GRENVILLE SERIES OF EASTERN NORTH AMERICA 625 



in grain, sometimes containing a certain amount of feldspar as an 

 accessory constituent, and elsewhere containing some pale-green 

 pyroxene, which is evidently derived from the alteration of a certain 

 amount of calcareous matter originally present. 



Under the microscope the quartz of these quartzites is seen to have 

 the form of elongated grains arranged in the direction of the foliation 

 and showing very marked strain shadows, sometimes passing into a 

 distinct cataclastic structure. There is every reason to believe that 

 these quartzites represent, in most cases at least, altered sandy 

 sediments. 



The gneisses of sedimentary origin {paragneiss). — These gneisses 

 differ distinctly in appearance from the foliated granite gneisses 

 already referred to, and which constitute the bathylithic intrusions. 

 They are fine in grain and show no protoclastic or cataclastic struc- 

 ture, the original material having been completely recrystallized. 

 They have an allotriomorphic structure, with a tendency of certain 

 of the constituent minerals to elongate themselves in the direction 

 of the original bedding. While quartz, feldspar, and biotite are 

 among the constituents present, the mica is usually more abundant 

 than in the granite gneisses, and, in addition, garnet, sillimanite, 

 graphite, and pyrite are very frequently present, the last giving 

 rise to a prevailingly rusty color on the weathered surface. These 

 gneisses occur in the form of well-defined beds and are usually found 

 intimately associated with the limestones. They resemble in many 

 respects the hornstones which are found in granite contact zones, 

 but are rather more coarsely crystalline than is usual in this class of 

 rocks. 



These gneisses, while presenting a general similarity in appearance, 

 show a considerable diversity in composition and are evidently derived 

 from the recrystallization of sediments which varied considerably 

 in character. The two following analyses show extreme cases of 

 this variation. 



No. I occurs interstratified with beds of white garnetiferous 

 quartzite at St. Jean de Matha in the Laurentian district to the north 

 of Montreal. It is composed of quartz, orthoclase, garnet (in large 

 amount), and sillimanite, with smaller amounts of rutile, biotite, 

 pyrite, graphite, and serpentine, the latter mineral resulting from the 



