Vol. 6S.'] PALEOZOIC EOCKS OF THE EILBEIDE PEmNSTILA. 95 



Phenocrysts of felspar are conspicuous in sections of (62) ; and 

 in (122) the weathered felspar is seen to be bordered by fresh 

 secondary material. 



The angite is, as a rule, granulitic in character ; but in some cases 

 (58, 62) large well-terminated crystals of augite occur. Less often, 

 as in (136), a specimen taken from the centre of the big dyke south- 

 east of Lough Mweelaun, the augite is ophitic. 



As for accessory minerals, magnetite is generally distributed. 

 Chlorite and calcite are common as alteration -products or j&lling 

 small vesicles, and epidote is occasionally met with (58). A specimen 

 (60) taken from the eastern end of the big dyke north of the Doon 

 Rock has a very fine-grained ground-mass, and its characters in 

 microscopical section are rather those of an augite-porphyrite than 

 of a dolerite. Its specific gravity, however, is 2-81. In addition to 

 the idiomorphic augites, it contains chloritized pseudomorphs, 

 probably after rhombic pyroxene. 



(ii) The mica- dole rites occur west of the Kilbride burial- 

 ground and north of the Doon Rock. They difi'er from the other 

 dolerites only in the presence of numerous fiakes of biotite. Dr. 

 Flett points out the presence of albite in these rocks, and that the 

 association of albite-diabases with spilites occurs in Mid Argyll 

 and is universal in Cornwall. Although these two rocks occur in 

 the Kilbride district, they can hardly be said to be associated, since 

 the spilites are of Arenig age, while the mica-dolerites are 'post- 

 Llandovery intrusions. Apatite is present as an accessory in the 

 rock from (66). 



(e) The Spilites (Pillow-Lavas). 



These are rocks of fine and uniform grain, consisting principally 

 of small lath-shaped felspars which often show fluxion-structure, or 

 are arranged in a divergent sheaf-like manner. Occasionally a 

 subvariolitic structure may be observed. The felspar is practically 

 always albite. Porphyritic felspars are rather rare ; they sometimes 

 (29) contain prehnite, and were probably not originally albite but 

 lime-bearing plagioclase. Occasionally pseudomorphs which seem 

 to be after pyroxene are present, but no unaltered ferromagnesian 

 minerals are preserved. Some of the felspars are replaced by 

 carbonate. Iron-ore (sometimes magnetite, sometimes ilmenite) is 

 often present, as small patches uniformly distributed over the 

 section and probably secondary. Vesicles are numerous, and may 

 be filled with carbonates, quartz, chalcedony, or chlorite. Dr. Plett, 

 to whom we are indebted for many of the observations recorded 

 above, alludes to the resemblance which the Kilbride rocks present to 

 the Ordovician pillow-lavas of the South of Scotland, Megavissey 

 and Mullion Island in Cornwall. The resemblance to the Devonian 

 and Carboniferous spilites of Devon and Cornwall is less marked. 



