44 NEW YORK STATE MUSEUM 



not seriously question the competence of contact influences to accom- 

 plish such differences of condition, or degree of reconstruction, but 

 they doubt the appropriateness of the explanation for the crystal- 

 linity of the Manhattan mica schist. 



In review of this point, therefore, it is proper to emphasize the 

 fact that the usual silication and silicification or induration effects, 

 as well as more elaborate and obscure recrystallizations, are in places 

 evident and doubtless are of contact origin. This is not the only 

 cause, however, of recrystallization, and it is therefore not always 

 possible to specify to what degree the various schists, gneisses and 

 limestones owe their present peculiarities of composition and struc- 

 ture to contact metamorphism. 



Dynamic influences or deformation. It is not the purpose at this 

 point to discuss the larger features of structural geology as 

 dependent upon the deformation history, but rather to enumerate 

 and compare the ways in which deformation has accomplished petro- 

 graphic variation or modification. It is the writers' belief that a 

 very ancient dynamic history is responsible for the original structural 

 quality of the oldest formation, the Grenville. It is our opini >n 

 that the deformation of that period developed foliated and folded 

 rocks. The same influence is likewise in control of the petrologic 

 quality of the Manhattan-Inwood series. To a much smaller degree 

 also, a tendency to this same sort of development is to be seen in 

 the Hudson River-Wappinger-Poughquag series. 



In connection with this deformational history, folding and fault- 

 ing have resulted, the effects of which give some of the most strik- 

 ing geologic features of the West Point area. In certain of the 

 weaker members, fine crumpling habit is developed and similar struc- 

 ture may rarely be seen in some of the gneissoid rocks believed to 

 be essentially igneous. In the latter case it is believed to be due 

 to deformation, perhaps of a regional sort, occurring when the rock 

 mass was only partly solidified. 



We are not able to determine whether these dynamic influences 

 referred to above as probably regional are connected instead with 

 the crowding action of invading and intruding magmatic masses. 

 Probably both have figured, but it seems to us that the regional 

 Influences are large and that the crowding and shouldering effects of 

 igneous masses are probably comparatively insignificant. It is not 

 clear, however, just how one could distinguish with certainty 

 between them. 



The larger deformation movements have at times been concen- 



