408 R. D. Irving—St. Peters and Potsdam Sandstones. 
only interlocking grains of quartz. These interlocking grains 
are of two very different sizes, the larger ones predominating . 
while the smaller here and there fill up spaces between the 
larger. Close study of this section in ordinary light brings out 
the fact that each of the larger ones of these areas, and here 
and there one of the smaller ones, is made up of a rounded, 
smoothly outlined worn grain, and a border of deposited quartz, 
the border and the worn grain within polarizing together. 
outlines of the bordering quartz are exceedingly irregular, the 
ifferent areas interlocking with one another more or less 1ntrl- 
cately. I have attempted to represent a portion of this section 
diagrammatically in fig. 7, the smoothly outlined areas of this 
figure representing as before the original worn grains and the 
different shading indicating the areas that are optically contin- 
uous. The lines marking the junction of the original grains with 
the deposited quartz are marked sometimes by a difference 1D 
the purity of the two quartzes, but more especially by the pres 
ence along the lines of flakes of ferrite and of numerous cavl- 
ties, the ferrite flakes evidently representing a ferruginous coat 
ing on the surfaces of the original grains, while the cavities are 
plainly produced by the great irregularity of the surfaces UpoP 
which the new quartz was deposited. Some of the smaller areas 
of interstitial quartz above alluded to may have been wholly 
produced by deposition, the infiltrated quartz in this case not 
coordinating itself with original grains. It would evidently be 
difficult, however, to prove this to be the case, since the out- 
lines of the original grains are now perceptible only when they 
were well coated with iron oxide or were rough enough to pro 
duce cavities in the deposited material. a) 
e next specimen is from the Archzean quartzite of Devil 8 
Lake, Wis. As I have indicated elsewhere* the larger portion 
of the quartzite of this region is without arenaceous appe ‘i : 
being usually of a non-granular, flakey texture, and of a color — 
* Geology of Wisconsin, vol. ii, p. 505. Bees 
