METHODS — GEOLOGICAL 7 



attitude, which disregard the minor irregularities of the bottom, 

 just as a deep snow buries the objects which he 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 7nust 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 inter- 

 mediate in age. This inference depends upon the simple prin- 

 ciple 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 com- 

 paring 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 oi 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 



