68 Dr. F. V. Hayden on the Primordial Sandstone 
Index of refraction of water, - ve ce - 1°336 
f = “ sulph. acid, - - - - 1-435 
~ r “ diatoms, - - - - 1435 
cn “ quartz, . - - - 1548 
“ “ Canada balsam, - - - 1548 
This table shows that the index of refraction of the diatoms is 
about half way between that of water and Canada balsam, thus 
explaining the fact that they appear about equally distinct in 
both of these media. 
Troy, Nov. 26th, 1861. 
Art. IX.—The Primordial Sandstone of the Rocky Mountains in 
the Northwestern Territories of the United States; by Dr. F. V. 
AYDEN. 
WE have attempted in this paper to present as clear and con- 
nected an account, as the known facts will permit, of the Pri- 
mordial rocks west of the Mississippi, more especially those of 
the northwest, west of longitude 96°. The Potsdam sandstone 
of the New York series is the division of the Primordial zone of 
Barrande, mainly represented in the Rocky Mountain district 
and is that part alluded to unless otherwise mentioned. 
In speaking of the geographical distribution of the Potsdam 
sandstone reference will be made to localities to the eastward 
where it has furnished most abundant and satisfactory testimony 
in regard to its age. We will in the first place, present more 10 
detail, such facts as we have been able to obtain by personal ob- 
servation in the field, and by the aid of these and the statements 
of reliable explorers we hope to give some idea of the geograph- 
ical extension of this wide spread formation in the West. t 
Our first knowledge of Primordial rocks west of the Missouri 
river, was obtained in the summer of 1857, during the explora- 
tion of the Black Hills of Nebraska, by an expedition under the 
command of Lieut. G. K. Warren, Top. Eng. The more impor- 
tant facts with the determination of the fossils, were published 
by Mr. Meek and the writer in March, 1858,* 
By reference to the general map of the country west of the 
Mississippi, recently published under the auspices of the Wat 
artment, we find that the Black Hills lie between the 43d 
and 45th degrees of latitude, and the 103d and 104th degrees of 
‘f 
. 
reenter eee 
CEN 
as teen atm 
