Chronology 



The commencement of the text of ^'ol. IV of Type Ammonites 

 offers the opportunity to introduce a new scheme of Jurassic chronology. 

 Some years ago I \vTote a paper on So-called ' Jurassic ' Time ; Quart. 

 Joum. Geol. Soc. LI\', 1898, p. 442. \Mien that paper was referreed, 

 " it was pointed out to the author that [the use of the same term, like 

 Bathonian, for Stage and for Age] would lead to confusion ; and he 

 then proposed to use, as distinctly chronological terms for Ages, names 

 taken from dominant Ammonite genera " (p. 442). Names were given 

 to certain Ages from Lower Lias to Combrash. However, the plan 

 seemed generally to be regarded as making undue complication : it was 

 argued that the same faunal term was used for the subdivision of an 

 Age — a hemera — and for the subdi\"ision of a stage — a zone ; also that 

 the same geographical terms were used for di\dsions of greater magnitude 

 — Jurassic System, Jurassic Period. 



Lately, the demand that there should be a separate nomenclature 

 for chronological as distinct from stratigraphical terms has been repeated 

 on various occasions. It seems advisable to meet it : not because a dual 

 system of terminology is altogether desirable, but rather with the hope 

 that in the future the system based on zoologs' will supplant that based 

 on strata! development and geography, at any rate for all those purposes 

 where chronology" and biolog}' are concerned. Other considerations have 

 also influenced this decision. 



There are various reasons why names taken from places — geograpliical 

 names — are unsuitable for chronological purposes. Chronologv depends 

 on the succession of phenomena, and when there are zoological phenomena, 

 as in the Jurassic Period, they are more rehable as time-indices than the 

 geological-geographical developments of strata : these are frequently 

 defective, either through poverty in original sedimentation, or b\- loss 

 from penecontemporaneous erosion. An ideal geographical naming of 

 geological strata would be taken from the places along some stretch of 

 coast where, owing to gentle dip, successively younger beds are met with 

 in a given direction — for instance, from L\Tne Regis (Lymian) to 

 Portland (Portlandian) along the Dorset coast. The geological succession 

 accords with geographical position — to remember the sequence is, 

 therefore, easy. But this ideal is impracticable : there are not enough 

 place-names to express the stratal (and faunal) developments, while the 

 strata! succession, grand though it be, is too frequently incomplete — 

 strata which are important elsewhere are either poorly developed or 

 entirel}' lacking here. Therefore it has become the custom to range 

 widely afield for names of Stages, there being in many cases several 

 localities with about equal claims to such distinction. But this has a 

 great disadvantage — the sequence of names given to stages becomes an 

 arbitrary' one, difficult to memorize, because there is no geographical 

 association as an aid. With few names this difficulty was not great ; 

 but with so large an increase in the names of Stages in a Period like 

 the Jurassic, due mainly to the discovery that local stratal developments 

 are so frequently defective, the difficulty becomes formidable. 



Local failures of strata, too, have introduced compUcations, tor as 



