598 Mr. J. Beete Jukes and the Eev. Samuel Haughton on the 



This formula represents the analysis pretty well, but is not so simple, nor so 

 exact as (5), which I prefer. 



The black mica of Ballyellin has been identified by me with the black 

 mica of the granite of Donegal, and is unquestionably a distinct and well- 

 marked mineral species. In most respects it appears to be identical with the 

 Lepidomelane of Soltmann, which is found at Petersberg in Wermland, in small 

 aggregations of minute scales, — a mode of occurrence very common with the 

 black mica of Kingstown, Scalloge Gap, and the black mica of Donegal. 

 Lepidomelane is also soluble in muriatic acid, and is probably uniaxal. Its 

 formula, as analyzed by Soltmann, is — 



3 HO, SiOs + 3 (R2O3, SiOs). (7) 



This corresponds with my formula (5), and I believe the two minerals to be the 

 same ; but it is evident that the Irish specimens are much finer and better 

 defined as to their optical properties. 



Having thus established the existence of four component minerals, viz.. 

 Quartz, Orthoclase, Margarodite, and Lepidomelane, it now remains to examine 

 the composition of the granite as a whole, with a view to determine the relative 

 proportions of each, or the mineralogical analysis of the rock, and also the 

 chemical composition of the felspathic paste remaining after the deduction of 

 the well-defined minerals. 



7. Granite Rock. — We have now to consider the chemical and mineralogical 

 composition of the granite rock considered as a whole. To the eye it presents 

 a very uniform appearance throughout the whole granitic axis from Rockabill 

 on the north-east, to Poulmounty on the south-west. Quartz, white felspar, 

 and white mica, are always present in distinct grains, and very frequently 

 black mica also, though in a smaller proportion than the white. The following 

 Table contains the analyses of the granite rock, taken as a whole, without 

 separating any of its constituent minerals : — 



