1856.] HAUGHTON — GRANITES OF IRELAND. 195 



closely allied ; a fact which explains the passage of one into the 

 other ; and also shows the necessity for some more certain criterion 

 of the true character of an igneous rock than either its physical 

 aspect or its mineralogical character, as both these may vary within 

 wide limits, although the ultimate analysis of the rock has changed 

 but little. 



Second Variety of Carlingford Granite* 



Specific gravity =2'632. 



Silica 71-41 ....1-552 



Alumina 12-64 0-246 



Protoxide of iron .... 4*76 



Lime 1-80 



Magnesia 0*63 J> 0-441 



Potash 5-47 I 



Soda 3-03J 



99-74 



The preceding granite was taken from Grange Irish, from the 

 main body of granite, within ten yards of the point where the granite 

 penetrates the carboniferous limestone in dykes, and is converted 

 into a crystalline syenite. I have considered its iron as protoxide, 

 for, as there is no mica, and the feldspar is a pure white, the iron 

 must belong to the hornblende, and therefore must be protoxide. 



If we suppose the alkalies and alumina to belong exclusively to 

 the feldspar, and consider the hornblende to be of the form 



RO, SiOg, 



we have from the preceding numbers 



1-552 = Q + 4F + H 

 0-246 = F 



0-441= F + H; 



&om which we obtain 



Q = 0-246 

 F = 0-373 

 H = 0-195. 



If we calculate the atomic weight of the hornblende, referring to 

 it the iron and lime, we find it to be 79 ; from which we may infer 

 the composition of the granite to be as follows : — 



Mineralogical Composition of the Granite of the Second Variety. 



Quartz 17*16 per cent. 



Feldspar 67*18 



Hornblende 15*40 



99*74 

 This is a potash-granite, similar in composition to the first variety, 



