The Winter in and around Lyell 



are present, while in certain parts of the north of 

 England the lowest beds belong to the Seminula 

 zone. 



The fossil remains have in this, as in many other 

 instances, enabled geologists not only to determine the 

 age of the rocks but to trace the geographical changes 

 which took place during their formation. 



The animals best suited for the zoning of rocks are 

 the members of groups that were in rapid evolution 

 during the time when the rocks were being formed, thus 

 one group is chosen for one formation, a different one 

 for another. 



The Carboniferous Limestone has been zoned, as 

 already stated, by means of the Corals and the 

 Brachiopods, while in the case of the Lias, a much newer 

 formation, the Ammonites were found to be the most 

 suitable. 



In a group of animals which are in rapid evolution, 

 a given form will persist for but a short time, and will 

 soon be replaced by another and usually more complex 

 type. Such a form will, on this account, be found only 

 in a thin series of beds, its modified descendants taking 

 its place in the next series. 



Some very interesting work has been done recently 

 on the modifications which certain corals have under- 

 gone during the Carboniferous Period, and it has been 

 shown that some of the forms in the higher beds are 

 the direct descendants of other and somewhat different 

 forms in the lower. 



Fossils, when carefully studied, not only enable us to 

 follow out the geographical changes which have occurred 

 in the past, but sometimes also to determine the par- 



187 



