Online Encyclopedia

IONA

Online Encyclopedia
Originally appearing in Volume V14, Page 726 of the 1911 Encyclopedia Britannica.
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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. R.*)
End of Article: IONA
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