50 TEOF. T. G. BO^^NEY AND THE REV. E. HILL: [Feb. I9I2, 



dykes. The ground-mass is cryptocrystalline to microcrystalline, 

 showing in many parts a tendency to spherulitic structure, in 

 crystallites more or less 'bunched'; but in parts it is almost a 

 micropegmatite. A little quartz is present, generally in clustered 

 granules ; some small rather altered biotite, similarly aggregated ; 

 also granules of decomposed iron-oxide, and one or two which may 

 be an impure sphene. Prof. Bonuey inclines to regard this rock 

 as a qu art z-porphy rite rather than a quartz-felsite, but a 

 chemical analysis alone could settle that point. 



The dyke on the shore (B) shows a crowd of very minute plagio- 

 clases (species indeterminable), perhaps set in a cryptocrystalline 

 * paste/ and rather numerous minute flakes of mica, white and 

 brown. Of larger minerals, there are felspars, several of fair size 

 and idiomorphic, mostly plagioclase with rather small extinction- 

 angles ; quartz-grains, not numerous and composite ; biotite, rather 

 altered, sometimes into a chlorite ; and a little iron-oxide, which 

 is associated occasionally with (possibly) a secondary sphene. This 

 rock is a quartz-porphyrite. 



The rather gneiss - like dyke (C) presents a cryptocrystalline 

 ground-mass, with phenocrystals of felspar, some rather rounded, 

 others idiomorphic, most of them certainlj^ plagioclase, with rather 

 low extinction-angles ; also some quartz and a moderate amount of 

 altered biotite, and some iron-oxide. This rock belongs to the 

 same group as the last one. It shows signs of having suffered 

 somewhat from pressure, both ground-mass and felspar being rather 

 cracked. 



A specimen from llocquaine Castle, described in 1884, much 

 resembles the grits from Pleinmont. Macculloch ^ speaks of ' a 

 stratum of argillaceous schist .... at the lower parts of the bay,' 

 and Ansted^ of 'a small patch of clay slate.' South of the Fort, the 

 shore-rocks below high-water mark are (nearest to the land) gneiss; 

 farther out is a fine-grained rock which includes patches and pockets 

 of a coarser one, apparently gneiss. Over a few square yards this 

 fine-grained rock resembles a slate. It extends seawards, and often 

 meets portions of coarse rock in a manner resembling igneous 

 contact. But the outcrops are much shattered, and in one clean 

 face the shapes of the coarser included patches, and a mottling in 

 the fine-grained ground-mass, strongly suggest crush. 



Two specimens in this locality were described and discussed in 

 1884^ with the result that, although the one was almost impossible 

 to distinguish from a grit, the other, in which a small part had an 

 almost identical structure, seemed in the rest to be a crushed 

 gneiss. We then inclined to regard both as much-crushed gneiss, 

 and this view has been confirmed by a third specimen, collected by 

 IMr. Hill in April 1911 : part of this slice much resembling the 

 Pleinmont grit, the rest almost certainly being crushed gneiss. 

 His field-work, as described above, suggests that this slaty rock 



^ Trans. Geol. Soc. ser. 1, rd. i (1811) p. 10. 



2 In a note, 1st ed. (ISGi) p. 257 ; ."rd ed. (1833) p. 208. 



3 Q. J. G. S. vol. xl, p. 427 (Nos. 40 & 47). 



