682 ABSTRACTS OF PAPERS 



metrical conceptions enumerated by Doctor Wright. For example, the average 

 student appears to grasp the reasons for the white light phenomena in a sec- 

 tion perpendicular to an optic ere more readily by a consideration of the ray 

 paths or the ware-surface normals. Again, the teacher's task involves the de- 

 velopment of tbe use of the literature by students. The student is offered the 

 dilemma of using the Euglish texts with a. b. z (or X, Y. and Z) or the 

 French texts with an indicatrix oriented in terms of the unfamiliar crystallo- 

 graphic notation of the French. For this reason it seems more advisable to 

 retain several of the geometrical conceptions in teaching in spite of a possible 

 initial confusion on the part of the student. 



IGNEOUS COMPLEX OF HIGH TITAXIUM-PHOSPHORUS-BEARIXG ROCKS OF 

 AMHERST-XELSOX COUNTIES, VIRGINIA 



BY THOMAS L. WATS0X AXD STEPHEX TABEB 



{Abstract) 



The area described is located along the western edge of the Piedmont Plateau 

 and near the southeastern slope of the main Blue Ridge, within the foothills, 

 about half way between Charlottesville and Lynchburg. Virginia. The rocks of 

 the area are igneous in origin, are intensely metamorphosed, and are divisible 

 into a number of types which show marked kinship chiefly in a combination of 

 particular mineralogical and chemical characters. The principal rock types 

 include (1) quartz-monozite gneiss. (2i syenite. C3) gabbro. (4) nelsonite. and 

 1 5 1 diabase. In general the rocks are characterized by the prominence of 

 apatite and the titanium minerals — rutile. ilmenite. and in places titaniferous 

 magnetite : by a peculiar blue opalescent quartz, and by pyroxene (chiefly hy- 

 persthene) or secondary hornblende derived from pyroxene as the dominant 

 femic silicate mineral. 



Tbe paper describes the general geology and petrology of the area in consid- 

 erable detail, and the relationships of the rock types are discussed on the basis 

 of microscopic study of thin sections and complete chemical analyses. The 

 norms of the rocks are computed and their positions in the quantitative system 

 of classification shown. Several new rock types are described and magmatic 

 names suggested for them. 



PETROLOGY OF A SERIES OF XEPHELIXE SYEXITE. CAMPTOXITE, MOXCHI- 

 QUITE, AXD DIABASE DIKES IX MIDDLE SHEXAXDOAE VALLEY. YIRGIXIA 



BY THOMAS L. WATSOX AXD JUSTUS H. CLUXE 



(Abstract) 



This paper embodies a petrologic study of a series of igneous dikes occurring 

 in Augusta, Rockbridge, and Rockingham counties. Virginia, which form a part 

 of tbe middle Shenandoah Valley. The rook types represented are diabase, in- 

 cluding both normal and olivinic forms, nephelire syenite, camptonite. and 

 monchiquite. Occurrences of diabase have previously been noted and described 

 in middle western Virginia by the Campbells. Fontaine. Darton. and Watson, 

 but recent investigations by the present writers have revealed numerous other 

 dikes of diabase in various parts of the three counties mentioned above. 



