the Counties of Mayo and Sligo in Ireland. 169 



tions, the plates of which are generally parallel, reflecting the light at the 

 same angle over the surface of the specimen. Quartz in hexagonal pyramids 

 also occurs, as well as green earth, lepidolite, and different zeolites. Near 

 the middle of the mass, which is about 1100 yards wide, the felspar becomes 

 opaque, yellowish grey, and regularly crystallized in rectangular prisms, the 

 augite diminishing in quantity, and the zeolites and quartz increasing ; but on 

 approaching the southern boundary the augite predominates, and the felspar 

 resumes its transparency and greyish colour. Near the hmits of the mass, 

 changes gradually take place, the grain becoming closer, and the crystals di- 

 minishing in size, till the trachyte can scarcely be distinguished from the 

 sandstone. The latter rock also becomes more compact and harder at the 

 junction, while the shale passes into Lydian stone, and the stratification is 

 much deranged, the dip being towards the trachyte on both the north and 

 south limits. A small dyke or vein is seen at the southern part of this bed, 

 having nearly an east direction. A few yards from the point where it emerges 

 from the bank, it is broken across and discontinued, but about three feet 

 to the south, occurs again in the same direction. This lateral shift or removal, 

 is twice repeated before it reaches low-water mark, and suggests the idea of 

 successive horizontal slips having taken place, from the north, after the consoli- 

 dation of this vein and the including rock. The trachyte is continued across 

 the lands of Ross to the Palmerstown River, where it forms the bank for some 

 distance, and its junction with the sandstone is well displayed under the old 

 church of Templemary on the opposite shore. Having been protruded late- 

 rally, it lies above, and at the plane of contact a substance is interposed 

 resembling fine clay slate, except in the lamellar structure. Could this inter- 

 vening layer have proceeded from fusion of the sandstone alone, or from its 

 component parts mingling with those of the liquid trachyte in the proportions 

 which form clay slate? Analysis might determine the question. Augite here 

 predominates, and the rock graduates into a coarse basalt in jointed columns. 

 Ascending to the cross-roads at Mullinacrush, the bed approaches more to 

 porphyry, being almost entirely composed of glassy felspar in thin tabular 

 crystals laterally compacted together, and in detached portions presenting a 

 schistose aspect, but in the mass rising in blocks like granite, without any 

 approach to stratification. It contains very sparingly, zeolite, chalcedony, 

 iron pyrites, and augite. It occupies the summit of the hill, not, however, ex- 

 tending far to the westward. On the opposite shore of Killalla Bay, I found 

 the trachyte on the lands of Cahirvove, bounded by limestone, upon which 

 it appears to lie ; but no seam divides the two rocks, which are mingled 

 irregularly at their junction, the limestone containing both felspar and 



VOL. V. SECOND SERIES. Z 



