NEW YORK STATE MUSEUM 



across the strike, which is northeast with a prevailing steep dip 

 to fhe son ill. It is hardly possible that this is anything but a 

 sedimentary formation, and in every way it is in striking con 

 trasl with the granite-gneisses, which are certainly igneous. 



As has been frequently slated in previous reports, such locali- 

 ties are important as standards of comparison in determining the 

 nature of rocks which, on account of their isolation or of poor 

 exposures, give no direct evidence of their origin. 



But even in this belt there are some rocks of doubtful char- 

 acter — certain light colored acid gneisses, which are massive and 

 look like granites, though in many respects they behave like con- 

 stituent parts of the sedimentary series. It is likely that both 

 igneous and sedimentary gneisses are here represh Lted, but their 

 differentiation will require much close work. 



In regard to the distribution of the granite-gneisses but little 

 need be said, since it has "been already stated that they occupy 

 all of the areas between the schists, not covered by the Potsdam. 

 A few of the gneiss localities, however, demand special mention. 

 Of these, perhaps the most important is about half a mile north 

 of the old military road leading from Redwood to South Ham- 

 mond, and beside the road running north to the river, 1J miles 

 east of the former place. This locality is important because it 

 shows clearly the complex nature of the granite-gneiss. A rather 

 coarse, dark, grayish red gneiss is cut and veined by a fine, light 

 pinkish red gneiss. If the two rocks occurred in distinct out- 

 crops but slightly separated, they would, almost without ques- 

 tion, be regarded as parts of one and the same rock, and it was 

 only after the careful examination of several outcrops that the 

 writer felt convinced of their difference in age. The exposures, 

 in an open field, are numerous and excellent, showing sharply 

 defined contacts between the two gneisses, with no gradation 

 between them, and with clearly marked dikes of the fine gneiss 

 cutting the coarse. With poorer exposures, such for instance as 

 generally occur in wooded districts, this difference in age would 

 probably be overlooked; and even where it has been fully estab- 

 lished, the actual separation of the rocks in mapping remains a 



