THE CRUST OF THE EARTH. 



the south, and which was terminated by crevasses from which 

 boiling- water issued. 



142. Although the crater-formed cavities which are so frequently 

 observed at various parts of the surface of the earth have been in 

 many cases preceded by volcanic eruptions, they are not invariably 

 to be ascribed to that cause, and indeed in many cases there is 

 abundant evidence that no such eruption could have taken place 

 at the moment of their formation. We have already explained 

 how the upheaving of the crust of the earth, produced by an earth- 

 quake shock, can produce not only radiating clefts, as in fig. 47, 

 but also an open cavity, as in fig. 51, the edges of which are sur- 

 rounded by divergent clefts. 



143. Mount Etna, the most remarkable of European volcanoes, 

 situated on the island of Sicily, and composed entirely of erupted 

 mineral substances, rises to a height above the level of the Medi- 

 terranean of nearly eleven thousand feet. The circumference of 

 its base is more than a hundred and eighty miles, and on a clear 

 day it may be distinctly seen from any elevated point of the island 

 of Malta, a distance of a hundred and fifty miles. Compared with 

 this volcano, Vesuvius is insignificant. While the streams of 

 lava from the latter never exceed seven miles in length, those of 

 Etna very often are from fifteen to thirty miles, being five miles 

 in breadth, and from fifty to a hundred feet in thickness. The 

 surface of Etna presents three distinct regions. Around the base 

 for an extent of twelve miles, the country is richly cultivated, and 

 abounds in vineyards and pastures, and is the site of many towns, 

 monasteries, and villages. The middle or temperate zone above 

 is covered with forests of oak and chestnut, and a luxuriant vege- 

 tation reaches to within a mile of the summit. Above this all is 

 sterility and desolation, and the highest point of the mountain is 

 covered with eternal snow. The crater is about a quarter of a 

 mile in height, and three-quarters of a mile in circumference, and 

 is situated in the centre of a gently inclined plain, three miles in 

 diameter. From the crater a column of vapour constantly issues, 

 emanating from the mass of incandescent mineral matter which 

 fills up the interior, and may be seen, in a state of ebullition, in 

 the fumarolles in some of the lateral crevices, of which there are 

 generally several accessible. 



144. Etna is recorded as having been in a state of activity 

 before the Trojan war; and ever since, at varying intervals, 

 violent eruptions have occurred. In an eruption of 1669, the 

 torrent of lava inundated a space of fourteen miles in length, and 

 four in breadth, burying beneath it five thousand villas and other 

 habitations, with part of the city of Catania, and at length fall- 

 ing into the sea. During several months before the lava burst 



108 



