244 PROCEEDINGS OP THE ACADEMY OF [1880. 



THE OPTICAL CHARACTERS OF SOME MICAS. 

 BY HENRY CARVILL LEWIS. 



For the determination of the true characters of the micas — a 

 class of minerals rapidl}" gaining in importance — a knowledge of 

 their optical characters is almost as necessarj^ as is that of their 

 chemical composition. The optical is certainly the most ready 

 method of determination. The investigation here recorded is but 

 a partial one, and it is hoped that in the future it may be extended 

 so as to include most of the American micaceous minerals. The 

 measurements have been made for the most part upon minerals 

 which have never been optically examined, and are chiefly Ameri- 

 can. A few foreign species have been introduced for comparison. 

 The micas examined are largely those in the collection of the 

 Academy. Others were either in the writer's collection or have 

 been kindly given him by friends. The source from which each 

 specimen has been obtained is noted in the tables given below. 



The polariscope used was made by Queen & Co., of this cit}', 

 and was described before this Section at its meeting last Maj-. 

 It reads to within 30'. The figures given below represent the 

 mean apparent optic-axial angular divergence for white light. As 

 the angle is somewhat different in different specimens and some- 

 times CA'en in different portions of the same plate, the figures must 

 be regarded as only approximate. In each case they represent 

 a mean of a number of separate measurements, and collectively' 

 are the result of over 1600 such measurements. 



Phlogopite. 



1. Sussex Co., N. Y. Hexagonal crystals, 3'ellow, 



transparent. (Acad. Nat. Sci.) 6°. 



2. Burgess, Oiit., Can. Clear brown. (A. N. S.) 6°45'. 



3. N. Shore of Rideau Lake, Burgess, Can. Angle 



varies in same piece. Clearbrown. (J.Willcox.) 6°-12° 



4. Hammond, St. Lawrence Co., N. Y. Clear 3'ellow. 



Hypei'bolas closer in the centre than thev are 

 near the edges of the crjj-stals. Crystals are 

 sometimes uniaxial in the centre and biaxial at 

 each end, while the plane of the optic axes at 



