Abstract

The writers note a convergence of opinion on a world-wide scale favoring placement of the Siluro-Devonian boundary at the base of the Gedinnian. This boundary is coincident with the base of the Monograptus uniformis and Icriodus woschmidti zones. It is recognizable in the brachiopod-coraltrilobite succession by the disappearance of pentamerids, Atrypella, Gracianella, halysitids, and Encrinurus and by the incoming of terebratulids, Cyrtina, and common Schizophoria. Insofar as the Pridoli-Lochkov boundary in Bohemia can be revised to correspond to this boundary, the Pridoli is recognized as the uppermost Silurian Stage.

In North America, the base of the Devonian, defined as the base of the Gedinnian, is located at what is probably the most satisfactory level because it lies at or near the base of the Helderbergian, which has traditionally been regarded as the lower-most Lower Devonian stage in eastern North America.

Seven regions in North America are chosen for discussion because they are representative and form standards of a sort, with which most other fossiliferous sections in North America can be compared. Only in one region does the Siluro-Devonian boundary appear to coincide with a formation boundary, namely, at the base of the St. Alban Formation of eastern Gaspé. It falls within the Stonehouse Formation of Arisaig, Nova Scotia; the Rondout Formation of New York and New Jersey; the Keyser Limestone of western Maryland and vicinity; the Roberts Mountains Formation of central Nevada; the Prongs Creek Formation of Yukon Territory; and the Devon Island and Cape Phillips Formations in the Canadian Arctic Islands.

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