GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 2/ 



Granites of many varieties, chiefly biotite or hornblende or 

 pyroxene granite, and soda granites. 



Pegmatite of great variety representing every extreme from pure 

 quartz pegmatite on one side to magnetite ore on the other. 



Dioritic rocks, both massive and gneissoid. 



Basic igneous rocks (not very abundant in the Precambrian). 



Dunite and gabbro and pyroxenite and hornblendite are representd. 



Granite gneiss and gneissoid granite with indeterminate gradations. 



Mica and quartz and hornblende schists. 



Crystalline and silicated limestones. 



Banded injection gneisses and impregnated gneiss of confused 

 relation. 



The chief geological problems are these two : first, to single out 

 the principal fundamental units and map their general distribution; 

 second, to determine the relation of these units to one another and 

 the origin of their petrographic variability. 



The structure of the region trends northeast and southwest 

 Most of the formations fall into this structural trend in their own 

 distribution. The principal control seems to be the original rock 

 structure of the region itself, that of the Grenville. Even the fold- 

 ing and faulting of later time conforms to the same orientation. 

 Such deviation as there is, arises chiefly from two causes, namely, 

 (i) portions of igneous intrusions which have not developed a 

 gneissoid habit often do not maintain the simplicity of the general 

 structural trend; (2) some faults of later date cut across this general 

 structure. Illustrations of the former cause of irregularity are such 

 masses as Dundefberg mountain and Bull hill, and representatives of 

 the later faulting are the Mahopac flexure and minor faults. 



In some places the original northeast trend of the structure has 

 been so disturbed by large intrusions that it is now almost perpen- 

 dicular to the normal direction. This may be seen at Fort Mont- 

 gomery, where the principal structure for a short distance trends 

 northwest and southeast instead of northeast and southwest. 



Method of mapping. On account of the obscurity of forma- 

 tional boundaries the difflculty of mapping can not be fully over- 

 come. It is impossible to draw sharp lines of demarkation between 

 certain formations. A method which more nearly represents the 

 actual conditions of affairs would be to map distinct formations 

 sharply where they are typically and clearly developed and allow them 

 to overlap in intermediate or transitional zones, so that the colors are 

 mingled in that portion where it is believed the actual rock materials 



