abstracts: petrology • 525 



tions be plotted in such a form that their changes can be represented 

 by straight Hnes. Eight plates, drawn on these principles, are included, 

 and furnish solutions for the following equations : 



sin i = n sin R, sin- / = n- sin- y, = sin ??i sin ??2, 



y — a 



1^ j_ 



^~ y' 



, cot A = sin B ■ cot C, and 



a- ~ 13- . 



F. E. W. 



PETROLOGY.^ — A graphical plot for use in the microscopical determi- 

 nation of the plagioclase feldspars. Fred. Eugene Wright. Am. 

 Jour. Sci. (4), 36: 540-542. 1913. 

 On this plot the changes in the optical properties of the plagioclase 

 feldspars with chemical composition are indicated by a set of curves, 

 the purpose being to furnish the petrologist, in convenient form and 

 on a single sheet, all the constants essential for the accurate determina- 

 tion of the plagioclase feldspars in thin rock sections. A new set of 

 curves for the extinction angles on sections showing symmetrical carls- 

 bad-albite twinning is included, the values having been derived graph- 

 ically from the best available measurements on plagioclase feldspars. 



PETROLOGY. — Oblique illumination i7i petrographic microscope work. 

 Fred Eugene Wrioht. Am. Jour. Sci. (4), 35: 63-82. 1913. 

 The study of interference phenomena resulting from oblique illumina- 

 tion between crossed nicols enables the observer to determine many 

 optical features in a given mineral plate. These phenomena are iden- 

 tical, so far as interference colors go, with the phenomena obtained in 

 interference figures from the same plate in convergent polarized light. 

 The study of mineral plates by the method of oblique illumination is of 

 value because it impresses the mind of the observer with the interde- 

 pendence of optical and crystallographic properties. It is, however, 

 highly important that the observer realize the essential agreement be- 

 tween the phenomena observed in oblique illumination and those seen 

 on interference figures in convergent polarized light. In the inter- 

 ference figures the interference color phenomena are seen at a glance, 

 and if they be studied with reference to the position of the mineral 



