FACIES OF THE SYENITE-GRANITE BODY 245 



are variations ranging through granitic syenite to true gj'anite. The 

 prevailing rock is syenite to granitic syenite. Brief descriptions only are 

 here given, the reader being referred to various papers and bulletins of the 

 Xew York State Museum for details. 



NORMAL SYENITE 



This rock shows a greenish-gray color when fresh and it weathers to a 

 light brown. Its weathered surface is seldom more than a few inches 

 thick. As regards structure and granularity, it is a quite variable rock. 

 The granularity ranges from fine to fairly coarse, with a medium grain 

 decidedly prevalent. A porphyritic texture is sometimes moderately de- 

 veloped. The structure ranges from only faintly gneissoid to very clearly 

 gneissoid to almost schistose, this structure being accentuated by the ar- 

 rangement (or flattening) of the dark-colored minerals with their long 

 axes parallel to the direction of the foliation. Evidence of crushing or 

 granulation is common, though it varies greatly, the feldspars showing 

 the effects of the granulation more than the other minerals. 



In mineral composition, too, the syenite is rather variable. Feld- 

 spars — microperthite, orthoclase, and soda-rich plagioclases — constitute 

 50 to 80 per cent of the rock. Quartz, in varying amounts up to 20 per 

 cent, is always present. Pyroxene or hornblende, or both, occur in 

 amounts up to 20 per cent. The pyroxene is mostly a green augite, with 

 sometimes a little hypersthene. Hornblende is generally more abundant 

 than pyroxene in the more quartzose syenites. From 1 to 5 per cent of 

 magnetite always appears. Small amounts of zircon, zoisite, and apatite 

 seldom fail. Garnet is much more sporadic in occurrence, though at 

 times it makes up several per cent of the rock. 



BASIC PHASES OF THE SYENITE 



So far as known, these rocks are of comparatively little extent in the 

 Adirondack region. By relative increase in plagioclase and decrease in 

 quartz the normal syenite shows transitions to rocks of dioritic or gab- 

 broic composition. At times the plagioclase is the only feldsj)ar present, 

 and quartz is either wholly absent or present in only slight amount. 

 Hornblende or pyroxene (augite or hypersthene), or both, always appear 

 in amounts up .to 20 per cent. From 1 to 6 per cent of magnetite nearly 

 always occurs. Smaller amounts of biotite, apatite, zircon, and zoisite 

 usually appear, while garnet is of sporadic occurrence. 



As regards color, granularity, and structure the statements made for 

 the normal syenite apply almost equally well here. 



