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170 GEOLOGY OF NEW YORK CITY
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studied at Kingsbridge. They lie in the river valleys, or more1 correctly,
the rivers have formed their valleys in the limestone depressions as
more easily eroded and dissolved. Tib-bit's Brook has worn its channels
partially in a limestone rock, the northern extension of the
Kingsbridge dolomite; the Bronx River has its head in limestone at and
north of Wil-liamsbridge; and it may be so with Westchester and West
Farms Creeks.
These
limestone beds were traversed by the Subway tunnel under the Harlem
River, and a deep, open cut made in them (Fig. 40), at 149th Street,
toward Third Avenue, brought in view their crystalline integrity, and
bedded structure. Limestone beds are seen at Jerome Avenue reservoir,
where actino-Mte, titanite, pyrite, pyrrhotite, orthoclase are
developed in a metamorphic seam. (J. H. Adams.)
Note.—Mr. Edwin W. Humphreys has furnished me with the following epitome of his observations in a Pleistocene or recent swamp in the Bronx Borough:
"
Situated in the valley which is just back of Claremont Park, Bronx
Borough, is a most interesting swamp. In it, a thick deposit of peat
has accumulated. The forcing up of the peat by the filling in of a
street across it, some years ago, revealed the following fossil
shells, Valvata tricarinata, Say, Planorbis parvus, Say, Planorbis bicarinata, Say, Physa heterostropha (?), Say, Anmicola limosa (Say)Hald, Pisidium virginicum (?) (Gmelin) Bourg."
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