part 3] MINOR iNTsrsiONs ii^ the island of mull. 243 



The colour of the crystals is deep blue in thick sections, and very 

 pale blue for the thickness of an ordinary rock-section. The pleo- 

 chroism is generally quite distinct, if not strong: — 



w=:pale blue. 6=pale greenish- blue to sea-green. 



Optical anomalies are rarel}^ met with, and twinning parallel to 

 the fundamental rhombohedron {1011}, although present in some 

 instances, is of rare occurrence. 



Inclusions are general, but almost invariably consist of brownish 

 to colourless glass, and this feature appears to be more or less 

 characteristic of the corundum of the metamorphic and non- 

 plutonic igneous rocks. 



As regards origin, the corundum of the sillimanite-buchite 

 represents the excess of alumina over that required for the forma- 

 tion of sillimanite and melt, and was the first solid phase to 

 separate out from the fused aluminous sediment. It rarely forms 

 as much as 1 per cent, of the buchite, and is generally below 0*5. 

 In the case of the corundum of the anorthite-corundum-spinel 

 assemblage that appears to interpose itself normally between 

 the sillimanite-buchite and the magma, we find that corundum 

 is more plentiful towards the buchite : that is to say, towards 

 the source of the alumina. It is invariably associated with 

 anorthite, and in a manner that points conclusively to crystal- 

 lization from a common solution of these two substances. In 

 some instances the lack of crystalline form of the corundum 

 in association with anorthite would suggest the simultaneous 

 crystallization of these minerals in equilibrium with each other 

 and with the naelt, and the maintenance by transfusion of an 

 anorthite-corundum eutectic condition. 



The later-formed corundum, which occurs associated with spinel 

 and oligoclase in a matrix of more igneous character, has been 

 formed by the separation of alumina from a partial magma 

 enriched in alumina through the resorption of anorthite containing 

 sillimanite (p. 244). A certain portion of the excess alumina has 

 combined with the available magnesia and ferrous iron of the 

 magma to form spinel (p. 248). Both corundum and spinel are 

 often enclosed by a zone of anorthite-crystals (PI. VIII, figs. 3 & 4), 

 a feature noted by Prof. Lacroix in xenoliths from the Cantal 

 region.^ 



Sillimanite. — This mineral is a common product in meta- 

 morphosed aluminous sediments ; in the xenoliths under description 

 it is, next to corundum, the earliest and most constant crystalline 

 substance to separate from the melt produced by the fusion of the 

 highly aluminous shales. It occurs mostly as minute needles about 

 1 to 2 mm. long, which are either felted together or arranged in a 

 parallel fluxional manner in a colourless or pale violet-tinted glass. 



^ ' Les Enclaves des Eoches Volcaniques ' p. 190 : 'On voit que lorsqu'un 

 grand cristal de corindon ou de spinelle se produit, il est entoure d'une zone 

 exclusivement feldspathique.' 



