36 



CHAPTER III. 



SUCCESSION OF FORMATIONS— CONTEMPORANEITY 

 OF STRATA— GEOLOGICAL CONTINUITY. 



Different Ages of the Aqueous Rocks. 



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 being sometimes afforded by mineral char- 

 acters. 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 compared 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 dealing, we have to appeal to other tests to learn more 

 than the mere order of succession in the particular region under 

 examination. Moreover, deposits formed in isolated basins, and 

 not in an area of continuous sedimentation, have necessarily no 

 stratigraphical relations to deposits laid down in other areas, and 

 their age can only be determined by palaeontological tests. This 

 difficulty, as pointed out by Professor C. A. White, is enhanced 

 when such isolated sediments have been produced within inland 

 fresh waters ; since such sediments, from their mode of formation, 

 can have no place in any observed order of superposition of marine 

 deposits, and would, in addition, necessarily contain wholly different 

 fossils as compared with beds laid down in the sea. 



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

 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, 



