ording 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 Canon 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
successive ages from below upwards in the order of their exposure. When
now he extends his explorations to another state like Utah or Wyoming, he
may find some but not all of the series exhibited in the Grand Canon,
overlaid or underlaid by other strata which in their turn can be assigned
to definite places in the sequence. By the same method, the geologist
correlates and arranges the rocks not only of different parts of the same
state, or of neighboring states, but even those of widely separated parts
of North America and of different continents. But he learns that he must
refrain from over-hasty conclusions, for he soon finds that the
sedimentary rocks have not been constructed at the same rate in different
places during one and the same epoch, and that rocks formed even at one
period are not always identical in nature. But his guiding principle is
sensible and reasonable, and by employing it with due caution he provides
the palaeontologist with the requisite knowledge for his special task,
which is to arrange the extinct animals whose remains are found as fossils
of various earth ages in the order of their succession in time.

CONDENSED TABLE OF PALAEONTOLOGICAL FACTS

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YEA