1858.] JAMIESON LIAS, SCOTLAND. 131 



Soltmann's lepidomelane, written in the same way, gives us, either 



[i(3R0)+fR,03]Si03t; 

 or, 



[25 (3E0) + 75 R, O3] + HO Si03. 

 A black mica, similar to that of BaUyellin, Co. Carlow, occurs in 

 the Poison Glen, leading to the Pass of Ballygihen, in the Co. 

 Donegal. It is uniaxal and soluble in muiiatic acid. It occurs in 

 granite, in |-inch plates, accumulated, at times, to |-inch thickness. 

 The following is its analysis. 



Black Mica of Ballygihen. 



Per centage. Atoms. 



Silica 36-20 . . 0-804 



Alumina 15-95 . . 0-307 1 p. ^ .„, 



Peroxide of Iron 27-19 .. 0-340 [ ^ 



0-515 



Lime 0-50 . . 0-018^ 



Magnesia 5-00 .. 0-250 



Soda 0-16 . . 0-005 



Potash 8-65 . . 0-184 



Protoxide of Iron 0-64 . . 0-017 



Protoxide of Manganese . . 1-50 . . 0-041 



Loss by ignition 3-90 . . ' 0-433 



99-69 

 If the result of this analysis be written as the former one, we obtain 

 for its formula 



[21 (3R0) + 79 R,03] + 98 Si03. 

 If written according to Dana's method, this would be 



[i-(3EO)+|EA]Si03. 



The black mica of Donegal is certainly identical with the black 

 mica of Carlow and Leinster, and probably the same as the black 

 mica described as " lepidomelane" by Soltmann. In the Ballygihen 

 district, after passing the Gap, the mica of the granite becomes 

 white mica, biaxal, in large plates, with angles between the axes 

 ranging from 62° 10' to 65° 10'. It is not associated with the 

 black mica, however, as is the case in the Leinster granites. 



4. On an Outlteb, of Lias in Abekdeenshtre. By T. F. Jamieson, Esq. 



(In a Letter to Sir Roderick I. Murchison, V.P.G.S., &c. *) 



In a cutting of the Banff, Macduff, and Turriff Extension Railway, 

 about four miles to the north of the latter town, close beside the 

 turnpike-road at the Plaidy toll-bar, there has been exposed a mass 

 of a very tenacious clay, blue iu colour, and of homogeneous texture, 

 which contains many Ammonites, Belemnites, and other fossils charac- 

 teristic of the Lias, such as Gryphcea, Plagiostoma, and possibly others ; 



* Dated Ellon, Aberdeenshire, March 17, 1858. 



k2 



