Becker — Determination of Plagioclase Feldspars. 353 



same index of refraction as quartz, and a higher index than 

 balsam, while albite has a lower index than either. 



Orthoclase microlites would be exactly square ; they would 

 not show polysynthetic twinning, and the index of refraction 

 is much lower than that of oligoclase, which, like orthoclase, 

 extinguishes at 0°. 



Extinction of nearly square sections of feldspar microlites cut within 10° of the 

 perpendicular to the edge (00 L) (010). 



Feldspar. Composition. Extinction. 



Anorthite Abu An 200 = 96$ An + 42£ ± 3| 



Calcium Labradorite Ab 3 An 4 = 60$ An + H8 ±3 



Sodium Labradorite Abi An x = 47$ An + 26£ ± 2| 



Andesine... Ab 5 An 3 = 34$ An + 16 ± 2 



Calcium Oligoclase Ab 3 An l = 28$ An + 4 ± 1| 



Sodium Oligoclase Ab 4 A^ = 18$ An + ± 1-J- 



Albite Ab = 0$ An —13 ± 2| 



It need not be said that in many rocks the determination of 

 the microlites is quite as important as that of the phenocrysts, 

 their united volume often equaling or exceeding that of the 

 larger crystals. It is also extremely interesting to compare the 

 character of the two generations. It is usually stated or 

 assumed that the microlites are all of one species. The exam- 

 ination of the square sections, of which a dozen or two are 

 often visible in a single slide, leads me to a different conclusion. 

 As a rule the majority do belong to a single species, but it is 

 seldom that two or three out of ten do not belong to a differ- 

 ent species; in short, the microlites in the slides I have exam- 

 ined show an irregularity of species of the same order as, and 

 often greater than, that of the phenocrysts. The square sec- 

 tions often show zonal structure, and nearly always the exterior 

 portion is more alkaline than the inner portion ; but in a couple 

 of cases this order was found to be reversed. Occasional' 

 reversals of this sort are to be found among phenocrysts also. 



