130 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 23^, 



wMte margarodite mica of the same locality, — ^plates of the white 

 mica being frequently met with containing large crystals of the 

 black mica, imbedded without any breach of continuity — and vice 

 versa. The angles of the crystalline plates of black mica are all 

 120° ; and, on examination by polarized light, it turned out to be 

 uniaxal. I examined with care the optical condition of the plates 

 of white mica which are continuous with the plates of black uniaxal 

 mica, and found that the plane of the optic axes of the white mica 

 was perpendicular to the common surface of the black and white 

 micas, and always contained the bounding line between the two 

 minerals. 



I found the angle between the optic axes of the white mica to 

 range, in different specimens, from 56° 30' to 71°. 



The black and white micas of Ballyellin occur in a coarse granite, 

 of which the other constituents are grey quartz and white orthoclase 

 in large crystals. 



In the neighbourhood of Ballyellin, at Scalloge Gap, between 

 Mount Leinster and Blackstairs, the black mica occurs in nests and 

 lenticular sheets, in a fine-grained granite, composed of white mica, 

 white felspar, and grey quartz. 



The following is the composition of the 



Black Mica of Ballyellin. 

 Per centage, 



SiKca 35-55 



Alumina 17-08 



Peroxide of Iron 23-70 



Lime 0-61 



Magnesia 3-07 



Soda 0-35 



Potash 9-45 



Protoxide of Iron 3-55 



Protoxide of Manganese . . 1-95 

 Loss by ignition 4-30 



Atoms. 



0-328 \ 



0-296 J 



0-021"^ 



0-153 



0-011 



0-201 



0-098 



0-054 



0-790 

 0-624 



VO-538 



0-477 



99-61 

 Ten grains of the mineral, acted on with muriatic acid, were found 

 to be completely decomposed, giving 3-52 grs. of slightly gelatinized 

 silex, much of which retained the skeleton form of the mica. 

 The result of the foregoing analysis, in atoms, is very accurately 



SiOg 9, 



^fi, 7, 



RO 6, 



differing from Soltmann's result by the addition of one atom to the 

 peroxides. If we compare directly the total quantity of oxygen in 

 all the bases with the oxygen of the silica, we see that they are 

 exactly equal ; so that the formula of this mica may be thus written : 



[f(3EO) + fR,OJSi03; 

 Or, as I would prefer stating it — 



[23 (3110) + 77 \ O3] + 100 Si03. 



