A moment’s consideration will show that, in any series of stratified rocks which have not been greatly disturbed from their original horizontal position, _the order of succession or superposition of the beds must necessarily be the chronological order of their formation_. (Fig. 1.) Obviously, the lowest beds must have been deposited first and therefore are the oldest of the series, while those at the top must be the newest or youngest and the beds intermediate in position are intermediate in age. This inference depends upon the simple principle that each bed must have been laid down before the next succeeding one can have been deposited upon it. While this is so clear as to be almost self-evident, it is plain that such a mode of determining the chronological order of the rocks of the earth’s crust can be of only local applicability and so far as the beds may be traced in unbroken continuity. It is of no direct assistance in correlating the events in the history of one continent with those of another and it fails even in comparing the distinctly separated parts of the same continent. Some method of universal applicability must be devised before the histories of scattered regions can be combined to form a history of the earth. Such a universal method is to be found in the _succession of the forms of life_, so far as that is recorded in the shape of _fossils_, or the recognizable remains of animal and vegetable organisms preserved in the rocks. [Illustration: FIG. 1.—Diagram section of a series of beds, illustrating superposition. A is the oldest, B, C, D, etc., succeeding in ascending order.]