OLDER AND YOUNGER FORMATIONS. 25 



As a general rule, the oldest formations will be the 

 deepest-seated, or, in other words, the strata that lie be- 

 neath must be older than those that lie above them. Gene- 

 rally speaking, too, the older rocks will be harder and more 

 crystalline in texture than the younger. There may be 

 isolated exceptions to this, but the great fact holds good, 

 that the older formations are really the more crystalline, 

 and that this characteristic becomes less and less marked 

 till we arrive at the recent and superficial layers of clay, 

 sand, and gravel. Again, all the stratified rocks are less 

 or more fossiliferous that is, contain the petrified remains 

 of plants and animals and these fossils, as they are called, 

 lead to pretty correct inferences not only as to the relative 

 ages of formations, but as to the conditions under which 

 they were deposited. And they do it in this way. Every 

 lake, or estuary, or sea, imbeds in its sediments the re- 

 mains of plants and animals that have either been drifted 

 from the land by rivers or have lived and grown in the 

 waters of deposit. As these remains get imbedded in the 

 sands, clays, and calcareous muds, and excluded from the 

 action of the air, they gradually undergo a change, become 

 impregnated with mineral solutions, and in course of time 

 are petrified, or converted into stony matter like the strata 

 that contain them. At the present day the plants and 

 animals entombed in the delta of the Mississippi differ 

 widely from those entombed in the delta of the Ganges j 

 and were these deltas subsequently converted into rock- 

 formations, those plants and animals would afford evidence 

 of the kind of life and climate that prevailed in their re- 

 spective areas. It is in this manner that the fossils found 

 in the earth's crust bespeak the conditions under which 

 they lived aquatic or terrestrial, fresh- water or marine, 

 inhabitants of a cold climate or inhabitants of a genial one. 

 They further afford the best of all evidence as to the rela- 



