Cole — Origin of the Orbicular Granite of Muilaghderg. 153 



graphite. This serves to illustrate their proposition that, as a rock-inclusion 

 melts, a zone of molten products forms about it, in which relics of the solid 

 nucleus survive. These particles gather towards one another and towards the 

 still unmelted core. "When melting has gone on up to the centre of the 

 inclusion, the origin of the structure becomes unrecognisable, as is usually the 

 case in orbicular granites. 



J. A. Phillips, 34 who was one of the first to study concretionary patches 

 and inclusions in granite in thin slices with the microscope, also made good 

 use of polished surfaces. In this he followed A. Delesse, 35 who drew with a 

 camera lucida from such surfaces of spherulitic rocks. Phillips notices that 

 in the granite of Dyce Quarry, near Aberdeen, a thin layer of mica envelops 

 a large mass of more finely grained granite, and that the quartzose sedi- 

 mentary rocks included in the granite of Goraghwood in Co. Down are 

 " darkened by the presence of a disseminated material of a nearly black 

 colour, to a depth of about an inch only." This altered portion is more 

 hornblendic than the central mass, and contains less lime ; the other chemical 

 differences pointed out by J. A. Phillips seem unimportant. 



At Swan Mount, Portnoo, in the county of Donegal, the granite contains 

 abundant closely set inclusions of the local Dalradian rocks. More prolonged 

 heating might have converted these into spherulites, like those of Slattmossa, 

 Kangasniemi, or Muilaghderg. When a polished surface is examined, dark 

 reaction-rims are seen round the aphanitic inclusions. The first step has 

 been taken towards zone-formation, by the diffusion outwards of certain 

 constituents, and the concentration of others in the relict zone. 



Tbe resorption-zones so often seen round amphiboles that are included 

 porphyritically in andesitic lavas are familiar illustrations of the same, 

 selective process. Magnetite is developed in these zones, and resists solution. 

 In some cases the original crystal is represented merely by an assemblage of 

 black grains. Since magnetite is the dark mineral in the concentric zones at 

 Muilaghderg, it is of interest to quote another case of its development in a 

 relict zone. E. A. Daly 30 has observed nodules of granular olivine in a 

 peridotite dyke, which probably developed at some place other than that to 

 which they have now been carried. These inclusions, as we may call them, are 

 3 to 6 cm. in diameter, and are surrounded by kelyphitic shells up to 15 mm. 

 thick. The photographs given show a darker zone between the nodules and 



34 Op. cit. (24), pp. 12 and 17. 



35 " Recherches sur lea roches globuleuses," Mem. Soc. geol. de France, 2me ser., 

 vol. iv (1851), p. 301. 



36 " Geology of the N. American Cordillera at the 49th Parallel," Geol. Surv. Canada, 



Mem. 3S (1912), p. 43, and plate 37. 



