CHRONOLOGICAL SUCCESSION. 39 



younger than all the strata upon which it rests, and older than 

 all those by which it is surmounted. 



It is possible, then, by an appeal to the rocks alone, to de- 

 termine in each country the general physical succession of the 

 strata, and this " stratigraphical " arrangement, when once de- 

 termined, gives us the relative ages of the successive groups. 

 The task, however, of the physical geologist in this matter is 

 immensely lightened when he calls in palaeontology to his aid, 

 and studies the evidence of the fossils embedded 'in the rocks. 

 Not only is it thus much easier to determine the order of suc- 

 cession of the strata in any given region, but it becomes now 

 for the first time possible to compare, with certainty and pre- 

 cision, the order of succession in one region with that which 

 exists in other regions far distant. The value of fossils as tests 

 of the relative ages of the sedimentary rocks depends on the 

 fact that they are not indefinitely or promiscuously scattered 

 through the crust of the earth, as it is conceivable that they 

 might be. On the contrary, the first and most firmly estab- 

 lished law of Palaeontology is, that particular kinds of fossils 

 are confined to particular rocks, and particular groups of fossils 

 are confined to particular groups of rocks. Fossils, then, are 

 distinctive of the rocks in which they are found much more 

 distinctive, in fact, than the mere mineral character of the rock 

 can be, for that commonly changes as a formation is traced 

 from one region to another, whilst the fossils remain unaltered. 

 It would therefore be quite possible for the palaeontologist, 

 by an appeal to the fossils alone, to arrange the series of sedi- 

 mentary deposits into a pile of strata having a certain definite 

 order. Not only would this be possible, but it would be found 

 if sufficient knowledge had been brought to bear on both 

 sides that the palaeontological arrangement of the strata would 

 coincide in its details with the stratigraphical or physical 

 arrangement. 



Happily for science, there is no such division between the 

 palaeontologist and the physical geologist as here supposed ; 

 but by the combined researches of the two, it has been found 

 possible to divide the entire series of stratified deposits into a 

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

 recognized order of succession, and each of which is charac- 

 terized 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, rep- 

 resents the life of the particular period in which the formation 



