Walker — Crystal Symmetry of the Mica Group. 201 



Phlogopites. 

 The measurements were made on the stage of an ordinary 

 petrological microscope and may be considered accurate to 

 degrees but not to fractions of a degree. The majority of the 

 specimens examined were cleavage sheets, either without crys- 

 tal edges or with edges which were too rough for exact 

 measurement. All those with sharp crystal edges showed that 

 the edge 001 a 010 is parallel to one of the rays of the percussion 

 figures, so that, where good edges were absent the divergence 

 of the optical plane from the commonly accepted position 

 was measured against that ray of the percussion figure which 

 was nearest to being parallel to it. The divergence for any 

 one specimen varied with different sheets and for different 

 parts of the same sheet. Cleavage fragments about half a 

 millimeter thick often showed no divergence whatever, but 

 when split to, say a fourth of that thickness, very considerable 

 divergence was noted, some thin cleavage fragments from one 

 and the same crystal showing very much more than others of 

 the same thickness. This, though apparently true in most 

 cases, is particularly plain in the case of small amber-colored 

 transparent crystals from crystalline limestone from Upper 

 Burma, which at a thickness of l mm generally show no diver- 

 gence, but thin plates, about a tenth of that thickness, show 

 a maximum divergence of 11° 30'. The position of the optical 

 plane on one of these Burmese crystals is shown in fig. 1, p. 204. 

 The maximum observed divergences on phlogopites are shown 

 below. 



Maximum 

 observed. 



Locality. Divergence. Angle x*- Material used. 

 Canada, 



Temple ton, P. Q. 9° Cleavage fragments. 



South Burgess, Ont _ 13 61° 53' 



United States. 



Winchester, Pa. 17 .... " " 



De Kalb, St. Lawrence Co., N. T. 6 20' 61 30 



Pierrepont, " " 4 20 



Rossie, " " 2 30 60 50 " " 



Macomb, " " 3 30 63 30 



Hammond, " " 10 30 Crystals. 



Pope's Mill, Jefferson Co., N. Y. _ 7 30 63 Cleavage fragments. 



Muscalonge Lake, N. T 8 30 Crystals. 



Finland. 



Pargas 14° 



„ 8 



India. 



Upper Burma 11°30' 60° 20' 



Travancore _____ - 14 63 14 Cleavage fragmeuts. 



Merzapur 3 59 20 " " 



Ceylon. 



Radigara 13° 63° 28' " " 



! 15 62 27 " " 



* The angle of the percussion figure opposite the clinopinacoidal edge.— Vide 

 this Journal, ii, 5, 1896. 



