228 PROCEEDINGS OF THE AMERICAN ACADEMY. 



6. Riebeckite granite. S. W. Houghnatten, Eftelot, Sanelsvar, W. 

 from Lougental, Southern Norway. 



7. Riebeckite granite, Ekona — Sungalc — Krater, Kamerum. 



8. Riebeckite, Acmite Granite, Dahamis, Insel, Sokotra. 



9. Arfvedsonite-Biotite granite, 2 7 Stony Brook Reservation, West Rox- 

 bury, Mass. Win. F. Hall, Analyst. 



cannot be made, nor if it could be done, could we apportion the various 

 radicles of the soda-iron pyroxene and hornblende accurately as we 

 do not know their exact composition. We may, however, proceed as 

 follows and arrive at a mineral composition which will give us a close 

 approximation to the relative amounts of quartz, the feldspars, the 

 sodic-iron silicates, and the oxide minerals. The albite has been 

 determined at least as sodic as Ab95 Aus and possibly more so. Using 

 this composition for the albite and making the assumption that 

 magnetite is not present in an amount over one percent, which is 

 certainly not far from the truth, further noting the fact that at least 

 one third of the CaO is present as CaCOs and disregarding the slight 

 excess of AI2O3 as being present in kaolin, we arrive at the following 

 composition calculated to 100 percent. The w^ater and fluorine have 

 been also disregarded though they certainly enter into the hornblende 

 present. The omission has the effect of making the hornblende 

 figure somewhat low. 



-^ . Feldspars , ^„ 

 Quartz 33.3 Ratio, — =1.67 



,,. ^, .^ ^\lbite Ab95An5 28.1 7f!^f^ 



Microperthite ■< , r- i- .t- - t^ • Albite , ^, 



( Alicroclme 2/.o Ratio. —; : — = 1.02 



Microcline 



Hornblende 1 



and \ 9.6 Albite 50.5 



Pyroxene ^licrocline 49.5 



Magnetite and ] Microperthite 100.00 



Ilmenite 



Zircon 



100.00 



These percentages agree well with approximate measurements of 

 the relative amounts of the constituent minerals made on thin-sections 

 and with Dale's Rosival measurements ^^ made on polished surfaces, 



27 F. Ba.scom, Journal Phil. Acad, of Nat. Sciences, 15, 2d series, p. 135. 

 (March, 1912). 

 281oc. cit., p. 



