KO.2220. COLORED VARIETIES OF QUARTZ— WATSOS & BEARD. 563 



suggested by Emerson and Perry/ who state that "a state of strain 

 has probably produced the blue color." They say: "Most of this 

 quartz is blue, and this color appears also in the contact zones and 

 even in the secondary quartz that is found in fragments of schist 

 which are inclosed in the granite and which have been greatly altered 

 by it. The fractured grains of quartz show with polarized hght 

 the strongest undulatory extinction, which indicates a state of strain 

 has probably produced the color." The authors do not mention at 

 this place (p. 46) the occurrence in the quartz of rutile inclusions, 

 but that they do occur is shown in a previous statement, where the 

 contact effect of the Milford granite on schists is described. They 

 say: "These grains are often full of rutile needles, like true granite 

 quartz." ^ 



The blue quartz of the Virginia rutile area is generally character- 

 ized macroscopically by fractures and microscopically in thin sections 

 by undulatory extinction, but the senior writer attributes the blue 

 color of the mineral to the behavior of light on the minute rutile 

 inclusions and not to any state of strain. This view is strengthened 

 by the fact that heating does not destroy the blue color of the quartz. 



1 Bull. 311, U. S. Geol. Survey, 1907, p. 46. 

 « Idem, 1907, p. 32. 



