374 HAROLD L. ALLING 



single thermal-diagram to illustrate the crystallization of the system. 

 Consequently a series of diagrams, one for each mode of origin, is 

 necessary. (Fig. i.) 



2. The proof of the dimorphism of KjVlSijOg and NaAlSiaOs is 

 exceedingly difficult to obtain. The "peg models" of the specific 

 gravities, indices of refraction and extinction angles, suggest two 

 series, one the orthoclase-barbierite and the other the microcline- 

 albite series. No entirely satisfactory series of diagrams that show 

 the dimorphism of both components can be made at the present time. 



3. The existence of nephelite in solid solution in feldspars is more 

 common than is supposed. When present it profoundly affects the 

 physical properties, so that determination by microscopic means 

 alone is very uncertain. It may be that the specimen from which 

 barbierite was named is a nephelite-bearing albite. 



4. Granting dimorphism, many rapidly cooled feldspars, such as 

 sanidines and orthoclases, possibly may be mixtures of two distinct 

 series: the so-called "monoclinic" series and the "triclinic" feld- 

 spars. This sometimes renders determination a difficult matter. 



5. Twinning is caused in part by pressure. It may be caused by 

 stress set up by the change in volume accompanying inversion. 

 "Scotch-plaid" twinning is not a reliable criterion for microcline. 

 The writer agrees with Hawes that plagioclase is not always striated, 

 and would put the reader on his guard. 



6. The decomposition of the unstable ferric feldspar, KFeSijOg, 

 may well account for pink microclines and other aventurine feld- 

 spars. 



7. Zonal-grown crystals are perfectly normal in rocks cooled at 

 the proper rate. Too rapid cooling prevents zonals while slow 

 cooling allows them to become homogeneous through reaction 

 between themselves and the still unfrozen liquid of the magma. 



8. The main contributions are the diagrams giving the specific 

 gravities, indices of refraction, and the extinction angles of the 

 potash-soda-lime feldspars. 



ACKNOWLEDGMENTS 



The writer is under obligation to many for helpful suggestions, 

 constructive criticism, assistance, and encouragement. Among 

 these he takes pleasure in expressing his indebtedness to Professors 



