410 &. D. Irving—St. Peters and Potsdam Sandstones. 
last described was taken, the rock has become hard, vitreous, 
urple quartzite, without trace of arenaceous appearance; in 
other words the ordinary quartzite of the region. The thin 
section of this rock, in polarized light, shows no trace what- 
ever of a fragmental origin, the grains being completely inter- 
locked, but in ordinary light, here and there, may be distinctly 
seen the rounded outlines of an original grain, polarizing 
with the deposited quartz surrounding it. As in the Gibraltar 
rock, ‘so also in this section, there is much of a fine interlocking 
interstitial quartz which may have been in part or wholly 
deposited. 
specimen shows numerous large quartz fragments embedded 
n a finer non-granular matrix, which is distinctly schistose, 
being apparently a clayey quartzite. At first sight the thin 
section of this rock seems to show no trace of fragmental origin, 
being made up of relatively large quartz particles embedded 1n 
a matrix composed of finer angular quartz particles and much 
of a kaolinic material. The quartz grains show throughout a 
tendency to have their longer axes in a common direction. 
Occasionally in the finer matrix are developed small but dis- 
tinct muscovite scales. Close study of the section brings out 
here and there the same feature as heretofore noted, namely, the 
presence in the interlocking angular quartzes of cores compose 
of rounded grains. The section of this rock is indistinguishable 
from a number that T have examined of the argillaceous quartZ- 
ites of the Lake Superior Huronian. 
It thus appears that the alteration which has produced from 
sandstone certain Archean quartzites and quartz-schists is of 
undisturbed and elsewhere wholly unaltered sandstones, and 
even as the surface induration produced in some sandstones by 
atmospheric waters upon occasional feldspar particles in “ 
sandstone. A similar origin may be assigned for the silica of 
+ s 
