PLACE OF ORIGIN OF THE MOON 37 
tematic difference in material and density, extending over large areas, 
would render the boundaries of the continents more subject to cracks, 
with their resulting volcanoes and earthquakes, than other portions of 
the Earth’s surface. A zone of territory subject to earthquakes 
extends around the Pacific. 
As is known from its rigidity, the interior of the Earth as a whole 
is solid. There cannot even be at present a continuous liquid sur- 
face between the center and the crust. Beneath every active volcano, 
however, there must be an area from which its lava is derived. In 
some way, without doubt by the contraction of the Earth, this lava 
is caused to approach the surface, and on the way it gradually changes 
from a viscous solid to a viscous liquid. There are only two ways 
in which this change can take place: one is by an increase in tem- 
perature, the other by a decrease in pressure. The latter is probably 
the actual one. 
Tangentially considered, the lower portions of what we may for 
convenience call the Earth’s crust are in a state of compression, the 
upper portions in a state of tension. Radially all are in a state of 
compression. Between the upper and lower portions is a neutral sur- 
face of no tangential strain. When a crack caused by the tangential 
tension reaches this neutral surface, the viscous rock oozes up through 
it, becoming more and more liquid as it approaches the surface and 
the pressure is diminished. As it melts and is relieved of pressure, its 
density diminishes, and, if it finally reaches the surface, the erupted 
lava will continue to flow till the pressure at its source is reduced 
to equality with the hydrostatic pressure at the base of the crack. 
The larger the opening and the shorter the distance from the surface, 
the sooner will this equality of pressure occur, and the shorter be 
the duration of the eruption. The expansion of the bubbles of 
steam near the top of the crack diminishes the hydrostatic pressure, 
and their escape obviously causes the explosions usually noticed. 
The violent manifestations are therefore all generated near the surface, 
as is the case of a geyser. 
The uprush and escape of all this material broaden the crack 
into a tube several hundred feet in diameter. After the lava has 
ceased to flow, the steam working its way up to the vent still keeps 
a somewhat narrowed passage open. It thus continues as a line of 
