HEAT. 



287 



The eruptions of Vesuvius are generally heralded by earthquakes. The 

 ejected lavas commonly bear evidence, in the various chlorides among the 

 ingredients deposited by vapors on the lavas, that the waters of the sea had 

 gained access to the fires. The accompanying projection of cinders is often 

 to great heights, and over a wide reach of country. Those of 1779, according 

 to Sir William Hamilton, were thrown to a height of 10,000 feet. 



The sketch of Vesuvius in Fig. 225, page 266, represents its condition a few weeks 

 before an eruption, when the crater was filled to the summit plain there shown, and a 

 cinder-cone on this plain (see sketch) was the most active feature ; but there was a slug- 

 gish stream of lava in the summit plain, and red heat was visible a foot down in cracks. 

 The eruption, as described by Abich, took place in August, 18.34 ; two streams of lava 

 flowed out, the chief one from the base of the old cone, and it was accompanied by flames, 

 which, according to Abich, were produced by hydrogen ; it was half a mile wide, 18 to 30 

 feet deep, and 9 miles long. It engulfed the village of Caporeco, sparing only 4 houses 

 out of 500. The old cone was laid open by the eruption, and the top plain, that was the 

 floor walked over by the author, had sunk into a deep abyss. (Abich, Vues Illustr. 

 sur le Vesuve et VJStna, Berlin, 1837.) 



6. Lava-streams. — (a) Their general characteristics. — Lava-streams sel- 

 dom make more than three miles of flow a day, and sometimes take a year 

 for 30 miles. This is true even of the basaltic kinds. They flow rapidly 

 when unobstructed, but often become dammed by coolings, especially at 

 the frequent interruptions. As the stream of basaltic lava moves, it be- 

 comes crusted over its exterior surface, and then flows on in the lava-tunnel 

 so made, which, at the end, it may leave empty. Owing to the obstruc- 

 tions, the lavas often break their bounds, and one stream becomes piled 

 over another. The surface of the stream has ropy lines and other marks 



249. 



View of the aa lava-stream, with a " bomb," a, 10 feet in breadth upon it. 1). '87. 



made by the flowing movement. This ordinary lava is called by the 

 Hawaiians pahoehoe, alluding to a relatively smooth and shining or satin- 

 like luster. Another kind, the aa, into which the pahoehoe sometimes 

 abruptly changes, shows over its surface no evidence of flow ; the stream 

 consists of broken, ragged masses, large and small, bristled all over with 

 points (Fig. 249); and, owing to the masses being piled loosely together, the 



