320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



CORRELATION 



The practice of dating rocks by the fossils they contain is based 

 upon the fact that during geologic time the complex of factors affect- 

 ing organisms has resulted in their evolution, migration, and extinc- 

 tion. Establishing the sequence of changes in individual categories 

 and assemblages of organisms provides a basis for a relative chronol- 

 ogy. The stratigraphic paleontologist can make correlations and age 

 determinations by comparison of fossils from beds of unknown age 

 with those from beds where the age is established. 



The suitability of pollen and spores for geologic dating arises 

 from several of the factors already discussed, namely, their small 

 size, taxonomic individuality, resistance to degradation, and wide- 

 spread distribution. They may be used in correlation either as a 

 means for identifying botanically the plants they represent or as 

 arbitrarily designated forms. In practice, a combination of the 

 two is often employed. The botanical approach takes advantage 

 of the fact that the times of appearance and disappearance of most 

 of the major plant groups are known. Thus Carboniferous plant 

 microfossils reflect the dominance of extinct arborescent lycopods and 

 horsetail relatives along with many ferns and seed ferns. Within 

 the Carboniferous, changes in generic and specific composition and 

 relative abundance with time are sufficient to make the numerous 

 plant microfossils in coal useful for correlating coal seams within a 

 basin (Kosanke, 1950). 



Permian and older Mesozoic rocks are characterized by the ab- 

 sence of many of the Carboniferous types and the increasing propor- 

 tion of winged gymnospermous grains and cycadophyte pollen. An- 

 giosperm pollen is not certainly present until early Cretaceous time 

 and is not abundant until late Cretaceous time. Pollen from Upper 

 Cretaceous rocks is predominantly that of extinct angiosperm genera ; 

 the floras assume an increasingly modern and more provincial aspect 

 in the Tertiary. 



Such floral changes are revealed, for example, within the Tertiary 

 sediments of the Great Basin, where fossil pollen assemblages record 

 major changes in the composition of the woody flora due to migra- 

 tion and to evolution. These changes are of the same nature and on 

 the same order as the regional floristic changes already outlined from 

 study of fossil leaves and fruits. As do the leaf floras, the Cretaceous 

 and early Tertiary pollen assemblages contain many strange uniden- 

 tifiable types, a few recognizable subtropical families or genera, coni- 

 fers, some of which are now extinct, and a few warm-temperate trees 

 that still grow on the North American continent but are no longer 

 present in the local flora. Middle Tertiary sediments show several 



