764 ME. F. E. C, EEED ON THE LOWEE PALEOZOIC [NoV. iSpQy 



surfaces, in connexion with a kind of concretionary structure 

 intimately related to a series of rectilinear cracks traversing the 

 rock in all directions, and not arranged with any regularity. In 

 each of the areas thus enclosed is a series of closely-packed 

 concentric perlitic cracks, often with subsidiary systems of con- 

 centric cracks. The whole appearance of the rock reminds one, on 

 a minute scale, of the spheroidal jointing in granite and other rocks. 

 With crossed nicols the perlitic structure becomes invisible, and the 

 groundmass is seen to consist of an indistinct microcrystalline 

 aggregate or irregular mosaic, with only a few microspherulites and 

 felspar-microlites. In some places the blurred-mosaic structure is 

 less evident, and the groundmass appears to consist of crowded 

 groups of incipient or ill-developed microspherulites. There are 

 no phenocrysts visible. 



This rock has been analysed for me in the University Laboratory, 

 Cambridge, by Messrs. H. 0. Jones & C. Eobinson, with the 

 following results : — 



Per cent. 



SiOs Silica 78-12 



AlgOg Alumina 13-61 



FcgOg Iron Sesquioxide 1-08 



CaO Lime 2-04 



MgO Magnesia -71 



KgO Potash 2-67 



Na^O Soda -24 



H^O Water -75 



99-22 



This analysis shows a type distinct from any described ^ from the 

 Waterford area, particularly in the proportion of the potash to the 

 soda. 



Another felsite [568] [13 w] from Newtown Head shows under 

 the microscope a minute distinct mosaic of irregular quartz -grains, 

 each containing many inclusions. Locally this structure appears 

 to be more of a granophyric than of a micropoikilitic nature. 

 Nests of clear granular quartz occur, probably representing original 

 vesicles, and one or two porphyritic plagioclase-felspars are also^ 

 distinguishable. 



A pale greenish felsite [501], from the same series, showing 

 in the groundmass loosely-packed clear circular spots which, with 

 crossed nicols, are seen to possess no radial structure but to enter 

 merely into the general granular character of the groundmass, 

 seems to be a variety of the nodular felsites which are common at 

 Eaheen. The nodules are frequently as large as peas, and stand 

 out on a weathered surface ; but sometimes [14 w], under the 

 microscope, they give no clear indication of their presence. 



In another case [502] they are visible under the microscope, 

 in ordinary light, as circular or slightly irregular areas free from 

 the semi-opaque microlites, granules, and dust which are aggre- 

 gated round their margins, and scattered less thickly throughout 



^ Hatch, Geol. Mag. 1889, p. 545 & references. 



