406 BR. D. Lreing—st. Peters and Potsdam Sandstones. 
the cloudiness and worn surface of the original grain and the 
pellucidity of the deposited quartz, or by the presence upon 
the surface of the original grain of a coating of oxide of iron. 
Only the smallest of the grains seem to show perfect crystalline 
faces, the supply of new quartz having been so great as to pro- 
duce some interference in most cases. This is indicated by the 
indentations observable in the crystalline outlines, as shown in 
figs. 1, 2 and 38, which represent grains of this sand as seen 
mounted in balsam. In this mounting the crystalline faces 0D 
the upper sides of the grains are not readily seen, but only the 
rough surfaces of the original grains, and the crystal outlines of 
the deposited quartz. The interference has, however, never 
extended very far, every grain showing some traces at least of 
the crystalline faces when viewed in a dry mounting. vb 
ently the small amount of induration in this sandstone 18 t 
be connected with this relatively slight amount of interference. 
As shown by Sorby,* the most perfect crystalline outlines are 
to be met with in quite unconsolidated sands, the quartz having 
then had full opportunity to develop perfect faces. In the 
figures drawn from the mountings of this sand, I have placed 
the three grains with their elasticity axes in a comme 
direction. 4 
The second specimen on the list is from the Potsdam sand- 
stone of the quarry at the depot at Black River Falls, Wiscon- 
sin. It is a white, much coarser-grained, and perceptibly wpa 
indurated rock, than the last, but many crystalline facets 0: 
some size are perceptible. The cause of the greater induration 
becomes readily apparent when the balsam mounting of crum- 
with that of some contiguous grain. 
* Op. cit., p. 37. 
