Page:A History of Land Mammals in the Western Hemisphere.djvu/31

Rh attitude, which disregard the minor irregularities of the bottom, just as a deep snow buries the objects which lie upon the surface.

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.

This principle was first enunciated by William Smith, an English engineer, near the close of the eighteenth century, who thus laid the foundations of Historical Geology. In the diagram. Fig. 2, is reproduced Smith's section across England from Wales to near London, which shows the successive strata or beds, very much tilted from their original horizontal position by the upheaval of the sea-bed upon which they were laid down. The section pictures the side of an imaginary gigantic trench cut across the island and was constructed by a