84 GEOLOGY. 



There are said to be 220 active volcanoes, of which 

 89 are in islands. 'In Europe there are hut Jour great 

 volcanoes. Asia has very few. The Continent of Africa 

 has none, although there are several in its islands. In 

 South America they are very numerous : in the range 

 of the Andes there are not fewer than 86. 



The cones and craters of extinct volcanoes still re- 

 main visible in various parts of the world, which, from 

 their appearance, may have been inactive for thousands 

 of years. 



Etna, the largest volcano in Europe, is about 6000 

 feet high, its cone ninety miles in circumference, its 

 crater is a quarter of a mile high, the mouth of the 

 crater is one mile in diameter, and it has about eighty 

 minor cones. Thirty-one great eruptions of this moun- 

 tain have occurred within the period of history, and 

 stones of an immense size have been hurled to an incre- 

 dible height and distance : small stones have fallen at 

 the distance of one hundred miles. The first eruption 

 of this mountain on record is mentioned by a Greek 

 historian as having taken place 480 years before Christ. 



Vesuvius is next in magnitude to Etna, and is of great 

 antiquity. It was in an inactive state for nearly 500 

 years until 1631, &nd its crater is said to have been 

 1000 paces in descent, and a mile and a half in diameter, 

 and was rich in wood and herbage : but since that time 

 it has had, every ten years, most destructive eruptions. 

 In the year 79, a tremendous eruption took place, which 

 destroyed and completely overwhelmed the cities of 

 Herculaneum and Pompeii. 



When the gases generated cannot escape, uplifting of 

 the earth, or earthquakes, must be the natural result. 



Of the Sedimentary or Stratified Rocks. 



GNEISS is of the same character, and composed of 

 the same materials, as granite ; the only difference is, 

 that one is stratified and the other is not. Gneiss is 

 rich in metallic veins, almost all the metals being found 

 in it ; particularly tin, copper, silver, and lead. It forms 

 extensive ranges in Norway, Sweden, Saxony, the Alps, 

 the Pyrenees, and in North and South America. 



MICA SLATE, or SCHIST, is composed of quartz and 



