CHRONOLOGICAL SUCCESSION. 37 



CHAPTER TIL 



CHRONOLOGICAL SUCCESSION OF THE 

 FOSSILIFEROUS ROCKS. 



The physical geologist, who deals with rocks simply as rocks, 

 and who does not necessarily trouble himself about what fossils 

 they may contain, finds that the stratified deposits which form 

 so large a portion of the visible part of the earth's crust are 

 not promiscuously heaped together, but that they have a cer- 

 tain definite arrangement. In each country that he examines, 

 he finds that certain groups of strata lie above certain other 

 groups ; and in comparing different countries with one another, 

 he finds that, in the main, the same groups of rocks are always 

 found in the same relative position to each other. It is pos- 

 sible, therefore, for the physical geologist to arrange the known 

 stratified rocks into a successive series of groups, or " forma- 

 tions," having a certain definite order. The establishment of 

 this physical order amongst the rocks introduces, however, at 

 once the element of time^ and the physical succession of the 

 strata can be converted directly into a historical or ch7'onologi- 

 cal succession. This is obvious, when we reflect that any bed 

 or set of beds of sedimentary origin is clearly and necessarily 

 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 



