Cole — Origin of the Orbicular Granite of Mullaghderg. 157 



accompanied, in all probability, by a rise of temperature, threatened the 

 spherulites with resorption. The streaks and swirls of granite rich in biotite 

 that appear in the more normal granite of Mullaghderg probably represent 

 flakes detached from the bounding walls of schist, which were carried about 

 in the magma under normal conditions, such as favour irregular diffusion. 

 They are too rich in basic magnesian matter to have been produced by the 

 resorption of the oligoclase-magnetite zones of the spherulites. 



The grey zones that occasionally occur, as described in Section I of this 

 paper, about a whole group of spherulites and their interstitial granite may, 

 however, be ascribed to the resorption stage. The included region has served 

 as a compound nucleus, from which matter has streamed out, until it was 

 precipitated along a saturated zone in the quiescent magma round about. 

 The closely packed spherulites have probably, as von Chrustschoff suggests, 

 prevented free flow and rapid interaction in a magma that was otherwise still 

 effective as regards temperature and possibilities of diffusion. 



The plasticity of the zonal portion of the spherulites during the resorption 

 stage is evidenced at Mullaghderg, as at Wirvik and Kangasniemi, by the 

 moulding of the spherulites against one another. Like the closely set rock- 

 inclusions at Portnoo, the spherulites are separated by thin sheets of the 

 encasing granite ; but none the less their outer surfaces have influenced one 

 another. The convex bulge of a spherulite produces a concavity in a neigh- 

 bour squeezed against it (Plate IV, C, D, and F). 



IV Summary. 



From examination of the orbicular granite of Mullaghderg in the field, 

 and in polished surfaces prepared for the Geological Survey of Ireland, 

 a close resemblance is noted between this rock and that described by 

 B. Frosterus from boulders at Kangasniemi in Finland. The orbicular 

 structure has developed round a number of close-set foreign inclusions of 

 biotite-schist, which may have been originally amphibolites. By diffusion, 

 basic material has spread from these inclusions outwards, and has in part 

 been precipitated as a zone of soda-lime felspar and magnetite. The granite 

 magma has been substituted for the dissolved portions of the inclusions, and 

 complete destruction of the inclusions has thus given rise to spherulites with 

 granitic cores. Where a relic of the schist remains, it is surrounded by 

 granite, outside which lie the radial and concentric zones described by 

 Dr. Hatch in 1888. Besorption-effects are noticeable, acting from without, 

 and also from within the spherulites, on the radial and concentric 

 envelopes. 



SCIENT. PEOC. K.D.S., VOL. XV., NO. XV. 2 



