600 



Mr. J. B. Jukes and the Rev. Samuel Haughton on the 



If we now take the atomic quotients of the average of the eleven analyses 

 of Table VI., we find the following : — 



Table VII. 



This Table affords us the means of forming three equations from the 

 atomic quotients of silica, peroxides, and protoxides present in the rock ; as I 

 have shown already (Journal, Geol. Soc. London, vol. xii. p. 178) ; but as we 

 have more than three unknown quantities, we must seek for other equations to 

 determine them ; and in doing so, there is a considerable variety of choice in 

 the methods which might be adopted. We shall first write down the three 

 equations already obtained. 



The granite is quinary, or composed of quartz, orthoclase, margarodite, 

 lepidomelane, and felspar paste, having the following mineralogical formula: — 



1. Quartz, . . . SiOj. 



2. Orthoclase, . RO, Si03 + AI2O3, 3 SiOj. 



3. Margarodite, . EO, SiO, + 2 [R2O3, SiOj] + 2H0. 



4. Lepidomelane, 2 [3R0, Si03] + 7 [R,03, SiOj] + 5H0. 



5. Felspar paste,. Unknown. 



(8) 



We cannot make a single step in the discussion without some assumption 

 as to the felspar paste, which is not known directly. The following assumption 

 is most probably correct: that the felspar paste is a tersilicated felspar, having, 

 therefore, the same chemical formula as orthoclase, pericline, and albite ; this 



