86 NEW YORK STATE MUSEUM 



shaly sandstones, limestone being absent. These are now gneisses 

 of various colors, from white to black, nearly always containing 

 pink garnets, and with the darker varieties often holding graphite 

 as well. 



What are supposed to have been shaly sandstones are now white 

 to gray, or greenish gray, gneisses, which are rather well banded^ 

 thus hinting at a sedimentary structure due to variation in com- 

 position of different layers. If this banding does represent orig- 

 inal bedding, then the present foliation conforms in direction 

 with it. 



To the eye these light colored gneisses appear very quartzose, 

 but the microscope dispels the impression. No case has been ob- 

 served in which the quartz constitutes so much as 50^ of the rock. 

 It commonly runs from 30^ to 40^, seeming always somewhat 

 subordinate to the feldspar in amount, the ratio between the two 

 varying from 3: 5 to 4: 5. Most of the feldspar appears to be 

 anorthoclase, as indicated by its faintly moir^ appearance, but 

 often a very considerable percentage of an acid plagioclase (ap- 

 parently between albite and oligoclase) is present in addition. 

 Other minerals than quartz and feldspar seldom constitute as 

 much as 10^ of the rock and often fall below 5^. Minute zircons 

 usually occur in considerable number, so much so as to form a 

 prominent feature of these Grenville rocks. Small garnets are 

 frequent. A little biotite, a little magnetite and an occasional 

 titanite are the other customary minerals. The silica percentage 

 must lie above 75^ in all cases, and it is believed that chemical 

 analyses would point strongly toward a sedimentary origin for the 

 rock, as suggested by its mineralogy and appearance. 



These quartzose gneisses contain bands of somewhat more basic 

 character, which differ from them mainly in the larger content of 

 garnet and biotite, and in usually holding graphite in addition, 

 mostly in minute scales and in no great quantity. One large gar- 

 net was noted full of inclusions of a green spinel, probably 

 pleonaste. The minerals other than quartz and feldspar make 

 from 15^ to 20js^ of the rock. The quartz percentage is nearly or 



