192 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



Composition of the Mourne Granite. 



The Mourne granite is very fine-grained, and would be called 

 elvan in Cornwall ; it contains distinctly quartz, orthoclase (both the 

 opaque variety and the translucent kind known as moonstone), albite, 

 and green mica. An average specimen from Slieve Corragh gave 

 the following results : — 



Composition of Granite. 



Specific gravity =2*595. 



Silica 75-00 



Alumina 13*24 



Peroxide of iron 2*52 



Lime 0*69 



Magnesia — 



Potash 4-33 



Soda 3-07 



Loss by ignition ; . . 0*80 



99-65 



A comparison of this analysis with the average analysis of the 

 potash-granites of Leinster shows a striking similarity in all the 

 constituents excepting the quartz, which is three per cent, greater in 

 the Mourne granite ; this excess of silica being accompanied with a 

 falling off in the lime and magnesia. Neglecting the small quantity 

 of mica in this granite, its mineralogical composition may be consi- 

 dered as — 



Quartz 28-0 per cent. 



Orthoclase 44*2 



Albite 27*8 



100-0 

 II. Granites op the Carlingford District. 



Description of the Igneous Rocks of Carlingford District. 



The granitic and other igneous rocks of the district south of Car- 

 lingford Bay, although only occupying a circular area of about five 

 miles in diameter, present to the geologist a variety and complication 

 which is at first perplexing. Although I propose to confine my atten- 

 tion to the granitic rocks of the district, yet it is necessary to give a 

 summary of all the igneous rocks, and a statement of what is known 

 of their relative ages. 



Igneous Rocks of Carlingford. 



1 . Medium-grained granite : quartz, feldspar, green mica. 



2. Fine-grained granite: quartz, feldspar, hornblende. 



3. Syenite : hornblende and anorthite. 



4. Hornblende-rock. 



5. Amygdaloidal or pockmarked greenstone. 



6. Fine-grained grey dolerite. 



