VOLCANOES 248 VOLGA furnishing' the elastic agent under con- ditions of temperature and pressure which are adequate to the result. The principal theaters of modern vol- canic action are the Chilean Andes, the Andes of Ecuador and Peru, Central America, the Dutch East Indies, and the Philippines, Iceland, and the Mediter- ranean. To these must be added the remarkable Hawaiian volcanoes, and perhaps also those of New Zealand. We have reason also to infer that many ex- tensive outbreaks occur beneath the ocean, but we know very little about them. The number of active volcanoes is usually reckoned as about 350; but it is difficult in many of these cases to de- cide whether the volcano is active or extinct. But the number now active is small indeed in comparison with those which have become extinct in compara- tively recent geological times. Many regions abound in volcanic piles which in the course of human history have given no sign of activity. This is espe- cially the case in the W. part of the United States, and most especially in the N. W. States and Territories. Here may be found gi-eat regions, tens of thousands of square miles in extent which have been overflowed with lava, and the successive sheets are thousands of feet in thickness. In many portions of the West the indications point to a very recent epoch for the eruptions, and it is by no means improbable that some of them have occurred since the Span- ish conquest. In all ages of the geological history of the planet, and as far back as we are able to discriminate the rela- tive antiquity of the strata, volcanic eruptions have been abundant and of the same general character as those which now prevail. Nor is it possible to s?y whether they were generally more extensive or less so than at present. Two of the most remarkable of active vol- canoes are Mauna Loa and Kilauea, in Hawaii; the former 13,760 feet in height and the latter, though only about 4,000 feet high, 8 miles in circumfer- ence. On the summJt of Mauna Loa is a circular crater 8,000 feet in diameter, its walls nearly vertical and many hun- dreds of feet deep. The eruptions of this volcano have been of great volume and at an average interval of eight years; each outflow representing more lava than Vesuvius has sent out since the de- struction of Pompeii. The Hot Lakes district of North Island, New Zealand, was the scene of remarkable volcanic disturbances in 1886. The effect ex- tended over 60 square miles, burying whole villages and causing much loss of life. The eruption was accompanied by violent earthquakes and hundreds at new geysers broke out. The fine vol- canic dust that settled over the province of Auckland proved to be beneficial to vegetation. The most destructive of modern volcanic eruptions was that of Mont Pelee in the island of Martinique in May, 1902. VOLE, the Arvicola, a genus of rodents typical of the sub-family Arvicolinse, which also includes the lemmings (My- lodes), the musk rats (Fiber), and sev- eral related genera. The genus Arvicola includes over 40 species distributed over Europe, Asia, and North America — all more or less like rats and mice. In Great Britain there are three — the water vole (A. amphibius) , the field vole {A. agres- tis) , and the bank vole (^4. glareolus). The water vole is about the same size as the brown rat, and rat it is often called. It has dark brown or black fur, a tail about half the length of the body, and very strong hind feet, with five rounded pads on their lower surfaces. It burrows by the banks of streams and feeds for the most part on vegetable food. In summer the female has three or four litters of two to seven young. The water vole does not occur in Ire- land. The field vole or short-tailed "field mouse," is about the size of a common mouse, but the body is plumper and the tail shorter. It has brownish-gray fur; its hind feet have six pads. It lives in fields and woods, feeds on vegetable food, is very prolific, and often does much damage. The bank vole is like the field vole, but has a "more or less rusty or rufous-colored back, larger ears, and a longer tail." Its habits are like those of the field vole. VOLGA, the largest river of Europe; rises in the central Russian province of Tver, near the Dwina, about 200 miles from the Gulf of Finland; flows S. E. through several small lakes, receiving the Selicharovka from Lake Seliger, then S. E. past Rsjev to Subzov, where it turns N. E., passing Tver and Ry- binsk, then S. E. through the provinces of Jaroslav, Kostroma, Nijni Novgorod, and Kasan, passing Nijni Novgorod and Kasan, and receiving from the right the Oka, and from the left the Mologa, Kos- , troma, Unsha, and Wetluga. At Kasan it turns S., receives from the left the great Kama and the Samara, and from inces of Simbirsk, Saratov, Samara, and the right the Sura, separates the prov- passes the towns of Simbirsk, Samara, Sysran, Wolsk, and Saratov. At Sarepta it turns sharply to the S. E. and flows in that direction to the Caspian, which it enters by 8 principal and 200 smaller
