194 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



quartz, feldspar, and green mica, and the other, fine-grained, of 

 quartz, feldspar, and hornblende. 



First Variety, of Carlingford Granite, 

 Specific gravity =2*593. 



Per cent. Atoms. 



Silica 70-48 ..1-532 



Alumina 14-24 



Peroxide of iron .... 3*72 



Lime 1 -48 



Magnesia 0-40 . 



Potash 4-26 f 



Soda 3-66J 



Loss by ignition .... 1-59 



0-323 

 0-281 



» 



99-83 



This granite was taken from the base of Slieve na glogh, is medium- 

 grained, and resembles the usual specimens of Dublin granites, 

 excepting that its mica is green and not grey silvery. Assuming,, 

 however, that the mica is margarodite, we easily deduce the foUowin 

 equations, from which its mineralogical composition may be inferred 



1-532 = Q+4F+3M, 

 0-323= F-f2M, 



0-281= F+ M. 



From these data we infer — 



Q =0-450 

 M=0-042 

 F =0-239 



If we assume the atomic weight of mica as before to be 304, we 

 can obtain the mineral composition of the rock ; in this calculation 

 the mica is slightly in defect, because there is more iron in this mica 

 than in the mica from which the number 304 is inferred. 



Mineralogical Composition of the Granite of the First Variety^ 



Quartz 20*70 per cent. 



Feldspar 66*37 



IVIica 12-76 



99-83 



From the preceding discussion we are entitled to draw the in- 

 ference that the first variety of granite of Carlingford is a potash- 

 granite, and has a mineral composition very like that of the main 

 chain of Leinster. 



The second variety of granite is very fine-grained, but distinctly 

 composed of quartz, white feldspar, and hornblende, without any 

 mica ; and the first and second varieties of granite pass into each 

 other. I shall show, that although physically, and mineral ogically, 

 these two varieties of granite are so distinct, yet chemically they are 



