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 centre outwards, caused local fusion of the rocks, though without forming distinct reservoirs of molten magma, and the fused matter charged with gases rose in liquid threads or tongues, which worked their way upwards, some reaching the superficial part of the earth and escaping through fissures in the zone of fracture, thus giving rise to volcanic phenomena. It is held that the explosive activity of a volcano is due to the presence of gases which have been brought up from the interior of the earth, whilst only a small and perhaps insignificant part is played by water of superficial origin.

Entirely new views of the origin of the earth’s internal heat have resulted from the discovery of radioactivity. It has been shown by the Hon. R. J. Strutt, Professor J. Joly and others that radium is present in all igneous rocks, and it is estimated that the quantity in the crust of the earth is amply sufficient to maintain its temperature. An ingenious hypothesis was enunciated by Major C. E. Dutton, who found in the radioactivity of the rocks a sufficient source of heat for the explanation of all volcanic phenomena. He believes that the development of heat arising from radioactivity may gradually bring about the local melting of the rocks so as to form large subterranean pools of magma, from which the volcanoes may be supplied. The supply is usually drawn from shallow sources, probably, according to Dutton, from a depth of not more than three or rarely four miles, and in some cases at not more than a mile from the surface. If the water in the local magma should attain sufficient expansive power, it will rupture the overlying rocks and thus give rise to a volcanic eruption. When the reservoir becomes exhausted the eruption ceases, but if more heat be generated by continued radioactivity further fusion may ensue, and in time the eruption be repeated. According, however, to Professor Joly, it is improbable that sufficient heat for the manifestation of volcanic phenomena could be developed by the local radioactivity of the rocks in the upper part of the earth’s crust.

 VOLCANO ISLANDS, three small islands in the western Pacific Ocean, S. of the Bonin Islands, forming part of the Japanese empire (annexed in 1891). They are also known as the Magellan Archipelago, and in Japan as Kwazan-retto (series of volcanic islands). They are situated between 24° and 26° N. and 141° and 142° E. Their names are Kita-iwo-jima (Santo Alessandro), Iwo-jima (Sulphur) and Minami-iwo-jima (Santo Agostino). Kita-iwo-jima—which, as its name (kita) implies, is the most northerly of the three—rises 2520 ft. above the water, and Minami-iwo-jima, the most southerly, to a height of 3021 ft. The islands are not inhabited. With this group is sometimes included another island, Arzobispo, nearer the Bonin group.  VOLCEI (mod. ), an ancient town of Lucania, 2128 ft. above sea-level, the chief town of the independent tribe of the Volceiam, Vulcientes or Volcentani, whose territory was bounded N. by that of the Hirpini, W. and S. by Lucania and E. by the territory of Venusia. Some pre-Roman ruins still exist (Not. Scav., 1884, 115). It became a municipium, and in 323 had an extensive territory attached to it, including the town of Numistro, the large Cyclopean walls of which may still be seen, 2 m. below Muro Lucano. Below the town is a well-preserved Roman bridge over the Tanager (mod. Tanagro).

 VOLCI, or, an ancient town of Etruria. The circuit of the walls measures about 4 m., and scanty traces of them and of Roman buildings within them still exist. The Ponte della Badia over the Fiora, a bridge with a main arch of 66 ft. span, 98 ft. above the stream, is also Roman. An aqueduct passes over it. The former wealth of the town is mainly proved by the discoveries made in its extensive necropolis from 1828 onwards—Greek vases, bronzes and other remains—many of which are now in the Vatican. By 1856 over 15,000 tombs had, it was calculated, had been opened. These were entirely subterranean, and little is now to be seen on the site but a great tumulus, the Cucumella, and a few smaller ones. The frescoes from the François tomb, discovered in 1857, illustrating Greek and Etruscan myths, are now in the Museo Torlonia at Rome. Volci was one of the twelve towns of Etruria. Coruncanius triumphed over the people of Vulsinii and Volci in 280, and the colony of Cosa was founded in their territory. This seems to have led to the decline of the city, and it does not seem to have been of great importance in the Roman period, though it became an episcopal see.

 VOLE, a book-name (invented by Dr J. Fleming, author of a work on British animals) for the water-rat and those species of field-mice which have cheek-teeth of the same general type. Although the British representatives of this group should undoubtedly retain their vernacular designations of water-rat and short-tailed field-mouse, the term “vole” is one of great convenience in zoology as a general one for all the members of the group. Systematically voles are classed in the mammalian order, in which they constitute the typical section of the subfamily Microtinae in the Muridae, or mouse-group. As a group, voles are characterized by being more heavily built than rats and mice, and by their less brisk movements. They have very small eyes, blunt snouts, inconspicuous ears and short limbs and tails, in all of which points they are markedly contrasted with true rats and mice. In common with lemmings and other representatives of the Microtinae, voles are, however, broadly distinguished from typical rats and mice by the structure of their three pairs of molar teeth. These, as shown in the figure, are composed of a variable number of vertical triangular prisms, in contact with one another by two (or one) of their angles. On the number and relations of these prisms the voles, which form an exceedingly large group, ranging all over Europe and Asia north of (and inclusive of) the