380 



From these results, and the assumption that the alkalies 

 are derived exclusively from the disintegration of the adjacent 

 granite, Sir Robert Kane went on to infer "that the felspar 

 of our Dublin granite is, on the whole, a soda or albitic felspar, 

 although, in particular spots, orthose or potash felspar may be 

 found." And in the succeeding paragraph of his paper he ob- 

 serves, that he found this conclusion corroborated by the re- 

 sults of numerous analyses of the granitic rocks of the same 

 locality. 



Having been present when this paper was read, I certainly 

 felt, relying merely on my memory at the time, that the 

 statement, alleging potash to be absent from the Dublin fel- 

 spar, or to be but casually present, and only in insignificant 

 quantity, was not in accordance with my experience. I have 

 had this mineral frequently analyzed as an exercise for pupils 

 in my laboratory, and, while recollecting that soda was inva- 

 riably found in it, I had also very distinctly on my mind that 

 the potash often preponderated, and was never present in very 

 small relative quantity. Upon referring to my notes, and 

 instituting some fresh analyses for the express purpose of 

 throwing light on this question, I find that the impressions I 

 previously entertained are in accordance with my experiments ; 

 and as the point under consideration is one of some practical 

 and scientific interest, I am anxious to be permitted to put on 

 record, through the Academy, the evidence in reference to it 

 which I have obtained. The following Table includes two 

 complete analyses of felspar, for some time in my possession, 

 and three partial analyses, made within the last fortnight, for 

 the sole purpose of determining the relative quantities of the 

 potash and soda. The composition of the Mourne felspar has 

 no immediate bearing upon the question under discussion, 

 and is given merely for the purpose of showing that a variety 

 of this mineral, generally considered as a true albite, includes 

 a large relative amount of potash. 



