Dec., 1912.] Ordovician Section, Lake Huron Area. 
39 
chotrema probably Rh. ainsliei, and a Dalmanella (. Pionodema ) 
belonging to the subaequata group. Among the bryozoans, Dr. 
E. O. Ulrich identified Escharopora ramosa, Phyllodictya labyrin- 
thica, Rhinidictya fidelis, Rh. nicholsoni, Rh. trentonensis , and forms 
of Rh. mutabilis and of Homotrypella instabilis, suggesting rela¬ 
tionship to the upper Platteville fauna of Minnesota and the Leray 
fauna of New York. This fauna is exposed also at a slightly 
higher geological horizon, immediately below the very fine grained 
“Birdseye” limestone, along the railroad about three quarters of 
a mile south of Swift Current. At a small quarried exposure along 
the same line of railway, but about a mile north of Swift Current, 
strands of some form of Tetradium occur, in the white limestones, 
which can not be identified with T. cellulosum. 
The very fine grained, white, “Birdseye” limestone, at the 
top of the Swift Current limestone series, firms a convenient 
lithological means of separating this series from the overlying 
part of the Black river beds. It is well exposed at several locali¬ 
ties within a mile going southward from Swift Current. Its 
thickness is about 11 feet. It is interbedded with a small amount 
of whitish clay, and contains but very few traces of fossils. 
Lithologically, the “Birdseye” limestone at the top of the 
Swift Current limestone section resembles the Tyrone limestone 
as exposed in Central Kentucky. This resemblance was noticed 
by Prof. Arthur M. Miller, who was a member of the party in 1911, 
and who made a thorough study of the entire Mohawkian group, 
giving the writer the benefit of his extended experience. It is 
probable that the entire Swift Current limestone section is to be 
correlated with the Tyrone, but this can not be determined from 
the meager fauna at hand. The total thickness of this section is 
unknown. Fifty feet probably is a moderate estimate. 
4. Cloche Island Beds; “Black River” Limestones. 
With the exception of the northern line of out crops on Cloche 
island, and those in the vicinity of Swift Current already described, 
almost the entire surface of Cloche island is formed by those 
darker limestones between the Leray member of the Lowville at 
the base and the Trenton limestones at the top to which it fre¬ 
quently has been customary to confine the term Black river. In 
the lower part of this Cloche island phase of the Black river sec¬ 
tion, fine grained limestones alternate with coarser grained layers 
for a vertical distance of about 30 feet. These strata are overlaid 
by coarser grained limestones in which finer grained layers are 
not conspicuous, and which attain a thickness of about 50 feet. 
These strata are well exposed along the railroad within two miles 
going south from Swift Current. The total thickness of the 
Cloche island beds may equal 150 feet, but no locality was found 
where this could be determined. 
