part 3] MiNOE iNTRrsiONs in the island of mull. 255 



V. SUMMAEY AND CONCLUSIONS. 



The western portion of Mull lying between Loch Scridain and 

 Loch Buie is remarkable for a series of minor intrusions that are 

 generally of a tholeiitic and andesitic character. Occasionally they 

 are composite, and one of the best examples of such a sill is that 

 which occurs at Rudh' a'Chromain and at Nuns' Pass west of 

 Carsaig. They frequently contain abundant xenoliths of both 

 accidental and cognate nature. The cognate xenoliths are glomero- 

 porphyritic masses of bytownite and hypersthene, or bytownite 

 and augite, and are frequently congregated in the lower portions 

 of the intrusions. The accidental xenoliths are both of siliceous 

 and aluminous types, the aluminous inclusions being generally by 

 far the more prevalent and ranging up to several feet in diameter. 

 The siliceous xenoliths show the usual type of alteration : quartz- 

 grains have developed fringes of tridymite, and in some instances 

 are surrounded by secondary augite. The aluminous xenoliths, 

 with which this paper deals more particularly, are characterized by 

 well-crystallized minerals, such as corundum (sapphire), spinel^ 

 sillimanite, cordierite, and anorthite. 



These aluminous xenoliths offer the clearest evidence of the 

 modification of a more or less pure aluminous sediment by permea- 

 .tion of magmatic matter, more particularly by the diffusion of 

 lime, ferrous iron, and magnesia. The aluminous sediment, when 

 simply altered by thermal agencies, takes the form of a sillimanite- 

 buchite (sillimanite and glass), and this by the chemical action of 

 the magma has most often been transformed, in whole or in part^ 

 into a coarselj^'-crj^stalline mass of anorthite that encloses silli- 

 manite, corundum, and spinel. Further action by the magma in a 

 modified form, in which it is relatively enriched by magnesia and 

 alkalies, has resulted in the formation of cordierite with a con- 

 comitant separation of corundum, spinel, and an acid plagioclase. 



It is held from the evidence afforded by the xenoliths that the 

 metamorphism was of a deep-seated character, and was produced 

 by a tholeiitic magma acting upon the lining of its basin, this 

 lining being constructed largely of aluminous sedimentary materiaL 

 The deep-seated character of the metamorphism is supported by 

 three facts: — (1) the xenoliths show practically no signs of modi- 

 fying, or of modification b}", the igneous rock now in contact with 

 them; (2) during the process of intrusion into their present 

 position they have suffered deformation, owing to their still plastic 

 state; and (3) they have developed vesicular cavities in their 

 contained glassy matter, consequent on the decrease of pressure by 

 Avhich dissolved volatile constituents were allowed to escape. The 

 vesicular cavities have in many instances been filled with low- 

 temperature minerals such as analcite, chalcedony, and calcite. 

 The mutual relations of the various xenolithic minerals, interpreted 

 in the light of recent work on fused mixtures of silicates and 

 oxides, suggest that the temperature of the magma in its basin 

 during the period in which the metamorphism was effected probably 



