GEOLOGY OP THE WEST POINT QtTADtlANGLE, NEW VORK ^1 



A walk across the West Point quadrangle with an understanding 

 of the geological principles represented in its great variety of rock 

 structure and composition is a better exhibit of structural and 

 dynamic geology than a whole museum of specimens. Some of the 

 principles represented, however, are so seldom encountered that it 

 seems to be advisable to discuss at greater length the different 

 processes involved in the petrogenesis. 



Original differences of composition of sediments. This is 

 clearly a possible cause of variation. The chief differences in the 

 Grenville metamorphics doubtless depend upon this point. All the 

 variety in the simpler metamorphosed sediments and much of it in 

 the more complex ones is directly related to the variety of the 

 original sediment with which the whole history began. Certain 

 beds of particular composition have maintained their identity fairly 

 well, others have recrystallized so as to be indistinguishable from 

 similar rocks of very different origin, and still others have yielded 

 to igneous attack to such extent as to have lost most of their original 

 character, even their composition. Doubtless there were originally 

 some thousands of feet of shales, sandstones and limestones in many 

 repetitions and variations of quality. It is these rocks that have 

 imposed their composition and structural influence on all subsequent 

 products. 



Metamorphism. There is hardly a formation in the region that 

 does not show at some point the effect of dynamic metamorphism. 

 The freest from it are the members of the Cortlandt series. Next to 

 these are certain large granite masses of the Highlands belt proper, 

 such as Storm King, Breakneck ridge, and Dunderberg mountain; 

 but even these are dynamically affected along certain zones to such 

 degree that the original character is wholly destroyed. 



All the sediments are extensively affected. The most modified of 

 all is the most ancient sedimentary representative, the Grenville; 

 next is the Manhattan-Inwood-Lowerre series and least of all the 

 Hudson River-Wappinger-Poughquag. 



In the last group are occasional occurrences of shales and slates 

 and beds of limestone and quartzite which maintain clear evidence 

 of their original bedding and composition ; but in many places the 

 sedimentary structure is completely eliminated, the minerals are 

 reorganized, the textural habit is entirely new, and even the com- 

 position has been altered. The chief change probably is the elimina- 

 tion of combined and interstitial water and soluble salts and the 

 introduction of silica in the more open-textured beds such as the 

 Poughquag quartzite. 



