GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 43 



a certain degree the process referred to above under the heading 

 " Igneous impregnation " is a part of a series of effects usually 

 assumed to 'belong to contact metamorphism. In so far as the 

 materials introduced in this way combine with those already in the 

 rock to make a new product, the classification holds strictly true, 

 but in so far as the impregnation results simply in the penetration of 

 the rock by the regular igneous rock minerals which crystallize there, 

 it is a question whether contact metamorphism is a suitable term. 



Tihe strong schistosity of certain parts of the Grenville suggest 

 that regional metamorphism had accomplished a great deal, and that 

 contact effects are supplementary rather than fundamental. If one 

 includes injection and impregnation phenomena with the contact 

 history, however, then certainly in many places contact effects are 

 dominant, and original regional recrystallization of secondary impor- 

 tance. But it is unlikely that the structural results now shovv-n would 

 have been attained if the older formation had not originally been 

 schistose, crystalline and folded. 



As usual the limestone beds show the best evidences of contact 

 metamorphism in this series, and extraordinary complexity of 

 appearance and composition is a common development. 



Contact metamorphism of earlier granites and gneisses by later 

 granite magmas . is very obscure and the later granites such as the 

 Peekskill-Mohegan have little apparent effect. Even the xenoliths 

 in this formation maintain their identity and appear to be of exactly 

 the same character as adjacent rock of the same formation not thus 

 exposed. The Cortlandt series accomplishes more on the xenolithic 

 masses of schist that have been engulfed. Some are not only 

 completely transformed in mineral constituents, but also show 

 selective elimination and addition of material from both the original 

 rock and the igneous host. Thus it happens that an abnormal com- 

 position results with corundum, emery and spinel, none of which 

 belongs typically to either rock. 



It is held by some geologists who have studied southeastern New 

 York that the coarsely crystalline habit of the Manhattan schist, 

 which differs so markedly in this respect from the Hudson River 

 phyllite, is due chiefly to the influence of invading igneous matters 

 or to subjacent magmatic masses which have induced more thorough 

 crystallization. This view, therefore, would credit the highly crys- 

 talline character of the Manhattan formation chiefly to influences 

 that may be classed with contact metamorphism. The writers do 



