The New York Rapid-transit Subway

6 PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. [Minutes of convenient to treat them as one, together with additions and extensions made during their construction. Topography and Geology. The portions of the City of New York whose topography and geology have a direct bearing upon the construction of the subways described in this Paper are chiefly the central part of Manhattan Island, adhering closely to the divide of the watersheds of the Hudson and East Rivers; the Harlem River, at a point midway in its length, a distance of about 1 mile on the north bank; the bottom of the East River, opposite the southern part of Man- hattan Island; and a line 14 mile long traversing a part of the Borough of Brooklyn. The upper portion of New York is composed of gneiss rock which, during the glacial period, was greatly affected by ice erosion. This rock, found everywhere as a surface outcrop north of 14th Street, disappears entirely from view south of this point, the rock surface being inclined downward, and reaching a measured depth of 170 feet from the surface of the ground, or at least 140 feet below mean high-tide level, thus forming the low point of the geological valley just south of Canal Street. The rock surface then rises on the south side of this valley and approaches the surface at the lower end of Manhattan Island; then, with a gradually descending slope, it attains a depth under the East River, on the line of the subway, of 70 feet and upwards below tide- level, except for a sharply-defined ridge of rock in the middle of the river, whose top is only 32 feet below the water-surface at low tide. Generally, the strike of the rock is substantially parallel with the streets running northward, which are usually spoken of as running north and south, although their true bearing is about N. 29° E. The dip of the rock from the east towards the west varies from perpendicular to a slope of about 30° from the hori- zontal. The quality of the rock not only varies greatly but is subject to frequent changes, as the mica, the biotite, and the felspars occur in greatly varying proportions. As regards hardness, about one-half of the rock was fit to be broken for concrete and was thus largely used on the work; the balance was so soft as to crumble into dust, on account of the presence of mica in small flakes.