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INTRODUCTION
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9
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covered),
Triassic shales or slates with Dolerite (trap), Cretaceous clays,
drift and shore sands. For the discussion, description and distribution
of these formations see following pages.
Class Directions.—The teacher should give a clear and simple account of the formations making the geological column, and explain the process of sedimentation by which they are formed. He should begin with the oldest rocks—the Archaean—the azoic. He will find immediately north of us in the Highlands an illustration of these rocks. Upon them, following south, are a series of crystalline
rocks, and slates with intrusive eruptive rocks (the Cortland series),
terminating in the schists of Manhattan Island and
its neighborhood. The Archaean rocks following the geological scale
are overlaid by the Cambrian beds, typically represented in
the Potsdam sandstones of northern New York, and supposed to have some
sort of equivalent in the Lowerre (Poughquag) quartzite or quartzose
gneiss at Yonkers. Over the Cambrian are laid the Ordovician beds,
represented in New York State by the Calciferous Sandrock, Chazy,
Bird's-eye, Blackriver and Trenton limestones, formerly known as Lower
Silurian. Of these the Inwood (Stockbridge) limestone is supposed to be the equivalent of the Calciferous Trenton groups. Above these again come the Hudson River beds, shales, clay limestones (Cincinnati), etc., which it is assumed are represented on our island by the Manhattan schists and gneisses, their much metamorphosed representatives,
THE CRYSTALLINE ROCKS
The
most significant and interesting formations in the New York City series
are the crystalline rocks. These are bodies of mineral aggregates in
which the component parts are separable minerals, and they are almost
exclusively gneisses, schists, granites, and limestones. The same rocks
and, it may be conceded, the same formations extend over western and
northern Connecticut, where the formations, as given by Prof. H. E.
Gregory, are the Becket gneiss, considered as a pre-Cambrian complex
equivalent to the Fordham gneiss of the! New York quadrangle; the
Poughquag quartzite (found
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