GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 59 



Although the rock shows abundant deformation effects especially 

 fracturing and rehealing, it is evident that the composition as well as 

 the distribution of minerals favors the theory of development from 

 a fusion or solution. It may very well be that it is not a simple 

 injection effect and this is indeed suggested by the presence of garnet 

 in rather prominent development (plate 28). The occurrence of 

 areas of quartz acting essentially as a host in poikilitic structural 

 habit with the other constituents adds also to the certainty that it is 

 not at all a metamorphosed quartzite in origin, but it is believed 

 that there are remains of an older rock and that some of the minerals 

 in it, such as garnet, are essentially syntectic products (plate 29). 



d Injection gneisses. The writers interpret many of the mixed 

 banded gneisses as injection effects. In the coarser and more sharply 

 defined occurrences there is no considerable doubt about this being 

 their origin, but in the fine structured varieties where the different 

 parts are not so well defined it is believed to be quite impossible to 

 distinguish material of different sources. 



It is certain, however, that injection on a fine scale is quite as 

 important in the gneisses of the Highlands as the coarse types, and 

 this fine injection process doubtless gives much of the variety to the 

 more obscure gneisses. If the injection habit is maintained distinctly 

 in these five types one ought to see it occasionally in the microscope. 

 A.S a matter of fact it is sometimes found very sharply defined on a 

 microscopic scale. No. 163 has furnished several good photomicro- 

 graphs for illustration (plates 30, 31 and 32). 



e Impregnation gneiss. A more intimate mixture than the type 

 just described may be called an impregnation gneiss. Typically it 

 ought to contain grains belonging to the original rock intimately 

 associated with grains of introduced origin connected with magmatic 

 invasion. Although the conclusion is fully justified that such rocks 

 occur in considerable abundance in this district, the finding of suitable 

 illustration material is difficult and in even the best occurrences it is 

 doubtful whether one could with certainty distinguish between the 

 two sets of components. It has seemed to us in the examination of 

 this field that the Canada Hill type of granite succeeds better in 

 accomplishing such impregnation than any other type although the 

 Reservoir granite is a close second and the dioritic magma, repre- 

 sented by the hornblende plagioclase gneiss or Pochuck, has intimate 

 penetrative habit also. One specimen taken frotn the Grenville 

 gneiss belt in proximity to the typical Canada Hill granite shows 

 sets of constituents that are believed to represent these two different 



