362 N. E. A. Hinds — An Alkali Gneiss 



General relations. 



The rarity of tlie primary alkaline orthogneisses has 

 been alluded to briefly in an earlier part of this paper. 

 Rosenbusch'^ says that rocks of this type in the form oh* 

 quartz-bearing arfvedsonite gneisses were first recorded 

 in the vicinity of Cervadaes, 8 kilometers north of Campo 

 Major, in the province of Alemtejo, Portugal. This 

 region has also yielded other varieties, including a quartz- 

 poor arfvedsonite gneiss and a nepheline-aegirine gneiss. 

 An alkaline gneiss, similar to the first-mentioned rock 

 from Cervadaes except that riebeckite and aegirine 

 replace arfvedsonite, has been recorded from Old Ped- 

 roso, 51.5 kilometers to the northeast of the above locality. 

 Eosenbusch also cites arfvedsonite gneisses from near 

 Vigo, Spain, and from near Baependy in the State of 

 Minas Graes, Brazil; an alkaline gneiss with riebeckite 

 and aegirine from near Gloggnitz, Austria; and an 

 umptekite gneiss (astochite gneiss) from West Green- 

 land. Gushing^ *^ has described an augite syenite gneiss 

 from Loon Lake, New York, which is rich in the alkalies, 

 due apparently to the abundance of orthoclase and albite. 

 This rock closely resembles certain of the alkali syenites 

 from Mt. Ascutney. The recent compilation of rock 

 analyses by Washington^^ has added still other types, — a 

 canadite gneiss from Sweden (Quensel) ; a nepheline 

 syenite gneiss from Madagascar (Lacroix) ; two gneis- 

 soid alkali syenites from Gellivare, Sweden (Hogbom) ; 

 and an aegirite-riebeckite gneiss from Ross-shire, Scot- 

 land (Flett). The Van Nest Gap rock adds another 

 example to this meager list. 



From his discussion of these rocks, Rosenbusch^^ 

 concludes (translated) that ^'alkali granites, alkali syen- 

 ites, and nepheline syenites are known in the form of 

 crystalline schists in the basement complex, and it may 

 be expected that further investigations will bring to light 

 monzonitic and essexitic types." From this summary it 

 is evident that the foliated equivalents of the alkaline 

 igneous rocks are decidedly unusual types, especially 



^ Eosenbiisch, H., Elemente der Gesteinslehre, 1910, p. 620. 



^° Gushing, H. P. ; Augite-svenite gneiss from near Loon Lake, New York, 

 Bull. Geol. Soc. America, voL 10, pp. 177-192, 1899. 



"Washington, H. S.; Chemical analyses of Igneous Eocks, U. S. Geol. 

 Survey, Prof. Paper 99, 1917. 



" Op. cit., p. 622. 



