Cole— Origin of the Orbicular Granite of Mullaghderg. 145 



rock as products of crystallisation from the granite magma, and Dr. Hoist 

 informs me that his opinion is still unchanged. Von Chrustschoff 8 believes 

 that in this case basic products of segregation have served as nuclei, and 

 have reacted on the magma like foreign inclusions, a zone of intermediate 

 composition arising on their surfaces. Personal experience over wide areas 

 has made me very sceptical as to these so-called basic segregations, 

 which prove in so many cases to be ill-digested foreign bodies. Von 

 Chrustschoff 3 holds that the magnificent spherulites of the granite of 

 Ghistorrai, near Fonni in Sardinia, which happen to show concentric 

 structure only, originated in fragments of various foreign rocks. These are 

 closely packed, as is the case at Mullaghderg, and it is pointed out that in 

 such conditions the zones of mixture during a process of absorption remain 

 longer in stationary contact with regard to the material of the inclusions. 

 Interaction is thus favoured, while complete absorption by diffusion outwards 

 is prevented. Von Chrustschoff remarks that orbicular structure is unlikely 

 to occur in a deep-seated (intratelluric) reservoir, where total destruction of 

 a foreign body will take place. Observations in recent years, however, on 

 blocks removed by " stoping " from the roofs or margins of such reservoirs 

 show that all stages of alteration may be traced in connexion with " batholites" 

 of vast extent. Stress may, however, be laid on von Chrustschoff'' s suggestion 

 that close packing is an important factor in the production of orbicular 

 spherulites from inclusions, and Carl Benedicks and Olof Tenow 10 show in 

 their experiments that melting is checked by the near approach of nuclei. 



Von Chrustschoff was not in a position to determine the presence of 

 foreign nuclei in the granite of Mullaghderg ; field-observation, however, as 

 we have seen, at once reveals them. In this and other respects the rock 

 closely resembles that occurring in boulders at Kangasniemi in Finland. 

 The admirable description of the latter example by B. Frosterus" leaves, 

 indeed, little to be said as to the main characters revealed at Mullaghderg. 

 The nuclei at Kangasniemi are inclusions of gneiss with dark mica; and 

 Frosterus 12 urges that magmatic action has changed some of these inclusions 

 into rocks with granitic structure. While the mother-rock contains biotite, 

 plagioclase (albite or oligoclase), orthoelase, microcline, and quartz, the 



8 Op. cit. (5), p. 229. 9 Op. cit. (5), pp. 128 and 239. 



10 "Kvinstliehe Naehbildung von Schmelz-und Kugelstructuren in Gesteinen," Geol. 

 Foren. i Stockholm Forhandl., vol. xxxiii (1912), p. 108. 



11 "Ueber einen neuen Kugelgranit von Kangasniemi in Finland," Bull, Coram, 

 geol. de Finlande, No. 4 (1896), Compare his account of the rock of Wirvik, Tscherm. 

 Mitt., vol. xiii (1893), p. 177. 



^Ibid. (1896), p. 9. 



