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



IV., V., VI., that the soda-granites are not so persistent in their 

 character and composition as the potash-granites of the same district. 

 This result is in accordance with that which I formerly deduced from 

 an examination of the potash- and soda-granites of Leinster, and 

 admits, as I think, of an explanation which I shall presently give. 



Assuming that the mica present, as green or black mica, in these 

 granites is the green mica of Mourne, I find the following equations 

 by the method described in Vol. xii. p. 201 : 



Q+4F + 5M= 1-429 

 F + 3M=0'367 

 F + 2M=0-294. 



From these equations, I find 



Q =0-472 

 F =0-148 

 M=0-073. 



And, using the atomic weights of quartz and green mica already 

 given, I obtain finally — 



The Miner alogical Composition of the Soda- Granites of the 

 North-east of Ireland. 



Quartz 21-24 



Felspar .. 41 -45 

 Mica 36-50 



99-19 



It may here be observed, that the quantity of free silica or quartz 

 in the soda-granites is comparable with that of the potash-granites ; 

 but it is much more visible to the ej^e than in the latter, although 

 from the analyses it appears that the soda-granites are deficient in 

 silica, as compared with the potash-granites, containing, in fact, 7 or 

 8 per cent, less ; this is due to the usually large quantity of mica 

 present in the soda-granites, as this mineral requires much less silica 

 than felspar. This example shows how necessary it is to check the 

 impressions of the eye by the severer test of laboratory analysis. 

 A geologist, speculating on the origin of these granites, and not 

 having the resources of chemistry at his command, might pronounce 

 them to be richer in silica than the potash-granites, whereas it ap- 

 pears that, like other soda-granites, they are really more basic, and 

 may have been formed from potash-granites by the addition of iron, 

 magnesia, lime, and soda. 



From the analysis of the pink elvan-dikes penetrating the granite 

 of Newry Quarry, it appears that the atomic quotients of silica, 

 peroxides, and protoxides, are as follow : — 



Atomic Quotients of Silica, Peroxides, and Protoxides in the 



Soda-Elvan. 



Silica 1-649 



Peroxides .. 0*231 

 Protoxides.. 0*321 



