THE MINERALOGRAPHY OF THE FELDSPARS 363 



face. Whether there are two distinct sets or not is a question still 

 to be investigated. However, their classification into two groups, 

 depending upon their height, did result in two contoured surfaces 

 which are reasonably regular. These diagrams are offered with the 

 full appreciation that they leave much to be desired. Slight modi- 

 fications of some of the values were necessary in order to make them 

 "fit." The majority of the analyses of plagioclase lacked the deter- 

 mination of K2O. In these cases the writer assumed the per cent 

 of the potash component to be from 2 to 4 per cent. Larsen^ 

 has measured the indices of the plagioclase series and recorded the 



results as . This curve has been modified so as to agree 



more closely with the curves of Rosenbusch-Wiilfing,^ Wright,^ and 

 Tsuboi.'' It was also found that Dittler's^ gamma values of syn- 

 thetic oranitic feldspars, when lowered to the probable beta values 

 were too low and they were arbitrarily raised by adding from .003 

 to .005 to them. In a similar way it was found that Wright's^ 

 molecular percentage curve when translated into the more usable 

 weight percentage form was lower than those for most natural feld- 

 spars. Even the curves of Iddings'^ and of the Winchells^ seem to 

 give more consistent results on natural specimens, though there is 

 every reason to believe that Wright's data on synthetic material is 

 more accurate. The real point is that the average petrographer 

 deals with natural and not with artificial minerals. The cause of 

 these discrepancies may be explained by the lack of ''impurities" 

 such as magnesia and iron in the synthetic material. It is to be 

 regretted that because of the paucity of accurate determinations of 

 the physical constants of synthetic minerals we are compelled to 



' E. S. Larsen, Amer. Jour. Sci., XXVIII (1909). 



^ As given by Albert Johannsen, Essentials for the Microscopical Determination 

 of Rock-Forming Minerals and Rocks, 1922. 



3 Fred E. Wright, Am. Jour. Sci. (4), XXXVI (November, 1913). 



^ Given by Albert Johannsen. Tsuboi, Jour. Geol. Soc. Tokyo, XXVII (1920), in 

 Japanese. 



5 E. Dittler, Tsch. Min. Petro. Mitt., XXIX, 39 and 387. 



^ Fred E. Wright, Amer. Jour. Sci. (4), XXXVI (November, 1913), 540. 



' J. P. Iddings, Rock Minerals. 



^N. H. and A. N. Winchell, Elements of Optical Mineralogy, 1909, p. 200. 



