116 The Rev. J. A. Galbraith on the Felspars occuiTing in 



Organic matter 23"30 



Carbonic acid 7*40 



Muriatic acid 17-99 



Sulphuric acid 6"34 



Silica 3-81 



Lime 3-03 



Magnesia 0*85 



Potash 2-86 



Soda 30-48 



Loss 3-94 



100-00 



In the discussion of this analj'sis, he considered that the 

 muriatic acid present existed in the form of common salt, de- 

 rived from the vapours of the adjoining sea, which leaves of the 

 soda 15-20 to be disposed of; he then allocated all the potash 

 to the sulphuric acid, of which it saturates 2'43, leaving 3'91, 

 which he supposed to be combined with soda, leaving finally of 

 this alkali 12-16 to exist in combination with silica and the 

 organic acids. This large amount of silicate of soda in the waters 

 heattributed to the decomposition of the felspathic materials of the 

 granite ; and in conclusion, drew the inference that the felspar 

 of the Dublin granites is, upon the whole, a soda or albitic fel- 

 spar, although in particular localities orthose or potash felspar 

 aay be found. This inference he also stated was verified by a 

 great number of analyses of specimens of granite taken from 

 various parts of the range which extends from Dublin into the 

 county Wicklow ; in all these specimens both alkalies were found, 

 but the soda in such excess, as to indicate that the potash should 

 be referred to the mica which the granite contained, and the 

 soda allocated to the felspar. 



By this interesting communication to the L-ish Academy two 

 important questions were raised; viz. 1, whether the felspar 

 which enters into the constitution of the granite is an albite ; 

 2, whether orthose or potash felspar is a rare mineral, to be 

 found only in special localities, such as Killiney. 



In a communication made at a subsequent meeting by Dr. 

 Apjohn, the first of these questions was fully disposed of, as 

 from the analyses which he furnished of specimens of felspars 

 taken from different localities, potash was invariably found in 

 notable quantity, constituting at least one-half of the alkalies 

 present, showing that the felspar could not in any sense be con- 

 sidered as albite or soda felspar. 



To the second of these questions I directed my attention, and 

 having procured specimens of felspar, taken from seven localities 



