26 Proceedings of the Royal Irish Academy. 



the internal part being granular. A noticeable feature of the rock is the 

 occurrence in it of irregular patches of crystalline material. These are of 

 a whitish colour, and are in marked contrast to the darker ground of the 

 rock. They are composed of olivine crystals completely altered, which are 

 notched by and include crystals of a pale pyroxene, the latter showing some 

 alteration into a green fibrous mineral. This appears to be an example of 

 the glomero-porphyritic structure first described by Judd 1 in the ophitic 

 dolerite of Fair Head, Co. Antrim, the associated olivine and pyroxene 

 having separated out at an earlier period from the magma, out of which the 

 enclosing rock was itself formed. A feature of these glomero-porphyritic 

 aggregates is the absence of titaniferous magnetite; although it occurs 

 abundantly scattered throughout the rest of the rock, these areas are 

 almost all entirely free from it. The individual grains of these porphyritic 

 aggregates are related to each other in the same manner as the grains in 

 an ordinary holocrystalline basic rock, such as a gabbro. They are allotrio- 

 morphic with regard to each other ; but in relation to the ground mass they 

 are sometimes idiomorphic in outline. 



The absence of titaniferous magnetite seems to point to an earlier 

 crystallization of these patches at some depth below the surface, while the 

 magma was still molten, the idiomorphic outline being due to a secondary 

 crystallization which has probably taken place during a later stage in the 

 process of consolidation. This is illustrated by some of the pyroxene crystals 

 which lie on the borders of these porphyritic areas. The crystals have a 

 distinct junction which divides them roughly into two equal portions, one 

 half being colourless, the other with the characteristic purplish brown tint. 

 The inner half, which is portion of the porphyritic area, is colourless, and 

 has no definite crystalline boundaries, while the outer portion, which projects 

 into the matrix of the rock, is idiomorphic, and is identical in colour with 

 that which occurs in the ground. The pale part of the crystal is free from 

 magnetite, while along the junction; between the two parts, which are in 

 optical continuity, a band of magnetite grains lie ; these grains are also 

 enclosed by the brownish portion of the crystal. Thus, the pale part of the 

 crystal was an earlier crystallization which had taken place in the magma 

 before intrusion, and before any of the titaniferous magnetite had begun 

 to crystallize, the other portion being added after intrusion when the 

 whole rock had begun to consolidate, and, as is usual with the pyroxene, 

 after the titaniferous magnetite had crystallized. There can be very little 

 doubt but that the porphyritic aggregates were formed under plutonic 



1 Quart. Jour. Gool. Soc, Lund., vol. xlii, 1886, p. 71. 



