Colk— Origin of the Orbicular Granite of Mullaghderg. 143 



(fig. 1), is crowded with the crystalline spheroids described by Hatch. A 

 block in the foreground of the photograph shows the character of the sea- 

 worn surfaces, and it is possible that some extension of the rock-type occurs 

 under the sand towards the bay. 



The joints break cleanly through the spherulites, 3 which give the im- 

 pression of a charge of round shot fired into the granite cauldron while its 

 contents were in a molten state. They lie close to one another in a crowded 

 group, divided by thin films of granite. Subsequent veins, also of red granite, 

 cut the mass, and occasionally cross the spherulites. 



The grey outer zone of most of the spherulites, composed of felspathie 

 matter arranged radially, together with associated grains of magnetite, has 

 been well described by Hatch. The nucleus within it, often consisting of 

 coarse-grained red granite, weathers away more easily than the radial zone. 

 Since the granite matrix surrounding the spherulites is similar in composition 

 to many of the cores, it also becomes worn down, and the grey zone stands 

 out on joint-surfaces as a raised ring. Here and there the radial zone con- 

 sists of red felspar, side by side with the more frequent grey examples. 



In many cases concentric shells of granitoid matter lie between the grey 

 zone and the nucleus, the latter exhibiting merely a granitic structure without 

 zoning. The most important point to be recorded in this paper is that some 

 of the cores consist of elongated, irregular, and ragged schistose matter, with 

 no resemblance to segregations. The foreign nature of these cores can be 

 still more clearly recognised in the polished slabs that are now placed in the 

 Geological Survey Collection in the National Museum, Dublin (fig. 3). A few 

 small inclusions of foreign rocks occur near a grey dyke referred to by 

 Hatch. 1 One on the east side of the dyke is somewhat granitised. I have 

 little doubt that these were derived from the Dalradian series invaded by the 

 granite ; but they are not surrounded by spherulitic matter. 



Here and there a group of the large spherulites, together with the 

 interstitial and environing granite, is surrounded by a grey zone with radial 

 structure, precisely similar to that round individual spherulites. The whole 

 group seems to have become so far differentiated in the cooling granite as 

 to promote a deposition of andesine and magnetite at its outer surface. 



3 The term spherulite was logically used by Vogelsang (" Etudes sur les cristallites," 

 Archives ne'erlandaises, vol. vii) for such forms, as well as for the more familiar small 

 examples in dykes and lava-flows, and it conveys|no assumption as to mode of origin. 

 Variole, though employed by von Chrustschoff in a general sense, properly refers to 

 spherulites in variolites. A. 0. Lawson (Univ. Calif., Bull. Depart. Geol., vol. iii, p. 396) 

 uses the word orbule. 



4 Op. cit. (1), p. 548. 



2a2 



