average rate of increase at one degree for every sixty feet of descent, and assuming that this rate, observed at the moderate distances open to observation, continues to at least thirty-five miles, the temperature at that depth must be more than three thousand degrees,—­a temperature at which all ordinary rocks would melt at the earth’s surface.  The rate of increase in temperature probably lessens as we go downward, and it may not be appreciable below a few hundred miles.  But there is no reason to doubt that the interior of the earth is intensely hot.  Below a depth of one or two score miles we may imagine the rocks everywhere glowing with heat.

Although the heat of the interior is great enough to melt all rocks at atmospheric pressure, it does not follow that the interior is fluid.  Pressure raises the fusing point of rocks, and the weight of the crust may keep the interior in what may be called a solid state, although so hot as to be a liquid or a gas were the pressure to be removed.

The interior of the earth is rigid and heavy.  The earth behaves as a globe more rigid than glass under the attractions of the sun and moon.  It is not deformed by these stresses as is the ocean in the tides, proving that it is not a fluid ball covered with a yielding crust a few miles thick.  Earthquakes pass through the earth faster than they would were it of solid steel.  Hence the rocks of the interior are highly elastic, being brought by pressure to a compact, continuous condition unbroken by the cracks and vesicles of surface rocks.  The interior of the earth is rigid

The common rocks of the crust are about two and a half times heavier than water, while the earth as a whole weighs five and six-tenths times as much as a globe of water of the same size.  The interior is therefore much more heavy than the crust. This may be caused in part by compression of the interior under the enormous weight of the crust, and in part also by an assortment of material, the heavier substances, such as the heavy metals, having gravitated towards the center.

Between the crust, which is solid because it is cool, and the interior, which is hot enough to melt were it not for the pressure which keeps it dense and rigid, there may be an intermediate zone in which heat and pressure are so evenly balanced that here rock liquefies whenever and wherever the pressure upon it may be relieved by movements of the crust.  It is perhaps from such a subcrustal layer that the lava of volcanoes is supplied.

The causes of volcanic action.  It is now generally believed that the heat of volcanoes is that of the earth’s interior.  Other causes, such as friction and crushing in the making of mountains and the chemical reactions between oxidizing agents of the crust and the unoxidized interior, have been suggested, but to most geologists they seem inadequate.