part 3] XENOLITHIC MI?fOE INTEUSIONS IN MULL. 231 



In 1912 Mr. Anderson discovered in the bed of a tributary of 

 Abhuinn nan Torr (Locality 2) an exposure of a rock that con- 

 tained abundant small plates of a deep-blue mineral. The rock 

 was submitted to me for examination, and, after isolation, the 

 blue mineral proved to be corundum of the sapphire-variety. 

 Further work on this locality, and some excavation, yielded a 

 series of most interesting specimens that proved clearly the abnormal 

 and intensely xenolithic character of the rock in question ; but the 

 relation of the xenolithic mass to the surrounding rocks was unfor- 

 tunately obscured by surface-accumulations. 



Later in the same year, Mr. G. V. Wilson & Mr. D. Tait^ 

 observed sapphire-bearing xenoliths among the beach-pebbles on the 

 shore of Carsaig Bay, south-west of Carsaig village ; following 

 this clue, they detected the parent source of these xenoliths in a 

 composite sill that cuts through the Jurassic sandstones and 

 Tertiary lavas about a mile and a half south-west of Carsaig, and 

 forms the low promontory of liudh' a'Chromain (Locality 1). 



As the survey of the western portion of Mull progressed, other 

 sills with sapphire-bearing xenoliths were encountered by E. M. 

 Anderson & C. T. Clough ^ : most of them lay, as before, in the 

 peninsular region south of Loch Scridain ; but a few others occurred 

 on the northern side of the Loch, in the neighbourhood of Tiroran. 



All the material collected by the officers of the Survey was 

 handed to me for examination, and was supplemented by collections 

 which I made when visiting the island in 1913 and 1914. 



The beauty of the xenoliths and the occurrence of such meta- 

 morphic minerals as corundum, spinel, sillimanite, cordierite, and 

 anorthite make these inclusions in themselves pre-eminently worthy 

 of petrographical description ; but, at the same time, an almost 

 unparalleled opportunity offers itself for the study of the progressive 

 metamorphic changes wrought in xenolithic material by an igneous 

 magma of basic composition. 



The object of the present communication is, therefore, twofold : 

 first, to describe in some detail the complex mineral assemblages 

 that constitute the xenoliths of these tholeiitic intrusions ; and, 

 secondly, to discuss the relation of the individual minerals to each 

 other and to the igneous magma that chemically and physically 

 controlled their formation. 



II. The Ii^TRrsiONs. 



General Description, Distribution^ and Petrography. 



All the xenolithic intrusions that form the subject of this paper 

 occur relatively close together in the south-western portion of the 

 Tertiary lava-field, in that part of the Ross of Mull which lies 

 between Carsaig and Penn3^ghael on the east, and Bunessan on the 

 west. Also, to a less extent, they have been met with north of 

 Loch Scridain, around Tiroran in the peninsula of Ardmeanach. 



1 'Summary of Progress for 1912 ' Mem. Geol. Surv. 1913, pp. 48 & 66. 

 ^ 'Summary of Progress for 1914 ' Ibid. 1915, p. 34. 



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