GEOLOGY OF THE NORTHERN ADIRONDACK REGION 301 



The other gneisses are black and consist essentially of feldspar 

 and hornblende, with or without pyroxene, in other words, they 

 are amphibolites. They have often a massive appearance and 

 are in general more abundant where the gray gneisses are scarce 

 or absent. The feldspar may be plagioclase or orthoclase or 

 both, but the former much predominates, and is usually ande- 

 sin or oligoclase. Pyroxene seldom equals the hornblende in 

 quantity and is often absent. The rocks seldom show any traces 

 of crushing but seem to have largely recrystallized. 



These rocks often occur in thick bands or masses, often in 

 thinner masses interbanded with red gneiss, and very similar rocks 

 are often found in thin bands or bunches in the red gneiss. 

 The latter may be either segregations or inclusions, it being 

 usually impossible to determine which. Very similar rocks are 

 also found as dikes cutting the red gneiss. These usually have 

 a more rusty appearance than the ordinary rock and may be 

 either offshoots from it or may represent a different and later 

 rock. 



These amphibolites are often found involved with red, granitic 

 gneisses which cut them intrusively, both cutting across the folia- 

 tion and sending a multitude of thin sheets into the amphibolite 

 along the foliation planes, producing a red and black gneiss 

 [pi. 1]. In such case the amphibolite is clearly the older, but 

 in all such the question arises whether the granite is merely a 

 phase of the ordinary red gneiss, or whether it is a different and 

 younger rock. Since in nearly all cases these granites are not 

 so foliated as the red gneiss, but retain distinct traces of igneous 

 textures, often abundantly, it is thought that they are likely 

 younger and not to be classed with it. Among other localities 

 such granites are widely shown around St Regis Falls in Franklin 

 county. 



Nearly all, if not all, of these doubtful gneisses seem to have the 

 composition of igneous rocks, though the question can not be fully 

 decided without a large amount of chemical work. The gray 

 gneisses are perhaps the most questionable in this respect. There 



