The Hamilton group in Pennsylvania, as in New York, is divisible into four formations: the Moscow, the Ludlowville, the Skaneateles, and the Marcellus. These are partially subdivisible. In studying the group, Cooper’s published data on the New York Hamilton 3 have been used extensively, supplemented by the senior author’s notes made while visiting the region of Hamilton, New York, with Dr. Cooper during the summer of 1932. Dr. Cooper must be credited for much of the information necessary in starting the detailed studies in Pennsylvania. The authors take this opportunity of expressing their appreciation of his exposition of the New York sequence, which has greatly assisted their studies. Reports by Prosser,4 Cleland,5 Grabau,6 Goldring,7 Clarke,8 and others have also proved particularly applicable. The junior author worked two field seasons in Perry County in the preparation of his dissertation in partial fulfillment of the requirements of the degree of Doctor . . .