are wont to have. Its banks are vast structures of adamant, piled
up in forms rarely seen in the mountains.

Down by the river the walls are composed of black gneiss, slates, and
schists, all greatly implicated and traversed by dikes of granite. Let
this formation be called the black gneiss. It is usually about 800 feet
in thickness.

Then over the black gneiss are found 800 feet of quartzites, usually in
very thin beds of many colors, but exceedingly hard, and ringing under
the hammer like phonolite. These beds are dipping and unconformable with
the rocks above; while they make but 800 feet of the wall or less, they
have a geological thickness of 12,000 feet. Set up a row of books
aslant; it is 10 inches from the shelf to the top of the line of books,
but there may be 3 feet of the books measured directly through the
leaves. So these quartzites are aslant, and though of great geologic
thickness, they make but 800 feet of the wall. Your books may have
many-colored bindings and differ greatly in their contents; so these
quartzites vary greatly from place to place along the wall, and in many
places they entirely disappear. Let us call this formation the
variegated quartzite.

Above the quartzites there are 500 feet of sandstones. They are of a
greenish hue, but are mottled with spots of brown and black by iron
stains. They usually stand in a bold cliff, weathered in alcoves. Let
this formation be called the cliff sandstone.

Above the cliff sandstone there are 700 feet of bedded sandstones and
limestones, which are massive sometimes and sometimes broken into thin
strata. These rocks are often weathered in deep alcoves. Let this
formation be called the alcove sandstone.

Over the alcove sandstone there are 1,600 feet of limestone, in many
places a beautiful marble, as in Marble Canyon. As it appears along the
Grand Canyon it is always stained a brilliant red, for immediately over
it there are thin seams of iron, and the storms have painted these
limestones with pigments from above. Altogether this is the red-wall
group. It is chiefly limestone. Let it be called the red wall limestone.

Above the red wall there are 800 feet of gray and bright red sandstone,
alternating in beds that look like vast ribbons of landscape. Let it be
called the banded sandstone.

And over all, at the top of the wall, is the Aubrey limestone, 1,000
feet in thickness. This Aubrey has much gypsum in it, great beds of
alabaster that are pure