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* 



hardly possible to distinguish any variation from symmetry, and 

 such crosses might be taken as proof of a uniaxial character. But 

 on rotating 45^, as in the case of the nitre crystal, we lose the sym- 

 metrical form of the cross, which opens out, as represented in fig. 4. 

 This deportment of the cross may be observed when, owing to 

 the depth of color, it is not possible clearly to discern the rings. 



A method given by Miiller* also depends upon this deport- 



ment of the dark bars; it consists in rotating the plate of mica 

 when on the table of M. Noremberg's apparatus (the ray being 

 extinguished by the eye-piece); if the mica be biaxial the light 

 is alternately, for every 45^ of rotation, transmitted or shut off, 

 if uniaxial^ no change is perceived. The image of the cross and 

 rings by this method is so much extended and dimly defined, 

 thai the outlines cannot be traced by the eye, and a slight open- 

 ing-out of the cross is not so readily noticed, even by a change in 

 the intensity of the light, as when its image is contracted and 

 sharpened in outlitie by the action of lenses. 



In the following table are given the localities and color of 

 several micas hitherto generally considered uniaxial, but which, 

 on examination with my arrangement, have given evidence of 

 being optically biaxial. 



76. Greenwood Furnace, Orange Co., N.Y.; olive green; rhombic plates.f 



77. « « « « « second specimen. 

 79. ** " " " " third specimen. 

 86, Easton, Pa. ; a white silvery mica. 



88. Topsham, Maine ; fine crimson red. 



89. Mount Vesuvius ; dark green. 



101. " " « « Anotlier specimen ; clearly biaxial. 



105. ** " " ** A thicker plate of specimen 101; clearly biaxiaL 



102. ** " transparent, with delicate shade of green ; banded with bands 



of a deep green, meeting at angles of 60^. 



* Lehrbuch der Physik und Meteorologie, Braunschweig, 1844, p. 588. 



f Greenwood furnace is in the town of Monroe, and this is the Monroe mica, ana- 

 lyzed by von Kob'jll (Kastn. Arch., xii, 29, and Dana's Min., 3d edit., 360) and pro- 

 nounced by him on optical grounds to be uniaxial, it giving a symmetrical cross. It 

 afforded the chemical fomiula R^ Si + (Xl, Pe) Si, coiresponding to the oxygen ratio 

 for the protoxyds, peroxyds and silica 1:1:2. The exact ratio as worked out by 

 Rammelsberg is 10*4 : 9-8 : 20-78, while the Vesuvian mica gave lO'l : 10*0 ; 20-65, 

 and one from Bodenmais 10*25 : 10"96 : 21*23, The ratio is in fact the sreneral ratio 

 of the so-called '' hex^rgonar nucas or Biotites, In the Greenwood furnace mica, we 

 have evidence tlierefore that one at least of these Biotites is not hexagonal; and the 

 same we believe true for some of the "' hexagonal" Y esuvian micas. The impor- 

 tance of a revised crystallographic examination of these micas is hence obvious. The 

 mica of Greenwood furnace is of a blackish green color, and occur? usually in very 

 acute oblique prisms, often of very large size; angle of prism (M:M) 71*^ to 72°, it 

 being oblique from an acute edge, and not from an obtuse edge as in ordinary biaxial 

 mica. The angle between the base mid a lateral plane (P : M) is 66^-67° and 103° 

 to 114°, according to the writer's measurement. The replacement of the acute solid 

 angle reduces the form to a rhombohedron with a basal cleavage plane, and thus it 

 was taken by von KobelL Tlie faces of the crystals are so imperfect that the dis- 

 crepancies obtained are usually one degi-ee, on similar parts of the same crystaL 

 Another Monroe mica (Xo: 81 above) is the only other mica which could possibly 

 have been Kobell's, and tlmt too is proved by ilr. Blake to be biaxiaL j, d. d. 



i 



8 On distinguishing Biaxial and Uniaxial Micas. ■ 



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