1856.] HAUGHTON — GRANITES OF IRELAND. 185 



It is not possible to obtain the mineralogical analysis of these 

 granites without assuming the composition of the black mica, which 

 has never been determined. If we assume, provisionally, that the 

 formula of the black mica is the same as that of the gray mica (as 

 seems probable from the manner in which it replaces it, isomor- 

 phously, in the crystals of the Three Rock Mountain), we can obtain 

 the relative atoms of quartz, feldspar, and mica in this granite from 

 equations (1) ; 



l'469=Q^4F-\-3M 



0'346 = F-\-2M. (1) 



0-335= F+M. 



From which we readily obtain as atoms of quartz, feldspar, and 

 mica : — 



Q=0-140 



F=0-324 



M==0-011. 



The per-centage mineral composition of the rock cannot be de- 

 duced from these figures without assuming the ultimate analysis of 

 either the mica or feldspar, both of which are unknown ; but we can 

 readily assign the limits of error. Let us suppose the mica to con- 

 tain no alumina. In this ease its peroxides are iron, and therefore 

 the atomic weight of the mineral is 356. If the mica contains no 

 iron, its peroxides are all alumina, and its atomic weight will be 299. 

 These are the extreme atomic weights of margarodite-mica, possible 

 on the supposition that its protoxides remain as in Table II. Multi- 

 plying the preceding numbers by M, we find, — 



Major per-centage of mica 356 X 0*01 1 =3'91 



Minor per-centage of mica 299 X 0*01 1 =3*29. 



Assuming the mean of these as the per-centage of mica, and deter- 

 mining the quartz as before, and the feldspar by difference, we ob- 

 tain the following 



Mineralogical Analysis. 



Quartz = 6-44 



Feldspar =89-69 



Mica = 3-60 



9973 



In the Cushbawn, or first group of isolated granites, the quartz 

 was diminished from 25 to 17 per cent, of the entire rock, as com- 

 pared with the granite of the main chain ; in the granites of the third 

 group of isolated granites, we observe a further reduction of quartz 

 to 6 per cent. 



This third group of granites agrees with the first, and differs from 

 the main chain in the following particulars : — 



1st. The diminution in per-centage of silex. 



2nd, The increase in iroD and lime. 



3rd. The preponderance of soda over potash. 



o2 



