6 Journal of the Mitchell Society [May 
from .25 to 1.1 mm. To the existence of these bands is due the 
laminated appearance of the rock in hand specimen. In some 
cases, also, there seems to have been a rough assortment of 
the larger feldspars in bands, although this occurrence was not 
observed to be universal. 
The feldspar is plagioclase, and occurs in crystals varying in 
size from the very minute particles of the groundmass up to .286 
mm. in diameter. The average diameter of the larger crystals is 
from .065 to .1 mm. The form is usually sub-angular, although 
rounded crystals are seen, suggestive of a clastic derivation. All 
the crystals polarize separately. Some crystals are intimately 
interlocked. Others have deep re-entrants into which the silica 
of the groundmass protrudes, suggesting a partial resorbtion of the 
feldspar by the groundmass. Albite twinning is universal, the 
maximum angle of the striations in the zone perpendicular to M 
lying between 10 and 16 degrees. Hence the plagioclase mixtures 
lie between basic oligoclase and andesine. The crystals show 
little if any decomposition . The largest feldspar noted, .286 mm. 
in diameter, is nearly round, is completely encased in a thin rim 
of greenish glass, and lies in a rubble of small angular feldspar 
fragments. 
The crystals of the groundmass all polarize separately and 
exhibit low interference colors. The grains are extremely irreg- 
ular in outline, and are closely intelocked. The average diameter 
is .015 mm. Many of these grains are seen to be plagioclase from 
the albite striations. Many are quartz, but owing to the diffi- 
culty of distinguishing between quartz and feldspar in very small 
angular fragments, it is not possible to state definitely the per- 
centage of each mineral. That much free silica is probably 
present, however, is indicated by the high total percentage of 
silica in the rock. 
Kaolin occurs in minute scales. In the narrow bands above 
noted, kaolin is by far the most abundant mineral, and the scales 
lie very close together. It is likewise found in less abundance in 
every part of the rock. The diameter of the scales varies from 
.0026 to .0052 mm. 
Epidote occurs rarely in minute grains in small clusters. The 
interference colors are of a low order. 
