592 Mr. J. Beete Jukes and the Eev. Samuel Haughton on the 



It will be observed that the protoxide bases are slightly in excess of the 

 usual formula for this mineral, viz.: — 



KO, Si O3 + AI2 O3, 3Si O3 ; (1) 



while the oxygen ratio gives very exactly the formula — 



understanding in both formulce that KO denotes all the protoxide bases, as well 

 as potash. 



4. Felspar of the Granitic Paste. — I have not made any direct examination 

 of tlie confusedly crystallized felspar of the granite paste, cementing the frag- 

 ments of quartz, orthoclase, and mica together, nor do I believe it has a precise 

 chemical composition constituting it a distinct mineral ; it contains more soda 

 than the orthoclase of the granite, in which we find only 2-75 per cent. 



So far as I am aware, no crystals of either albite or oligoclase have ever 

 been found in the Leinster granite, and this is easily accounted for by the fact, 

 that the granite nowhere contains enough soda to admit of the formation of 

 such crystals. There is not sufBcient potash to form a felspar exclusively 

 monoclinic in its form, nor is there sufScient soda to form the triclinic felspars; 

 consequently, the orthoclase has crystallized out from the molten mass, leaving 

 a confused residue of felspar to form the paste ; this residue does not crystal- 

 lize freely, as it is undecided whether to assume the monoclinic or triclinic form, 

 and it, therefore, remains as a saccharoid paste of periclinic felspar. 



The following partial analyses of this felspathic paste have been pub- 

 lished : — 



As all these specimens were taken from the same mountain, — the Three 

 Rock Mountain, county of Dublin, — they are comparable with each other, and 



* Proceedings of Royal Irish Academy, vol. v., p. 381. 

 t Phil. Mag., Fourth Series, vol. ix., p. 513. 



1 



