Abstract

INTRODUCTION
STRATIGRAPHIC LIMITS OF THE PORTAGE GROUP

The Portage group in Pennsylvania is here defined as including all beds between the subjacent Hamilton and the overlying Chemung, recognized by the advent of Spirifer disjunctus. Where, in eastern portions, marine Chemung is absent, supplanted by the Catskill continental facies, the top of the Portage is indefinite.1 Nevertheless, occasionally this upper limit can be approximately fixed. The top of the Hamilton is often defined through the presence of a fossiliferous zone characterized by Vitulina pustulosa.2 A sharp lithologic change is recognized at the same point. Along the Allegheny Front the Portage group is introduced by the Tully limestone, which contrasts sharply with the Hamilton shales. Where the Tully is absent, the Burket (“Genesee”) black shale usually occurs and is faunally and lithologically distinct. Only in the most eastern sections is it difficult, because of lithologic and faunal convergence, to separate the Portage . . .

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