600 WILLIAM J. MILLER 



granulated than the Grenville series. How is this fact to be 

 explained if both series have been subjected to strong regional 

 compression after the intrusions ? 



THE SYENITE-GRANITE SERIES AND ITS FOLIATION 



Character of the syenite-granite series. — The main bulk of syenites 

 and granites in the Adirondacks are regarded by the writer as facies 

 of a single great body intrusive into the Grenville, the intrusives 

 being much more extensively exposed than the Grenville. Perfect 

 gradations from basic (dioritic) facies of syenite to true granite are 

 commonly shown, a quartz syenite being the prevailing rock. As re- 

 gards granularity, structure, and mineral composition, the members of 

 the syenite-granite series are very variable. The granularity ranges 

 from line to coarse grain, with medium grain decidedly prevalent. 

 Aporphyritic texture is sometimes well developed. Granulation is 

 common, especially in the more acidic rocks, the feldspars generally 

 being notably more crushed than the other minerals. In structure 

 the rocks range from very faintly gneissoid to very clearly gneissoid 

 or sometimes almost schistose, the foliation being accentuated by 

 the roughly parallel arrangement of the dark-colored minerals. 

 The minerals, especially quartz and feldspar, often show more or 

 less flattening or elongation parallel to the foliation. In general 

 the more highly foliated rocks appear to be most granulated. In 

 mineral composition the range is from dioritic types rich in plagi- 

 oclase, orthoclase, pyroxene, and hornblende; to syenite rich in 

 microperthite, orthoclase, and hornblende or augite together with 

 some quartz and plagioclase; to granite rich in microperthite, 

 quartz, orthoclase, and microcline together with some plagioclase, 

 hornblende, and biotite. Various accessory minerals in smaller 

 amounts also occur. The color of the typical fresh syenite is 

 greenish gray which weathers to light brown, while the fresh 

 granite colors vary from greenish gray to light gray and pinkish 

 gray to almost red. In common with the Grenville, the 

 syenite-granite foliation shows a tendency toward a northeast- 

 southwest strike, with parallelism of syenite-granite and adja- 

 cent Grenville quite common, though there are many notable 

 exceptions. 



