I 



QUARRY MATERIALS OF NEW YORK I37 



THE MANHATTAN SCHIST 



The Manhattan schist which underhes the island of Manhattan 

 and extends northward into the Bronx and Westchester county 

 has no great importance as a quarry stone. Its foHation, variable 

 composition and thinly jointed character are against its general use 

 for architectural purposes or for cut stone, though it has been em- 

 ployed quite extensively for walls and rough masonry where 

 readily available. In a few places, specially in the vicinity of plu- 

 tonic intrusives which have invaded and injected the schist, thereby 

 rendering it more massive and compact, it has found some sale for 

 building stone. 



The schist, like the Fordham gneiss, is a metamorphosed sedi- 

 ment ; in its original form probably a shale. In the field there 

 is a close resemblance between the two, though stratigraphically 

 they are separated by both the Lowerre quartzite and the Inwood 

 limestone. A comparison of typical samples of the schist and 

 gneiss shows, however, that the former is more micaceous and 

 carries less of the feldspars than the Fordham. The mica in both 

 is mostly biotite, but in the Manhattan schist there is also con- 

 siderable muscovite. The feldspathic constituents are generally 

 subordinate to the cjuartz. 



The color of the ^lanhattan schist is gray, medium to dark, the 

 lightest being the injected phases. Foliation is marked, owing to 

 the abundance of mica, and follows apparently the original stratifi- 

 cation. Crumpled and thin- jointed types are common. 



The schist is intruded by dikes and small bosses of granite and 

 occasionally of diorite and more basic rocks. In their vicinity, 

 but especially near the granitic intrusives, it is likely to change con- 

 siderably in appearance and composition. Through the injection 

 by granite, it develops into a feldspathic rock which resembles a 

 banded gneiss or, when the schist is more thoroughly absorbed, 

 it becomes fairly uniform and cjuite massive, not unlike the granite 

 itself. vSuch mixed phases are too numerous to require separate 

 mention. Merrill has noted their occurrence also in connection with 

 the diorite intrusions north and east of New Rochelle. 



Besides mica, quartz and feldspar, the schist contains a number 

 of accessory minerals like garnet, sillimanite, titanite and mag- 

 netite. The texture is generally fine, even granular, but may be- 

 come porphyritic near igneous conta-ts through the development 

 of large feldspars'. The rock possesses no features that are objec- 

 tionable to its general employment for construction purposes, ex- 

 cept its somewhat variable appearance and foliation. The mica 



