the Granites of the South-east of Ireland. 



29 



tity of a mineral wliicli is either hornblende or chlorite. Calcu- 

 lating the per-centages of quartz, felspar, and mica of the two 

 granites, which may be represented as composed of these mme- 

 rals, we find, — 



Table VI. — Per-centages. 



Whatever doubt may attach to the preceding calculations, 

 owing to the hypothesis on which they are of necessity founded, 

 no such doubt or uncertainty can belong to the results of direct 

 experiment contained in Tables I. and IV., which have a positive 

 value, independent altogether of the inferences which may be 

 deduced from them. From these tables I deduce the following 

 as the average composition of the granites of the main chain and 

 of the isolated granites: — 



Table VII. — Average Composition of Granites. 



On examining Tables I. and IV., several interesting diflferences 

 present themselves in the composition of the two groups of gra- 

 nites, in addition to the important difference to which I have 

 drawn attention with respect to their alkaline constituents; 

 but I content myself at present with establishing this funda- 

 mental distinction between the two groups, and thus furnishing 

 an additional proof of the service which may be rendered to 

 geological science by the more exact and experimental sciences. 



