1856.] HAUGHTON — GRANITES OF IRELAND. 201 



peroxide of iron replaces alumina to a considerable extent, or not ; 

 I have been unable to determine this point by direct experiment, 

 either chemically or optically, from the difficulty of procuring a 

 sufficient quantity of pure mica for examination. 



If the green mica be not the margarodite of the Leinster granites, 

 but similar in composition to the green mica of the Mourne Moun- 

 tains already described, we shall then have, taking the mean of the 

 atoms of the Wellington Inn and Slieve na glogh granites : — 



1-540=Q+4F + .5M 

 0-322= F + 3M 



0-277= F+2M. 



Solving these equations, for Q, F, M, we find : — 



Q=0-567 



r=0-187 

 M=0'045 



In calculating per-centages from these results, we must use the 

 atomic weight of the mica found from the analysis of green mica in 

 page 191, which is found to be 508 instead of 304. Hence we ob- 

 tain finally : — 



Potash-Granite of Newry and Carting ford. 



Quartz 26*08 per cent. 



Feldspar .... 50*76 

 Green mica . . 22*86 



99*70 



It is worthy of remark, and confirms the preceding view of this 

 granite, that, if we suppose the peroxide of iron of the granite (which 

 is 3*54 per cent.) to belong exclusively to a mica similar to the green 

 mica of Mourne, we shall obtain 20*07 per cent, of mica in the gra- 

 nite ; a result which is sufficiently near the per-centage just given. 



I have noticed as a curious fact that the mica of the potash- 

 granites of Mourne and Carlingford is the green mica, without any 

 transparent grey mica, and that the mica of the soda-granites of 

 Newry is a jet-black mica, identical in appearance with the black 

 mica which is found in the soda-granites of Oulart and Wexford ; 

 and that this black mica, both in Newry and Wexford, is unaccom- 

 panied by any of the grey margarodite of Leinster. 



Soda-Granites of the Newry District. 



As I have not as yet satisfied myself as to the true mineralogical 

 character of the type of granites to which I propose to apply the 

 term " Sod a- Granites," I shall here confine my attention exclusively 

 to their chemical composition, which is sufficient to identify them 

 with th€ soda-granites of Wicklow and Wexford. The soda-granites 

 of the Newry district are characterized by the presence of black 

 mica, of the exact composition of which I have no means of forming 

 an opinion at present. 



p2 



