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CHAPTER III. 







SUCCESSION OF FORMATIONS CONTEMPORANEITY OF 

 STRATA GEOLOGICAL CONTINUITY. 



DIFFERENT AGES OF THE AQUEOUS EOCKS. 



THE two principal tests by which the age of any particular 

 bed, or group of beds, may be determined, are superposition 

 and organic remains a third test sometimes being afforded 

 by mineral characters. The first and most obvious test of 

 the age of any aqueous rock is its relative position to other 

 rocks. Any bed or set of beds of sedimentary origin is 

 obviously and necessarily older than all the strata which 

 surmount it, and younger than all those upon which it 

 rests. It is to be remembered, however, that superposition 

 can at best give us but the relative age of a bed as com- 

 pared with other beds of the same region. It cannot give 

 us the absolute age of any bed ; and if we are ignorant of 

 the age of any of the beds with which we may be deal- 

 ing, we have to appeal to other tests to learn more than 

 the mere order of succession -in the particular region under 

 examination. 



The second, and in the long-run more available, test of 

 the ages of the different sedimentary beds, is that afforded 

 by their organic remains. Still, this test is also by no 

 means universally applicable, nor in all cases absolutely 

 conclusive. Many aqueous rocks are unfossiliferous through 

 a thickness of hundreds, or even thousands, of feet of little 

 altered sediments ; and even amongst beds which do contain 

 fossils, we often meet with strata of a few feet or yards in 



