Cole — Origin of the Orbicular Granite of Mullaghderg. 14fr 



dark mica after an episode of melting. They represent the reconsolidation of 

 basic matter that was unable to diffuse away completely from the central region 

 of attack. The resemblance of this patch to the recrystallised relics of schist- 

 layers in composite gneisses is suggestive, and, indeed, convincing. Where 

 the nucleus of a spherulite consists of granite, this nucleus often contains more 

 biotite than is common in the granite of the cauldron, a feature that is, 

 of course, explicable if the granitic matter has entered the spherulite as a 

 replacement of an originally basic nucleus. 



More than this, the grey concentric zones of oligoclase and magnetite 

 have been attacked by resorption, and they have been penetrated here and 

 there, like those of Kangasniemi, 10 by films from the still fluid magma. They 



Fig. 2. — Thin section of central portion of spherulite from granite of Mullaghderg, showing 

 arrangement of hiotite, which represents the residue of an inclusion, around crystals of felspar, x 14. 

 Geological Survey specimen. 



have also suffered from stoping and solution at their inner surfaces (Plate IV, 

 D, and Plate, V, I). A molten and highly siliceous (unclersaturated) zone has 

 worked against them, as well as against the relics of the basic nucleus. In so 

 doing, its temperature has been raised, 17 and it no doubt remained liquid 

 after the consolidation of the magma in the cauldron. Its primary efficiency, 

 however, depended on the supply of material from without, and on the 

 possibility of the diffusion of absorbed material in a reverse direction 



10 Op. cit. (11), p. 20. 



17 See S.J. Shand, "On Saturated and Unsaturated Igneous Rocks," Geol. Mag., 

 1913, p. 510. 



