416 PEOCEEDINGS OF THE^EOLOGICAL SOCIETY. [Jime 18, 



felspar of this rock has been already given ; that of its hornblende 

 is as follows : — 



Hornblende of Donegal Syenitic Granite. 



Per-centage. Atoms. 



Silica 47-25 1050 



Alumina 5-65 108 



r protoxides ] 



Iron (peroxide) 19-11 i of iron and I 551 ] 



( manganese J 



Lime 11-76 420 



Magnesia 11-26 563 



Soda 0-98 31 



Potash 1-04 22 



Iron (protoxide) .... 0-94 

 Manganese (protoxide) 1-70 



1587 



99-69 



The rock, of which this hornblende is a constituent, has the follow- 

 ing composition : — 



Syenitic Granite of Donegal. 



Snica 58-04 



Alumina 16-08 



Iron (peroxide) 8-27 



Lime 6-52 



Magnesia 2-94 



Soda 4-65 



Potash 2-21 



Iron (protoxide) 0*45 



Manganese (protoxide) 1-12 



100-28 



B. Accidental Minerals. 



1. Sjahene. — This mineral is very like the clove-brown sphene of 

 Norway : it is found in the granite, when the latter becomes basic, 

 containing much black mica and oligoclase ; but it is principally 

 found in a rock formed of a paste of quartz and felspar, that often lies 

 between the granite and limestone of the metamorphic rocks of 

 Donegal. This is especially observable at Anagarry and Barnesbeg, 

 where this rock is so abundant as to become entitled to the name of 

 sphene-rock ; and it cannot be distinguished from similar rocks from 

 Norway. 



2. Schorl. — This mineral accompanies orthoclase in veins, and is 

 often curved and cracked, showing the wider openings of the fissures 

 next the convex side, and filled with quartz, as if the curvature of 

 the schorl, and the filling of its fissures with quartz, were the result 

 of an action that took place after the deposition of the mineral. 



3. Beryl. — The only known locality for beryl in Donegal is She- 



