42 NEW YORK STATE MUSEUM 



is more than half granite. That formation has not been so affected 

 within this quadrangle but very elaborate granitized efifects are pro- 

 duced in what was once the Grenville. 



A peculiar form of this impregnation jirocess appears in connec- 

 tion with the Cortlandt series, where xenoliths were partly absorbed. 

 It is believed that those are the places that have developed corundum, 

 emery and spinel and are the sources of the emery deposits of the 

 Peekskill district. The process of impregnating the original rock 

 together with the selective removal of material from it has developed 

 these unusual constituents. None of them appears in either the 

 original schist or the original norite, but they are found where traces 

 «f syntexis and impregnation are in evidence. 



It is possible that other impregnation effects are selective in their 

 nature, that in most cases part of the country rock has been com- 

 pletely removed while new material has come in, and that the minerals 

 finally left as the additional constituents are only a part of those 

 whicli passed through the particular spot. 



Granitization of limestones is apparently a much more difficult 

 thing- to accomplish, and it is seldom observed in this or adjacent 

 districts. Farther to the south in the Harlem quadrangle, however, 

 where thin limestone beds are associated with gneisses, and where 

 both injection and impregnation gneisses are developed, a case has 

 been found where a limestone layer was transformed into a slightly 

 banded granite gneiss, the change taking place within a distance of 

 30 feet. It is therefore difficult to escape the conviction that granite 

 impregnation has been a very important process in the transforma- 

 tion of many of the ancient rocks of this district. The older 

 granites have apparently been the most efficient, and the youngest 

 ones decidedly less so. The Peekskill-Mohegan ty]:)e is practically 

 free from important effects of this kind. 



Contact effects. In a district where iyneous invasion has been 

 developed on as large a scale as in this West Point area, one would 

 expect to find some evidences of contact metamorphism. Such 

 effects are found, but not on as large a scale or as clearly defined 

 as other districts have produced. Perhaps it is somewhat more 

 accurate to say that most of the contact metamorphism that has 

 been effected is either so intimately involved with the processes 

 already enumerated, or the connection of the metamorphism with 

 any particular igneous influence is so obscure, that one feels uncertain 

 about the amount of modification to be attached to this cause. To 



