VOLCANIC CONES AND CRATERS. 
495 
whether the rocks reduced to fusion in the subterranean re¬ 
gions happened to have contained more or less silica, pot¬ 
ash, soda, lime, iron, and other ingredients. We are best ac¬ 
quainted with the effects of eruptions above water, or those 
called subaerial or supramarine; yet the products even of 
these are arranged in so many ways that their interpretation 
has given rise to a variety of contradictory opinions, some of 
which will have to be considered in this chapter. 
Cones and Craters .-—In regions where the eruption of vol¬ 
canic matter has taken place in the open air, and where the 
surface has never since been subjected to great aqueous den¬ 
udation, cones and craters constitute the most striking pecul¬ 
iarity of this class of formations. Many hundreds of these 
cones sCre seen in central France, in the ancient provinces of 
Auvergne, Velay, and Vivarais, where they observe, for the 
most part, a linear arrangement, and form chains of hills. 
Although none of the eruptions have happened within the 
historical era, the streams of lava may still be traced dis¬ 
tinctly descending from many of the craters, and following 
the lowest levels of the existing valleys. The origin of the 
cone and crater-shaped hill is well understood, the growth 
Fig. 585. 
Part of the chain of extinct volcanoes called the Monts Dome, Auvergne. (Scrope.) 
of many having been w^atched during volcanic eruptions. 
A chasm or fissure first opens in the earth, from which great 
volumes of steam are evolved. The explosions are so vio¬ 
lent 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 lara usually ascends through the 
chimney or vent by which the gases make their escape. Al¬ 
though extremely heavy, this lava is forced up by the ex¬ 
pansive power of entangled gaseous fluids, chiefly steam or 
aqueous vapor, exactly in the same manner as water is made 
to boil over the edge of a vessel when steam has been gener¬ 
ated at the bottom by heat. Large quantities of the lava 
are also shot up into the air, where it separates into frag¬ 
ments, and acquires a spongy texture by the sudden enlarge- 
