332 ELEMENTARY GEOLOGY 
to the Rockies, in which there seems never to have 
been extensive volcanic action. Such places are those 
from which mountain growth has also been absent. 
Hence the association between volcanoes and moun- 
tains seems to be intimate. There is good reason for 
believing that in past ages, especially in those imme- 
diately preceding the present, voleanic action was more 
violent than now. This was the epoch in which the 
larger chains of mountains in the world were being 
formed; and hence, at this time, it 1s possible that 
mountain growth was more active than at present. 
Products of Eruption. — Water, ash, and lava are the 
materials which are most abundantly ejected from vol- 
canoes; but various kinds of sulphurous and other 
gases also burst from the vent. The aqueous portion 
of the eruption escapes as steam, rising in great volume 
and extending high into the air (Fig. 203). Much of 
it passes into the atmosphere as vapor, but consider- 
able falls back to the earth in the form of rain. This 
may so deluge the side of the cone, that the water, 
rushing down the mountain side, takes up such quanti- 
ties of loose ash as to become a flow of mud. These 
mud flows are very destructive, and some of the buried 
towns on the flanks of Mount Vesuvius have been cov- 
ered by such a flow. 
The Java usually escapes without great violence, and 
flows down the side of the cone, sometimes from the 
