258 DE. H. H. THOMAS ON XENOLITHIC [vol. Ixxviii, 



Fig. 6. Aluminous xenolith, shoAving anorthite full of minute crystals 

 of corundum and invaded by magma. The magma has resorbed 

 part of the original felspar with its excess of alumina, bringing 

 about the groAvth of a less basic felspar in optic continuity with 

 that previously existing, and the precipitation of well - formed 

 plum - coloured spinels. A large crystal of spinel occupies the 

 centre of the field. Eiidh' a'Chromain. S. 16605. X 12. (See 

 p. 245.) 



Plate IX. 



JFig. 1. Xenolith invaded by tholeiitic magma. The mass consists of anorthite 

 full of sillimanite-needles which have been partly re-dissolved, with 

 the subsequent growth of secondary felspar on the remaining 

 anorthite. The later felspar is free from sillimanite, and consists 

 partly of anorthite, partly of oligoclase, which forms the outer- 

 most zone and sends skeleton growths out into the glassy matrix. 

 The later formation of the felspar of more acid character has been 

 accompanied by the precipitation of the excess of alumina as 

 sapphire and spinel. The sapphires (of which several are shown 

 in the figure) are surrounded by feathery growths of oligoclase. 

 Eudh' a'Chromain. S. 17177. X 13-5. (See p. 245.) 



2. Xenolith invaded by tholeiitic magma. The section shows anorthite 



full of sillimanite, and a silhmanite-buchite invaded by dark tho- 

 leiitic matter (glass). Resorption of anorthite with its contained 

 sillimanite has taken place, followed by the crystallization of large 

 dark-green spinels (and cordierite enclosing spinel). A less basic 

 felspar, following solution of the sillimanite-bearing anorthite, has 

 grown outwards from the undissolved anorthite, and forms a 

 noticeable zone between it and the magmatic glass. Shore a quarter 

 of a mile from the mouth of Allt Coille Moire. S. 17405. X 16. 

 (See p. 247.) 



3. Original anorthite of a xenolith full of sillimanite undergoing partial 



resorption by tholeiitic magma, -svith the separation of the excess 

 of alumina as corundum and spinel, and the feathery crystallization 

 of a less basic plagioclase (oligoclase) in the matrix. Eudh a'Chro- 

 main. S. 17178. X 13-5. (Seep. 248.) 



4. Junction of normal tholeiite with sillimanite -buchite, showing a re- 



action-zone : this zone consists of basic plagioclase and cordierite. 

 Cordierite occurs in relatively large crystals growing into the 

 buchite, is enclosed in the anorthite, and forms a narrow band 

 between the anorthite and the tholeiite. A few crystals of cord- 

 ierite occur within the tholeiite itself near the junction. Old road 

 100 yards north-west of Feorlein Cottage, Carsaig. S. 20289. 

 X 9. (See p. 245.) 



5. Original sillimanite-bearing anorthite invaded and resorbed by magma, 



with the consequent crystalhzation of a sillimanite-free felspar of 

 less basic composition on the undissolved nuclei. Eudh' a'Chro- 

 main S. 16603. X 45. (See p. 245.) 

 -6. An example of the breaking-up of the primary anorthite- sillimanite 

 xenolith and resorption by magma. The excess of alumina fur- 

 nished by the resorbed anorthite and sillimanite has been precipitated 

 as a deep plum-coloured spinel and corundum. A regrowth of 

 basic plagioclase edged with oligoclase has taken place on the un- 

 dissolved felsijar-fragments, and shows white in the figure. The 

 melt surrounding the large spinels finally consolidated as oligoclase, 

 skeleton spinels, and glass. Nuns' Pass, Carsaig. S. 20271. 

 X 7. (See p. 254.) 



