192 HP rr'SHixG — vugttk-syf.xite gneiss near loon lake 



contrast to one another that i^ eN:hihited by the Christiana and Ekersund- 

 Soggendal di«?tricts of Norway> 



Summary 



1. A <|uartz-augite <^yenite gneiss near Loon lake is described ti^^ regards 

 its field relations and megascopic and microscopic characters. 



2. Chemical analysis shows it to be a member of the syenite group 

 and an acid representative of the variety called akerite by Brogger 



3. It is shown to be nearly related to the anorthosites m age, inas- 

 much as it is intrusive in the Grenville series, but much older than the 

 ]>re-Potsdam diabases of the region. 



4. Other Adirondack localities are briefly described, and the rock is 

 shown to vary within quite wide limits, ranging from a granite to syenites 

 more basic than the one analyzed. 



5. The relations of the syenites to the anorthosites are discussed, show- 

 ing a lack of decisive evidence, but indicating that syenites are in part 

 a result of differentiation in the anorthosite magma after reaching its 

 place of final coolmg and in part are somewhat later in date. 



6. Comparison is made with the similar petrographic provinces of 

 Canada north of Monti*eal and of Ekersund, Norwa}^ followed by a dis- 

 cussion of the order of eruption of the Adirondack eruptives, which is 

 anorthosites, norites and diabasic norites, syenites, and granites, followed 

 later by diabases and syenite porphyries. 



7. A brief comparison with the Es«ex county, Massachusetts, province 

 suggests that the original magmas in the two districts were quite different, 

 yet another instance is added by them to the many already known of 

 very similar rocks produced by the differentiation of quite dissimilar 



magmas. 



*see Koldeiup, lot < it , pp 191-104 



