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Ch. 1: Introduction

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6                GEOLOGY OF NEW YORK CITY
Rocks ; predominatingly slates and sandstones :
Note.—The Trenton epoch has been separated into three for­mal strata, viz., Black River, Bird's-eye, and Trenton limestones. Ophileta is a fossil gasteropod or coiled univalve shell ; Moclurea-, a fossil gasteropod, flat surface, with an operculum; Asaphus, a trilobite ; Graptolites, fossil hydrozoans occurring as black glis­tening notched threads; Modiolopsis, a fossil bivalve, mussel­like.
Rocks ; predominatingly limestones, with slates and shales in the Utica and Hudson River :
Note.—This era is quite sharply divided by the Salina beds. The upper division, the Lower Helderberg, has a great thickness in the east, and disappears westward; the zoological character of the Niagara and Lower Helderberg periods is contrasted.
Rocks ; sandstone, conglomerate, shales, slate, limestone, the latter especially. in the Niagara:
Ch. 1: Introduction Page of 281 Ch. 1: Introduction
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