PART II. CHAPTER XIII. 163 



Chronological Classification of Aqueous Rocks. 



not from the destruction of what once existed, by denudation, but 

 because no strata of an intermediate age were ever deposited on c. 

 They were not formed at that place, either because the region 

 was dry land during the interval, or because it was part of a sea 

 or lake to which no sediment was carried. 



In order, therefore, to establish a chronological succession of 

 fossil iferous groups, a geologist must begin with a single section, 

 in which several sets of strata lie one upon the other. He must 

 then trace these formations, by attention to their mineral char- 

 acter and fossils, continuously, as far as possible, from the start- 

 ing point. As often as he meets with new groups, he must ascer- 

 tain by superposition their age relatively to those first examined, 

 and thus learn how to intercalate them in a tabular arrangement 

 of the whole. 



By this means the German, French, and English geologists 

 have determined the succession of strata throughout a great part 

 of Europe, and have adopted pretty generally the following 

 groups, almost all of which have their representatives in the 

 British Islands. 



Groups of Fossiliferous Strata observed in Western Europe, 

 arranged in what is termed a descending series, or beginning 

 with the newest. 



A glance at the above table will show that the three great 

 sections called primary fossiliferous, secondary, and tertiary, are 

 by no means of equivalent importance, if the eighteen subordi- 

 nate groups comprise monuments relating to equal portions of 



* For tertiary, Mr. De la Beche has used the term " supracretaceous," a name 

 implying that the strata so called are superior in position to the chalk. 



