the Granite of the Dublin and Wicklow Mountains. 41 



of granite rock, that is to say, of a mineral occurring in the sixth 

 system, whose rational formula is 



(KO, NaO)Si0 3 + Al 2 3 , 3Si0 3 . 

 I mention this because there has existed a good deal of confusion 

 as to the use of the terms orthose and albite; the former having 

 been commonly appropriated to minerals of the felspathic family 

 in which the alkali potash exists in excess, the latter to those in 

 which soda preponderates. The true distinction unquestionably 

 rests on crystallographic form, as instances are to be found of 

 minerals included under the formula already given, occurriug in 

 the fifth system, in which soda preponderates, and to which 

 therefore we should give the name orthose. And vice versa of 

 specimens containing potash in excess occurring in the sixth 

 system, and which should therefore be called albite. 



It was very commonly believed until lately that this case of 

 the Mourne Mountains was a singular instance in Ireland. In 

 the Proceedings of the Royal Irish Academy, vol. v. p. 351, I 

 find that Sir Robert Kane announced as a fact, that the felspar 

 of the Dublin and Wicklow granites " was almost exclusively an 

 albitic or soda felspar, containing only in some cases a small 

 quantity of replacing potash." This conclusion he derived from 

 the analyses of some surface waters taken from the Three Rock 

 Mountain in the neighbourhood of Dublin, in which he found a 

 great preponderance of soda, but at the same time stated that 

 " the fact was verified by a great number of analyses of speci- 

 mens of granites taken from various parts of the great mass which 

 extends from Dublin into the county Wicklow. In all the analvses 

 made, which included both ordinary granites and elvan or gra- 

 nite porphyries, both potash and soda were found present ; and 

 the latter almost always so preponderant, as to lead to the con- 

 clusion that the potash should in most cases be considered to 

 belong to the mica which the granite contained ; and the felspar 

 was almost exclusively an albitic or soda felspar, containing only 

 in some cases a small quantity of replacing potash." 



These analyses, which he stated had been made in the Museum 

 of Irish Industry, he did not present to the Academy, nor have 

 they ever, as far as I know, come before the public. 



The statement of a fact so interesting and at the same time 

 so novel excited much interest ; and as it was suggested at the 

 time by a very eminent authority, Dr. Apjohn, Professor of Che- 

 mistry and Mineralogy in the University of Dublin, that a direct 

 investigation of the mineral in question would be of essential 

 service in determining so important a geological fact, I under- 

 took the task of making a systematic investigation of the felspars 

 of the range of mountains which runs in a south-westerly direct- 

 ion from Dublin to within a few miles oT New Ross. As a first 



