Microscopic Descriptions of the Schists. 
215 
lower portion of the series is coarser, containing a large pro¬ 
portion of fine conglomerate and grit interbedded with finer 
clastic rocks. In the upper portions the rocks are prevailingly 
fine-grained. The coarse materials occur frequently in lenticu¬ 
lar masses which may extend several hundred yards, and finally 
give place to a different rock. The finer rocks are all schist¬ 
ose. Those which contain a large proportion of grit are not, 
although the grains of sand and the small pebbles which make 
up the bed are usually flattened parallel to the schistosity in 
the finer rocks. The black schists are made up primarily of 
feldspar, with a large percentage of quartz at times; also flakes 
of iron oxide, mica, hornblende, chlorite, etc. 
MICROSCOPIC DESCRIPTIONS OF THE SCHISTS. 
The dark schists .—Microscopically the schists are found to be 
made up principally of quartz and feldspar in fine grains, the 
latter generally altered to sericite and chlorite. Flakes of bio- 
tite magnetite and ilmenite are common, metamorphic minerals 
especially garnet arid andalusite occur frequently. The biotite 
shows secondary change often, due to the leaching out of the 
coloring matter. Numerous patches of yellow iron oxide, con¬ 
taining yet unaltered cores of magnetite. The most noticeable 
feature of these rocks is the abundance of graphite, it occurs 
heaped together in masses, not uniformly distributed, portions 
of the section are without this coloring matter except as it occurs 
in scattered flakes. In this respect the arrangement resembles 
knotenschiefer, but the knots of graphite are not well defined, 
they are irregular in shape and possess no sharp boundary line. 
The proportion^ of quartz to feldspar is generally small in the 
finer schists, the grains of which the schist is composed are small 
averaging 0.1 mm. They occur interlocked closely as a rule, 
though sometimes in juxtaposition. 
Frequent alternation of fine clastic material and coarser grit 
are found even in microscopical sections, giving the section a 
banded appearance. By the increasing admixture of quartz grains 
the rock passes into a grit, which frequently becomes sufficiently 
coarse to be denominated a conglomerate. 
