330 THE GRANOPHYRES OF STRATH (SKYE). [May 1 896, 



he had pointed out that Sir Archibald Geikie's conclusions with 

 respect to the granophyre of the Western Islands of Scotland being 

 intrusive into the gabbro, and therefore younger than the latter, 

 were strikingly confirmed by the precisely similar phenomenon in 

 the Island of Elba, where the Tertiary granite traversed the gabbro, 

 diabase, and serpentine dykes. Mr. Harker had now shown the 

 same phenomenon to exist also in Skye, and the speaker therefore 

 wished to emphasize the analogy of Elba, the more so as Gen. 

 M c Mahon had brought forward similar evidence with reference to 

 the Himalayas. 



Mr. Barrow drew attention to the strong contrast between the 

 evidence adduced by the Author to show actual absorption of part 

 of these inclusions, and that published some time ago in his and 

 Mr. Marr's work on the metamorphic aureole surrounding the Shap 

 Granite. In the latter case, the Authors selected the amygdules 

 in certain altered igneous rocks, and produced exceptionally clear 

 evidence of the extremely limited migration of material that accom- 

 panied the development of new minerals in these bodies. Experience 

 has shown that some of our best data are obtained in searching for 

 the cause of such widely differing results. At present the evidence 

 seems to suggest that the * initial depth temperature ' may be one of 

 the chief factors in determining the amount of change produced. 



Mr. Rtttley asked for further information regarding the geological 

 structure of the area represented in the diagram. It was suggestive 

 of proximity to a centre of eruption, and the irregular character of 

 the patches mapped as gabbro-bearing granophyre, and their asso- 

 ciation with volcanic agglomerates and bedded lavas, favoured this 

 belief. Cases such as this, of the absorption of fragments of a 

 basic rock by an acid one, were, he thought, of comparatively rare 

 occurrence. The reverse was more frequently the case. The ex- 

 posures, described as dykes, might possibly be portions of buttress- 

 dykes cutting irregularly through the agglomerates. The paper 

 dealt with several points of great interest. 



The President also spoke. 



The Atjthor briefly replied, thanking the speakers for their 

 appreciation and criticism. Though the phenomena of incorporation 

 of basic material by an acid magma appeared to be uncommon, at 

 least on such a scale as in the examples described, the converse case 

 of acid rocks absorbed into basic was illustrated by very many dykes 

 in the Skye district. A considerable difference in acidity between 

 the absorbed and the absorbing rock seemed to be as essential a 

 factor in the process as that of temperature ; hence the absorption 

 of gabbros by granophyres observed in several districts. The 

 peculiar intrusions described were possibly somewhat younger than 

 the normal granophyres of Skye ; but wherever these latter were 

 seen in junction with gabbros in the area as yet mapped by the 

 Author, the same relation, namely, the acid intrusion succeeding 

 the basic, was invariably verified. 



