part 3] MINOE INTRUSIONS IN THE ISLAND OF MULL. 237 



although they often have a more or less vitreous centre (leidleite) 

 with stony marginal portions. The xenoliths are generally con- 

 centrated near the margins ; and their containing matrix, where 

 investigated, consists of tholeiite similar to the lateral members of 

 the Rudh' a'Chromain intrusion. 



The intrusion of Mullach Grlac an t'Sneachda, in which sapphire 

 was first detected, calls for special notice on account of its unusual 

 character. It is unfortunately but indifferently exposed, and its 

 relations to adjoining rocks are somewhat obscure. It appears as 

 a dark-grey amygdaloidal rock of fine texture and igneous aspect. 

 It is studded with small lustrous plates of sapphire, and packed 

 with aluminous and siliceous xenoliths of all sizes. A mass of 

 siliceous material 6 feet long exposed by the side of the burn is 

 clearly a xenolith. Cognate xenoliths are unrepresented, except in 

 the form of isolated and often broken crystals of dark-green 

 hypersthene and less frequently of augite, indicating an early 

 crystalline separation from a tholeiitic magma similar to that 

 which gave rise to the cognate xenoliths of Rudh' a'Chromain. 

 The chief anomaly is the dark matrix, which, under the micro- 

 scope, is seen not to be a true igneous rock, but to be a sillimanite- 

 buchite (p. 240) modified by more or less complete admixture with 

 igneous material. The apparent j)orphyritic crj^stals are all broken 

 bytownite and anorthite, derived from highly crystalline aluminous 

 xenoliths (p. 241), and, therefore, themselves of xenolithic nature. 



That the rock was intruded as a molten or semi-molten mass is 

 proved by its numerous vesicular cavities, now filled with zeolites 

 and other low- temperature minerals. 



The amount of xenolithic material carried by this intrusion is 

 very considerable : for, apart from the abundant accidental xeno- 

 liths held by the pseudo-igneous matrix, the matrix itself is largely 

 composed of fused sedimentary material (sillimanite-buchite), 

 modified to some extent and doubtless rendered more fluid by 

 admixture with a tholeiitic magma. It will be seen in the sequel 

 that such an intrusion would result from the squeezing-upwards of 

 the viscous semi-fused material that formed the wall of the magma- 

 basin (p. 250). 



A similar type of intrusion would appear to be represented in 

 the kersantite of Michaelstein,i which forms a dyke of obviously 

 mixed igneous and sedimentary material full of a variety of 

 minerals foreign to the rock. 



Generally speaking, the tholeiitic intrusions have produced little 

 thermal alteration of the rocks into which they have been ulti- 

 mately injected, although the sill at Traigh Bhan na Sgurra is a 

 notable exception. The intrusions are seldom of large dimensions, 

 their type of crystallization indicates rapid consolidation, and we 



^ M. Koch, ' Die Kersantite des Unterliarzes ' Jahrb. K. Preusg. Geol. 

 Landesanst. 1886, p. 44. 



