246 DE. H. H. THOilAS 02i XEXOLITHIC [vol. Ixxviii, 



still fluid matrix of a sillimanite-biichite which gives rise to 

 cordierite and spinel, or cordierite and corundum, according to 

 the amount of available magmatic magnesia (see analyses, p. 236). 



In the former case of direct and complete absor|)tion of aluminous 

 material b}^ the magma, the cordierites formed are either colourless, 

 rectangular, single crystals with distinct outline, or small triple and 

 complex twins. In magma contaminated with aluminous xenolithic 

 matter, the percentage of alumina may vary within widely- separated 

 limits, and there may be no excess of alumina beyond that necessary 

 to form cordierite with the available magnesia and silica. The 

 result, in such a case, is the formation of a cordierite-buchite that 

 consists solely of cordierite and brown glass (PI. A^II, fig. 3). Such 

 cordierite must be regarded as of pyrogenetic, rather than of meta- 

 morphic, origin. When there is a greater proportion of xenolithic 

 matter, sillimanite is the earlier product of crystallization, and this 

 is followed by cordierite, giving rise to a cordierite-sillimanite- 

 buchite similar to that figured in PL YII, fig. 2. The formation 

 of cordierite under such conditions has been dealt with and described 

 by many authors. 



When the tholeiite-magma reacts upon an ah'eady-formed silli- 

 manite-buchite, bringing about mutual modification by the trans- 

 ference of alumina from the xenolith to the mao^ma, andof mag^nesia 

 from the magma to the xenolith, large crystals of cordierite grow 

 outwards from the magmatic side into the buchite, keeping pace 

 with the diffusion of magnesia. Much of the sillimanite of the 

 l)uchite is generally resorbed, but such cordierite-crystals commonly 

 enclose a considerable undissolved portion in the form of the usual 

 slender needles. These needles are often to be seen passing across 

 the boundary between cordierite-crystals and buchite (PL X, fig. 1). 



Material collected from Locality 6, 3500 feet north of west of 

 Ormsaig, shows beautifully the result of interaction between silli- 

 manite-buchite and magma, and is illustrated by figs. 2 & 3 of PL X. 

 The sillimanite-buchite may be seen to pass, in the direction of the 

 igneous rock, into a sillimanite-cordierite-buchite of somewhat 

 coarsely crystalline character, the relatively large size of the crystals 

 being attributable to sustained elevated temperature. Adjoining 

 the true sillimanite-buchite the cordierites are of large size, and, 

 owino^ to their remoteness from the source of mao-nesia, contain no 

 inclusions of spinel ; but nearer the magma, where more magnesia 

 und less silica were available, the cordierites are full of minute 

 brownish-green crystals of spinel (PL TII, fig. 4). 



As is occasionally the case with cordierite of a pyrogenetic 

 nature, the mineral is colourless, exhibits no pleochroism, and in no 

 instance shows pleochroic halos around inclusions such as are usual 

 in the cordierite of the crystaRine schists and metamorphic rocks. 

 The formation of cordierite in the xenoliths appears in all cases to 

 belong to a relatively late stage in their metamorphism, and to be 

 the natural product of a magma that had become relatively richer 

 in silica by the normal process of differentiation, acting on highly- 

 aluminous material such as the sillimanite-buchite. The proportion 



