This contribution to stratigraphy redefines the Medina formation and describes the character of its strata and fauna in the typical area in the State of New York. The new Cataract formation of Ontario is also described in detail and its fauna listed. Some of the most characteristic species of the latter formation were formerly ascribed to the Clinton, but it is here shown that the Cataract is equivalent to the Medina—the typical Medina as seen at Medina, New York. Finally, these two formations are compared with the Brassfield of Ohio, which has also been correlated with the Clinton, but is now seen to be of about the same geologic time as the Cataract and the Medina.

The following account of the Cataract, Medina, and Brassfield is divided into two parts. Part I contains a general discussion of the formations, their faunas, and their interrelations. In the second part . . .

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