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 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