146 Scientific Proceedings, Royal Dublin Society. 



felspar of the granitic kernel, where alteration has gone so far, is oligo- 

 clase, 13 with some andesine posterior to it, and a very little orthoclase 

 and microcline. .Frosterus concludes, that a fluid zone formed round the 

 inclusions, as a mixture of their material and the granite magma, and that 

 plagioclase-constituents moved from this towards the centre, while more 

 acid matter (potassium felspar) moved outwards. By diffusion, the molten 

 material of the mixed zone became divided into concentric layers, the acidity 

 of which increases outwards. Analyses show that the chemical composition 

 of the mixed zone is intermediate between that of a schistose inclusion and 

 of the granite. 14 



Frosterus observes that at Kangasniemi potassium felspar is more pro- 

 minent in the felspathic zone round about the kernel of the spherulites when 

 this kernel has undergone the greatest alteration, and he looks on this as a 

 replacement of plagioclase by resorption. Nuclei of granitic structure occur 

 both at Kangnasiemi and Mullaghderg, side by side with less modified nuclei 

 of schist. If the replacement suggested by Frosterus can go on within the 

 kernel as well as in the mixed zone immediately round it, ice have the 

 ■possibility of a pseudomorphic replacement by granite of the original included 

 block of biotite- gneiss, schist, or amphibolite. 



The evidence at Mullaghderg is very strong in this direction. Hatch, 15 

 working on very limited material, described the nucleus of the spherulite 

 examined by him as consisting principally of triclinic felspar, together with a 

 little quartz and possibly orthoclase. He observed that it was not so basic as 

 the marginal portions. In the field or on our polished slabs, a fragment of 

 schist is often seen within the felspathic nucleus (Plate IV, E and F). The 

 foreign schistose rock is thus the original nucleus. Where it has been 

 preserved, it is surrounded by a granite zone, rich in quartz and felspar. 

 The concentric and radial zones in turn surround this granitic region, and 

 beyond them lies the general granite of the cauldron (fig. 3). 



Not only has the zone around the schistose nucleus an essentially granitic 

 structure, but it is rich in potassium felspar. The felspars of this zone, 

 moreover, occasionally penetrate the schistose nucleus, and indicate the 

 commencement of pseudomorphosis. In a thin section cut from a highly 

 granitic nucleus (fig. 2), a cluster of biotite crystals, separated from one 

 another, but sufficiently grouped and concentrated to suggest their former 

 association, remains as the last trace of the inclusion. The biotite crystals 

 are associated with magnetite and with a little sphene. They are arranged 

 round crystals of felspar, as if their constituents had separated out to form 



13 Ibid., pp. 18 and 7. u Ibid., p. 31. l6 Op. cit. (1), p. 555, 



