1856.] HAUGHTON — GRANITES OF IRELAND. 187 



Calculated Feldspar of Carnsore Granite. 



Silica 65-68 



Alumina 14*92 



Peroxide of iron. ... 4' 93 



Lime 3*30 



Magnesia 



Potash 6-07 



Soda ., 3-90 



Loss by ignition. . . . 1*20 



100-00 



From the analysis of the granite and its calculated feldspar, the 

 following inferences may be drawn : — 



1st. The Carnsore granite, as respects its ultimate analysis, is ab- 

 solutely identical with the average granite of the main chain, in the 

 constituents, silica, lime, magnesia, potash, and soda. 



2nd. In the Carnsore granite, peroxide of iron to some extent 

 replaces the alumina, giving to the feldspar its reddish-pink colour. 



3nd. The free quartz in the Carnsore granite is nearly equal in 

 amount to that of the main chain. 



Type-Granites of the South-east of Ireland. 



From the preceding facts, the following inferences appear to me to 

 follow : — 



1st. In the south-east of Ireland, the granites may be classified by 

 the preponderance of potash over soda, or vice versa. 



2nd. The granites of the main chain and of Carnsore, are potash- 

 granites. 



3rd. The granites of the intermediate groups are soda-granites, 

 and are reducible to two types : — 



a. The Croghan Kinshela granite ; 



b. The soda-granite proper. 



The former of these is exceptional, and of rare occurrence ; the latter 

 is common, and reappears in the County Down and County Armagh 

 granites. 



4th. The potash- and soda-granites, properly so called, or type- 

 granites, differ from each other in a regular manner, in respect to 

 the other constituents, as well as in respect to the alkalies ; the most 

 striking differences being the deficiency of silica in the soda-granites, 

 this deficiency being made up by the addition of peroxide of iron % 

 and lime ; and the increase in specific gravity of the soda- granites. 



Note. — At the Meeting of the Geological Society at which this 

 paper was read, I was requested to state whether there was any 

 geological evidence of difference of age between the potash- and soda- 

 granites of Leinster. I know of no conclusive evidence on this point ; 

 but the age of the potash-granites is known within certain limits ; it 

 is subsequent in time to the Silurian rocks of Wicklow and Wexford, 



