STRATIGRAPHY 'A I. I Al 1 ONTOLOGY 



the idea that a /one is a set of beds characterised by the occurrence 

 of one particular species, and that every bed which yield* this 

 index -species must belong to the zone. It mu.-t be remembered 

 that this particular species is only one of several or many, and 

 that it is this assemblage of species which is the guide to the 



"The value and importance of a zone depends partly on tin- 

 number of restricted species and partly on the extent of country 

 over which it can be traced. The thickness of a zone is of course 

 a stratigraphical accident, depending merely on the amount of 

 sediment deposited at any locality during the lifetime of the zonal 

 fauna. Thus a zone may be in one area only a few feet in thick- 

 ness, in another it may be 20 or 30 feet, and in a third it may 

 expand into 200 or 300 feet. 



"The limits of a zone may be definite or indefinite. Where 

 sedimentation has been continuous and fairly rapid, the limits of 

 the zones will naturally be indefinite ; some of the characteristic 

 species of one zone may survive into the overlying zone, and others 

 which are characteristic of the latter may make their first appearance 

 in the former. Thus in many cases it is impossible to say exactly 

 where the one zone ends and the other begins, and though by 

 careful collecting one may fix it with fair certainty in one section, 

 one cannot be certain of taking exactly the same plane of separation 

 in another quarry or cliff which is a few miles away from the first 

 Thus where a zone is complete its limits are indefinite, but the 

 succession of zones will be the same in both places. 



"Where a zone has definite limits it is generally because 

 sedimentation ceased for a time, and in many cases because an 

 erosive current swept away some of the sediment which had 

 previously been accumulated, thus destroying the continuity of tlu- 

 record and producing what is called a ' surface of contemporaneous 

 erosion.' Even if the current was only strong enough to prevent 

 the accumulation of sediment, it causes a break in the record, 

 because for a time there is no embedment and preservation of 

 organic remains. In this way, therefore, a zone or a sub-zone may 

 die out when traced in a certain direction." 



In some cases still smaller zonal subdivisions may be recognised, 

 consisting of one or two beds which are characterised by one or 

 more peculiar species ; these may be called tub-zone* or tomra, but 

 there is not necessarily a complete succession of such sub-zone*, nor 

 can they be expected to have so wide a geographical extension as 

 zones ; they are merely special horizons, and equivalent to the 

 niveaux of French geologists. 



Synchronism and Homotaxis. It was stated in Chapte 

 that when two districts are physically separated from one another 



