Gold ''Pockef Deposits in NorthcriL California. — Hcrshcy. 43 
the concentration of the debris into the small depressions in the 
surface of the diabase, it has been subjected to great heat, 
many of the grains of gold having combined into larger par- 
ticles and perhaps sometimes even a great many of them being 
thoroughly liquified and run together into large nuggets. This 
abnormal heating may have been locally produced by friction 
through movement of the rock on either side of the contact, of 
which there is evidence. Wherever the black slates are cut by 
dykes of "birdseye" porphyry, they have lost their dull color 
and regular lamination for a space of 10 to 50 feet from the 
dyke walls, and been converted into a peculiar schistose slate 
of a very bright shining lustre. Evidently heat cavised this 
metamorphosis. Precisely similar schistose bands are found in 
places immediately above the contact with the diabase, espe- 
cially where the strata show greatest disturbance from the 
mountain-building forces; and it is in these districts that the 
pockets of coarse gold are found. 
Many of the gold-bearing quartz veins of northern Califor- 
nia are of later age than has been postulated for the "pocket" 
deposits. The main gold-depositing period occurred after the 
Jurassic black slates were completed, probably during the time 
of orographic disturbance when the sea bottom was thrust up 
into a mountainous land area, and in great fissures rose suc- 
cessively the melted rock to form the dykes and magmas of 
greenstone, dioryte, "birdseye" porphyry and granite which are 
so characteristic of Trinity county. During this period the 
"pocket" deposits, being buried deep in the strata, must have 
suffered great changes, but the original concentration of the 
gold, if I have read the evidence aright, dates from an earlier 
period, the early Jurassic, and so far as known, they are in all 
probability the oldest workable gold deposits of the state of 
California. 
May, 24, i8gg. 
