268 Scientific Proceedings, Royal Dublin Society. 



In thin slices the rock is seen to consist of a microgranitic basis, 

 with crystals of zonal, and other felspar scattered through it, as well 

 as occasional plates of chlorite, and associated epidote, magnetite, 

 and zircon. The basis consists of minute crystals, grains of felspar, 

 muscovite, and quartz. The zonal felspar crystals have, in many 

 cases, been broken and cemented together again by a subsequent 

 deposit of frequently non-zonal felspar. About the margins of the 

 felspar crystals a fringe of smaller and irregularly-shaped ones 

 sometimes occur. 



The chlorite complexes are evidently derived from biotite, and 

 represent the biotite of the previously described granite. 



The minute muscovite crystals of the basis are frequently 

 arranged more or less in parallelism, especially in the neigh- 

 bourhood of the larger felspar crystals, and when two of these occur 

 in close proximity, the narrow band of matrix lying between them 

 presents exceptionally large and parallel crystals of muscovite. 



No trace of microcline is to be detected. 



The history of this rock would thus appear to have been the 

 same as that of the main mass of granite from which it was 

 derived up to a certain point, i.e. as far as the formation of the 

 zonal felspars. When this stage had been reached, an eruption 

 into schists took place, leading to rapid cooling, and, as a conse- 

 quence, to the microcrystalline character of the matrix. 



Soda-Granite of Coolboy. — In thin slices this presents very much 

 the same characters as the soda- granite from Aughrim. According 

 to Dr. Haughton's analyses it is, however, about 7 per cent, poorer 

 in silica, and in accordance with this the difference between the 

 angle of extinction of the core and the outermost envelope of the 

 zonal felspar is on the whole greater than in the case of the 

 Aughrim rock. Microcline, though present, is less abundant. 



Soda-Granophyre of Croaghan-Kinshella. — This remarkable rock 

 contains, according to Dr. Haughton's analyses, a larger proportion 

 of soda and a smaller proportion of potash than any other of the 

 rocks of this group. Its mineral composition, as calculated by Dr. 

 Haughton, is as follows : — Albite, 62 per cent. ; quartz, 38 per cent. 

 If we calculate the percentage of different felspar molecules present, 

 directly from the percentage of lime, soda, and potash present, we 

 obtain the following :— Albite, 47-12 per cent.; anorthite, 4-41 

 per cent. ; orthoclase, 2*37 per cent. Disregarding the potash, and 



