1856.] HAUGHTON GRANITES OF IRELAND. 179 



It follows from these expressions, and from the definitions given 

 of a, /3, y, that, if Q, F, M denote the number of atoms of quartz, 

 feldspar, and mica respectively contained in the granite, 



a = Q + 4F + 3M1 



/3 = F + 2M y (1). 



y = F + M J 



Substituting in these equations for a, /3, y their numerical values 

 from the last column of Table IV., we find 



1-567 = Q + 4F + 3M 

 0-311 = F + 2M 

 0-267 = F + M. . 



From these equations, we find easily 



M = 0-044 

 F =0-223 

 Q = 0-543. 



These numbers express the atomic quotients of mica, feldspar, 

 and quartz existing in the granite, and if m, /, q denote the atomic 

 weights of these minerals, then 



The per-centage of mica = M x m"^ 



feldspar = F x/>. . . (2). 

 „ quartz = Q x q J 



And consequently, if our hypothesis be correct, the following 

 equation must be satisfied : — 



99-53 = Mm + F/+ Qg (3). 



The left-hand side of the equation being taken from the last 

 column of Table III. 



But here a difficulty presents itself; we know the atomic weight 

 of quartz, but not of margarodite or feldspar, unless we assume the 

 per-centage composition of these minerals, or in other words assume 

 the composition of the constituent minerals of the granite. 



I shall assume the composition of the grey mica from Table II., 

 from which and the subsequent list of atomic quotients, we may 

 calculate the atomic weight of the grey mica of the granite as 

 follows : — 



The mineralogical formula of margarodite is in atoms = 3 silica 

 -f 2 peroxides -H 1 protoxide + 2 water. The atomic weights of 

 silica and water are known, and are 46 and 9 respectively ; there- 

 fore, if X and y denote the atomic weights of the peroxides and pro- 

 toxides respectively, we have 



Atomic weight of mica = 3x46-f2.r-f2/+2x9. , 



