216 A. J. Jukes-Broume — Zones and Maps. 



V. — Zones and ' Chronological ' Maps. 

 By A. J. Jukes-Browne, B.A., F.G.S. 

 ll/TODERN stratigraphical geologists, both those who study the 

 JjJL Palaeozoic and those who are occupied with the Neozoic rocks, 

 are largely concerned with the establishment of zones ; in other 

 words, the zonal method of subdivision has been recognized as the 

 best means of correlating the stratigraphical succession in one 

 area with that of another. But when it comes to giving an exact 

 definition of a zone there does not appear to be the same unanimity 

 of opinion, and consequently it seems desirable that some well- 

 considered definition of a zone should be put forward and discussed. 



The conceptions of a zone which have from time to time been 

 expressed by different English writers may, 1 think, be arranged in 

 two sets, according as they give prominence to one of two different 

 ideas. One set of writers have regarded the essential idea of a zone 

 to be an assemblage of fossils which could be traced over a more or 

 less extended geographical ai"ea ; other writers consider a zone to be 

 an actual belt or band of stratified rock characterized by a certain 

 number of fossil species. 



Most of the earlier English writers on this subject used the term 

 ' zones of life,' and it is probable that this imparted to some minds 

 an erroneous impression that such zones were zoological assemblages 

 or faunas, and were only useful as aids in determining a sequence 

 which was established on a different basis. 



Another phrase frequently used was * Ammonite zones,' which 

 naturally led to the fallacy that zones were established solely on the 

 occurrence of certain species of Ammonites without reference to 

 the distribution of other kinds of fossils. 



One of the earliest investigators of zones in this country was 

 Dr. Thomas Wright, and though he makes use of the terms ' life- 

 zones ' and ' Ammonite zones,' yet it is quite clear from his writings 

 that by a zone of the Lias he means an actual subdivision of the 

 Lias, a group of beds containing an assemblage of fossils, some of 

 which are restricted to the zone.^ 



Mr. E. Tate, writing in 1867 of Liassic zones, says : " The zone is 

 a zoological one and signifies an assemblage of species, and whether 

 you call it Lias a or Lias ^, or the zones of Ammonites planorbis 

 and Ammonites angnlatus, the succession of faunas in the Lower Lias 

 still remains patent, and it is to these several faunas that the term 

 Ammonite zone is applied." * 



This passage is somewhat confused, for he defines a zone as a fauna, 

 and yet says it might equally well be called Lias a, /3, etc., which are 

 stratigraphical subdivisions. In 1875, however, he expresses himself 

 much more clearly, for he wi'ites that " the strata [of the Lias] admit 

 of a grouping into zones after their palgeontological characters, and 

 further into regions of Ammonites, a classification as much in 

 harmony with facts as that of the Oolitic rocks into formations." ^ 



1 Quart. Journ. Geol. Soc, vol. xvi (1860), p. 374 ; and " The Lias Ammonites," 

 Mon Pal. Soc, 1878-9. ^ Quart. Journ. Geol. Soc, -vol. xxiii, p. SOU. 



^ Quart. Journ. Geol. Soc, vol. xrsi, p. 493. 



