574 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



Chabacteeistic shapes of the eutile inclusions in mica. 

 Greatly maonified. 



accompanying figure). Numerous angular measurements were made, 

 but the values were not constant, due doubtless to the restricted 

 quarters of growth, so that the form is no absolute criterion for iden- 

 tification. The mineral 

 is brightly and delicately 

 colored, showing shades of 

 green, purple, blue, red, 

 and yellow, with often a 

 gradation in colors along 

 the length of a single 

 crystal. The pleochroism 

 is usually weak, but oc- 

 casionally strong: c = dark 

 green to brown; n = pale 

 gray. The extinction is 

 parallel and the crystals 

 positively elongated. Ab- 

 sorption IS c>a, which corresponds to £>co; birefringence high. The 

 index of refraction, determined by immersion,*^ is considerably greater 

 than 1.82. 



These features and the resistance to chemical attack eliminate 

 other possibilities and concur in identifying the mineral as rutile. 



Analogous inclusions in mica have been described from several 

 localities, but the conclusions regarding their nature are diverse. The 

 much discussed inclusions causing asterism in mica from South Bur- 

 gess, Canada, were described and figured in detail by Rose,^ who 

 originally considered these to be cyanite, but later concurred in the 

 opinion of De Cloizeaux that they represented an uniaxial mica. An 

 examination of specimens from this locality, and a consideration of 

 Rose's abundant figures, leaves no doubt that his inclusions are also 

 rutile. 



Tschermak,'' in 1878, studied the needles causing asterism in phlo- 

 gopite from Perth, Canada, and from their refraction and form con- 

 cluded that they could not be a mica, but was unable to determine 

 their real nature. 



Sanberger^ in 1881 noted rutile needles in a dark mica, altering 

 to chlorite, from Bordenmais, and in the following year'^ described a 



a The writer is indebted to Dr. F. E. Wright, of the Carnegie Geophysical Laboratory, 

 for kindly making this determination. 



^G. Rose. Ueber die regelmassigen Verwachsungen der verschiedenen Glimmer- 

 arten unter einander sowie mit Pennin und Eisenglanz. Monatsb. kon. preuss. Acad. 

 Berlin, 1869, pp. 339-362. 



cG. Tschermak. Die Glimmergruppe. Sitzungb. Akad. Wiss. Wien, vol. 76, 

 1877, p. 125; Zeitschr. f. Kryst., vol. 2, 1878, pp. 14, 15. 



dF. Sanberger. [Note.] Neues Jahrb. Min. Geol. Pal., vol. 1, 1881, pp. 258, 259. 



«F. Sanberger. Ueber Rutile in Phlogopite, etc. Neues Jahrb. Min. Geol. Pal., 

 vol. 2, 1882, pp. 192. 193. 



