Vol. 59.] TOKBIDONIAN, ETC. OF THE ISLE OF EUM. 215 



the other elements which enter into the complex may be interpreted 

 as representing other known members of the Tertiary suite, dis- 

 guised by the metamorphosing and corroding action of the acid 

 magma, fused by it, and forming with it various xenolithic and 

 hybrid rocks. 



In their paper on the banded gabbros of Skye, Sir Archibald 

 Geikie & Mr. Teall, after demonstrating the origin of the gneissic 

 structure there by fluxion in a heterogeneous rock-magma, urged the 

 application of the same principle to some gneissic rocks of much 

 greater antiquity. The facts now recorded suggest a certain extension 

 of the idea there thrown out. It appears that the requisite hetero- 

 geneity, which in some cases arises from imperfect differentiation, 

 may in other cases be brought about by admixture; and that there 

 may be produced in this way banded rocks which, although of 

 purely igneous origin, present unusual mineralogical associations, 

 and do not readily find a place in any systematic scheme of normal 

 igneous rocks. Whether this principle also may have an application 

 beyond the particular case described above is a question which I 

 shall not presume to decide. 



EXPLANATION OF PLATE XIV. 



Geological sketch-map of the Isle of Bum, reduced from the field-maps to a 

 scale of 1 inch to the mile. In order to avoid needless complication the inland 

 lochs and streams are omitted, as well as the less important hills ; also all 

 geological details not relevant to the subject of the paper; in particular, the 

 very numerous small plutonic intrusions, dykes, and sheets of Tertiary age. 



The area not yet surveyed, in the south and south-west of the island, is 

 occupied wholly by Tertiary igneous rocks, excepting only the southern 

 termination of the disturbed Torridonian belt. 



Discussion. 



Prof. Jtjdd congratulated the Author on the splendid work which 

 he had been doing in the Inner Hebrides, and especially in the 

 little-known island of Rum. He had himself confined his neces- 

 sarily rapid traverse of that island (nearly thirty years ago) to the 

 great igneous masses, and had paid little attention to the Torridonian 

 rocks, but the account given by the Author of the disturbances 

 undergone by these ancient rocks, as well as of gneissic structures 

 formed in igneous masses near their planes of junction, was of a 

 most interesting and suggestive character. 



Prof. Bonney said that he was not likely to differ from the 

 Author's interpretation of the gneisses, because he had already, on 

 more than one occasion, called attention to the formation of banded 

 gneisses and hornblende-schists by the intrusion of an acid into 

 basic rocks, as in Cornwall and Sark. He thought that sometimes 

 there was, though not very commonly, local melting of a more 

 basic and solid by a more acid and liquid material, and sometimes 

 a large mass at a very high temperature broke into and mixed with 

 other large masses, which also were extremely hot and perhaps not 



