PRELIMINARY CONSIDERATIONS. 3 



of life. On the one hand, when we know the successive ages 

 of fossil forms, these become to us, like medals or coins to the 

 historian, evidences of periods in the earth's history. On the 

 other hand, we are obliged in the first instance to ascertain the 

 ages of the medals themselves by their position in the succes- 

 sive strata which have been accumulated on the surface. The 

 series of layers which explorers like Schliemann find on the 

 site of an ancient city, and which hold the works of successive 

 peoples who have inhabited the place, thus present on a small 

 scale a faithful picture of the succession of beds and of forms 

 of life on the great earth itself. 



Our leading criterion for estimating the relative ages of rocks 

 is the superposition of their beds on each other. The beds of 

 sandstone, shale, limestone, and other rocks which constitute 

 the earth's crust have nearly all been deposited thereon by water, 

 and originally in attitudes approaching to horizontality. Hence 

 the bed that is the lower is the older of any two beds. Hence 

 also, when any cutting or section reveals to us the succession 

 of several beds, we know that fossil remains contained in the 

 lower beds must be of older date. 



We can scarcely walk by the side of a stream which has 

 been cutting into its banks, or at the foot of a sea-cliff, or 

 through a road-cutting, without observing illustrations of this. 

 For instance, in the section represented in Fig. i, we see at 

 the surface the vegetable soil, below 'this layers of gravel and 

 sand, below this a bed of clay, and below this hard limestone. 

 Of these beds a is the newest, d the oldest ; and if, for 

 example, we should find some marine shells in d, some fresh- 

 water shells in c, bones of land animals and flint arrow- 

 heads in d, and fragments of modern pottery in a, we should 

 be able at once to assign their relative ages to these fossils, 

 and to form some idea of the succession of conditions and of 

 life which had occurred in the locality. 



On a somewhat larger scale, we have in Fig. 2 a section of 

 the beds cut through by the great Fall of Niagara. All of 



n 2 



