12 PRINCIPLES OF GEOLOGY. 



granite and mica slate. Since, then, among the secondary rocks, some 

 contain, and others do not contain, organic remains, they may thus be 

 sometimes distinguished. But when we consider the immense variety 

 of organic remains, and learn that in a very limited district of England, 

 many hundreds of species can be collected, and in the whole kingdom 

 several thousands, it becomes evident that a more important branch 

 of the inquiry remains : viz. In what manner the different species are 

 distributed in the interior of the earth. Whether, for instance, they 

 are arranged according to geographical position, as is partly the case with 

 existing races, or according to the order of the different rocks, or mixed 

 confusedly together. 



That they are not mixed confusedly together, is decisively proved 

 by many cases like the following : the fossils of the chalk cliffs near 

 Bridlington, are numerous and well known ; so are those of the lias shale 

 in the cliffs near Whitby ; and also those of the mountain limestone near 

 Skipton ; and on comparison, it becomes evident that no one fossil of the 

 whole number is found in two of the strata enumerated ! each of these 

 three strata has its own peculiar fossils distinct from those in the others. 

 By prosecution of such comparisons, Mr. Smith discovered that organic- 

 fossils are distributed in the earth, not in proportion to depth from the 

 surface, nor even according to chemical composition, but according to 

 the order and succession of the rocks. He has the great merit of esta- 

 blishing the facts, 



That different strata contain generally different fossils ; but that the 

 same stratum over a very large extent of country, contains generally the 

 same fossils. Hence he deduced the important conclusion, 



That strata may be discriminated and identified by their organic con- 

 tents. 



Since, then, rocks of different antiquity contain different fossils, it is 

 possible to class the organic remains according to their respective periods 



