Dec, 1912.] Ordovician Section, Lake Huron Area. 39 



chotrema probably RIt. aiiisliei, and a Dalmanella (Pionodema) 

 belonging to the subaequata group. Among the bryozoans, Dr. 

 E. O. Ulrich identified Escharopora ranwsa, Phyllodictya lahyrin- 

 thica, Rhinidictya fidelis, Rh. iiichohoni, Rli. trcntonensis, and forms 

 of Rh. mutabilis and of HomotrypcUa 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 Tctradium 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, fomis 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 probal^ly 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. 



