PALEONTOLOGY. 153 



This generalization first made by the " Father of 

 English Geology/' William Smith leads directly to 

 the doctrine that " strata may be identified by fossils," 

 for if these groups of characteristic species have been 

 changing throughout all time, and were different in 

 every geological period, we see at once that the relative 

 age and position of beds may be known from their em- 

 bedded fossils. 



To take a simple instance : Ammonite shells and the 

 bones of an Ichthyosaurus have been discovered in some 

 of the Arctic Islands, and we at once conclude that the 

 rocks containing them belong to some member of the 

 Secondary system ; moreover, we guess that they belong 

 to the Jurassic formation, since the Ammonite more re- 

 sembles an Oolitic form than any known in Cretaceous 

 rocks. 



Another more detailed instance may be given : It is 

 known that there are among the Upper Silurian rocks 

 three distinct beds of limestone ; where these occur in 

 a disturbed and faulted district, we- might discover a 

 -quarry in limestone, forming part of an area which had 

 been faulted up or down, and thus separated from beds 

 with which it was originally continuous. In such a case 

 the stratigraphical evidence might be very obscure, but 

 the palaeontological evidence would at once decide the 

 matter, for on finding among the fossils which had been 

 collected from the limestone specimens of the large 

 Brachiopod, Pentamerus Knightii, we should recognize 

 in it a fossil characteristic of the Aymestry limestone, 

 .and have little hesitation in referring the beds to that 

 formation ; if, on the other hand, we found such forms 

 as Acervularia luxurians, Euomphalus discors, and 



