1858.] 



HArGHTON GEAJflTES OF rEELAND. 



129 



3. On the Black Mica of the Gea]S"ite of Led^stee and Donegai, ; 

 and its probable identity with Lepidomela]s:e. By the Rev. Samuel 

 Haughton, F.E.S., F.G.S., FeUow of Trinity CoUege, Dubhn, and 

 Professor of Geology in the University of Dublin. 



We owe our knowledge of Lepidomelane to a single analysis, made 

 by Soltmann, of a specimen from Petersberg, Wermland. He 

 describes it as occurring in an aggregate of minute black scales, in 

 small six-sided tables, with perfect basal cleavage — either hexagonal 

 or trimetric, easUy dissolved in muriatic or nitric acid, leaving a 

 skeleton of silica. 



Its composition is as follows — 



Lepidomelane. 







Per centage. 

 SiHca 37-40 . 



Atoms. 



0-831 

 . 0-2231.™ 



0-346 J ^ ^^^ 

 . 0-345 ^ 

 . 0-010 I 0-551 

 . 0-196 



3 



Peroxide of Iron 27-66 . 



Protoxide of Iron 12-43 . 



Tiirae and Magnesia .... 0-26 . 



Potash 9-20 . 



Water 0-60 



2 



2 



99-15 







This analysis gives very nearly, in atom 

 SiOg .... 3 .. 

 RA .... 2 .. 

 RO .... 2 



LS 



. 9 

 .. 6 

 .. 6 





Soltmann does not appear to have ascertained whether his mica 

 were uniaxal or biaxal, as he is in doubt whether it is trimetric or 

 hexagonal. 



In my first paper* on the Granites of Leinster, I have mentioned 

 the black mica which is found accompanying the white margarodite 

 of the Leinster granite, in small flakes, and noticed the curious fact 

 that these flakes are sometimes physically imbedded in the plates of 

 white mica, without injuring their fissility or lustre, but always 

 effectiag a reduction of about 20° in the angle between the optic 

 axes of the latter. 



Since the publication of that paper, I have ascertained the 

 existence, in large quantities, of a similar black mica in the 

 Co. Donegal, both in granite and gneiss ; and also, through the 

 kindness of Mr. Cotton, C.E., obtained specimens of the black mica 

 of the Leinster granite, found in large crystals in the cuttings of 

 the Bagenalstown and Wexford Railway, at BaUyeUin, in the 

 Co. Carlow. 



The black mica of BallyeUin is found in crystalline plates, 2 in. 

 by J in. ; it is not only associated with, but physically united to, the 



* Quart. Joum. Geol. See, toI. xii. p. 175. 



VOL. XV. PART I. 



