tube, or conduit, in the earth's crust, through which the molten rock is
forced to the surface. The conduit penetrates the cool and rigid rocks
forming the superficial portion of the earth, and reaches its highly
heated interior.

"The length of volcanic conduits can only be conjectured, but, judging
from the approximately known rate of increase of heat with depth (on an
average one degree Fahrenheit for each sixty feet), and the temperature
at which volcanic rocks melt (from 2,300 to 2,700 degrees Fahrenheit,
when not under pressure), they must seemingly have a depth of at least
twenty miles. There are other factors to be considered, but in general
terms it is safe to assume that the conduits of volcanoes are irregular
openings, many miles in depth, which furnish passageways for molten
rock (lava) from the highly-heated sub-crust portion of the earth to its
surface. . . ."

ERUPTIONS OF QUIET TYPE

"During eruptions of the quiet type, the lava comes to the surface in a
highly liquid condition--that is, it is thoroughly fused, and flows with
almost the freedom of water. It spreads widely, even on a nearly level
plain, and may form a comparatively thin sheet several hundred square
miles in area, as has been observed in Iceland and Hawaii. On the Snake
River plains, in Southern Idaho, there are sheets of once molten rock
which were poured out in the manner just stated, some four hundred
square miles in area and not over seventy-five feet in average
thickness. When an eruption of highly liquid lava occurs in a
mountainous region, the molten rock may cascade down deep slopes and
flow through narrow valleys for fifty miles or more before becoming
chilled sufficiently to arrest its progress. Instances are abundant
where quiet eruptions have occurred in the midst of a plain, and
built up 'lava cones,' or low mounds, with immensely expanded bases.
Illustrations are furnished 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 poured out, without danger or serious
inconvenience.

"The quiet volcanic eruptions are attended by the escape of steam or
g