342 ELEMENTARY GEOLOGY 
erupt basalt, while from some of the Mexican cones 
acidic rhyolite or trachyte lavas are always erupted. 
On the other hand, some volcanoes erupt one kind at 
one period and another later, as in the case of Vesuvius. 
Reasons for Differences in Eruptions. — The reasons 
for these differences can be partly stated. Those vol- 
canoes which always erupt one kind of lava, evidently 
reach down to the’ same reservoir at all times; but 
those that vary in the nature of the rock erupted, must 
tap different reservoirs at different times, for in vari- 
ous parts of the earth the chemical composition of the 
molten rock must certainly vary. 
The explanation of the other variable features of 
eruption, is to be traced to the steam which is the 
immediate cause of outbreak. If the melted rock is 
very liquid, like the basaltic lava of Kilauea, the steam 
readily escapes; but if it 1s more pasty, the steam is 
partly prevented from escaping. When the steam does 
pass through the pasty lava, it necessarily blows holes 
in some of the rock, just as a thick fluid durme boiling 
is spattered by the bubbles of rismg steam. If a 
crust of lava has formed in the crater, the steam may 
become so confined, that when it gathers power enough 
to escape, it blows away the rock, causing a violent 
eruption. 
It is a fact that the most tremendous outbursts take 
place after long periods of quiet. This evidently means, 
