Lower Palaeozoic Rocks of the South-East of Ireland. 601 



assumption I consider nearly certain, from the fact of the presence of abun- 

 dance of free quartz, which would have converted any nionosilicated or bisili- 

 cated felspar originally present into a tersilicated felspar, taking its mineralo- 

 gical character from the predominant alkali. 



Let Q, F, M, L, denote the number of atoms of quartz, felspar, margaro- 

 dite, and lepidomelane present in the granite ; from the formula of these mine- 

 rals just given, and from Table VII., we find, as in my former paper, 



l-601 = Q + 4i^+3J/+9Z; 



0-313= F+IM-VIL; (A) 



0-273= F+ il/-f6Z. 



In these equations we have/owr unknown quantities, and only three equa- 

 tions ; but a fourth equation may be found as follows : — 



Let q^f m, I denote the per-centages of quartz, felspar, margarodite, and 

 lepidomelane in the granite ; then q, f, m, I will be equal to Q, F, M, L 

 respectively multiplied by their atomic weights. 



The atomic weights of Q, M, L are known, and may be found as fol- 

 lows : — 



The atomic weight of quartz is 45 ; that of margarodite may be thus 

 found: — 



The mineralogical formula of margarodite is in atoms = 3 silica + 2 per- 

 oxides -1- 1 protoxide -)- 2 water. The atomic weights of silica and water are 

 known, and are 45 and 9 respectively ; therefore, if x and y denote the atomic 

 weights of the peroxides and protoxides respectively, we have 



Atomic weight of margarodite =Sx4:5 + 2x+y + 2 x9. 



The values of a: and y are found from Tables III. and IV., by the equa- 

 tions, 



36-62 = 0-674 X J-, 



13 23 = 0-325 xy. ^ ^ 



The left-hand numbers being the per-centages of peroxides and protoxides 

 respectively ; and the right-hand numerical coefficients denoting the atomic 

 quotients of peroxides and protoxides respectively. From these equations we 

 find the values of x and y to be 54 and 40. And finally — 



