On the Potash and Soda Felspars of the Dublin Mountains. 511 



or ascertain precisely to what system they lead ; but the discovery 

 by Descloiseaux and Brooke, tbat leucophane shows distinctly 

 two systems of rings* when examined by polarized light in a 

 direction perpendicular to the face of principal cleavage (c), ren- 

 ders the reading of this crystal a matter of less uncertainty ; and 

 I am inclined to consider tbat leucophane belongs to the pris- 

 matic system, having a right rectangular prism for its primitive 

 form. This view supposes that a6 = 90°, as well as cb and ac. 



I have observed three cleavages of leucophane. One parallel 

 to c is the most readily obtained; a second is apparently parallel 

 to i ; and tbe tbird, t, truncates the edge between a and b, at an 

 angle of about 135° with each of those faces. The tbree cleavage 

 planes are in one zone, ci being = 126° 25', and c/ = 90°. 



LXX. On an Analysis of the Potash and Soda Felspars of one of 

 the Dublin Mountains. By Francis M. Jennings, M.R.I. A., 

 F.G.S.f 



AT the January meeting of the Geological Society, Dublin, 

 1855, a discussion took place on a paper, by the Rev. 

 Joseph Galbraith, Professor of Natural and Experimental Phi- 

 losophy in the University of Dublin, entitled " The Composition 

 of the Felspars of the Granite of the Dublin and Wicklow Moun- 

 tains," which appeared in the January Number of the Philoso- 

 phical Magazine. 



I regret much that the reverend essayist was not present, as 

 all those who spoke, except myself, appeared to agree in the 

 opinion expressed in the paper, namely that the granite of the 

 Dublin and Wicklow Mountains was a potash granite, the ana- 

 lyses of the felspars by the author appearing conclusive to them. 



I expressed doubts, not as to the correctness of the analyses, 

 but as to the opinions of those who, from the analyses of the 

 felspars, drew such inferences. It appeared to me a great defect 

 in a paper drawing such important conclusions, that portions of 

 the specimens which were analysed were not exhibited to the 

 meeting, for it to judge if they were selected with that required 

 discretion and care that would justify him in drawing from them 

 the conclusions he had come to. 



To ascertain the relative proportions of potash and soda of the 

 felspar of those mountains, and thence to infer the composition 

 of the granite, I consider it necessary that the specimens to be 

 analysed should be selected so as to represent as closely as pos- 



* According to M. Descloiseaux, exactly resembling those exhibited 1>_\ 

 topaz. 



t Communicated by the Author ; having been read at the meeting of 

 the Geological Society of Dublin, May '.), 1855. 



