210 REPORT OF THE AMERICAN COMMITTEE. 
upper Mississippi valley have been known widely as the equiva- 
lent of the Potsdam sandstone. 
3. In 1&72 the writer called attention to the discrepancy in 
this supposed parallelism and suggested the name St. Croix for 
the fossil iferous formation that had furnished the so-called Pots- 
dam fossils, and has also since then shown the fact that this group 
of fossils, while pertaining to the upper part of the primordial 
zone, is not found at all in the true Potsdam horizon, but pertains 
rather to the succeeding epoch. 
4. Recognizing the discordance between the fauna of the true 
Potsdam of New York and some fossils found in the Calciferous 
of the same State, but their alliance with the fauna of the so- 
called Western Potsdam, Mr. Walcott in a published paper called 
attention to the alliance that thus was shown of the Calciferous of 
New York with the so-called Potsdam of the Northwest. 
5. Since that time much more evidence has appeared, pub- 
lished both by Mr. Walcott and by Mr. Dwight, which has con- 
firmed the opinion of the writer published in 1872 ; and this goes 
to demonstrate that the fossils of the Western so-called Potsdam 
are distributed mainly through strata that lie above the New- 
York Potsdam sandstone, and principally in the recognized Cal- 
ciferous sandrock. 
6. Further, this sub-fauna was discovered in Canada by Mr. 
Billings. He placed it in the Quebec group, which he supposed 
was between the Potsdam of New York and the Calciferous, but 
embracing much of the Calciferous. 
7. There is a great sandstoue-quartzite formation comparable 
to the New York Potsdam lying below the St. Croix beds, 
found in Wisconsin and Minnesota separated from the St. Croix 
beds by a great unconformity. This great formation plays a 
conspicuous part in producing the marked topographic features 
of several places in Wisconsin and Minnesota. 
8. Professor R. D. Irving, late of the Wisconsin geological 
survey, placed the principal fossil-bearing stratum of the St. 
Croix beds, under the name "Mendota limestone," in the Lower' 
Magnesian (Calciferous) horizon, considering that it was no part 
even of the St. Croix [so-called Western Potsdam] beds, but 
that the Potsdam is represented by some lower sandstone. 
9. Tlie result has made it apparent that by the use of the terra 
Potsdam for this sub-fauna the two terms come into direct colli- 
