120 VOLCANIC PHENOMENA. 



flux to the large quantity of silex, which would 

 otherwise prove very difficult of fusion. It is re- 

 markable that, though volcanic rocks abound in si. 

 lex, they are generally deficient in quartz, which is 

 chiefly silex. Most of the volcanic rocks produce 

 a fertile soil by their disintegration. We explain 

 this by supposing that their component ingredients, 

 silica, alumina, lime, potash, iron, and the rest, are 

 in proportions well fitted for vegetation. Thus, in 

 Italy, in the neighbourhood of Mount Vesuvius, the 

 fertility of the soil is astonishingly great, producing 

 the finest grapes in the world. 



The manner in which volcanic rocks are produ- 

 ced by internal heat, is thus described by Mr. Lyell. 

 " A chasm or fissure first opens in the earth, from 

 which great volumes of steam and other gases are 

 evolved. The explosions are so violent as to hurl 

 up into the air fragments of broken stone, parts of 

 which are shivered into minute atoms. At the same 

 time, melted stone, or lava, usually ascends through 

 the chimney or vent by which the gases make their 

 escape. Although extremely heavy, this load is 

 forced up by the expansive power of entangled 

 gaseous fluids, chiefly steam or aqueous vapour, 

 exactly in the same manner as water is made to 

 boil over the edge of a vessel where steam has been 

 generated at the bottom by heat. Large quantities 

 of the lava are also shot up into the air, where it 

 separates into fragments, and acquires a shaggy 

 texture by the sudden enlargement of the included 

 gases, and thus forms scoria, other portions being 

 reduced to an impalpable powder or dust. The 

 showering down of the various ejected materials 

 round the orifice of eruption gives rise to a conical 

 mound, in which the successive envelopes of sand 

 and scoriae form layers, dipping on all sides from 

 a central axis. In the mean time, a hollow called a 

 crater has been kept open in the middle of the 

 mound by the continued passage upward of steam 



