40 The A?nerica?i Geologist. July, isoo 
the local variations, it may be said that the green igneous rock 
forms the foundation of the entire region, outcropping in the 
valleys, passing through under the mountains to reappear in a 
distant valley or gulch. The black slates form the bulk of the 
mountains, capping every ridge and always above the green 
rock. The valleys are merely gigantic ditches cut by nature 
through the black slates and into the green rock below. 
The contact plane between the massive green igneous for- 
mation and the black slates has so far proven to be the "pock- 
et" horizon of the territory. Some gold in quartz ledges of low 
grade has been found in the diabase formation and perhaps a 
little may be scattered through the black slates above, but all 
the "rich strikes" have been made at or in the immediate vicin- 
ity of the contact. By reason of a long period of weathering 
in some ancient time, the green rock near the contact with the 
black slates has been softened and semi-decomposed and 
changed to a dull brown in color. It is popularly known as 
porphyry, but is as far from being a true porphyry as it can 
well be. Immediately above the contact, the black slate fre- 
quently has a shining lustre, a coal black color and a schistose 
structure quite unlike its usual characters. Sometimes there is 
a thin vein of quartz between the so-called porphyry and the 
black, slaty material, but generally they are separated merely 
by a thin seam of ferruginous dirt or by nothing at all. The 
dirt seam often carries a little free gold, but the "pockets" are 
found near or where seams of quartz penetrate the "porphyry" 
downward from, the contact. The gold lies in a thin, fiat sheet 
upon the igneous rock and under the slate, and in some cases 
extends a short distance into the former formation, rarely into 
the latter. It is in the form of coarse and line grains, rarely 
threads, and sometimes nuggets of considerable size. The par- 
ticles have a peculiar smooth and rounded surface, quite un- 
like the free gold in quartz veins. Indeed, so characteristic are 
the pocket "colors," that an experienced California miner can 
readily distinguish an isolated particle of "pocket" gold from 
both "river" gold and "quartz" gold. 
The limited nature of this paper will not permit me to dis- 
cuss the various theories that have been advanced in explana- 
tion of these curious "pocket" deposits, and I shall be obliged 
to content mvself with a mere statement of what I conceive to 
