22 STEATIGEAPHICAL GEOLOGY 



Beyond all this there can be traced a distinct and decided 

 progress in the types of life as we ascend the geological scale. 

 It is true that in the earliest fauna yet known we start with a 

 number of more or less specialised invertebrate types, but the 

 more highly organised orders of Gastropods, Cephalopods, Crustacea, 

 and Echinoderms do not appear till later periods ; Fish are not 

 known earlier than Silurian times ; Amphibia commence in the 

 Carboniferous ; Reptiles in the Permian and Trias ; Birds in the 

 Jurassic ; Mammals of low organisation in the Trias ; higher 

 Mammals, including Lemurs, appear in the Eocene, but true 

 monkeys are not yet known from beds of greater age than the 

 Miocene. 



Zones. It only remains to indicate how the succession of life- 

 forms may be utilised for the purposes of subdivision and correla- 

 tion, when we have to deal with a continuous succession of deposits, 

 and more especially with a great thickness of one kind of deposit, 

 such as shale or limestone. During the accumulation of such a 

 mass of sediment it would seem that small changes were continu- 

 ally taking place in the personnel of the fauna, new forms taking 

 the place of old ones, some species dying out and others coming 

 in from elsewhere ; so that when the fossils are carefully collected 

 from each successive bed it is found that they form assemblages of 

 species, each extending through a certain thickness of beds and 

 thus characterising a band or zone within the formation. 



A zone, therefore, is a certain thickness of stratified material 

 which contains a certain assemblage of fossil species, some of these 

 species being either restricted to the band or particularly abundant 

 in it. Moreover, it is not merely a specially fossiliferous band in a 

 thick mass of sediment, but is one of a succession of zones, each 

 stage in a fossiliferous series being usually divisible into two, three, 

 or more zones. 



The following remarks on zones are quoted from the author's 

 memoir on the Cretaceous Rocks of Britain. 1 " By the fauna of a 

 zone we generally mean all the fossils which have been or can be 

 found in the beds which are recognised as belonging to the zone. 

 But what may be termed the critical fauna of a zone is the much 

 smaller assemblage of fossils which are either restricted to it or are 

 specially abundant in it. One of these species is then chosen as 

 the index of the restricted assemblage of species, and the name of 

 this fossil is given to the zone, so that we speak of it as the zone of 

 this fossil, e.g. the zone of Ammonites varians, or the zone of Belem- 

 nitella mucronata. 



" The reader must be cautioned against becoming possessed with 

 1 "The Cretacecras Eocks of Britain," vol. i. p. 34, Mem. Geol. Survey. 



