20 NEW YORK STATE MUSEUM 



which have been recrystallized and stretched out into rude 

 parallelism with the sedimentary beds as a result of severe 

 metamorphism. 



Though of somewhat variable nature, they present three main 

 types : 



1 Red gneisses which have the mineralogy of granites, and are 

 thought to have corresponding chemical composition also, though 

 they have not been analyzed. They are usually fine grained and 

 with quartz of a pronounced leaf type. 



2 Black, hornblende gneisses, sometimes with pyroxene also 

 and usually with black mica (biotite), which have the composi- 

 tion of gabbros or diabases. 



3 Greenish gray gneisses, which have somewhat the color and 

 appearance of very gneissoid varieties of the syenite previously 

 described, and are very difficult to distinguish from them when 

 occurring alone. They are commonly very quartzose, more so 

 than the usual syenite, and very distinctly of the " leaf gneiss " 

 type. They are very like the red gneisses of the first type under 

 the microscope, have the mineralogy of granites, or of quartz 

 syenites, and are regarded as igneous rocks. Their possible re- 

 lationship with the Little Falls syenite is an exceedingly difficult 

 problem, not as yet satisfactorily solved, though they are hesi- 

 tatingly regarded as distinct and as older. 



Though all these rocks are usually found interbanded with the 

 Grenville sediments, so that there can be little doubt as to their 

 close association, they may occur elsewhere unaccompanied by 

 the sedimentaries, or with these in very minor quantity, and such 

 areas have been given a separate coloration on the map, though 

 the distinction is -not a sharp one, and there is some question as 

 to its wisdom. 



Syenite gneiss. There is a considerable area shown in the north- 

 eastern part of the map where the rock is of the same sort through- 

 out. The exposures are all in the woods and of the unsatisfactory 

 sort that obtain there. The rock is thoroughly gneissoid, of a 

 greenish color and weathers rapidly to a dingy brown. Most of 

 the exposures show nothing but the brown rock, though usually 



