22 NEW YORK STATE MUSEUM 



don's discussions of correlation problems therefore are of special 

 interest in connection with the West Point studies. 



On the south side, Charles R. Fettke^^ has made a special, very 

 detailed study of the Manhattan schist. His contribution therefore 

 supplements that of Mr Gordon. 



The latest contribution^^ bearing directly on the geology of this 

 district is that by R. J. Colony, which is confined to the Poughquag 

 formation and its petrographic and chemical character. 



Geologic Formations 



The quadrangle is composed almost entirely of Precambrian 

 gneisses, schists, and limestones (the Highlands gneiss, Inwood lime- 

 stone, Manhattan schist) and their associated intrusives. 



In the northwest corner a small area of the Hudson River series 

 (Ordovician) appears and in the southwest is another small area 

 of the Cambro-Ordovician quartzite, limestone and shale. The 

 Manhattan schist is cut by the Peekskill granite and Cortlandt 

 gabbro-diorite series of uncertain age which lie east of Peekskill, 



The Highlands gneiss, which makes about 70 per cent of the total 

 area, is considered to be the age equivalent of the Grenville gneiss 

 and associated series of the Adirondacks and Canada, but here it is 

 so penetrated and replaced by granites that its original character 

 can be distinguished in few places. The chief belt which approaches 

 the Grenville in character lies along the Hudson river where it 

 shows as a series of thin calcareous, micaceous and quartzose beds 

 cut by granites and pegmatites, and is much sheared and distorted 

 by faulting. Almost all the remainder of the Highlands area is 

 either granite or gneiss so granitized that it could readily be taken 

 for a granite. The chief proof of the original sedimentary character 

 of the country is found in the interbedded limestones, the largest of 

 which underlies the valley of Sprout brook. 



There has been extensive granitic intrusion and replacement 

 throughout the area, and so intimate a mixture of granite and gneiss 

 has been formed that it is usually difficult to determine the origin 

 of any given outcrop. The simplest igneous rock of the region is 

 the Storm King granite which forms Breakneck ridge and Bull hill 

 on the east side of the Hudson, and practically all the mountainous 

 area on the west side. 



The general strike of the gneisses is northeast-southwest and the 

 dip of the principal structure is steep to the southeast. 



" The Manhattan Schist of Southeastern New York State and its Associated 

 Igneous Rocks. Annals N. Y. Acad. Sci. 23:193-260 (1913). 



'" High Grade Silica Materials, N. Y. Slate Miis. P)nl. 203, 204. 



