54 NEW YORK STATE MUSEUM 



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resenting fragments of all three series occur in the gabbro. Some 

 of the metagabbro ( ?) is quite certainly older than the syenite- 

 granite.^ 



Pegmatite dikes and certain small aplite dikes have been observed 

 as sharply defined intrusions in the gabbro. The diabase dikes are 

 later intrusions than the gabbro as proved by their distinctly finer 

 grain, and the fact that one actually cuts the gabbro in the northern 

 part of the North Creek quadrangle. It is therefore evident that 

 at least three very distinct minor intrusions succeeded the gabbro 

 intrusions of the quadrangle. 



Megascopic features. The typical nonfoliated gabbro is 

 readily distinguished from all the other rocks of the quadrangle. 

 Such rock makes up the main or interior masses of nearly all the 

 stocks, especially the larger ones. It is medium to moderately 

 coarse grained, dark gray to almost black where fresh, and it 

 weathers to a deep brown. The plagioclase feldspar varies in color 

 from a light gray to a dark bluish gray. A diabasic texture is 

 usually more or less well developed, this being particularly striking 

 in the coarser grained facies. Minerals recognizable with the 

 naked eye or hand lens include plagioclase, pyroxene, hornblende, 

 ilmenite (or magnetite) and nearly always biotite and red garnet. 



Variations from the typical nonfoliated gabbro just described 

 are common, one of the most abundant being highly foliated border 

 facies (usually amphibolite) which do not show a diabasic texture. 

 Taken by themselves, some of the amphibolitic border facies are 

 very difficult to distinguish from certain Grenville hornblende 

 gneisses, or even from certain very gneissoid gabbroid facies of 

 the Whiteface anorthosite. Nearly always, however, the mode of 

 occurrence or the gradation into more typical gabbro renders cer- 

 tain the recognition of the gneissoid border facies of the gabbro 

 (figure 4). Even the more typical inner portions of the gabbro 

 stocks show many notable variations in structure, texture and 

 mineralogical composition as pointed out belaw in the special 

 descriptions. 



Microscopic features. In the following table the mineral con- 

 tents of the gabbro is represented by thin sections of samples from 

 several of the areas. 



1 Recent work by the writer in the Lyon Mountain quadrangle has shown 

 that most of the gabbro' and metagabbro are there older than the syenite- 

 granite series which leads to the suspicion that some of the Schroon Lake 

 quadrangle gabbro may also be older but definite evidence is lacking. 



