234 ^/^<? American Geologist. April, looi. 
cases vertical and even overturned. This folding somewhat 
obscures the fact that the greenstone is always under the 
slates and never intruded into them. 
The whole black slate country between Trinity Center and 
Lewiston has been thrown into a series of about ten anticlinal 
folds, striking east to west through the Trinity mountain and 
across the courses of Trinity river and Clear creek. Near the 
center of the series the folds are closely oppressed and the 
slates along the sides vertical. Now, if we follow the contact 
between the greenstone and slates up the limb of the anticlinal 
we do not have to climb more than 500, or at most 1,000, feet 
above the Trinity river until we find the slates curving up over 
the greenstone, and the mountains above are entirely of slate. 
The maximum vertical range of the folding is probably 2,000 
feet, and the more open folds may be included within 1,000 feet. 
Thus the greenstone rises into view in the axis of each anti- 
cline, and sinks beneath the valley under each syncline, 
The greenstone formation is made up of a variety of de- 
posits of a volcanic nature, but all having something in com- 
mon so that it appears over wide areas as a massive, fine- 
grained dull green rock, outcropping in ragged cliffs and 
weathering down into a reddish clay soil. Much of it is of a 
detrital character, chiefly diabasic tuffs and ashes, although in 
places it is brecciated and occasionally it has a conglomerate 
structure. This latter is peculiar. It consists of rounded peb- 
bles -oi diabase cemented by similar material. Among the dia- 
basic tuffs are undoubtedly sheets of lava, in places containing 
amygdaloids. The formation has been much altered and 
abounds in secondary minerals of which epidote is macroscop- 
ically the most prominent. 
Running through the greenstone areas are long broad belts 
of a fine-grained white rock resembling certain quartzytes, bvit 
apparently a felsyte or a devitrified rhyolyte. Commonly asso- 
ciated with them are zones of impregnation of iron and copper 
pyrites, the sulphides sometimes concentrating into large solid 
bodies, such as the Iron mountain and Copper city deposits. 
Along certain zones of shearing which trend usually east- 
west and have a width of ten to hundreds of feet, the diabase 
has been converted into a light-greenish schistose rock which 
appears to me to answer the description of certain phases of 
