OBSERVATIONS ON THE BROOKLYN INJECTION GNEISS AND RELATED FORMATIONS

Abstract

AbstractThe bedrock under the western end of Long Island, N. Y. has been known as the Brooklyn Injection Gneiss, and regarded as the Fordham Gneiss mixed with various amounts of intrusive Ravenswood Granodiorite. A water tunnel constructed in this bedrock encountered gneiss of Fordham type, gneissoid granodiorite, laminated mica schist, and interbedded limestones (impure marbles). Except for cross‐cutting dikes of amphibolite and of pegmatite, contacts between the different rock types are conformable and gradational, as well as conformable with the banding of the Inwood and Manhattan formations farther north.These observations lead to the conclusion that all the metamorphic rock formations in the New York City area originated as one great sedimentary sequence, interrupted by only local unconformities. The work of others suggests that the entire sequence may range in age from Late Precambrian to Ordovician.The Ravenswood Granodiorite may not be igneous but may represent merely certain sedimentary strata having a composition slightly different from that of the rest of the Fordham Gneiss. As some of the interbedded limestones are lenticular, even the Inwood Marble may have been a lenticular deposit, which may explain its variation in thickness and its absence in the east Bronx. The laminated mica schist occurs associated with the Inwood Marble at Rikers Island, but it also occurs extensively in Brooklyn without marble. Numerous cross‐cutting amphibolite dikes are present in Brooklyn, although they are not found in the normal Fordham, Inwood, and Manhattan formations in this area.