part 3] MINOR INTRUSIONS IN THE ISLAI5D OF MULL. 241 



produced by the simple fusion and subsequent consolidation of 

 sedimentary material without or with admixture of magmatic 

 matter supplied by the invading igneous rock. Such buchites 

 have been described frequentl}^ especially by Prof. A. Lacroix and 

 Dr. Alfred Harker ; and, later, by Dr. J. S. Flett, in association 

 with the Tertiary dykes of the Oban and Dalmally district. ^ 



The sillimanite-buchites, of which many examples have been 

 collected from the various localities, are compact, vitreous, grey- 

 blue or lilac-coloured rocks, consisting of glass in which the 

 abundant minute needles of sillimanite are usually arranged in a 

 parallel manner (PL VII, figs. 1 & 2). 



The cordierite-buchites are most often black, and cordierite is 

 either the chief or the only crystalline constituent (PL VII, tig. 3). 



(2) What has been termed the anorthite-corundum-spinel 

 assemblage is an almost holocrystalline mixture of these minerals, 

 and is extremely prevalent in the intrusions under description. It 

 is clearly of xenolithic nature, and is well illustrated by PL VIII, 

 fig. 2. 



(3) The cordierite-sillimanite-spinel assemblage is less 

 common than Group 2, and results presumably from somewhat 

 special conditions that are discussed later. 



The relation and interdependence of the anorthite-corundum- 

 spinel assemblage and the sillimanite-buchites are clearly indicated. 



In some of the more symmetrical xenolithic masses it can be 

 demonstrated that the innermost portion is a dark-grey or dull 

 lilac-coloured rock of vitreous character (sillimanite-buchite). This 

 vitreous mass, followed towards its outer margin, is seen first to 

 contain scattered felspar-crystals, often skeletal in form, and then, 

 by the felspars becoming more frequent and better formed, to 

 develop into a more or less completely crystalline felspathic zone. 

 This zone, often several inches thick, and rich in corundum and 

 spinel, normally s(iparates the buchite from the enclosing tholeiite. 



The felspathic portion (anorthite-corundum-spinel assemblage) 

 is roughly divisible into two — an inner sub-zone in which corundum 

 in the form of sapphire is abundant and spinel subordinate, and an 

 outer sub-zone in which the ratio of these two minerals one to the 

 other is reversed. 



The line of demarcation between the holocrystalline portion of 

 the xenolith and the surrounding igneous rock is usually quite 

 sharp, and the igneous rock rarely shows any compositional 

 variation that can be attributed to direct action of the foreign 

 inclusion. 



In practically every xenolith of any size that has been examined 

 a zonal arrangement can, in some measure, be made out, but not 

 always of a symmetrical character (p. 249). Sometimes the inner 

 vitreous portion (buchite) is reduced to a minimum, and at other 

 times the outer crystalline zone is thin or of varying thickness. 



1 ' Geology of the Country near Oban & Dalmally ' Mem. Geol. Surv. 1908, 

 pp. 129-31 ; see also ' Geology of Colonsay & Oronsay, with part of the Ross 

 of Mull ' Mem. Geol. Surv. 1911, pp. 93-95. 



