32 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



tropical regions; those with non-entire margins in mesophytic cold temperate 

 regions." 



They show also that entire leaves occur in tropical and subtropical 

 regions and in frigid regions, while non-entire leaves occur in cold temperate 

 regions. A study of Tertiary and Cretaceous dicotyledon leaves leads them 

 to state that this generalization "affords a simple and rapid means of 

 gauging the general climatic conditions which existed in regions where these 

 plants flourished." 



Another observation has been made which should be kept constantly 

 in mind — the effect of altitude upon both animal and plant life. This has 

 been in part discussed above. All degrees of humidity are found at the 

 different levels, the factors of temperature and pressure alone decreasing 

 constantly as the elevation increases, and the character of the flora with 

 reference to those, aside from the more variable factors, must be determined 

 as an isolated fact.^ 



V. CORRELATION OF THE UNIT WITH OTHER BEDS. 



The correlation of the beds is a most necessary step in the solution of 

 the problem and also one of the most difficult in many cases. The criteria 

 of correlation as given by Ulrich include diastrophic movements; evidence 

 of sea-filling and tidal flats; by fossils; by lithologic similarity; by prob- 

 abilities depending on rhythm of movement; by unconformities, overlaps, 

 and hiatuses.- To these should be added the tracing of actual continuity. 



When the surface is obscured by vegetation or soil the separation of 

 exposures even for limited distances may lead to error, especially when the 

 unit to be traced is variable in character, as a delta or flood-plain deposit. 

 In many places where the beds by reason of aridity and exposure are laid 

 bare to the eye for miles it is very difficult to follow a deposit because of the 

 rapidly changing character of its inorganic content. Individual beds in the 

 Permo-Carboniferous, Triassic, and Jurassic Red Beds, even when exposed 

 on the face of a naked cliff or on a bare flat, may not be followed for more 

 than a short distance in many places. When, as in Kansas, Oklahoma, or 

 Texas, these exposures are interrupted by areas of grassland or soil, the 

 difficulty is enormously increased. Many of the older deposits in the 

 eastern United States are of the same character, and any attempt to strictly 

 correlate separate exposures in a region of heavy soil, grassland, or thick 

 woods is sure to lead to very questionable results. 



Similarly, but in even higher degree, efforts to correlate beds by samples 

 from drilled or bored wells are open to question. The persistence of similar 

 characters in a unit over great areas must be established before isolated 



* Seward, A. C, Fossil Plants as Tests of Climate. London, 1892. 



2 Ulrich, E. O., Revision of the Paleozoic Systems, Bull. Geol. Soc. Amer., vol. 22, p. 394. 

 (Index in vol. 24, p. 625, 1913.) 



