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On distinguishing Biaxial and Uniaxial Micas. 



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lO.S. Mount Vesuyius ; from same specimen as 102. but thicker: biaxial 

 104. « 



142. 



" " from same specimen, still thicker ; gives good elliptical hnage, 



" " beautiful green, like No. 101, is in small hexagonal crystals 



and associated with Idocrase. 



111. Franklin Furnace, Sussex Co., N.J. ; deep copper red ; G. J. Brush to B. SQliman, Jr. 

 109. St. Jeron)e, Canada; coppery, reddish; angle, estimated .10^. 



114. Moors Slide, Ottawa, Canada; dark bottle green'; T. S. Hunt to B. Sillinian, Jr. 

 116. St. Lawrence Co., N. Y.; dark brownish green; opaque, over -062 nnn. thick. 



84. Hammond, St. Lawrence Co., N. Y. ; rich sniuky jellow ; from Saml. W. Johnson. 



^S. " « dark brownidi red ; black by reflected hght. 



106. Gouvemeur, K Y.; (boulder) dark brown. 

 81. Monroe, Orange Co., N. Y. ; dark green. 



^C>S. « « clark bottled-green ; from S. R. Horton to B. Silliman, Jr. 



107. Locality ? Cambridge Cabinet; rich brown; in beautiful rhombic plates; 



112. Locality ■ ? Cambridge Cabinet; deep green ; like Greenwood Furnace mica. 

 63. Moriah, Essex Co., N. Y.; tier}' red; Lederer Cabinet; angle measm*able. 



139. Warwick, Fa.; olive green (brownish); angle estimated over 10^; this much 



resembles the Vesuvius specimens, Nos. 102 and 103. 



With the exception of the dark micas mentioned in the follow- 

 ing table, I have not yet examined any that do not give evidence 

 of being optically ifaxial; and, it is probable, that these exceptions 

 would give the modifications of the cross, &c., if their dark color 

 did not make it impossible to observe them in plates sufficiently 

 thick. But if biaxial, the angle for the mica of Sussex, New 

 • Jersey, No. 115, must be very small. 



-? dark olive green; resembles K'o. 81. 

 ? Yale Coll. Cabinet ; color intensely green. 



98. Locality — 



83. Locaiity ■. ^ctic v^^i.^. v^c^^n.^^., ^y.^^^ »..v.^^wj ^ 



J.15. Sussex Co., N. Y. ; dark green ; opaque when in plates over '222 of 1 mm. thick. 



113. Locality ~ ? IST. Y. Lye. Nat. Hist. ; opaque in plates over "042 mm. thick 



148. Middletown, Ct. ; very dark green. 



146. N. Y. Island ; very dark olive-green; in granite veins traversing gneiss. 



147, " " " « « ti 



To Prof. B. Silliman, Jr., I am indebted for the free use of his 

 specimens for these investigations: the biaxial character of many 

 of them was suspected by him, when he first examined them, 

 as is mentioned in his communication in this Journal, vol X; No- 

 vember, 1850. 



In specimens of mica referable to the species Phlogopite, hav- 

 ing the inclination of the resultant axes between 5° and 20^, — 

 when the thickness is so much reduced that the systems of rings 

 around poles are not distinctly separated, the angular inclina- 

 tion of the axes cannot be determined in the ordinary manner. 

 Farther observations upon this point I reserve for a future com- 

 munication. 



Second Series, YoL XII, No. 34.— July, ISr! 



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