GEOLOGY OF THE LUZERNE QUADRANGLE 29 



very irregular, being wavy to almost contorted. No. i6 of the table 

 gives the mineralogical composition of a thin section of this rock 

 which happens to show no garnet. In the quarry the rock is very 

 similar to that just described except that the feldspar is all nearly 

 white, and the garnets rise to several per cent. No. 17 of the table 

 shows the minerals in a thin section which happens to contain but 

 little of the garnet. In the northwestern part of the area some 

 ledges show variations to medium-grained and even aplitic facies. 



In the southern of the two areas there are good exposures in the 

 middle portion which show the rock to be essentially like that of 

 the northern area with garnets, but only moderately foliated. 



The writer ventures to suggest that this granite porphyry is not a 

 pure magmatic differentiation phase of the syenite-granite like the 

 coarse-grained granite above described, but that it probably repre- 

 sents Grenville dark gneiss (or possibly metagabbro) which has 

 been thoroughly injected and more or less altered and possibly 

 assimilated by pegmatitic granite magma. This view is strength- 

 ened by the character of the Grenville-granite mixed rocks below 

 described as occurring in the quarry near the road one-half of a 

 mile north of Luzerne. 



Grenville-granite Mixed Rocks 



Eighteen areas of Grenville-granite mixed rocks are shown on 

 the accompanying geologic map. These are, next after the syenite 

 and granite, the most extensively developed rocks of the quad- 

 rangle. They are by no means uniform in appearance. Some of 

 them are practically pure intact masses of Grenville strata cut by 

 numerous dikes of granite and pegmatite; some are rather intri- 

 cately involved Grenville and granite; while still others apparently 

 represent Grenville strata so highly injected, altered and probably 

 assimilated by granite or aplitic granite magma that the main char- 

 acteristics of both the Grenville and granite have been destroyed. 



Much mixed rock of the kind last mentioned occurs in the areas 

 in the southern third of the quadrangle. In fact such rock is 

 by far the prevailing type in these areas. This type of Grenville- 

 granite mixed rock is much like that described as occurring in the 

 southwestern part of the Russell, St Lawrence county, quadrangle. 

 An excellent outcrop (see plate 7B) by the main road i mile east- 

 southeast of Lin wood school well illustrates very typical rock of 

 this sort. It is highly banded in almost perfectly straight layers 

 strongly resembling a true stratified rock when viewed from a dis- 



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