Becker — Determination of Plagioclase Feldspars. 351 



some slight changes in the diagrams. He has given a new 

 diagram for the extinctions in the zone of symmetry, showing 

 the zero angle for the same orientation in all varieties. He 

 does not give a new set of stereographic projections for the 

 several species, and the means are therefore not at hand 

 for plotting the extinctions at 10° from the zone of sym- 

 metry. For this reason I have not redrawn the diagram 

 given above of the extinctions. The changes which would be 

 involved appear inconsiderable, and I have found the diagrams 

 for what may be called latitude ±10° too useful to be willing 

 to abandon them. It should be noted that Mr. Michel-Levy's 

 new fascicle gives stereographic diagrams for microcline.* 



While the zone of symmetry is the most useful one, and is 

 usually sufficient where material is abundant, cases also arise in 

 which other parts of the stereographic projections are indis- 

 pensible. When needful, a skillful use of the stereographic 

 projections will suffice to determine almost any doubly twinned 

 phenocrysts, however cut, and sometimes mere albite twins. 

 With the aid of the Fedoroff or Klein stages any phenocrystic 

 albite twin can be determined, but the use of such a table 

 involves the application of low powers only. 



While Mr. Michel-Levy's method of Carlsbad twins, referred 

 to above, is usually sufficient to determine the species of the feld- 

 spars of primary generation in porphyritic rocks, it is not 

 easily applicable to the microlitic feldspar of secondary consoli- 

 dation. Such microlites are twinned according to both the 

 albite and Carlsbad laws, less frequently than are the pheno- 

 crysts ; they are also often entirely embedded in groundmass 

 which obscures the extinctions, and relatively high powers 

 must be employed in examining them. Nevertheless, with 

 patience and good eyesight, determinations can often be made. 

 Such determinations have been used as a check upon another 

 method which presents no difficulties, and which will now be 

 described. 



In studying the groundmass of lavas from Alaska and Cali- 

 fornia, I have observed many minute, nearly square, sections of 

 plagioclase microlites. f These sometimes show albite twin- 



* Taking albite as NaAlSi 3 8 , molecular weight 263-36, and anorthite as 

 CaAl 2 Si 2 8 , molecular weight 279-09, I find the following percentages of anorth- 

 ite in the several feldspars. Michel-Levy gives somewhat different values in his 

 second fascicle, page 107. 



Percentage of 

 Feldspar. Symbol. anortliitc. 



Albite Ab 



Sodium 01 igoolase Ab 4 An! 20-95 



Calcium Oligoclase Ab^An! 26*11 



Andesine Ab 6 An 3 38-88 



Sodium Labradorite AbxAn! 51-45 



Calcium Labradorite ._ Ab 3 An 4 5855 



Anorthite Ab u Ana o 95*06 



\ The length of a side is usually less than two one-hundredths of a millimeter. 

 Sometimes there are two generations of microlites. 



