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Rh cleavage, but presents parting planes due to the structure of the crystal, which have been studied by Prof. J. W. Judd.

Next to diamond, corundum is the hardest known mineral. Its hardness is generally given as 9, but there are slight variations in different stones, sapphire being rather harder than ruby, and ruby than common corundum. The colours are very varied, and it is probable that iron is responsible for many of the tints, though chromium is a possible agent in certain cases. The transparent varieties are often distinguished as “Oriental” stones. (See and .) Corundum is used largely for watch-jewels, and for bearings in electrical apparatus.

The coloured corundums fit for gem-stones come chiefly from Ceylon, Burma, Siam and Montana. Coarse dull corundum is found in many localities, and usually has higher commercial value as an abrasive agent than emery, which is less pure. The coarse corundum, however, is often partially hydrated or otherwise altered, whereby its hardness is diminished. In India, where the native lapidaries use corundum-sticks and rubbers formed of the powdered mineral cemented with lac, it occurs in the Salem district, Madras, in Mysore and in Rewa. Large deposits of corundum exist in the United States, especially in N. Carolina and Georgia, where they are associated with peridotites, often near contact with gneiss. The mineral has been extensively worked, as at Corundum Hill, Macon county, N.C., near which, in 1871, were discovered numerous rubies, sapphires and pebbles of coarse corundum in the bed of a river. Corundum occurs also at many localities in Montana, where the crystals are often of gem quality. They are found mostly as loose crystals in gravel, but are known also in igenous rocks like andesite and lamprophyre. Prof. J. H. Pratt, who has studied the occurrence both in Montana and in N. Carolina, considers that the alumina was dissolved in a molten magma, from which it separated at an early period of consolidation, as illustrated by the experiments of J. Morozewicz. Corundum occurs also in Canada in an igneous rock, a nepheline-syenite, associated with Laurentian gneiss. Important deposits were discovered by the Geological Survey in 1896, in Hastings county, Ontario; and corundum is now worked there and in Renfrew county. New South Wales, Queensland and Victoria are other localities for corundum. The mineral is found also in the Urals and the Ilmen Mountains, in the Alps (in dolomite), in the basalts of the Rhine, and indeed as a subordinate rock-constituent corundum seems to enjoy a wide distribution, being found even in the British Isles.

See Joseph Hyde Pratt, “Corundum and its Occurrence and Distribution in the United States,” ''Bulletin U.S. Geol. Surv., No. 269 (1906); T. H. Holland, Economic Geology of India'' (2nd ed.), Part i. (1898).

CORUNNA, a maritime province in the extreme north-west of Spain; forming part of Galicia, and bounded on the E. by Lugo, S. by Pontevedra, W. and N. by the Atlantic Ocean. Pop. (1900) 653,556; area, 3051 sq. m. The coast of Corunna is exposed to the full force of the Atlantic; it forms one succession of fantastically shaped promontories, divided by bays and estuaries which often extend for many miles inland, with reefs and small islands in their midst. Though well lighted, this coast is very dangerous to navigation, gales and fogs being frequent in winter and spring. The most conspicuous headlands are Cape Ortegal and Cape de Vares, the most northerly points of the Spanish seaboard, and Capes Finisterre and Toriñana in the extreme west. The principal bays are those of Santa Marta, Ferrol and Corunna, on the north; Corcubion, Muros y Noya and Arosa, on the west. Wild and rugged though this region appears to travellers at sea, the mountains which overspread the interior are covered with forests and pastures, and watered by an abundance of small rivers and streams. The climate is mild and singularly equable, but the rainfall is very heavy. All the fruits and vegetables of northern Europe thrive in the sheltered valleys, and the cultivation of cherries, strawberries, peas and onions, for export, ranks among the most profitable local industries. Heavy crops of wheat, rye, maize and sugar-beet are raised. The wines of Corunna are heady and of inferior flavour. Cattle-breeding, once a flourishing industry, had greatly declined by the beginning of the 20th century, owing to foreign competition. All along the coast there are valuable fisheries of sardines, lobsters, cod, hake and other fish. Copper, tin and gold are procured in small quantities, and other minerals undoubtedly exist. The exports consist chiefly of farm produce and fish; the imports, of coal and textiles from England, petroleum from the United States, marble from Italy, salt fish from Norway and Newfoundland, and hides. The principal towns are Corunna, the capital and chief port (pop. 1900, 43,971); Ferrol (25,281), another seaport; Santiago de Compostela (24,120), famous as a place of pilgrimage; Carballo (13,032); Ortigueira (18,426) and Ribeira (12,218). These are described under separate headings. Along the coast there are numerous trading and fishing stations of minor importance. Railway communication is very defective. From Corunna a line passes south-eastward to Lugo and Madrid, and from Santiago another line goes southward to Vigo and Oporto; but the centre and the north-west of the province are, to a great extent, inaccessible except by road; and many, even of the main highways, are ill-constructed and ill-kept. Very few Spanish provinces have so high a birthrate, but the population increases very slowly owing to emigration. For a description of the peasantry, who are distinguished in may respects from those inhabiting other parts of Spain, see.

 CORUNNA (Span. La Coruña; Fr. La Corogne; Eng. formerly often The Groyne), the capital of the province described above; in 43° 22′ N., and 8° 22′ W.; on the bay of Corunna, an inlet of the Atlantic Ocean. Pop. (1900) 43,971. The principal railways of north-western Spain converge on Corunna, and afford direct communication with Madrid and Oporto. Corunna consists of an upper and a lower town, built respectively on the eastern side of a small peninsula, and on the isthmus connecting the peninsula with the mainland. The upper town is the more ancient, and is still surrounded by walls and bastions, and defended by a citadel; but it has been gradually outgrown by the lower, which, though at first a mere fishing village, as its name of Pescaderia implies, is now comparatively well built, and has many broad and handsome streets. There is little remarkable in the public buildings, although the churches of Santiago and the Colegiata date respectively from the 12th and 13th centuries, and there are several convents, two hospitals, a palace for the captain-general of Galicia, a theatre, a school of navigation, an arsenal and barracks. The harbour is on the east. Though difficult to approach in stormy weather, it is completely sheltered, and accommodates vessels drawing 22 ft. It is defended by several forts, of which the most important are San Diego, on the east, and San Antonio, on the west. These fortifications are of little practical value on the landward side, as they are commanded by a hill which overlooks the town. The so-called Tower of Hercules, on the north, has been increased by modern additions to a height of nearly 400 ft., and is surmounted by a fine revolving light. Many foreign steamers call here, for emigrants or mails, on their way to South America. Upwards of 1200 merchant ships, mostly British, entered the port in 1905. The exports are chiefly agricultural produce, wine and fish; the imports are coal, colonial products, and manufactured goods. Chief among the industrial establishments is a state tobacco factory; the sardine and herring fisheries also employ a large number of the inhabitants.

Corunna, possibly at first a Phoenician settlement, is usually identified with the ancient Ardobrica, a seaport mentioned by the 1st-century historian, Pomponius Mela, as in the country of the Artabri, from whom the name of Portus Artabrorum was given to the bay on which the city is situated. In the middle ages, and probably at an earlier period, it was called Caronium; and this name is much more probably the origin of the present designation than the Latin Columna which is sometimes put forward. The harbour has always been of considerable importance, but it is only in comparatively modern times that it has made a figure in history. In 1588 it gave shelter to the Invincible Armada; in 1598 the town was captured and burned by the British under Drake and Norris. In 1747, and again in 1805, the bay was the scene of a naval victory of the British over the French; and on the 16th of January 1809 a battle took place