IONA  , or IconmmLL, an

island of the Inner Hebrides, Argyll-shire, Scotland, 62 m . S. of Staffa and 14 m . W. of the Ross of Mull, from which it is separated by the shallow Sound of Iona . Pop . (1901) 213 . It is about 31- M. long and 11 m. broad; its area being some 2200 acres, of which about one-third is under cultivation, oats, potatoes and barley being grown . In the rest of the island grassy hollows, yielding pasturage for a few hundred cattle and sheep and some horses, alternate with rocky elevations, which culminate on the northern coast in Duni (332 ft.), from the base of which a dazzling stretch of white shell sand, partly covered with grass, stretches to the sea . To the south-west the island is fringed with precipitous cliffs . Iona is composed entirely of ancient gneisses and schists of Lewisian age; these ('loe, "violet "; XLBos, "stone") . It is generally called by petrographers cordierite, a name given by R . J . Hauy in honour of the French mineralogist, P .

L . Cordier, who discovered its remarkable dichroism, and suggested for it the name dichroite, still sometimes used . The difference of

colour which it shows in different directions is so marked as to be well seen without the dichroscope . The typical colours are deep blue, pale blue and yellowish grey . While the crystal as a whole shows these three colours, each face is dichroic . lolite is a hydrous magnesium and aluminium silicate, with ferrous iron partially replacing magnesium . It crystallizes in the orthorhombic system . In hardness and specific gravity it much resembles quartz . The transparent blue or violet variety used as a gem occurs as pebbles in the gravels of Ceylon, and bears in many cases a resemblance to sapphire . The paler kinds are often called water-sapphire (saphir d'eau of French jewellers) and the darker kinds lynx-sapphire; the shade of colour varying with the direction in which the stone is cut . From sapphire the iolite' is readily distinguished by its stronger pleochroism, its lower density (about 2.6)- and its inferior hardness (about 7) . Iolite occurs in granite and in true eruptive rocks, but is most characteristically developed as a product of contact metamorphism in gneiss and altered slates .

A variety occurring at the contact of

clay-slate and granite on the border of the provinces of Shimotsuke and Kodzuke in Japan has been called cerasite . It readily suffers chemical change, and gives rise to a number of alteration-products, of which pinite is a characteristic example . Although iolite, or cordierite, is rather widely distributed as a constituent of certain rocks, fine crystals of the mineral are of very limited occurrence . One of the best-known localities is Bodenmais, in Bavaria, where it occurs with pyrrhotite in a granite matrix . It is found also in Norway, Sweden and Finland, in Saxony and in Switzerland . Large crystals are developed in veins of granite running through gneiss at Haddam, Connecticut; and it is known at many other localities in the United States . (F . W .