582 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 53. 



From a chemical and microscopical examination into the cause of 

 the blue color of quartz in Nelson County, Virginia, Robertson* 

 concluded that "in view of the color of some of the varieties of 

 titanic oxide, when seen by reflected light, it appears possible that 

 the partial reflection of light by the surfaces of these microscopic 

 crystals occasions the color m question, or the latter may be in a 

 measure due to the interference of light occasioned by these crystals," 



Blue quartz occurs as a constituent of the quartz members of the 

 charnockite rock series of India. On microscopic examination 

 Holland found the blue quartz to be crowded with minute hairlike 

 inclusions, presumably rutde, arranged with crystallographic regu- 

 larity. Concerning the cause of the blue color of the quartz he 

 says: "I conclude that the hairlike inclusions, to which probably 

 the blue color of the quartz is due, are arranged with crystallographic 

 regularity." 3 



In his studies of the quartz-feldspar porphyry in which are 

 developed phenocrysts of sky-blue opalescent quartz from Llano 

 County, Texas, Iddings' states that the blue color of the quartz 

 "is undoubtedly due to the reflections of blue hght waves from the 

 minute colorless prisms, whose width is a fraction of the length of 

 light waves. It is similar to the blue color of the sky. It is prob- 

 able, however, that there is also blue light produced by interference 

 of the light reflected from both sides of the minute tabular crystals 

 whose width is also of the order of a fraction of a hght-wave length; 

 so that both kinds of phenomena occur within these quartzes." 



In the comagmatic area of titanium-bearing rocks of Amherst and 

 Nelson Counties, Virginia, deep blue opalescent quartz both in 

 minute grains and in large masses is an abundant constituent. The 

 blue color of the quartz is pronoimced even in tliui section, and 

 pressure effects are exhibited chiefly in granulation, fractures, and 

 wavy extinction. The most pronounced microscopic character of 

 the quartz is the presence of abimdant closely crowded, minute 

 hairlike inclusions of rutile, which are distributed rather unevenly 

 through each grain. After a detailed study of the quartz from this 

 area by Watson and Taber,* the following conclusion as to color was 

 stated: "It seems probable that the blue color of the quartz char- 

 acterizing the rocks of the Amherst-Nelson Counties area is to be 

 attributed to the multitude of hairlike inclusions as explained by 

 Robertson, Iddings, and Holland." 



A different cause of the color of the constantly present blue quartz 

 in the MiKord granite of Massachusetts and Rhode Island has been 



> The Virginias, 1885, vol. 6, pp. 2-.3. 



« Memoirs, Geol. Surv. of India, 1900, vol. 28, pt. 2, pp. 138-139. 



» Journ. ol Geolosy, 190t, vol. 12, p. 227. 



< Bull. Ill-A, Virginia Geol. Survey, 1913, pp. 214-215. 



