_ ton, Columbia Co 
ER. D. Irving—St. Peters and Potsdam Sandstones. 407 
grains of this rock are shown in contact, the line drawn on 
each indicating the position of its elasticity axis. On the upper 
side of the upper one of the two grains the 
pyramidal outline is apparent, but below the * 
deposited quartz of this grain interferes with 
that of the next one, upon which no linear out- 
lines whatever are visible, the interference with 
the deposited quartz of contiguous grains hay- 
ing evidently been complete. 
€ next specimen is one from an outlier of 
St. Peters sandstone, in the town of Arling- 
, Wis. The larger part, of 
the specimen shows a fine-grained, very loose, 
saccharoidal sandstone, in which there is almost 
no trace of induration, but in which numerous flashing points 
t 
indicate the presence of crystal coatings to the grains. ou 
one-fourth of an inch, however, on the weathered side of the 
Specimen, presents the appearance of a completely vitreous 
quartzite. Seen under the microscope a thin section of this 
vitrified crust shows plainly the original rounded grains of the 
Sandstone, but everywhere between them a deposited quartz 
which is divided off into areas codrdinating optically with the 
Original grains. The interference between the different areas of 
this deposited quartz has been nearly always too great to allow 
of the formation of crystalline outlines. Figs. 5 and 6 repre- 
Sent portions of the section of this crust. The smooth curved 
lines of the figures show the outlines of the original grains, 
while the shading indicates the way in which the original and 
deposited quartz polarize together. 
The next specimen is one of St. Peter’s sandstone from Gib- 
raltar Bluff, a very bold and prominent point on the south 
side of the Wisconsin River in the town of West Point, Colum- 
bia Co, Wis. This rock is one which, if found among the 
crystalline schists, would undoubtedly be classed as a quartzite. 
Itisa very much indurated, light-colored rock, in which a very 
fine arenaceous texture is perceptible only on the closest inspec- 
tion. The thin section of this rock in polarized light shows 
