part 3] MINOE IlS^TRUSIONS IJN" THE ISLAND OF MULL. 257 



Fig. 2, Sillimanite -cordierite - buchite, 500 yards north-east of 

 Abhuinn Bail' a'Mhuilinn, 1000 yards north of west of the bridge 

 over Allt a'Mhuchaidh. The rock contains a few large and many 

 small crystals of anorthite, in a glassy matrix that is partly mag- 

 matic and felspathic and partly a sillimanite-cordierite-buchite, as 

 figured. The cordierite-crystals are particularly well-formed. The 

 rock suggests the incomplete solution of aluminous shale in a 

 tholeiitic magma. S. 18529. X 131-5. (See p. 245.) 



3. Cordierite-buchite, Allt a'Mhuchaidh, 1000 yards above the 



bridge. Composed of beautifully-formed crystals of cordierite, 

 many showing complex twinning, in a matrix of brown glass. The 

 rock is due to the local solution of aluminous xenolithic material 

 by the tholeiitic magma. S. 17997. X 10. (See p. 245.) 



4. Sillimanite-cordierite-buchite, Ormsaig. Large well-formed 



rose-pink crystals of sillimanite, and crystals of cordierite crowded 

 with minute brownish-green spinels, in a brown glassy matrix. 

 The striking feature of the rock is the size and colour of the 

 sillimanite - crystals : their large size suggests long - continued 

 heating and relatively- slow crystallization. S. 18001 a. x 11- 

 (See p. 245.) 



5. Siliceous xenolith of partly dissolved sandstone, Feorlein Cottage, 



Carsaig. The individual quartz-grains show beautiful tridymite- 

 fringes, now reverted to quartz. Large plates of tridymite, similarly 

 changed, occur in the glassy matrix. S. 20283. X 20, (See 

 p. 240.) 



6. Siliceous xenolith, south-east of Gortein Driseach. The quartz 



has undergone resorption by a melt of mixed character that has 

 consolidated as a streaky glass. Tridymite -fringes occur around 

 the quartz-grains, and cordierite is to a small extent developed in 

 the matrix. The rock has been produced by the action of a 

 tholeiitic magma on a quartzite. S. 16067. X 11. (See p. 239.) 



Plate VIII. 



Fig. 1. Small crystals of anorthite, showing evidence of rapid growth, forming 

 in a glassy magmatic matrix. They are the result of reaction 

 between the magma and an aluminous xenolith, and are accompanied 

 by spinel. Allt a'Mhuchaidh. S. 17998 a. X 15. (See p. 244.) 



2. Anorthite-sapphire-spinel assemblage. The normal crystal- 



line aggregate that appears to result from the initial action of the 

 tholeiite -magma on an aluminous xenolith. S. 16611. X 12*5. 

 (See p. 240.) 



3. This shows the crystallization of sapphire and deep-green spinel in 



brown magmatic glass. Both minerals are separated from the 

 glass by zones of anorthite-crystals. The manner in which the 

 crystals of anorthite are planted upon the spinels is particularly 

 striking ; see also fig. 4. Allt a'Mhuchaidh, 1000 yards above the 

 bridge. S. 17999. X 7. (See p. 247.) 



4. Another portion of the same section with higher magnification. Note 



the well-formed crystals of anorthite growing in brown glass, and 

 planted on an elongated crystal of greenish-brown spinel. The 

 spinel is possibly pseudomorphous after sillimanite. S. 17999. 

 X 42. (See p. 247.) 



5. Anorthite-spinel assemblage of an aluminous xenolith, showing 



green spinel and anorthite in eutectic relationship. The structure 

 probably results from the reheating of the ordinary spinel-anorthite 

 aggregate of an aluminous xenolith — a point emphasized by the 

 occurrence of pseudomorphs after cordierite within the anorthite 

 of another part of the slide. Old Road, 100 yards north-west of 

 Feorlein Cottage, Carsaig. S. 20286. X 29. (See p. 247.) 



