FETTKE, MANHATTAN SCHIST OF NEW YORK 195 



localit}^ just north of the New York City limits tliat it was first described. 

 This quartzite can be studied well at Sparta, a mile south of Ossining 

 along the New York Central Eailroad tracks, and also just east of Hast- 

 ings-on-Hudson. It seldom exceeds one hundred feet in thickness and 

 apparently grades into the underlying gneiss. None of the outcrops can 

 be followed for any considerable distance laterally and they are not of 

 common occurrence. 



The Inwood limestone follows the Fordham gneiss, but in some locali- 

 ties a thin bed of quartzite inter^^enes, as is mentioned above. Typically, 

 this limestone is rather coarse-grained and crystalline, reaching a maxi- 

 mum thickness of about eight hundred feet. Locally, where the original 

 limestone was impure, tremolite and diopside occur in it and certain beds 

 are quite micaceous, containing large amounts of phlogopite. Much of it 

 runs high in magnesia and grades into a dolomite, but comparatively pure 

 limestone beds are also present. 



The exact nature of the contact of this limestone with the underlying 

 gneiss affords a problem which is difficult to solve. Apparently, the bed- 

 ding planes of the limestone are parallel to the banded structure of the 

 gneiss, and, if this banded structure represents the stratification planes 

 of former sediments from which the gneiss was derived, it would appear 

 that no marked unconformity exists between the two. This relationship 

 has been particularly well brought out by the contacts in the tunnels of 

 the Catskill aqueduct across the formations. In the tunnel underneath 

 the Harlem Eiver just below High Bridge, no deviation from parallelism 

 in the banding of the gneiss and the bedding of the overlying limestone 

 at the contact, which is a sharp one, could be detected. No quartzite is 

 present here, but a thin seam of pegmatite occurs along the contact. It 

 seems that a slight amount of faulting movement has taken place along 

 the contact, which would naturally be a weak zone, but no brecciation 

 could be detected. A short distance beyond this contact, a bed of light 

 gray gneiss several feet thick was encountered in the limestone. Its 

 foliation is also parallel to the bedding of the limestone. 



Under the microscope (PI. XII, Fig. 1), this gneiss is seen to be made 

 up largely of feldspar, quartz and mica. The feldspar is largely micro- 

 cline, with some orthoclase and plagioclase present. Biotite is the most 

 prominent mica present, only a little muscovite appearing now and then. 

 The biotite is a deep brown variety showing intense pleochroism from 

 light 3^ellow to deep brown. Occasional minute rounded grains of zircon 

 and isolated calcite crystals were noticed. The mica shows more or less 

 parallel orientation, thus giving the rock its gneissoid structure, while the 

 feldspar and quartz occur in interlocking grains of medium texture and 

 uniform size. 



