302 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 20, 



quantity of green mica, hornblende, or chlorite is small ; and, if it be 

 neglected, we find, from the column of average atomic quotients, the 

 following number of atoms of quartz and felspar in the potash- 

 granites of the Newry district : — 



Q = 0-358 

 F = 0-306. 



From these numbers, we find the per-centages of quartz and 

 felspar, 



Quartz = 16*11 percent. 

 Felspar = 83-91 „ 



100-02 



If we take into consideration the mica or hornblende present in 

 the granite, and consider it to be the same as the green mica of the 

 Mourne range, described in page 1 9 1 of my former paper, we shall 

 have the following equations to determine the number of atoms of 

 quartz, felspar, and mica : — 



Q+4F+5M= 1-582 

 F+3M=0-311 



F + 2M=0*301. 



From these equations, we readily obtain 



Q =0-408 

 F =0-281 

 M=0-010. 



Assuming the atomic weights of quartz and the green mica as 45 

 and 500 respectively (see p. 201. vol. xii.), we obtain finally the 



following :- 



Miner alogical Composition of Pot ash- Granites. 



Quartz = 18*36 



Felspar = 76*66 



Green mica. . = 5*00 



100*02 



II. Soda-Granites of the North-east District. 



The soda-granites of the North-east of Ireland occur, as I have 

 already observed, to the north of Newry, and are characterized for 

 the most part by pinkish or reddish translucent felspar and black 

 mica, — circumstances indicating the presence of iron. 



The following Table contains the analyses of four of those granites. 



No. 1. Granite from Newry Quarry, medium-grained, composed of quartz, white 

 felspar passing into pale pink, translucent, and black mica. 



No. 2. Elvan-granite from Newry Quarry, intersecting the former in dikes and 

 veins, fine-grained, pink. Besides the elvan-dikes, dikes of dark green- 

 stone also penetrate the granite (No. 1) in Newry Quarry. 



