64 Proceedings of the Royal Irish Academy. 



upwards, so far as I have observed, before reaching the zone of 

 altered sediment, as if this flinty layer was already in existence in its 

 metamorphosed state, and provided a tough obstacle, while the basalt 

 below still shrank, settled, and split open. 



Sir A. Geikie ^ has carefully described these veins and those at 

 Fair Head, and has urged that the complete dovetailing of the 

 crystals at their edges in between those of the main dolerite shows 

 that the latter rock was still plastic at the period of the intrusion of 

 the veins. A high temperature seems certainly to have prevailed in 

 the invaded mass, since it is difficult otherwise to account for the 

 coarseness of the crystallisation within the veins. Eut the renewed 

 growth of crystals in an invaded rock, and the interchange of 

 constituents at high temperatures with those of its invader, may 

 cause an interlocking of crystals to arise between rocks of very 

 different ages.^ 



Sir A. Geikie observed also how the felspar at Portrush is 

 collected in the central part of the veins, the dark constituents being 

 gathered on the margins. To this it may be added that the pyroxene 

 and magnetite are distinctly more conspicuous in the lower part of 

 the horizontal veins, though they are also grouped towards the upper 

 surface. Sometimes the augite crystals grow out in the upper part, 

 where they have more play, approximately at right angles to the 

 surface of the sheet; in the flower zone they are more closely 

 matted together. The marginal aggregation gives us, as it were, a 

 model, in one small sheet after another, of the Hauptmassiv and 

 Grenzfaciesglieder of Brogger.^ Even in microscopic sections of 

 veinules a millimetre across, traversing fine-grained basalt, it is 

 possible to trace a gathering of granular pyroxene on the margins, 

 representing the first deposit from the magma against the bounding 

 walls. At this stage, then, the conditions were clearly not such as 

 Avould produce the ordinary ophitic structure. Such marginal 

 differentiation in veins has of course been noted in other areas. 

 Professor R. B. Young^ has thus recently described a similar darkening 

 of the sides of small basic veins at Corby Craigs. The gravitational 

 separation towards the lower surface in the Portrush examples is 



^ Ancient Volcanoes of Great Britain, vol. ii., pp. 300 and 303. 



- Cf. Cole, " On a Hillside in Donegal," Science Progress in the Twentieth 

 Century, A'ol i. (1906), pp. 3,51 and 353. 



Die Eruptivgesteine des Kristianiagebietes ; I. Die Gesteine der Grorudit- 

 Tinguait-Serie (1894), p. 179. 



^ Op. cit., Trans. Edin. Geol. Soc, vol. viii., p. 334. 



