464 NEW YORK STATE MUSEUM 



usual rock which borders the Grenville belts. The exposures ex- 

 hibit a fairly uniform mass of gneiss, uniform in that it has a certain 

 facies which is readily recognizable. It is not uniform in comiposi- 

 tion, since it varies from a red, granitic gneiss to a black, gabbroic 

 one, both kinds occurring in many exposures. But the bulk of the 

 gneiss consists of these two sharply contrasted rock varieties. 

 Frequent intermediate varieties occur, and the granitic gneiss shows 

 considerable minor variation; but the group as a whole consists of 

 alternating masses of granitic and gabbroic gneiss. 



The granitic gneisses show a twofold facies ; most commonly they 

 are finely and evenly granular and quite gneissoid; but mingled 

 with these are many masses of quite coarse, granitic make-up, 

 vastly less gneissoid than the other. In a few cases very quartzose 

 granites of fhe Morris type, shortly to be described, occur, and they 

 distinctly cut the other and are therefore younger. But in the 

 majority of instances no such relationship is observable, and the 

 distinct impression is created that the one rock is merely a phase 

 of the other; or in other words that the coarse material differs from 

 the fine merely in having locally escaped the excessive granulation 

 which that has experienced. 



A red to brown feldspar is always much the most prominent con- 

 stituent of this rock, comprising from 60 per cent to 80 per cent 

 of the whole. ^ Quartz forms on the average from 15 per cent to 

 20 per cent, but runs both higher and lower. Black mica (biotite) 

 is the next mineral in importance, though usually accompanied 

 and frequently wholly replaced by hornblende. Both the granites 

 and the granitic gneiss have essentially the same composition, 

 though the latter are usually richer in the black, ferro-magnesian 

 minerals. 



Black, amphibolitic gneisses constitute from 20 per cent to 30 per 

 cent of the general mass of the Long lake gneiss. vSometimes they 

 occur in bands only a few feet in thickness with red gneisses above 

 and below, and here they usually appear interbanded, or in other 

 words the contacts are parallel with the general foliation of the 

 mass. Fromi these smaller bands there are all gradations up to 

 very thick masses of large areal extent. For the most part these 

 are hornblende feldspar gneisses, or amphibolites, the feldspar 

 being mainly plagioclase, ranging from andesin to basic labra- 



I The mineralogy of this and the succeeding rocks will be described in detail in a later 

 portion of this report. 



