the Granites of the South-east of Ireland. 25 



Then, on the hypothesis that the granite is composed exclu- 

 sively of quartz, felspar, and mica (raargarodite), since 



Quartz =Si03 



Felspar =R0, SiO^ + RSQ^ SSiO^ 



Margarodite =110, Si03 + 2(R2 0^ SiO=^)+2HO, 



we find, if Q, F, M denote the number of atoms of quartz, fel- 

 spar, and margarodite present in the granite, the following rela- 

 tions : — 



* = F+2M I (1) 



c = F + M J 



In these equations, a, b, c arc given by the analysis; and 

 from these equations, Q, F, M maj- be found. Having deter- 

 mined Q, F, M, we can obtain the per-centages corresponding 

 to them by multiplying Q, F, M by their respective atomic 

 weights. The atomic weight of quartz is known, and is 46; 

 but the atomic weights of felspar and mica vary with the rela- 

 tive proportions of the ingredients composing these minerals. 

 Assuming the average of the analyses of micas from this granite 

 range already given by me (Phil. Mag. vol. ix. p. 272), it is easy 

 to infer from it an atomic weight of mica equal to 305. This 

 atomic weight of mica has been used by me in the calculations 

 made in this paper, and the per-centages of felspar found by 

 difference. 



The calculations just mentioned do not prove that the gra- 

 nites to which they are applied are composed of quartz, felspar 

 and mica, as combinations of other minerals might equally well 

 represent the analyses ; but on the hypothesis that the granites 

 are composed of these three minerals, they give their per-centages 

 with a close degree of approximation : and further, if the equa- 

 tions cannot be satisfied with positive values of Q, F, M, it may 

 be considered as proved that the granite under consideration 

 cannot be simply a compound of quartz, felspar and marga- 

 rodite. 



It is to be observed, that if a granite be composed of four or 

 more minerals, it is not possible to find by the process just de- 

 scribed, the per-centages of each mi]icral, because ii»this case the 

 number of unknown quantities is greater than that of the equa- 

 tions from which they are to be calculated. 



The following Table contains the analyses of eight granites 

 taken from north to south along the; ])rincipal granite chain, and 

 the notes appended contain a few interesting particulars respect- 

 ing each granite, 



