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VOLCANIC AND EARTHQUAKE ACTION 391 
occurred in the midst of a plain, and built up ‘lava cones,’ or low 
mounds, with immensely expanded bases. Illustrations are fur- 
nished in Southern Idaho, in which the cones formed are only three 
hundred or four hundred feet high, but have a breadth at the base 
of eight or ten miles. In the class of eruption illustrated by these 
examples, there is an absence of fragmental material, such as 
explosive volcanoes hurl into the air, and a person may stand within 
_a few yards of a rushing stream of molten rock, or examine closely 
‘the opening from which it is being oa ses out, without danger or 
serious inconvenience. 
“The quiet volcanic eruptions are attended by the escape of 
steam or gases from the molten rock, but the lava being in a highly 
liquid state, the steam and gases dissolved. in it escape quietly and 
without explosions. If; however, the molten rock is less com- 
pletely fluid, or in a viscous condition, the vapors and.gases con- 
tained init find difficulty in escaping, and may be retained until, 
becoming concentrated in large volume, they break their way to 
the surface, producing violent explosions. Volcanoes in which the 
lava extruded is viscous, and the escape of steam and gases is 
retarded until the pent-up energy bursts all bounds, are of the 
explosive type. One characteristic example is Vesuvius. 
“When steam escapes from the summit of avolcanic conduit— 
which, in plain terms, is a tall vessel filled with intensely hot and 
more or less viscous liquid—masses of the liquid rock are blown 
into the air, and on falling build up a rim or crater about the place 
of discharge. Commonly the lava in the summit portion of a con- 
duit becomes chilled and perhaps hardened, and when a steam 
_ explosion occurs this crust is shattered and the fragments hurled 
into the air and contributed to the building of the walls of the 
inclosing crater,