ON A HILLSIDE IN DONEGAL 357 



banding has arisen by lit par lit injection of rocks already solid, 

 but with well-marked foliation-planes which provided easy 

 passage for the invader ; and it may be urged that, if we accept 

 the differentiation-view in the case of Skye, 1 other cases may 

 arise where a gneiss may result from the intrusion of one 

 igneous rock into another of far older date. 



The study of these relatively modern fluidal gneisses in 

 Skye led the authors named to point out the importance of 

 original flow in the structure of the pre-Cambrian gneisses of 

 the west of Scotland ; 2 and Harker's work in Rum, 3 to which we 

 now return, is an equally important contribution to the study 

 of gneisses of all ages. 



The gneisses of Rum, " with well-marked parallel banding 

 and foliation, and frequent alternations of different lithological 

 types, are perfectly characteristic gneisses in the ordinary 

 descriptive sense of the word. Indeed, their appearance led 

 Sir Archibald Geikie to assign them to an Archaean age." They 

 are related to a highly disturbed belt of country, and are 

 believed to represent material forced into place under consider- 

 able pressure. But their main masses do not show evidence 

 of crushing, and their foliation is produced by igneous flow. 

 A granitic magma, the Tertiary granite of the Inner Hebrides, 

 has, in fact, flowed into pre-existing basic rocks. " Given a 

 granitic magma enclosing debris of more basic rocks, an irregular 

 distribution of the debris such as is seen where the xenoliths " 

 (derived fragments) " are still traceable, reactions between the 

 basic rocks and the acid magma of a kind familiar in many 

 other districts, and that drawing out of the whole by flowing- 

 movement which is proved by the banded structure, we have 

 a complete explanation of the principal part of the Rum 

 gneisses." 4 The " basic rocks appear to have been of the 

 nature of gabbros, now transformed by metamorphism, and in 

 some measure by interchange of material with the acid magma." 

 Sollas's work at Carlingford 5 on the interaction of granite and 

 gabbro, which was often regarded as a localised instance, or 

 even as a misreading of the field-evidence, has been more than 



1 See Harker's discussion of these rocks, Igneous Rocks of Skye , pp. 90 and 119. 



2 See also Geikie, Ancient Volcanoes, vol. i., p- 116. 



3 See also Harker, "Tertiary Crust-Movements in the Inner Hebrides," Trans. 

 Edinburgh Geol. Soc. vol. viii. (1904), p. 346. 



4 Harker, Quart. Journ. Geol. Soc, 1903, p. 212. 



5 Trans. R. Irish Acad., vol. xxx. (1893), P- 477- 



