JURASSIC. 



255 



There is a marked succession in the forms of life that characterize 

 the divisions of the Jurassic rocks, and this is notably the case with 

 the Ammonites of the Lias and Inferior Oolite. Hence certain 

 species have been taken to mark zones or epochs in the life-history 

 of the strata, because as a rule their vertical range is more restricted 

 than that of other equally abundant fossils ; but while species of 

 Ammo7iitcs have been generally selected as indices to zones, " their 

 presence alone (as Prof. R. Tate has remarked) does not warrant 

 us in assigning this limestone to the zone of Ammonites attgulaiiis, 

 or that clay to that of Ammonites Turneri, as the case may be ; as 

 they may range through several zones. For the Ammonite which 

 gives its name to a zone is but one of many which mark a de- 

 terminate stage in the life-history of the formation. Thus the zone 

 is a zoological one, and signifies an assemblage of species, and not 

 the range of an Ammonite T ' Owing to the rarity of fossils in some 

 localities, it may be impossible to determine the zones, while the 

 particular species of Ammonites or other fossils taken to indicate 

 the zone may be absent. Moreover, zones characterized by the 

 same species, may not be precisely synchronous in different parts 

 of the country: they are useful as marking the succession of life- 

 forms, but they must not be regarded as definite stratigraphical 

 divisions. 



The Jurassic system is divided as follows •} — 



Jurassic. 



Upper. 



Middle. 

 ^Lower. 



Purbeck Beds. 

 Portland Beds. 

 K^imeridge Clay. 

 Corallian Beds. 

 Oxford Clay, with 

 Kellaways Rock. 

 Great Oolite Series. 

 Inferior Oolite Series. 

 Lias. 



Upper. 



Middle. 



Loiver. 



^Oolites. 



The mapping of the Jurassic rocks for the Geological Survey was done in the 

 south-west of England, partly by Sir A. C. Ramsay and John Phillips, but chiefly 

 by H. W. Bristow and E. Hull, with revisions in places by J. H. Blake, W. A. 

 E. Ussher, and the writer ; in the midland counties, the work was done by H. 

 H. Howell, A. H. Green and J. W. Judd ; and in the north-eastern counties 

 (Lincolnshire and Yorkshire), by C. Fox Strangvvays, W. H. Penning, W. A. E. 

 Ussher, W. H. Holloway, C. Reid, and G. Barrow. 



^ Q. J. xxiii. 300. 



^ Mr. Jukes-Browne (1S84) has proposed the name Glez'onian (from Gleva or 

 Glevona, the old name of Gloucester) for the Middle Jurassic series ; and 

 Clavinian (from Clavinium, the old name of Weymouth) for the Upper Jurassic. 

 G. Mag. 1884, p. 525. 



