144 MESSRS. C. I. GAEDINER & S. H. REYNOLDS [Feb. 1 898, 



There are a few instances, however, where the gronndmass 

 shows the structure known as * micropoikilitic' Mr. Alfred 

 Harker, in his paper on ' The Gabbro of Carroek Fell,' ^ and Mr. F. 

 E,. Cowper Reed, in his notes on ' The Geology of the Country 

 around Fishguard,' ^ notice this structure, the former in lavas 

 metamorphosed by the gabbro, the latter in felsites which he con- 

 siders metamorphosed. In the last-named paper a very full account 

 of previous notices of this structure, which seems generally to 

 have been seen in felsitic rocks, is given. On Lambay Island, how- 

 ever, it occurs in andesitic rocks and in the coarse porphyrite. It is 

 to be seen in five of the slides which we have had cut ; three of these 



Fig. 3. — Vesicular andesite from Freshwater Bay. 





r. 



\ 



y 



\ 



-J" 



X 18. 



[The rock is very much altered, calcite and otber minerals having come in with 

 the alteration. In the vesicles the calcite often assumes a spherical 

 arrangement, as shown in the figure, the spheres of calcite being separated 

 by spherical shells of haematite.] 



are from andesites, and two from the coarse porphyrite. Two of the 

 slides from the andesites are cut from rocks close to the junction 

 with the limestone at Kiln Point. This rock in a hand-specimen is 

 seen to have an appearance somewhat different from that of the usual 

 fine andesite of the island, being more compact and horny, and 

 the sections show the alteration of the rhombic pyroxene which 

 they contain and of the gronndmass, the small felspars in which 

 have to a large extent disappeared, ragged-edged plates of quartz 

 having been formed, giving rise to the micropoikilitic structure. 



1 Quart. Joum. Geol. Soc. vol. 1 (1894) p. 311. 



2 Ibid. vol. li (1895) p. 149. 



