Dana's account of the limestone beds as the most importiint feature and value which geology points out for the county. Of course we had to abridge as much a.s possible in order to keep within the limits of our work.
'- Prof. I. S. Newberry, in Popular Sci«Ht« Monlhly, for October, 1878.
73"45
73°30'
45'
73 •45 '
73 '30'
Geoloi>ir Map of VV^est Chester Coimty, N.Y.
MILES
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GEOLOGY.
ed horizontally -- sandstones, shales and limestones -- but now, upheaved and set on edge, are by nietamorphism converted into compact crystalline strata, with the obliteration of all fossils-- if fossils they contained. The age of these rocks has not yet been accurately determined, although they have been supposed to be Lower Silurian, and a continuation of those which contain the marble beds of Western Massachusetts and Vermont. There are some reasons, however, why they should be regarded as still older. That they do not form the southern prolongation oi the marble belt of Vermont is indicated by the facts that both the marble beds and the rocks associated with them are so unlike in the two localities that they can hardly be parts of the same formation.
In Vermont the marbles occur in what is essentially a single belt, are fine-grained, unusually banded and mottled, are nearly pure carbonates of lime, and the rocks immediately associated with them are gray siliceous limestones, quartzites and slates. In Westchester County and on New York Island, on the contrary, the niarbles are very coarsely crystalline dolomites (double carbonates of lime and magnesia), which occur in, a number of parallel belts, are generally of uniform white or whitish color, and have no rocks associated with them that can represent the quartzites and argillites of Vermont.