J. IMBRIE 143 



quate field work: nondeposition, erosion, migration, nonpreserva- 

 tion, and inaccessibility. Together, these factors account for the 

 prevalence of transient species. 



Since the publication of the classic paper by Barrell (1917), 

 stratigraphers have generally admitted that very few local sedi- 



B 



» »"'». l . ^ • ' .■^ ' . ' . l ' ^•.T T' ■ ^ ^. , • * : ^^f^/?^^ T ^ T ^* *• * •• 'T ^'. M ' ' . ^ ■ ' ■'... " ^ " 



Gap 



V T .-. ■ . ' ■ ' . ' . ' ■ i , ' .' Ty7r r T ...... \. I 



Gap 



/ \ 



\ 



'• ' .-- ' . ' ■'. ' • ' . ' . ' .'.-• ' .■.•■ ' •■•■ •* > - > >w ^rT^' -'- ' --- J -- ^ V. ' • ' ■ ' . ' !'.'.- ' •. ' .-• • 



1 ■ ■ { ' ' *-• • ' 



N. 



Gap 



E 



Trend 



Fig. 11. Diagram to illustrate how gaps in the stratigraphic record re- 

 sult in a meager fossil record of a continuously evolving lineage. Gradual 

 evolution of a lineage is indicated by a series of shifting normal frequency 

 curves. Stippled areas represent preserved portions of the stratigraphic rec- 

 ord. Solid frequency curves represent four transient species collected from 

 layers A-D. Gaps in the fossil record, indicated by dashed curves, cannot 

 be eliminated by collecting in this area. (After Newell, 1956.) 



mentary sequences contain a direct, continuous record of the total 

 time span represented in a given stratigraphic sequence. This is 

 because many fossiliferous strata were formed in water so shallow 

 that the permanent accumulation of a sedimentary layer (with 

 entombed fossils) was a rare event, possible only when the sur- 

 face of sedimentation was below the depth to which waves and 



