VOLCANOES AND IGNEOUS INTRUSIONS 



303 



long distances by the wind. Pompeii, at the foot of the mountain, 

 wis buried beneath 25 or 30 feet of ash, and Herculaneum beneath 

 60 feet of mud and ash, the latter being covered by a layer of lava 

 during a later eruption. So completely were these cities hidden that 

 their sites were unknown for more than 1600 years. After this first 

 historical eruption (79 A.D.), which is well described by the younger 

 Pliny in a letter to Tacitus, 1 the volcano was occasionally eruptive 

 until 1 139. Then, for a period of almost 500 years, with the exception 

 of one feeble eruption, the volcano seemed again to have become 

 extinct, and the crater was choked with rubbish and covered with 

 trees. In 163 1 another violent eruption 

 occurred (Fig. 295) ; fissures opened in the 

 side of the mountain, through some of 

 which steam and ash were thrown, and 

 four streams of lava poured from the 

 crater, three of which reached the sea. 

 During this eruption the cone was reduced 

 about 525 feet in height. During the 

 eruption in 1906, the main lava stream 

 flowed at a rate of a little less than two 

 miles an hour where the slope was steep, 



but more slowly when passing over a lower _ 18 n 68 . 



, -r TT1 it- 1 Fig. 29c. — Section through 



grade. When wooden objects, such as Vesuvius, showing the changes 

 trees, were encountered by this lava in the shape of the volcano 

 stream, they were charred but not burned ; ^™ een 6s A,D - and l868, 

 some were broken off by the weight of the 



lava and carried on the surface of the stream. When a large object 

 was reached the lava piled up behind it until it was moved aside, 

 overflowed, or the stream moved around it. 



A summary of the sequence of events in a recent eruption is as follows : In 1904 

 Vesuvius was almost quiet, but soon explosions occurred of sufficient force to throw 

 fragments short distances above the crater's. rim. In 1905 a narrow stream of lava 

 flowed from a fissure in the cone throughout the year. On April 4 of the following year 

 a great cauliflower-shaped cloud of dust and gas rose from the crater, and lava streamed 

 in small quantities from successively lower openings in the side of the cone. On April 

 7 an explosion occurred which sent a column of dust-laden gas four miles vertically 

 into the air, and new and larger fissures opened through which lava flowed. The 

 dust so weighted the roofs of the houses as to cause them to collapse with loss of life. 

 One lava stream destroyed the town of Boscotrecase. 



1 Translation in Shaler's Aspects of the Earth, pp. 50-56, or in Lyell's Principles oj Geology , 

 p. 603. 



CLELAND GEOL. — 20 



