From these and similar facts, the naturalist finds how agencies of the
present construct new rocks and alter the old; and so in the light of this
knowledge, he proceeds with his task of analyzing the remote past,
confident that the same natural forces have done the work of constructing
the lower geological levels because these earlier products are similar to
those being formed to-day. After learning this much, he must immediately
undertake to arrange the strata according to their ages. This might seem a
difficult or even an impossible task, but the rocks themselves provide him
with sure guidance.

Wherever a river has graven its deep way through an area of hard rocks, as
in the case of Niagara, the walls display on their cut surfaces a series
of lines and planes showing that they are superimposed layers formed
serially by deposits that have differed some or much at different times
according to the circumstances controlling the erosion of their
constituent particles. A layer of several feet in thickness may be
composed of compact shale, while above it will be a zone of limestone, and
again above this another layer of shale. Successive strata like these,
where they are parallel and obviously undisturbed, are evidently arranged
in the order of their formation and age. But by far the most impressive
demonstration of the basic principle of geology employed for the
determination of the relative ages of rocks is the mighty Cañon of the
Colorado. As the traveler stands on the winding rim of this vast chasm,
his eye ranges across 13 miles of space to the opposite walls, which
stretch for scores of miles to the right and left; upon this serried face
he will see zone after zone of yellow and red and gray rock arranged with
mathematical precision and level in the same order as on the steep slopes
beneath him. Plain common sense tells him that the great sheets of rock
stretched continuously at one time between the now separate walls, and
that the various strata of sandstone and limestone were deposited in