358 HAROLD L. ALLING 



perhaps due to volume change on inversion. Or it may be that 

 microcline and plagioclase twinning are often produced by stress 

 in making thin sections. Incipient twinning may always be present 

 in plagioclase or microcline as a result of atomic arrangement. If 

 it is very minute it may be exaggerated by stress to a point where it 

 can be seen. 



Many years ago .... George W. Hawes^ noticed in slides of ... . 

 fanorthosite] some feldspars which failed to afiford the twinning striations, yet 

 which he suspected of being plagioclase. Analytical tests demonstrated that 

 they were. The same untwinned character may reappear so that the observer 

 must be on his guard, but it is also true that a chance section parallel to the 

 twinning plane would be without striations.^ 



In some cases ordinary slides of normal andesine, labradorite and 

 especially bytownite show no twinning at all. The feldspars are 

 clear and so similar to orthoclase under the microscope that unless 

 extinction angles or, better still, the indices of refraction are deter- 

 mined it may well pass for that mineral. This development of 

 twinning by pressure is more noticeable in albite ranges and decreases 

 in intensity as the lime range is approached. The bytownite from 

 Crystal Bay, Minnesota^ (Mi2.oAb24.7An73.3) when thin-sectioned 

 with great care is entirely free from twinning, while an ordinary 

 slide prepared by Tomlinson exhibited broad albite twinning. 

 Thus the writer would voice a word of caution: All feldspars 

 may occur in thin section absolutely untwinned and hence the 

 presence or absence of twinning is not a reliable means of identi- 

 fication. 



POTASH-SODA-LIME FELDSPARS 



In Part I the writer maintained that all natural feldspars are sys- 

 tems of three components and should, when elaboration demands, be 

 recorded as such (cf . the composition of the feldspars here discussed) . 

 Professor A. N. Winchell has kindly called my attention to the fact 

 that Sabof reached that conclusion in 191 5. Sabot says (in an 

 abstract in translation) : 



^ George W. Hawes, "On the Determination of Feldspars in Thin Sections of 

 Rocks," U.S. Nat. Mus. Proc. 1882, IV, 134-36. 



'J. F. Kemp, New York State Mus. Bull. 138 (igio), 30. 



3 Specimen 969B. 



t R. Sabot, Compte Rendu des Seances de la Societe de Physique et de I'Histoire 

 Naturelle de Geneve, XXXV (1918), 72; XXXVII (1919), 51. 



