plain has been cut down and carried away, leaving these picturesque
hills, the survival of which is as much a mystery as the disappearance
of the balance of the plateau of which they were once a part.

The fold that forms a typical mountain ridge has a cone of granite, the
original rock foundation of the earth, and on this are layers of
stratified rock, ancient deposits of sediment carried to the sea by
streams. When exposed to wind and rain, the ridge is gradually worn
down. In some places the water cuts away the soft rock and forms a
stream-bed, that cuts deeper and deeper, using the rock fragments as its
tools. Often the layers of aqueous rocks are cut through, and the
granite exposed.

Sometimes the hardest stratified rock-beds resist the water and the wind
and are left as a series of ridges along the sides of the main range.
The crumpling forces may crack the ridge open for its whole length, and
one side of the chasm may slip down and the other go up. The result is a
sheer wall of exposed rock strata, layers of which correspond with those
that lie far below the top of the portion that slid down in the great
upheaval and subsidence that parted them. These slips are known as
_faults_.

THE LAVA FLOOD OF THE NORTHWEST

We know little about the substance that occupies the four thousand miles
of distance between the surface and the centre of our earth. We know
that the terrible weight borne by the central mass compresses it, so
that the interior must grow denser as the core is approached. Scientists
have weighed the earth, and tell us that the crust is lighter than the
rest. The supposition is that there is a great deal of iron in the
interior, and possibly precious metals, too.

Our deepest wells and mines go down about a mile, then digging stops, on
account of the excessive heat. But the crumpling of the crust, and the
wearing away of the folded strata by wind and running water, have laid
bare rocks several miles in thickness on the slopes of mountains, and
exposed the underlying granite, on which the first sedimentary rocks
were deposited. On this granite lie stratified rocks, which are
crystalline in texture. These are the beds, sometimes miles in depth,
called _metamorphic_ rocks, formed by water, then transformed by heat.

The wearing away of rocks by wind and water has furnished the materials
out of which the aqueous rocks have been made. Layers upon layers of
sandstone, shales, limestone, and the