1856.] HAUGHTON GRANITES OF IRELAND. 181 



Calculated Composition of Granite-Feldspar. 



Per cent. Atoms. 



Silica 67-30 1*464 .. 1-464 ..4 



Alumina 16-85 0-328 \ r. oa. , 



Peroxide of iron 2-87 ... . 0-036 J ♦ • " '^'^^ • • ^ 



Lime 2-60 .. ..0-093^ 



Magnesia .... — .... — I n-'^fio i 



Potash 5-43 ....0-115 f-"'^^^ --^ 



Soda 4-71 0-152J 



Loss by ignition 0*24 



100-00 



This is a genuine tersilicated feldspar ; from which fact we may- 

 draw the conclusion, that our original hypothesis as to the granite is 

 admissible, viz. that it is composed of quartz, margarodite-mica, and 

 tersilicated feldspar ; if other minerals enter into its average compo-. 

 sition, they must do so to such a slight extent as practically to have 

 no influence on the composition of the granitic mass. On comparing 

 the calculated feldspar just determined with the average feldspar of 

 Table I. the following interesting difference may be observed ; that 

 the feldspar of the main body of the granite contains more soda, and 

 lime replacing potash, than the crystals of feldspar, whose compo- 

 sition is given in that Table. It would appear from this fact, that in 

 cooling from a molten condition, the large crystals of feldspar, in 

 assuming their crystalline state, had appropriated to themselves more 

 than their share of potash. It is possible, inasmuch as the typical 

 orthoclase of the fifth system contains a large proportion of potash, 

 that the excess of potash may be an essential condition for the forma- 

 tion of such large and distinct crystals. In fact, in a molten mass, 

 like that under consideration, if no alkali were present besides potash, 

 all the crystals of feldspar would belong to the monoclinic system ; 

 and if the only alkali present were soda, all the feldspar crystals 

 would be found in the triclinic system. There must therefore be 

 some intermediate mixture of potash and soda for which the crystals 

 of feldspar are, as it were, undecided in which system to crystallize ; 

 or, in other words, the feldspar will remain in a confused crystalline 

 mass, as in the body of the granite, while the large crystals which 

 occur take their character from the predominant alkali, and consist, 

 as in the case before us, exclusively of orthoclase crystals, of typical 

 composition, in the monoclinic system. 



II. Isolated Granites, 



The isolated granites of Wicklow and Wexford are shown on the 

 Map, fig. 1, p. 1 72; they appear in the form of isolated detached ranges 

 A, B, C, having an elongated form, running N.N.E. and S.S.W., or 

 parallel to the axis of the main chain. The granites composing these 

 detached hills are frequently associated with and penetrated by dykes 

 of greenstone and other forms of trap-rocks. Mineralogically they 

 are distinguished by the presence of black mica, chlorite, dark-green 

 mica, and hornblende. These minerals do not enter into the compo- 



