LR en rena a de aU Seninscy Senge an 2 
ss 
Lt. D. Irving—St. Peters and Potsdam Sandstones. 405 
_ Were indisputably of that nature, while the proof that such a 
thing is possible as the deposition of quartz upon a quartz sur- 
face, in such a manner as to be erystallographically continuous 
with it, was also forthcoming. At this stage of the investiga- 
may now describe briefly a series of specimens illustrating 
these different degrees of alteration, beginning with the least 
indurated. 
The first specimen is of the Potsdam sandstone from the 
quarries at New Lisbon, Wis.—the same as described by Mr. 
- A. Young in this Journal for July, 1881. It is a very fine- 
rained, pink- and white-mottled sandstone, from which the 
i i i The indura- 
readily in the fingers. The crumbled sand, mounted in balsam, 
shows every grain edged with more or less of the deposited 
quartz, which is always optically continuous with the original 
grain. The line of junction between the new quartz and the 
old is always strongly marked, either by a contrast between 
