W. T. Schaller — Dumortierite. 213 



The imperfect prismatic cleavage is present and divides the sec- 

 tion up into a multitude of irregular bodies. Occasionally a 

 short crack is observed which is parallel to the brachypinacoid, 

 and it may be that the mineral possesses an imperfect inter- 

 rupted brachypinacoidal cleavage. 



The prismatic cleavage was found to be parallel to the prism 

 (210). A large number of measurements gave as the angle 

 (010) : cleavage cracks 62°-67°, averaging 64°-65°. The angle 

 (010) : (120) is 65° 23'. The cleavage cracks are irregular and 

 it was difficult to obtain any accurate measurements, but the 

 ones made are sufficient to determine the cleavage. They were 

 measured against the trace of the optic axial plane (= (010)). 



Polysynthetic twinning lamellae were observed a number of 

 times and in one particular section (the same on which the 

 prismatic cleavage was measured) were determinable to a cer- 

 tain extent. The alternate lamellae extinguished together, the 

 difference in angle of extinction of two adjacent lamellae being 

 31°. The line joining these lamellae is parallel to the cleavage 

 or to the prism (210). 



The pleochroism of the mineral is beautiful, especially if 

 the section be not too thin, c = a is colorless, B = o is color- 

 less to very faint pink, a — c is deep red purple. JSTone of the 

 sections entirely possess the ordinary blue pleochroism. In 

 some slides, however, there are certain small areas of varied 

 and irregular shape which do show the ordinary pleochroism. 

 These small blue areas in the rich red-purple background make 

 a most beautiful combination. On some sections a large area 

 will have a faint bluish purple color, as if intermediate between 

 the deep blue and the red-purple. 



The axial angle on the California dumortierite is small. The 

 following measurements were made with the microscope and 

 micrometer ocular and represent only approximate values. 



2E £l - = 33° 

 <2E Na = 37° 

 2E Cu = 42° 



The dispersion is p <] v. 



The dumortierite from Skamania Co., Washington, was first 

 described by Ford.* Mr. Brereton, of Woodstock, Oregon, 

 kindly sent the writer some specimens on which the following 

 description is based. 



The rock is a light-colored fine-grained one, composed of 

 about equal parts of andalusite, quartz and muscovite (deter- 

 mined in thin section). 



The dumortierite occurs in small spherulites scattered 



throughout the rock and occasionally bunched together to form 



a large patch of blue. On an average they reach a diameter of 



from \ to l mm . Their shape, while in general circular, is often 



* This Journal (4), xiv, 426. 



