314 NEW YORK STATE MUSEUM 



Garnet occurs only sporadically and then always corrosion zone 

 fashion, between the magnetite and feldspar. But little horn- 

 blende is found in the Loon lake rock. 



In the type rook quartz occurs only sparingly, though quite 

 quartzose varieties occur in the immediate vicinity. It is mainly 

 in rather coarse, elongated spindles or lenses. It is also found as 

 small inclusions in the feldspar, sometimes rather numerously and 

 with a tendency to the production of micrographic growths. 



The rock has a cataclastic structure, ranging from rather coarse 

 varieties to those which are thoroughly gneissoid, and the granu- 

 lation pretty complete. In other words, it shows the same 

 variations in texture which the anorthosites exhibit, except for 

 the lack of the very coarse varieties.^ 



Variahility of the syenite. While this description will answer 

 for the usual rock in many places, it shows great variability. On 

 the one hand, the amount of quartz varies widely, rocks which 

 contain as much as 20^ of it being not at all uncommon. Increase 

 in quartz is commonly accompanied by decrease in the amount of 

 pyroxene and hornblende present, and hence by disappearance of 

 well marked foliation, it being replaced by a linear structure due 

 to the spindle form of the quartzes and their parallel alinement. 

 This structure is quite characteristic of some of the igneous rocks 

 of the region. This quartzose variety is usually coarsely granular 

 and seems to weather even more rapidly than the ordinary rock, 

 so that it is very difficult to obtain in fresh condition, and usually 

 only the rusty brown rock can be foun,d. It is the great similarity 

 of this variety, which can be traced into the normal rock through 

 all gradations, to the brown, quartzose gneiss north of Franklin 

 Falls which Kemp regards as a possible Grenville conglomerate 

 and which has already- been referred to, that causes the writer's 

 hesitation in accepting that origin for the rock. It may be also 

 added that, whereas this Franklin Falls rock is adjoined by Gren- 

 ville sediments on one side, it is also adjoined by augite syenite 

 on the other, so that areally the connection with one sort of rock 

 is no closer than with the other. 



Another common variation in the rock is brought about by 

 changes in the relative amounts of pyroxene and hornblende. In 



1 For a more detailed description of these rocks, see Geol. Soc. Am. Bui. 

 10:177-82. 



