dillek] COPPER DEPOSITS OF REDDING REGION, CAL. 125 
the lavas noted above, and are themselves intersected locally by 
dikes of diabase, so that in order of age the granitic rocks come 
between the great mass of older lavas and younger dike rocks. 
Dike rocks. — The dikes are of a large variety of rocks, and range 
in size from a few inches to a hundred feet or more in width. They 
intersect older igneous rocks as well as sedimentary rocks, and are 
widely distributed throughout the field. Some are decidedly porphy- 
ritic, but the majority are fine grained and compact, without promi- 
nent crystals. 
Of the porphyritic type some contain prominent crystals of both 
quartz and feldspar, and are closely related to the granitic rocks, 
with which they may be connected. They may hold an important 
relation to the ore bodies, but the relation can not be fully determined 
without detailed investigation. Dikes of this sort occur most abund- 
antly in the western portion of the field, where they may be seen in 
places directly connected with the granitic rocks. 
A decidedly porphyritic type, containing prominent crystals of 
feldspar only, occurs near the Uncle Sam mine of Squaw Creek and 
at a number of points about Bear Valley, but deposits of ore have 
not been noted in their vicinity. 
The most abundant dike rock is an altered variety of basalt or 
metabasalt in which the feldspars usually have that ophitic arrange- 
ment which characterizes diabase. It is generally not porphyritic 
like the other diabasic rocks, but compact and greenish in color, espe- 
cially on fresh fracture. Large areas of it occur about the Carbon- 
iferous limestone from Gray Rock northward, and dikes of it cut 
through the limestone, giving rise to interesting and important con- 
tact deposits of ore unlike any others in the region. South and east 
of Bass Mountain is a large mass of this ancient igneous rock, and 
along the Sacramento River there are numerous dikes of it cutting 
the older lavas. 
Folding and displacement of the rocks. — The rocks of the copper 
region are folded and faulted, but the extent in both cases is limited, 
and varies with the kind of rock and locality. The shales, sand- 
stones, and tuffs are usually soft rocks with little rigidity. They show 
many sharp folds and faults, but an attempt to trace them reveals 
their very local character and small extent. To determine whether 
large folds and faults are present the Triassic, Carboniferous, and 
Devonian limestones afford the best horizons for observation. Owing 
<> their light color these rocks may be seen from a long distance in 
tracing structure, and each, having its own characteristic fossils, 
nay be identified with certaint}^ The general course of these rocks 
o«||icross the region is nearly north and south, but in the Furnaceville 
listrict, as well as about the head of Squaw Creek, the Triassic lime- 
ullstone turns easterly, sending a synclinal point in the one case southwest 
4 ;o Bear Mountain and in the other northwest to the head of Claiborne 
