596 Mr. J. B. Jukes and the Rev. Samuel Haughton on iJm 



which gives exactly, — 



[i(3R0) + ^(R.Oj)] SiOai + xHO, (4) 



the formula of Daua ; and proves its identity with the mineral described by 

 him as Margarodite. 



6. Blach Mica. — In addition to the white pearl mica, or Margarodite, just 

 described, and which is always found in the granite of the Leinster axis, a 

 variety of black mica is very commonly found in small grains, diffused through 

 the mass of the granite in particular localities, as at Dalkey, Glendalough, and 

 other places. 



This black mica is occasionally found in nests, and much more rarely in 

 long crystals, associated Avith the white mica. 



It was discovered in great abundance by my friend and former pupil, Mr. 

 Cotton, C. E., on the cutting of the Bagnalstown and Wexford Railway at 

 BallyelUn, in the county of Carlow. The granite of this locality consists of 

 white orthoclase, quartz, white and black mica, half and half, in long crystals, 

 lying side by side and fortuing physically the same sheet of mineral, but sepa- 

 rated from each other by a well-defined straight line. This line coincides with 

 the trace of the plane of the optic axes of the white mica. The black mica 

 proved on examination to be uniaxal, its optic axis bisecting the angle between 

 the optic axes of the white or biaxal mica. I examined carefully the angle 

 between the optic axes of the white mica associated with the black mica, and 

 found it to vary in difierent specimens from — 



56° 30' to 71° 0'. 



The crystals of black mica are 2 in. by ^ in., and they fit into the crystals 

 of white mica readily, in consequence of the angles of the primary lozenge of 

 the latter being 60° and 120° ; the regular hexagonal base of the black mica 

 crystal fitting, without physical dislocation, into the angles of the white lozenges. 

 At Scalloge Gap, between Mount Leinster and Blackstairs, this black mica is 

 found in nests and lenticular sheets in the fine-grained gneissose granite, which 

 is itself composed of gray quartz, white felspar, and white mica, being a ter- 

 nary granite, except where the occurrence of the black mica renders it 

 quaternary. 



