329 



which white mica has been developed. This, however, only occurs in rocks 

 in which the schistosity is imperfect. 



The granites in question have marked affinities with those of the 

 West of England. They differ, however, in age, inasmuch as they are 

 undoubtedly pre-Carboniferous. They are intrusive in lower Paloeozoic 

 rocks. 



Isolated masses of granitic rock occur on a line running roughly 

 parallel to the main range and some distance to the east. As most of these 

 contain an excess of soda over potash Dr. HAUGHTON refers to them as 

 soda-granites. The white rock of Croghan Kinshela has a very remarkable 

 composition. It is supposed by Dr. HAUGHTON to be composed essentially 

 of quartz and albite (38% of quartz and 62 of albite). A small amount 

 of chlorite is occasionally present. 



I. Extremes. II. 



Si0 2 ... 72-084 ... 70-28 ... 73-24 ... 80-24 



A1 2 3 ... 14-459 ... 12-64 ... 16-44 ... 12-24 



Fe 3 3 ... 2-399 ... 1-40 ... 3-47 ... 0-72 



CaO ... 1-762 ... 0-96 ... 2-84 ... 0-89 



MgO ... 0-105 ... 0-11 ... 0-53 ... tr. 



K 2 ... 4-803 ... 5-90 ... 3-95 ... 0-40 



Na 2 .. 3-014 ... 2-31 ... 3-53 ... 5-58 



Loss 0-906 0-96 1-39 



99-532 100-07 



Sp. Gr. 2-634 2-616 2-652 2 629 



I. Average of nine analyses, the _extremes of which are quoted, of the granite of the 

 main chain. 



II. Soda-granite of Croghan Kinshela. It was in the alluvial deposits of the streams 

 flowing from Croghan Kinshela that gold was found in considerable quantities at the end of the 

 last century. No gold was ever found in situ. 



North-east of Ireland. These granites occur in three isolated districts 

 of considerable size on the borders of the counties of Down, Louth and 

 Armagh. They are very different in character from those of Leinster. 

 That of the Carlingford district is certainly of post-Carboniferous age. 

 The granites of the Mourne mountains have decided affinities with those 

 of Arran, while the granites of the Carlingford district (including the 

 granophyre of Barneveve) resemble those of Mull and Skye. These petro- 

 graphical resemblances are probably an indication of contemporaneity. 



The granite of the Mourne Mountains is composed of two felspars, 

 quartz and a dark mica. It is usually fine in grain but often contains drusy 

 cavities in which the constituents occur in a well-crystallized form and 

 attain considerable dimensions. The principal minerals of these cavities are 

 white orthoclase, albite and smoky quartz. The accessory minerals are 

 beryl, chrysoberyl, octahedral fluor, topaz and peridote (HAUGHTON). A rock 

 from Rostrevor is a fairly coarse-grained, grey granitite. Idiomorphic 

 quartz occurs as inclusions in the felspar and much of the quartz is seen 

 to be idiomorphic with respect to the felspar. The dark mica is very 

 strongly pleochroic and often contains minute grains of zircon. The 



