226 Scientific Proceedings, Royal Dublin Society. 



very irregular in structure ; but associated with them are horn- 

 blendytes or schistose rocks, more or less regularly bedded. 



In various parts of the Cos. Gralway and Mayo the slates or 

 shales, and grits or sandstones of Ordovician age, graduate into 

 schists and quartzytes respectively ; in Co. Mayo a portion of the 

 Silurians in the neighbourhood of Louisburg is changed into schists. 



Most of these schists, as they rise in flat-bedded stones, are very 

 suitable for walling, and are very generally used in the different 

 areas. 



Outlying Tract. A few miles north-east of Dunmore, Co. 

 Galway, at the north-east extremity of Slieve Dart, there is an 

 exposure of felstone. This stone is of a flaky character, which 

 unfits it for building purposes ; but it is useful as road metal. 



About nine miles eastward of Westport, south of Ballyhean, 

 there is a small hill of metamorphic rocks {Ordovician). These are 

 in no place well exposed ; yet among them we can find an ophytic 

 hornblendic rock, or perhaps an eklogyte. This stone may possibly 

 be suitable for ornamental purposes, as a specimen of it, obtained 

 with a great deal of trouble, cut and polished easily and well. It 

 is of handsome and unique green and brown shades of colour. 



EAST MAYO, SLIGO, ROSCOMMON, AND LEITRIM. 



The Granitic rocks are found associated with the schists, in the 

 hills known as the Slieve Graniph and Ox Mountains. The largest 

 exposure is in the former, constituting a long, wide, south-west and 

 north-east tract, extending from the country north of Castlebar, 

 past Lough Cullin and Foxford, into the Co. Sligo ; while there is 

 a much smaller tract further north-east in the Ox Mountains in the 

 vicinity of Lough Easky : the former being on comparatively low 

 ground, and the latter at a higher elevation. In the schist country, 

 between the large mass of the granite and the Carboniferous rocks 

 of the Castlebar district, there are small intrudes, courses, and 

 veins of granite. 



All the above granites are foliated, and now appear as a coarse 

 gneiss, more or less similar in appearance to the Laurentian gneiss. 



[There is a marked distinction between this gneiss and the granitic gneiss of the 

 Co. Galway. The latter, although in places somewhat similar in aspect to that of 

 north-west Mayo and Sligo, is evidently the result of the extreme metamorphism of 



