FETTKE, MANHATTAN SCHIST OF NEW YORK 227 



The igneous origin of the Harrison granodiorite gneiss has never been 

 questioned by those who have made a study of this rock. Its chemical 

 composition is clearly that of a medium basic igneous rock, as a compari- 

 son with analyses of massive igneous rock of similar composition will 

 show. Its uniformity of texture, structure and mineral composition over 

 large areas is another point in favor of such an origin. It probably en- 

 tered the older strata in the form of a large irregular laccolith and when 

 they were shales. There was thus a period of igneous activity antedating 

 the folding and dynamic metamorphism of the sediments. We reach this 

 conclusion because of the very marked foliated structure, clearly of sec- 

 ondary origin, which has been developed in the granodiorite. The strike 

 of the foliation is parallel to that of the mica schist which surrounds the 

 intrusive. 



MASSIVE BASIC IGNEOUS ROCKS 



In addition to the highly metamorphosed foliated basic igneous rocks 

 which occur as intinisives in the Manhattan schist, there is another series 

 which shows only slight or no effects of dynamic metamorphism. The 

 series appears as normal, massive, igneous rocks of basic composition and 

 of granitoid texture intrusive in the schist and as massive serpentine 

 derived from such rocks. 



CortJandt Series 



The largest of these intrusive masses of basic igneous rocks occurs just 

 south and east of Peekskill, covering an ai*ea of about twenty-eight square 

 miles in the township of Cortlandt, the most northwesterly in Westchester 

 County, from which the series has taken its name. 



It consists of an igneous complex made up of a great number of differ- 

 ent varieties of intrusive igneous rocks mostly of a basic nature which 

 grade into one another, often by almost imperceptible transitions. G. S. 

 Rogers in a recent paper^^ has discussed the geology of this intiTisive mass 

 in detail. He has shown that there is a centrally located no rite area 

 flanked on both sides by pyroxenites. The western pyroxenite probably 

 continues beneath the Hudson Eiver, since these rocks outcrop again at 

 Stony Point. Between this western area of pyroxenites and the norites, 

 lies a diorite area. To the extreme northeast, the basic rocks are adjoined 

 by an area of granite. The order of intrusion seems to have been first 

 pyroxenite, followed closely by the norites, the diorite and finally granite. 

 It is among the basic members of the series that gradations from one into 

 another appear, producing a large number of intermediate t}^es. 



^ "Geology of the Cortlandt series and its emery deposits." Ann. N. Y. Acad. Sci.. Vol, 

 XXI, pp. 11-86. 1011. 



