like, are exposed when a river
cuts a canyon through a plateau. The layered deposits of debris at the
mouth of the river make new aqueous rocks out of old. Every sandy beach
is sandstone in the making. This work is never ended.

In the early days the earth's crust often gaped open in a mighty crater
and let a flood of lava overspread the surface. The ocean floor often
received this flood of melted rock. In many places the same chimney
opened again and again, each time spreading a new layer of lava on top
of the old, so that the surface has several lava sheets overlying the
aqueous strata.

If the hardened lava sheet proved a barrier to the rising tide of molten
lava in the chimney it was often forced out in sheets between the layers
of aqueous rocks. Wherever the heated material came into contact with
aqueous rocks it transformed them, for a foot or more, into crystalline,
metamorphic rocks.

A chimney of lava is called a _dike_. In mountainous countries dikes are
common. Sometimes small, they may also be hundreds of feet across, often
standing high above the softer strata, which rains have worn away. Dikes
often look like ruined walls, and may be traced for miles where they
have been overturned in the mountain-making process.

The great lava flood of the Northwest happened when the Coast Range was
born. Along the border of the Pacific Ocean vast sedimentary deposits
had accumulated during the Cretaceous and Tertiary Periods. Then the
mighty upheaval came, the mountain ridge rose at the end of the Miocene
epoch and stretched itself for hundreds of miles through the region
which is now the coast of California and Oregon. Great fissures opened
in the folded crust, and floods of lava overspread an area of 150,000
square miles. A dozen dead craters show to-day where those immense
volcanic chimneys were. The depth of the lava-beds is well shown where
the Columbia River has worn its channel through. Walls of lava three
thousand feet in thickness rise on each side of the river. They are made
of columns of basalt, fitted together, like cells of a honeycomb, and
jointed, forming stone blocks laid one upon another. The lava shrinks on
cooling and forms prisms. In Ireland, the Giants' Causeway is a famous
example of basaltic formation. In Oregon, the walls of the Des Chutes
River show thirty lava layers, each made of vertical basalt columns. The
palisades of the Hudson, Mt. Tom, and Mt. Holyoke are examples on the
eastern side