144 Scientific Proceedings, Royal Dublin Society. 



Beyond this surface, spherulites have arisen independently, like those in the 

 surrounded group. 



The impression made upon me in the field was that a portion of the roof 

 covering the granite cauldron, to which possibly some already chilled granite 

 adhered, had fallen in upon unconsolidated material below, and that the 

 blocks had promoted a rapid cooling, with consequent spherulitic crystallisa- 

 tion round them. It was left for subsequent examination of polished surfaces 

 and thin slices to show how far interaction had gone on between the included 

 materials and the granite magma in which they had become immersed. 



Hatch quotes observations by Kilroe to the effect that the occurrence at 

 Mullaghderg lies at a considerable distance from the margin of the granite. 

 This point is of importance if the spherulites are held to have originated in 

 undigested inclusions. Mica-schist forms the eastern boundary of the granite 

 cauldron four miles away at Crolly Bridge ; but it is clear that we have to 

 consider in such cases the possible nearness of the roof. Mica-schist and 

 limestone occur as part of the roof, or a roof-pendant, at Annagarry Bridge, 

 and limestone and quartzite are associated as a patch in the granite at 

 Lough Ibby, still nearer to Mullaghderg. 



II. The Nature and Oeigin of the Nuclei of the Spherulites 



of Mullaghderg. 



K. von Chrustschoff (Khrushchov), 5 in 1891, classed the rock of Mullaghderg 

 among those in which orbicular structure originated around "inclusions," 

 which might be more acid or more basic products of segregation from the 

 magma, or which might be, on the other hand, foreign bodies. His point is that 

 something that was already cooled before the individualisation of the main 

 crystalline mass has 'served in such cases as a nucleus for spherulitic growth. 

 He places in the same group the orbicular rocks of Wirvik (Finland), Kortfors 

 (Karlskoga, Orebro, Sweden), and Slattmossa (Sweden) in distinction from 

 those of Corsica and Bomsas, in which he believes the spherulites to be 

 " endomorphe Kontaktbildungen." All these, like the rock of Mullaghderg, 

 show radial structure in their spherulitic growths; and von Chrustschoff 

 connects this 6 with the close packing of the nuclei. 



N. 0. Hoist and F. Eichstadt 7 described the spherulites in the Slattmossa 



6 "Uber holokrystalline makrovariolitischeGesteine " ; sub-title, "Uber einige neue 

 mid weniger bekannte holokrystalline Kugelgesteine " ; Mem. Acad. Imp. Sci., St. Peiers- 

 bourg, vii' ser., tome xlii (1891-4), p. 231. 



Q Ibid., p. 232. 



7 " Klotdiorit fran Slattmossa, Kalmar Ian," Geol. Foren. i Stockholm Forhandl., 

 toI. vii. (1884), p. 134. 



