spubr.] DESCRIPTION OF THE ROCKS. 13 
discovered under younger rocks in a number of different mines, such 
as the Montana Tonopah, the North Star, the Ohio Tonopah, the 
California Tonopah, the Wandering Boy, part of the Fraction at 
least, and probably the West End and MacNamara, the Midway, etc. 
Nowhere is it in even an approximately fresh state, but, as before 
noted, it has altered largely to calcite and chlorite, and (more com- 
monly in the vicinity of the veins) to quartz and sericite. In the 
former condition it has a fairly deep-green or blue color, while in the 
latter phases it is lighter green or blue, and very often nearly white, 
or assumes, near the surface and along faults or fractures, a pale 
yellowish, brownish, or reddish color. This common quartz-sericite 
type has the appearance of rhyolite rather than of anything else, and 
does not at all resemble andesite, while the calcite-chlorite phase has 
more nearly the rightful aspect of a highly altered andesite. 
It is difficult always to distinguish this early andesite with certainty 
on account of its many different phases. It is most likely to be con- 
founded with the later andesite on one hand and with certain types 
of rhyolite on the other. The main points of difference will be out- 
lined shortly. 
Distribution and characteristics of the later andesite. — The later 
andesite outcrops over a considerable region in the northern part 
of the district. The upper portions, and in some cases the whole, of 
the following shafts are in it: Montana Tonopah, North Star, Mid- 
way, Golden Anchor, Little Tonopah, Boston Tonopah, Halifax, etc. 
It varies greatly in state of preservation, being frequently found 
nearly fresh, and again highly decomposed. In its fresher phases 
it is a dark-colored, hard rock, with large crystals of pyroxene, feld- 
spar, and biotite. It has usually been more or less altered, and 
as secondary minerals calcite, chlorite, serpentine, etc., have been 
formed, giving the rock a dark-green color. Frequently, also, the 
altered rock has a deep-blue color. Near the surface the red of the 
oxidized iron combines with these colors to produce a characteristic 
rich purple. In some places the rock has been thoroughly altered to 
calcite, chlorite, and pyrite, with other secondary minerals, and again 
has been so completely leached as to be soft and white. 
In these highly altered phases the later andesite becomes with great 
difficulty distinguishable from the early andesite, and where it is fine 
grained the resemblance may become almost exact. 
For the distinction of the two rocks in general it may be noted that 
the early andesite is characteristically finer grained than the later 
andesite, and that in the former the feldspar crystals are slimmer and 
rectangular, while in the latter they are stouter and often of complex 
shape. In some of the hornblende-andesite (though not generally) the 
long dark crystals of altered hornblende may be distinguished, while 
in the later andesite the crystals of biotite are usually plainly visible, 
even in the considerably altered phases. The characteristic later 
andesite has a coarse, mottled appearance, and even when highly 
