rsoME.] METAMORPHISM OF THE COUNTRY ROCK. 117 
j microscope it is seen that the alteration has been more thorough 
in in the specimens previously studied. The forms of some of the 
L-nocrysts are preserved by pseudomorphous aggregates — sericite, 
h some chlorite, calcite, and rutile, apparently after biotite, and 
cite, quartz, sericite, and chlorite in varying proportions after augite 
1 plagioclase. The groundinass is entirely recrystallized as a fine 
»regate composed chiefly of quartz and sericite, with less chlorite, 
casional larger grains of quartz with rounded and embayed outlines 
: probably original phenocrysts. A little apatite and a few minute 
tins of pyrite occur in the groundinass. The dominant minerals of 
i rock are quartz and sericite. A specimen taken from the same 
sscut at a distance of only 10 inches from the west wall of the vein 
embles the others closely. It does not, however, show breccia 
ucture, and is sparingly sprinkled with small crystals of pyrite. 
der the microscope the rock is seen to be essentially an aggregate 
sericite, quartz, calcite, and chlorite, named in the order of relative 
mdance. The former feldspar phenocrysts are now pseudomorphs 
sericite, calcite, and quartz, and biol ite has been altered to chlorite, 
icite, and rutile. The groundinass consists chiefly of quartz, chlo- 
i, calcite, and sericite. Although this facies is nearer the vein than 
■ preceding, the character of its metamorphism is perhaps even less 
'erentiated from the more general kind of propylitic alteration 
ich the country rock in the mine, even at a distance of several hun- 
<d feet from the principal lodes, has undergone. Lastly, a specimen 
s taken from the cast wall of the Iowa vein in the main stope above 
• fourth level. At this point the vein contained good ore, and the 
senee of gouge allowed the specimen to be taken immediately in 
itact with the ore. It differs from those thus far described in its 
it-gray color and more evident alteration. It is sprinkled with 
"ite and a little galena, and is traversed by minute veinlets of 
irtz. Its breccia structure is st i 1 1 < I iscernible in the hand specimen. 
eroscopically examined, the rock, while retaining traces of clastic 
gin, is wholly recrystallized. The former phenocrysts of feldspar 
replaced by pseudomorphous aggregates of quartz and sericite. 
e quartz in such cases is often rather coarsely crystalline (grains 
to O.o millimeter in diameter), the sericite being included as minute 
!ips, or gathered into bunches in the interstices between the allotrio- 
rphic quartz grains. Crystals of pyrite are sometimes inclosed in 
quartz. Of augite no trace remains, but some sericite inclosing 
ile is apparently pseudomorphous after biotite. The groundinass 
i finely crystalline mosaic of quartz and sericite. The notable fea- 
e of this wall rock is the absence of calcite and chlorite, 
'he alteration studied in the foregoing set of specimens, collected 
that purpose in the Iowa mine, has been traced with similar 
ults in collections from the neighboring Silver Lake mine. Cal- 
i and chlorite are as a rule prominent constituents of the altered 
