GEOLOGY OF THE VICINITY OF LITTLE FALLS 87 



quite as high as in the more acid gneisses, the basic minerals 

 increasing at the expense of the feldspar. 



There is also much of a more basic, heavily garnetiferous rock 

 interbanded with the lighter colored one. Its mineralogy is much 

 the same as that of the white gneisses in respect to the minerals 

 present, but there is a great change in quantity. There are the 

 same abundant small zircons, quite a little graphite, pyrite and 

 apatite are present, a little magnetite, quite a lot of biotite and 

 abundant garnet. All together these make from 30^ to 50^ of the 

 rock, garnet alone constituting from 20 to 30^. The remainder of 

 the rock is made up of feldspar, nearly all of which is microper- 

 thite, only a little acid plagioclase being present. The rock is 

 practically free from quartz, all there is being found as inclusions 

 in the large garnets. It would seem to have the composition of 

 a calcareous shale, yet is not at all the sort of rock customarily 

 produced from such shales by metamorphism, as amphibole of 

 some sort usually develops in quantity. In fact, the rock may not 

 have been calcareous, since, if present, the lime is now in the gar- 

 net, and it has not been analyzed. If it be a lime garnet the deep 

 seated conditions prevailing during metamorphism may account 

 for the character of the rock. 



There is another variety of the above rocks which is character- 

 ized by abundant pyrite, roughly some 5j^. It also has a consider- 

 able quartz content, some 25^, and more than half of the feldspar 

 is plagioclase, an acid oligoclase with maximum extinctions of 

 10°. There is also considerable of a thoroughly rotted bisilicate. 

 Biotite is present in large quantity and garnets are sparing or 

 absent. With pyrite decay the rock weathers rusty. All sorts of 

 gradations between all these types occur, but taken as a whole 

 they characterize the sedimentary Grenville of the district. 



Along with the foregoing are occasional bands of a rusty weath- 

 ering gneiss which, when fresh, is seen to be thoroughly gneissoid, 

 foliae of glassy quartz grains alternating with black leaves of 

 more basic mineral fragments, the whole making a rather dark 

 colored rock. Yet it shows a higher quartz content than any 

 other, that mineral making fully two thirds of the rock. It is of 



