GEOLOGY OP THE VICINITY OP LITTLE FALLS 89 



biotite, with sometimes hornblende also. These taken together 

 often constitute no more than 5^ of the rock, and seldom exceed 

 10^. There are occasional coarser feldspar fragments present 

 which may represent crystal remnants of the original rock that 

 have escaped the prevailing recrystallization. 



The syenites are gray to greenish gray rocks, commonly rather 

 quartzose, which approach granites on the one hand and gabbros 

 on the other. Except for their close association with the Gren- 

 ville sediments, they are not to be distinguished from the 

 thoroughly gneissoid phases of the later syenite, whose descrip- 

 tion will serve equally for them. 



The original gabbros are now converted into hornblende or 

 pyroxene gneisses. On the one hand, are hornblende, biotite, 

 plagioclase gneisses; on the other, augite and bronzite (or hyper- 

 sthene) appear instead of hornblende. In the pyroxene gneisses 

 garnet often occurs, magnetite always and pyrite sometimes. 

 The feldspar ranges from andesin to labradorite; but not in- 

 frequently a large part of it is not plagioclase at all but of inter- 

 growths, either of microperthitic or of micrographic habit, and 

 such portion may make more than 50^ of the whole, giving the 

 rock more of a monzonitic than of a gabbroic make-up. 



Syenite gneiss. The area given the syenite coloration in the 

 northeast portion of the geologic map is constituted of quite 

 homogeneous rocks, of thoroughly gneissoid character, gray to 

 greenish gray color, rapidly weathering brown, and of syenitic 

 make-up. They vary somewhat in coarseness of grain and con- 

 siderably in their quartz percentage, some of them being very 

 acid. There are occasionally to be seen slightly larger feldspar 

 fragments which seem to be of the nature of augen, and around 

 which traces of cataclastic structure appear. But these are 

 small fragments at best, the structure traces are obscure, and 

 the evidence of igneous origin from this standpoint very slender. 

 If, however, the writer be correct in referring the augen character 

 of the syenite at Little Falls to, an original porphyritic structure 

 in the rock, then the absence of that character here may have no 



