BASAL GNEISSES OF THE HIGHLANDS 363 



Petrographic range 



No single type of rock can be selected as a constant for this 

 formation. Certain varieties are more abundant than others 

 and thereby give the complex series a certain character that 

 serves to distinguish it from all others of the region. But in 

 isolated outcrops of limited extent an abnormal or unusual type 

 may cause great uncertainty of identification. 



Broadly, though, the formation includes banded granitic, 

 hornblendic, micaceous and quartzose gneisses ; mica, horn- 

 blende, chlorite, quartz and epidote schists; garnetiferous, pyri- 

 tiferous, graphitic, pyroxenic, tremolitic and magnetic schists 

 and gneisses ; crystalline limestone, serpentinous limestones, ophi- 

 calcites, serpentine, tremolitic limestone and quartzite ; pyrite 

 and magnetite deposits ; granite and diorite gneisses ; true granite, 

 diorite and gabbro bosses; numerous dikes, stringers and lenses 

 of pegmatite ; and occasional basaltic, diabasic, and andesitic 

 dikes. All of these occur with many variations and gradations 

 such as can be seen only in an area of extreme metamorphism 

 and many dynamic disturbances. 



Of these the most abundant are the banded hornblendic, 

 micaceous and quartzose gneisses, and the more massive granitic 

 gneisses. The banded type is the most characteristic for the 

 whole region. The most suggestive as to origin and interpreta- 

 tion are the quartzose rocks and the limestones, while the most 

 deceptive and difficult are the granites and granite gneisses. 

 Among these rocks there are numerous representatives of meta- 

 morphosed sediments of such character and relationship that 

 their origin can not be mistaken. Almost as abundant are the 

 igneous types, chiefly granites and their metamorphosed deriva- 

 tives. Frequent small members are found whose origin is 

 uncertain ; but in almost every case similar varieties occur 

 under conditions of relationship that usually leave no doubt 

 as to their position and interpretation. 



General structural features of the formation 



Almost all of the various schists and banded gneisses, together 

 with all of the quartzites, quartzose gneisses, graphitic schists, 

 and limestones form an interbedded series. At all contacts 

 between those members they are conformable. Most of them 

 are thin beds. Several of the limestones are only 3 to 10 feet 



