part 3] MIFOE INTRUSIOIfS IN THE ISLAND OE MULL. 239 



IV. The Accidental Xenoliths (Enclaves Enallogenes). 



The greater number of accidental xenoliths are carried by tho- 

 leiitic sheets, or in the basic tholeiitic portions of the composite 

 intrusions. They are, however, not entirely restricted to the more 

 basic rocks, and may be found in less numbers and in a somewhat 

 different state of metamorphism in the more acid portions of some 

 of the sills, especially that of Kudh' a'Chromain. 



The}'' may be divided into two classes, according to their nature, 

 which we may designate respectively as (a) siliceous and (b) alu- 

 minous. The former class includes such xenoliths as arise from 

 sandstones and granitic or gneissose rocks, while the latter class is 

 exemplified by xenoliths that appear to have had their origin in 

 aluminous shales of fairly constant composition. The aluminous 

 xenoliths, besides occurring in greater quantity, also have greater 

 average dimensions, and it is with these that we are chiefly 

 concerned. 



(a) The Siliceous Xenoliths. 



These present no particularly new features, and the majority 

 of the chancres that thev have undergone have been dealt with in 

 a most complete manner by other authors, more particularly by 

 Prof. A. F. A. Lacroix.i A few points, however, are worthy of 

 mention, especially those indicating the selective fusion of the 

 felspathic constituents and the almost universal formation of tri- 

 dymite around grains of undissolved quartz when the interstitial 

 melt is acid. The separation of augite and rhombic pyroxene 

 around undissolved quartz from a sufficiently basic melt, or from 

 an interstitial melt modified by transfusion of basic igneous 

 material, is too well known to call for further comment. 



Sandstones, granitoid rocks, and what presumably have been 

 gneisses are all represented among the xenoliths, and all show a 

 similar and relatively simple type of metamorphism. Sandstones 

 are the most prevalent, being distinctly common in the intrusions 

 of Localities 1 & 2. 



The Carsaig Sandstone, or some similar rock, has furnished a 

 considerable number of xenoliths to the sills, and the nature of the 

 metamorphism is identical with, but greater in degree than, that 

 exhibited by the Carsaig Sandstone in contact with the intrusive 

 tholeiite of Rudh' a'Chromain. The metamorphism is usually 

 unaccompanied by any transfusion of igneous material, and has 

 been produced by purely thermal agencies. Felspathic constituents 

 at the elevated temperature thus originated have given rise to an 

 interstitial melt, which attacked the surfaces of the quartz-grains 

 and thus became enriched with dissolved silica. Crystals of 

 tridymite separated from the melt, and attaching themselves at 

 all angles to the undissolved grains, formed fringes. The residual 

 melt consolidated as glass. The tridymite-crystals have now 



^ ' Les Enclaves des Roches Volcaniques ' Ann. Acad. Macon, vol. x, 1893. 



