10 INTRODUCTION. 



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 sometimes being afforded by 

 mineral characters. The first and most obvious test of the 

 age of any aqueous rock is rts 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 remem- 

 bered, 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. 



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 univer- 

 sally 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 thickness, which are 

 wholly destitute of any traces of life. Many fossils, again, 

 range vertically through many groups of strata, and in some 

 cases even through several formations. Such fossils, there- 

 fore, if occurring by themselves, or considered apart from 

 other associated organisms, are not conclusive as to the age of 

 any particular set of beds. As the result, however, of com- 

 bined palaeontological and geological researches, it is now pos- 

 sible for us to divide the entire series of stratified deposits into 

 a number of definite rock-groups or formations, each of which 

 is characterised by possessing an assemblage of organic 

 remains which do not occur in association in any other 

 formation. Such an assemblage of fossils, characteristic of 

 any given formation, represents the life of the particular period 

 in which the formation was deposited. It follows from this, 

 that whenever we can get a group or collection of fossils from 

 any particular bed or set of beds, there is rarely any difficulty 

 in determining the precise geological horizon of the beds in 

 which the fossils occur. 



With certain limitations, however, we may go much further 

 than this. Not only are the great formations characterised by 



