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Proceedings of the Roj/al Irish Acadeni;/. 



various angles in one layer; small crystals, which are virtually 

 colour-spots, occurring in the next layer ; and a third layer being 

 practically free from mica. These zones coincide, in the specimens 

 in my hands, with the bedding-planes of the shale ; but I suspect 

 that examples will be found where the zonal arrangement will be 

 seen to be parallel to the surface of contact with the intrusive mass, 

 Avhether this runs along or transverse to the bedding-planes. 



In Portlock's specimens, it is not clear if the coarse development 

 of the mica occurred in actual contact with a dolerite sheet, since the 

 specimens are free from igneous matter where the large crystals are 

 displayed. But I think it highly probable that a more massive sheet, 

 or the main igneous mass, lay towards this surface, the mineralising 

 agents of Levy, the " crystallisers" of Doelter,^ operating there most 

 freely. For the production of biotite or hornblende, it is now 

 recognised that the presence of some such stimulus is required.^ 



The brown mica, however, does not represent the first product 

 of metamorphism in the calcareous shale of Portrush. It is well 

 known that the rock has become flinty, and shows the streak of 

 steel when we attempt to scratch it with a knife. This is largely 

 due to the formation of minute grains and prisms of pale yellow- 

 green to yellow-brown pyroxene throughout the ground. Small 

 granules of quartz, apparently also secondary, and sometimes includ- 

 ing spherulites of chlorite, occur here and there ; there are also a 

 few nests of zeolites ; but the essential mineral is pyroxene, which 

 sometimes replaces a whole zone in closely packed and fairly uniform 

 granules. The brown mica in consequence includes granular pyroxene 

 in abundance, and has developed, indeed, first as colour-spots in the 

 interstitial material, and then as more defined crystals, with continuous 

 cleavage-planes, but without proper bounding edges. 



In a specimen collected by myself, the mica, so long as it is 

 minute, looks as if it had arisen simultaneously with pyroxene 

 granules of similar size ; but this effect is probably deceptive, 

 since all the larger crystals include pyroxene. A black mineral, 

 in feathery aggregates of small rods, with the appearance of 

 magnetite by reflected light, occurs in the zone in which mica is 

 best developed, and also in less degree on either side of it. This 

 suggests rutile : but even in very small prisms it is not translucent. 



1 retrogeuesis (1006), pp. 22 and 21. 



~ The biotite-calcipLyres of Monte Somma are well known. For a small Irisli 

 example, see Cole, " On the Geology of Slieve Gallion," Sti. Trans. 11. Dublin Soc, 

 vol. vi. (1897), p. 224. 



