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, vjhich 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 scorics, 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 
