184 ri. r. cushing — augite-syenite gneiss near loon lake 



festJy im])Ossil)le liore. The only way of iiiterpretino; the analysis that 

 suggests itself to the writer is to assume that the augite is essentially a 

 lime, ferrous-iron, alumina silicate, unusuall}^ high in the last named 

 oxide. 



Culcidalion of Anuh/siii I 





INIolecu- 

 lar ratio. 



INIagne- 

 tite. 



Ortho- 

 clase. 



Albite. 



Anor- 

 thite. 



Hj'per- 



sthene. 



Augite. 



Quartz. 



SiO., 



10575 



9 



1795 



26 



494 



8 



523 



87 



547 



816 



9 



26' 



o5 



3282 



4896 



318 



171 



745 



1160 



TiO., 



AUG, 



Fe.O'o 





547 



816 



159 





372 





FeO 









87 



372 





BaO 







8 

 151 





CaO . 









87 



372 





Mo'O 











K,0 



Ka.O 





547 



"'816 































Per cent . 



0.73 



30.39 



42.70 



4.51 



2.02 



12.02 



7.07 









This result agrees quite Avell with that obtained by separation with 

 heav}' solutions, and can not be far from the actual composition of the 

 rock. As it stands, the plagioclase is albite, Ab^^ An,, and the micro- 

 perthite is approximately Or, Abj. The plagioclase is in all ]irobal)ility 

 not quite so acid as this would imply, as the augite undoubtedly contains 

 a little soda, which would displace some of the lime calculated in that 

 mineral, and both lower the albite and raise the anorthite ]>ercentage. 

 It is thought that this change is only slight, not materially affecting the 

 calculation. 



The agreement between analyses I and II is exceedingl}^ close. The 

 Diana rock is even more feldspathic than that from Loon lake, which 

 accounts for the increased silica and diminished lime and magnesia of 

 the former; but the rock at Loon lake is quite variable, and specimens 

 could be selected which would tally almost exactl}^ with the Diana 

 analysis. No doubt also the converse is true. 



Of the other available published analyses of American syenites, the 

 two which stand nearest the Adirondack rock are quoted in columns III 

 and IV of the table. Of the Mount Ascutney s^'enite, no published petro- 

 graphic description is available.* A hand specimen and slide in the 

 writer's })Ossession show a green feldspathic rock very similar to that 



* Dr Daly participated in the discussion following the reading of the paper before the Society, 

 stating that the relations of the rocks of mount Ascutney had been carefully worked out and were 

 in preparation for publication, and empliasizing the similarity of the Loon Lalce rock. 



