Geology 



In the first place it has been divided into two main 

 sections, an older, known as the Tournaisian, and a newer, 

 the Vis^an. These are subdivided into the following 

 zones by means of corals and brachiopods, each zone 

 being characterized by an assemblage of fossils peculiar 

 to itself. In each case a single species is taken as the 

 index fossil of the assemblage, and its name is given 

 to the zone : 



5. Dibunophyllum zone. 

 4. Seminula zone. 

 3. Syringothyris zone. 

 2. Zaphrentis zone. 

 i. Cleistopora zone. 



In England the Carboniferous Limestone rests on a 

 very irregular floor of older rocks an ancient land 

 surface of hills and valleys. So we know that in that 

 part of the world the Carboniferous Period was preceded 

 by a time when the surface was above sea-level and was 

 subject to ordinary denudation, possessing its brooks and 

 rivers, though, of course, these ran in channels very 

 different in direction from those of the present day. 



In the extreme south of England and on the 

 neighbouring coasts of Europe, this period immediately 

 before the Carboniferous the Devonian was a marine 

 period, and the waters of its sea appear to have spread 

 slowly towards the north during the early part of the 

 Carboniferous time. 



Thus it happens that the lowest beds of the Carboni- 

 ferous Limestone in the south of England are older 

 than those in the north, a fact which was discovered by 

 a comparison of their fossil contents. 



Thus at Bristol all the five zones enumerated above 



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