1856.] 



HAUGHTON GRANITES OF IRELAND. 



173 



districts in the counties of Wicklow and Wexford, forming small 

 islands, as it were, of granite, which have penetrated and broken 

 through the Silurian slate of tliose counties. The general axis of 

 each of these granitic outbursts is parallel to that of the main chain ; 

 these secondary granitic rocks are found at intervals for a distance of 

 forty-three miles, from Ballinaclash, county Wicklow, to Camaross 

 Hill, county Wexford. 



I propose to give a short mineralogical sketch of these two groups 

 of granitic rocks, which, considered as groups, are distinguished from 

 each other by well-marked chemical differences, not hitherto observed. 



I. Granites of the Main Chain. 



Elementary Minerals composing the Granite of the Main Chain. 



The granite of the main chain varies less than might be supposed 

 from its great extent, and specimens from the southern end of the 

 chain might easily be mistaken for northern specimens. This simi- 

 larity of appearance arises from the prevalence of the same consti- 

 tuent minerals, blended together in not very dissimilar proportions. 

 The elementary minerals, which may be seen distinctly crystallized 

 in the granite of this whole range, are 



1. Quartz. 



2. Orthoclase. 



3. Silvery grey biaxial mica. 



4. Black mica. 



1. Quartz. — The quartz of this district is grey transparent, and 

 presents no variety of colour or appearance ; smoky quartz is, so 

 far as I am aware, unknown in this range, although so abundant in 

 the Mourne granites. The mean specific gravity of the quartz, taken 

 from the granite of distant localities, is = 2*645. 



2. Orthoclase. — The feldspar of the granite of the main chain is 

 invariably white and opaque, and occasionally crystallizes out from 

 the mass in large crystals ; these crystals were examined from seven 

 different localities by Professor Galbraith of Trinity College, and 

 found to be, without exception, pure Orthoclase. I here subjoin, in 

 a tabular form, the results of his examination of this mineral in the 

 granite-range. 



Table I. — Analyses of Feldspars. 





1. 



2. 



3. 



4. 



5. 6. 



7. Average. 



Silica 



64-00 

 1811 



0-57 



12 73 



300 



0-55 



65-40 

 17-71 



*l-77 



10-68 



3-26 



0-69 



65-44 



18-36 



0-80 



12-34 

 2-73 

 0-52 



6505 



17-72 



0-23 



13-42 

 2-75 

 0-36 



6419 63-60 



64-48 

 19-04 



1-02 



10-74 



2-64 



0-78 



64-59 

 18-31 

 0-25 

 0-58 

 12-23 

 2-75 

 0-58 



Alumina 



18-39 

 0-70 

 0-34 



11-39 



18-84 



o-To 



U-.S3 



Lime 



Magnesia 



Potash 



Soda 



2-95 ; 1 92 

 0-58 0-60 



Loss by ignition 



Total 



98-96 



99-51 



100-19 



99-53 



98-54 99-69 



98-70 



9929 





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