748 WALTER HARVEY WEED 
rapid one. More often such rocks occur as intrusions in the 
altered sediments about the border of the batholith. The first 
analysis is that of a rock that might be called a diorite, though it 
hardly comes under that name. It consists mainly of green horn- 
blende, which is stringy and appears to be derived from augite, 
and of small zonally built basic plagioclase feldspars and a very 
little quartz. It is a nearly black, quite coarsely crystalline rock, 
and is an unusual type. It occurs intrusive in phyllites and 
schists on the summit of Red Mountain, ten miles south of 
Butte. 
The second analysis is that of a rock fairly typical of the 
batholith contact at many localities. It is a dark gray granular 
rock, rather finer grained than the granite into which it can in 
some places be traced by insensible gradations. It is a very 
basic diorite which approaches a hornblende-gabbro. The horn- 
blende is quite stringy and of uralitic appearance, pale green 
passing into deeper green and into colorless forms. Brown bio- 
tite is rare. Orthoclase is present in small amount and only in 
interstices between more idiomorphic plagioclase. The latter 
shows an extremely fine zonal structure and varies between 
labradorite and albite, and will perhaps in a majority of cases 
have an average composition corresponding to andesine. A very 
little quartz is also present, together with apatite and iron ore 
as accessories. 
The rock grades into one consisting mainly of zonally built 
basic plagioclase in small idiomorphic crystals, equal in amount 
to that of the combined dark colored constituents, brown pleo- 
chroic biotiteand hornblende derived from light colored pyroxene, 
remnants of which still remain. 
For the purpose of determining what changes, if any, are 
accomplished in the ordinary weathering of the granite, an analy- 
sis of a coherent but quite triable rock has been made. The 
material, which is quite typical of that commonly seen in natural 
exposures, is rather lighter in color than the perfectly fresh rock 
and has a clayey odor when moist. The greenish plagioclase 
and pinkish feldspar has been bleached to a dull white or waxy 
