Lower Palceozoic Bocks of the South-East of Ireland. 



b°i1 



The following analysis gives the chemical composition of the black mica 

 of Ballyellin : — 



Table V. 

 Black Mica of Ballyellin. 



Silica, 



Alumina, 



Peroxide of iron, . . 



Lime, 



Magnesia, 



Potash, 



Soda, 



Protoxide of iron, . 

 Protoxide of Manganese, 

 Water, 



Total, 



Per-centage. 



35 '55 

 17-08 

 23-70 

 0-61 

 3-07 

 9-45 

 0-35 

 3-55 

 1-95 

 4-30 



99-61 



Atoms. 



790 



328- 



296. 



021 



153 



201 



Oil 



098 



054 



477' 



024 



^538 



Oxygen 

 ratio. 



790 

 624 



179 



The preceding results -^vere obtained by fluxing with carbonates of soda 

 and potash, and with carbonate of lime and sal ammoniac ; and afterwards, on 

 acting upon lOgrs. with hydrochloric acid, it was found to be completely 

 decomposed, giving 35-20 per cent of gelatinous silex. The black mica of 

 Ballyellin is, — 



1. Uniaxal. 



2. Decomposable by muriatic acid. 



From the oxygen ratios of this mica it is easy to deduce the following 

 formula — 



3R0r 



(5) 



There is no simple relation between the peroxides and protoxides in this 

 analysis ; the following is the nearest approach to a formula, founded on their 

 ratio, that I can make — 



VOL. XXIII. 



2 [3R0, SiOs] + 7 (RjOa, SiOa) + 5 HO. 

 4i 



(6) 



