part 3] MINOE IT^TETJSIOI^S IN THE ISLAND OF MULL. 233 



margin is chilled against an earlier intrusion of bostonite, which 

 separates it from the Carsaig Sandstone in that direction. 



The details of the intrusion are illustrated in the sketches 

 (figs. 2 & 3, p. 234) made by Mr. D. Tait. 



The central portion of the sill is a pale-grey rock, finely crystal- 

 line for the greater part, without any marked porphyritic con- 

 stituents. Locally, it is distinctl}?" hyaline in texture, and, like 

 many of the pitchstones of Mull, has developed the characteristic 

 ' sheath-and-core ' structure of these rocks. ^ 



Xenoliths occur sparingly throughout its mass, but perhaps are 

 more prevalent in the somewhat less acid selvages that adjoin the 

 basic members on each side. The xenoliths are mainly of sand- 

 stone, and range up to 6 inches or so in longest diameter. Large 

 xenoliths of both shale and sandstone occur in a more central 

 position, and may measure several feet in diameter, with an average 

 of about 2 feet. 



The basic marginal members of the sill clearly antedate the acid 

 interior. They differ somewhat one from the other in relative 

 thickness, and in the number and character of the contained xeno- 

 liths. In regard to texture, they appear to be fine-grained dark- 

 grey to dark-brown rocks, exhibiting a more compact facies at 

 their outer contacts with the Carsaig Sandstone and the bostonite 

 respectively. 



The upper basic member is divisible into two zones. The outer 

 zone, about 4 to 6 feet thick, contains abundant cognate xenoliths 

 of gabbroid character. These xenoliths are of all sizes, from quite 

 small dimensions to a foot or so in diameter. The inner zone of 

 similar material, ranging from 2.| to 5 feet in thickness, is densely 

 crowded with accidental aluminous xenoliths of all shapes, which 

 range in size from an inch to 4 feet in greatest dimension. The 

 lower basic member of the sill is from 2 to 5 feet thick, and is 

 characterized particularly by the size and abundance of its cognate 

 xenoliths and the relative rarity of accidental xenoliths. The 

 concentration of cognate xenoliths in the lower portion of the 

 Traigh-Bhan-na-Sgurra sheet is a point worthy of notice in this 

 connexion. 2 



The acid interior of the sill is, as a whole, a compact pale-grey 

 to brownish- grey rock, which, towards its external margins, for a 

 foot or two, darkens in colour, apparently becomes slightly more 

 basic, and carries numerous xenoliths of sandstone. These more 

 basic and relatively thin marginal portions are of the nature of 

 inninmorite, a type of Mull pitchstone characterized by the 

 presence of uniaxial augite and described in 1915 in the paper 

 already cited by Mr. E. M. Anderson & Mr. E. G. Eadley.^ 



^ E. M. Anderson & E. G. Radley, 'The Pitclistones of Mull & their 

 Genesis ' Q. J. G. S. vol. Ixxi (1915-16), p. 210. 



^ J. S. Flett, ' Geology of Colonsay & Oronsay, with part of the Ross of 

 Mull ' Mem. Geol. Surv. 1911, p. 92. 



^ ' The Pitchstones of Mull & their Genesis ' Q. J. G. S. vol. Ixxi (1915-16) 

 p. 209 ; see also A. F. Hallimond, ' Optically Uniaxial Augite from Mull ' 

 Min. Mag. vol. xvii (1914) p. 97. 



