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VOLCANO. 205 VOLGA. cagua, Mistf, Colopaxi, Saliama. Cliiinborazo, Tolima) ; the Lesser Antilles (Soiifiii>re, Pelee, and many peaks on .Saint Lucia, Uominiea, Guaileloupe, etc.) ; Central America and Mexico ( Irasfi, Cosegiiina, Izaleo, Santa Maria, .^gua, Fuego, Popocatepetl, Orizaba, Colima, .loniUo) ; the west coast of Xortli America ( Shasta, Ilood, Rainier, Baker, VVrangel. Tllianma, Sheslialden, Makushin) ; Kamchatka, the .Fapanese Islands, (Fujiyama, Aso-San, Kan<lai-San, .sainayama) ; the Philippines (Mayon) and Sunda Sea Islands (Gelunggung, Papadayang, Tengger, Temboro, Krakatoa) ; New Zealand (Egniont); and Vic- toria Land (Antarctica: Erebus and Terror). All of these seem to define a nearly continuous line or belt of weakness or disruption in the earth's crust. (2) A Mediterranean-Caspian region is defined by the positions of the Lipari Islands ( with Stromboli ) , Etna, Vesuvius, the jEgean Islands (with Santorin), Demavend, and Ararat. (3) An Atlantic region, defined by the mainl}' quiescent volcanoes of Iceland (Hecla, Skajitar .liikulU.the Azores (Pico) , Madeira, Ca- naries (Tcnerifi'e) , Cape de Verde Islands, Tristan da Cunha, etc. (4) A Pacific region, with the numerous volcanoes on the small islands of the Polynesian group and the three main volcanoes of Hawaii' (Mauna Loa, Mauna Kea, and Kilauca). The Cause of Volcamc Er.uPTiONS. There is still much obscurity regarding the causes that condition volcanic eru]itions — that permit of the generation of steam within the earth's interior, of its explosive escape through the 'crust,' and of the formation or existence of the molten m.ag- matic mass that reaches the surface as lava or in its disrupted form, as scorife and ash. The ac- cessible facts seem to point to the conclusion that the steam or vapor of water is derived from the hydrated rocks of the earth's superficial mass, and its sudden evolution is due to release from pressure of the rock or magma with which it is inbound, or to steady and increasing superheat- ing. These conditions, as well as that of the uplifting and outthrow of lava, might be supplied as the result of deep rock displacements, which, by squeezing, would bring the more highly heated deeper crust nearer to the surface or to the critical point of dismemberment. And these dis- placements can readily be ascribed to vast subsi- dence, notably of the sea bottom. BiBLiOGKAPiiY. Lyell, Principles of Geology, vol. i. (New York, i872) : Geikie, Text-Book of Geolof/n (London, ISO.S) ; Sorope, "Considerations on Volcanoes," Ueolofiirnl Magazine (London, 1870) ; Fuchs, Tulhine vnd Erdbeben (Leipzig, 1875) ; Mallet, "Volcanic Energy," Philosoiihical Transactions of the Royal Society (London, 1873) : Dana, Characteristics of Volcanoes (New York, 1800); Judd. Volcanoes (ib., 1891): Eussell, Volcanoes of North America (ib., 1897); Hcil- prin, ^font Pelee and the Tragedy of Martinique (Philadelphia, 1903) ; and various memoirs by Rcyer, Lapparenty, Bonney, Supan, Humboldt, Von Buch, and Darwin. See Geology ; Ve.suvius; Etna; PeliSe, Mont; etc. VOLE. An English name, little used in America until .recently, for the smaller members of the rat familv (Murids) of the genera Arvi- cola, Evotomys, ' Calomys. and their near allies. The species are numerous and widely distributed, being found in Europe, Asia, Africa, and North and South America. Some of them are com- pletely terrestrial in their habits, others are aquatic. See !MousE; Meadow iMou.SE. VOL'GA. The largest river of Russia as well as of tlu' wliole of Europe. It rises among a cluster of lakes and marshes in the Valdai Hills in the Government of Tver, 20.5 miles southeast of Saint Petersburg, and flows in a generally ■soutlieastward course, though it describes a large zigzag with sharp bends at Samara and Tsaritsyn, between which its course ru)is to the southwoat. It empties through a large delta into the Caspian Sea, after a course of about 2300 miles (Map: Russia, G .5). Its total fall from its source to its mouth is less than 900 feet, and its current is throughout tranquil and regular, wholly without falls or rapids, so that the river is navigable almost to its source. The extreme upper portion of the river flows between wooded hills, but after leaving the Valdai plateau the entire remainder of its course lies within the great plains. The right baidv is generally higher than the left, and for a considerable distance along the middle course it is lined by a limestone escarpment with steep, wooded cliffs, a spur of which deflects the river at Samara in a long, narrow loop ."50 miles to the eastward. The left bank .is low, and in some places along the middle course rt is lined with great marshes and treml)ling forests, in which the trees are supported by soil floating on soft mud. In the lower course both hanks are low, but dry, owing to the rapid evaporation. Here the river receives practically no tributaries, and flows through saline steppes lying below sea level. Below Tsaritsyn the river begins to divide into a number of parallel arms, but the true alluvial delta begins only a short distance above Astrakhan. The delta is about 70 miles wide, and consists of eight large and numerous smaller arms which are all merged together under a single sheet of water during the floods. The main channel near its mouth is several miles wide ; below the confluence of the Kama the width of the river is 1500 3'ards, and at Tver it is about 200 yards. In some places the channel is nearly 100 feet deep, in others it is scarcely more than four or five feet in the lower and two or three feet in the upper reaches. The quantity of sediment deposit- ed seems to increase from year to year; shoals and sandbars are continually formed, and a num- ber of di'edging machines have to be kept con- stantly at work. The chief tributaries of the Volga are the Kama from the cast, a large river in itself, and the Oka from the west. The whole system waters twenty-two of the Russian gov- ernments, and has a total navigable length of 9090 miles. Steamers ascend the main stream for more than 2000 miles. Though the river is ice- bound more than one-third of the year, the traftie is enormous, amoimting to over 14.000.000 tons of goods annually, or more than one-fourth of the total traffic on all the Russian railroads. The principal articles carried are timber, grain, salt, and naphtha. The chief ports on the river are, in the order of their shipping. Astrakhan, Nizhni- Novgorod, Tsaritsyn, and Saratov. Connected witir the Volga is a vast canal system, through which communication is established between the