296 HAROLD L. ALLING 



K-, Na-, and Ca- feldspars. If the sum of the three feldspars 

 was appreciably below or above one hundred, the analysis was con- 

 sidered inferior, or the specimen abnormal, and it was consequently 

 omitted. Thus there has been some selection of the available data 

 by elimination. Upon a triangular base each recast analysis was 

 located according to the composition and indicated by a point. On 

 each point a pin, whose length (height) represented the value of the 

 specific gravity, was erected. In this manner a "peg model" was 

 constructed. It is very evident from the model, or three-dimen- 

 sional graph, that there are two groups, one set of pins being longer, 

 standing higher, than the other group. As the majority of the pins 

 giving the values of specific gravities of feldspars called in the litera- 

 ture "orthoclase" are longer than those called "microcline," the 

 obvious interpretation is that orthoclase, soda orthoclase, "mono- 

 cUnic" anorthoclase, as well as orthoclasic phases in perthitic feld- 

 spars, are heavier than the microclinic equivalents of these. This 

 idea was suggested in Part I (Fig. 6, p. 228) but the writer, through 

 the construction of the peg model, feels that such an interpretation 

 can now be entertained with more confidence. 



A single and constant physical or chemical difference between or- 

 thoclase and microcline is sufficient to establish a case of dimorphism. 

 Apparently here the specific gravities furnish the desired evidence. 



The relation of specific gravity to composition is taken up in 



detail later. 



Albite and "harhierite.'' — Barbier,^ Prost,^ Clarke,^ Shaller,^ and 

 others have reached the conclusion that there is a monocHnic modi- 

 fication of the soda component which the latter named barbierite. 

 A chemical analysis of the "barbierite" from Kragero, Norway, is as 



follows: siO^ 67.00 



AI2O3 19-12 



CaO 0.78 . 



K,0 I. IS 



Na^O 11-74 



99-79 



^ Barbier and Prost, "Sur I'existence d'un feldspath sodique monoclinic isomorphe 

 de I'orthoclase," Bull. Soc. Chem., Ill (1908), 894. 



2 F. W. Clarke, U.S. Geol. Surv. Bull. 588, 35. 



3 W. T. Shaller, Bull. Soc. Min., XXXIII (1910), 320; Zeitschr.f. KrysL, L (loii), 

 347; Jour. Wash. Acad. ScL, I (1911), i77; U.S. Geol. Surv. Bull. 509 (1912), p. 40; 

 Barbier and Gonnard, Bull. Soc. Min., XXXIII (1910),. 81. 



