The ledges of the valley of our illustration are of sandstone.
Looking closely at the rock we see that it is composed of myriads
of grains of sand cemented together. These grains have been worn
and rounded. They are sorted also, those of each layer being about
of a size. By some means they have been brought hither from some
more ancient source. Surely these grains have had a history before
they here found a resting place,--a history which we are to learn
to read.

The successive layers of the rock suggest that they were built one
after another from the bottom upward. We may be as sure that each
layer was formed before those above it as that the bottom courses
of stone in a wall were laid before the courses which rest upon
them.

We have no reason to believe that the lowest layers which we see
here were the earliest ever formed. Indeed, some deep boring in
the vicinity may prove that the ledges rest upon other layers of
rock which extend downward for many hundreds of feet below the
valley floor. Nor may we conclude that the highest layers here
were the latest ever laid; for elsewhere we may find still later
layers lying upon them.

A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as
these are of kinds whose nearest living relatives now have their
home in the sea, we infer that it was on the flat sea floor that
the sandstone was laid. Its present position hundreds of feet
above sea level proves that it has since emerged to form part of
the land; while the flatness of the beds shows that the movement