged slopes of Chipper Mountain present a great variety 
of strong colors, mostly yellow and brown, due to the oxidation of 
the tuffs and other rocks. 
‘The rocks of Clipper Mountain are 
principally tuffs and agglomerates, inter- 
led with many thick flows of light- 
colored lava (rhyolite) and penetrated by 
a number of thick stocks of this and other 
seeiigrs rocks. The tuff and ash beds 
ear the top are capped by a thick sheet 
2 mor lava (basalt), which dips north- 
westward. Small masses of this kind of 
lava se appear at several points in the 
plain south and southwest of the moun- 
tain, but these are probably of later age 
than the summit cap. The agglomerate, 
tuff, and ash of this succession were 
ejected from volcanic vents and probably 
accumulated very rapidly, with intervals 
The 
final eruption was the sheet of black lava 
(basalt) now capping Clipper Mountain, 
which probably flowed out at the same 
time as the sheets capping Signal Peak, 
north of Goffs, and Black Top, southwest 
of Goffs. The same succession also con- 
stitutes the high ridge extending through 
Vontrigger to the east slope of the Prov- 
idence Mountains northwest of Fenner. 
So 
outbursts were so recent that they appear 
to have been almost within the historic 
period. The largest bodies of lava, how- 
“hig sid accumulated in middle and 
Tertiary time, when a vast amount of 
fragmental material was thrown out of 
numerous vents of various kinds and 
spread over a wide area. These formed 
thick deposits of breccias, which consist 
mostly of fragments of lava, tuff, or finer- 
grained ejected material of the nature of 
ash and cinders, in part mixed with large 
volumes of fine volcanic ash. Most of 
this ejected matter was piled up as it fell, 
but in some places water had a part in its 
distribution, and from some of the vents 
there also came extensive mud _ flows. 
At intervals and from place to place there 
were great outflows of lavas of various 
kinds, which spread widely over the sur- 
face of the deposits of fragmental material, 
and subsequently were buried beneath 
accumulations of breccia, tuff, and ash. 
In general, the order of rocks erupted in 
Tertiary time has been latite, rhyolite, 
diabase rhyolite, and several varieties of 
asalt. The configuration of the region 
was probably much smoother at that time 
than it is now, for in general the old rock 
surface on which the volcanic deposits 
lie appears to be smooth at most localities. 
In places, however, ridges of older rocks 
protruded which were not covered by the 
volcanic materials. After the main period 
of volcanic action in Tertiary time the 
region was uplifted and the beds broken 
and tilted. It is from the erosion of this 
i ar surface by streams and other 
agencies that most of the present land 
forms are derived. 
= 
laying bare the underlying older rocks. 
Several times after the uplift there were 
extensive eruptions of later lavas, some 
of them accompanied by the ejection of 
tuff, ash, and other fragmental material. 
