148 Scientific Proceedings, Royal Dublin Society. 



outwards. In many cases a granitic nucleus has finally resulted, with no 

 trace at the centre of the former basic core. The material of this core must 

 now be sought in the outer shells of the spherulite. Continued action has 

 threatened even these outer zones ; and in some cases the original inclusions 

 have left merely dark fluidal bands, which impart to the granite here and 

 there the aspect of a gneiss (compare p. 157). 



The granite nuclei are thus localised instances of a process of pseudo- 

 morphosis that has become familiar to us on a large scale in the field. The 

 cleanness of the contact-zone which is seen in many instances where a granite 

 magma has invaded superincumbent or surrounding rocks has been explained 

 by the draining off of the absorbed matter to other portions of the cauldron. 18 

 The bands of granite that often predominate in the material of a composite 

 gneiss, such as that on the south side of the Gweebarra Kiver in the county 

 of Donegal, are not merely injections between layers of schist, but take the 

 place of bodies of schist or associated rocks that have disappeared by inter- 

 change with the advancing granite. Such a pseudomorphosis obviously 

 implies removal of material. Professor Schwarz 19 speaks of a replacement of 

 the slate at Sea Point, near Cape Town, by granite, and of felspathic matter 

 as dissolving out the slate and crystallising " in the cavity it had eaten out 

 for itself." He urges that, " of the various substances in solution, felspar is 

 the most mobile, and gains admittance to the slates first." Though he does 

 not specify an opposite process of diffusion, he no doubt contemplates the 

 passing out of aluminous and ferromagnesian matter from the slates. I have 

 seen reason 28 to differ from his views as to the production of the porphyritic 

 orthoclase, which is accompanied in the slate by more fine-grained and 

 diffused material from the granite, and which may have been floated into the 

 softening slate in an already formed condition ; but my own examination of 

 the historic Sea Point section enables me thoroughly to agree with Professor 

 Schwarz's well-expressed proposition that it reveals " a pseudomorph in 

 granite of the sedimentary rocks." Whatever view is taken in such cases as 

 to the origin of large crystals of felspar within the attacked rock, their 

 resemblance to those in the adjacent granite furnishes evidence of the 

 introduction of matter from the invader. The softening of the rock that is 

 attacked, whether it is a bounding mass or an inclusion, allows of its ready 



'*> G. A. J. Cole, " Geology of Slieve Gallion," Sci. Trans. R. Dublin Soc, vol. vi (1897), 

 p. 242. 



19 E. H. L. Schwarz, "The Sea-Point Granite-Slate contact," Trans. Geol. Soc. 

 S. Africa, vol. xvi (1914), pp. 37, 35, and 36. 



20 G. A. J. Cole, " A Composite Gneiss near Barna (County Galway)," Quart. Journ. 

 Geol. Soc. London, vol. lxxi (1910), p. 186. 



