The Monoclinic Pyroxenes of New York State. 13*7 



These crystals were first called by Prof. Williams hemimorphic,, 

 but subsequently the term hemihedi-al was used by him. 



Optical Properties. 



With the exception of an approximate determination of the ex- 

 tinction angles of the pyroxenes occurring in the igneous and 

 metamorphic rocks of New York State, the only optical determi- 

 nations which have been made are those by K. Zimanyi on De- 

 Kalb diopside (Ref. 64), and by C. Doelter on augite from Green- 

 wood Furnace. (Ref. 14.) 



This ma}' be due to the fact that New York does not abound in 

 fresh glass}^ crystals whose condition would be such as to en- 

 courage optical investigation. 



Relation between the optical and chemical properties. 



The researches of Wiik, Doelter and Herwig have already 

 been mentioned in the introductor}^ portion of this paper, and in 

 order to determine how far the writer's results agree with theirs 

 the following table was made, giving the extinction angle, and the 

 percentages of FeO, Fe^Og and AI2O3. 



Locality. e.'\t 



Eussel 37° 



DeKalb 40° 



Sing Sing 40° 



Port Henry 41° 



Pitcairn 41° 30' 



Diana 41° 30' 



Greenw. Furn...42° 20' 



Warwick 45° 15' 



Cheever 52° 



An examination of the above table shows that the extinction 

 angle does not increase with the percentage of FeO, thus showing 

 a disagreement with Wiik's results, although it should be added 

 that Wiik found several exceptions to his rule. Comparing the 

 extinction angles with the corresponding sum of the ferrous and 

 ferric iron gives no better results. If, however, the combined 

 percentages of FeO, FegOg and AI2O3 be taken, a more regular 

 series is obtained, the greatest variation being the pyroxene from 

 Russel. If, furthermore, those containing under 3% of AI2O3 be 

 excluded from the list as belonging more properly to the diop. 









Sum 



FeO. 



Fe^Og. 



AlA- 



of Three. 



1.29 





2.42 



3.71 



1.12 





.40 



1.52 



.80 





4.11 



4.91 



1.80 





1.32 



3.12 



1.301 





3.09 



4.391 



8.63 



.50 



4.94 



14.07 



2.55 



5.05 



5.09 



12.64 



6.86 



.57 



5.82 



13.25 



5.98 



3.53 



7.49 



27.00 



