366 



HAROLD L. ALLING 



of both end members (minals) and (4) by the probable mixture of 

 these two series. 



KAIS13O8 



Triclinlc'Series 



NaAISiaOa ^^ ^ ^^CaAlaSigOe 



KAISisOs 



Triclinic" Series 



NaAISiaOs ^ 



5 



o i* ^ 4? CaAlaSiaOs 



Fig. 6. — Plot of extinction angles for potash-soda-lime feldspars. Upper figure: 

 (010) face. Lower figure: (001), base. Both for " Triclinic " series. The monoclinic 

 series not attempted tentative, subject to revision. 



The writer wishes to correct an error in Part I. On page 251 

 the statement is made that in using the conventional curves for 

 extinction angles of the soda-lime series "it is not possible to deter- 



