THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 33 



outcrops can be considered as establishing continuity. For well-established 

 units of known and persistent character and in regions of continuous exposure 

 the method leads to incontrovertable results. 



Correlation by the fossil content is by far the most commonly applicable, 

 but the method of applying the evidence is still in dispute. For the corre- 

 lation of closely adjacent exposures the identity of faunae or florae is an 

 unquestioned proof of contemporaneity, but the farther the exposures are 

 separated the more shrewdly the evidence must be questioned. The ques- 

 tion of contemporaneity lersus homotaxy has been thoroughly discussed, 

 and one would hesitate to connect far separate areas as parts of the same 

 unit by the simple presence of even closely similar fossils. The character 

 of the fossils must of course be considered ; floating forms readily dispersed 

 by currents may spread over a large portion of the earth within the time of 

 the deposition of even a thin unit. So much has been claimed for the 

 graptolites, and Ulrich has shown how relatively rapidly even mollusks may 

 be dispersed.^ 



Another phase of the question of correlation by fossil content is the dis- 

 cussion as to the relative importance of unique or common species. Is 

 contemporaneity to be judged by the common occurrence of a large propor- 

 tion of similar species, "matching species," or by the common occurrence 

 of a few peculiar species? The pros and cons of this important question 

 are taken up in papers by Ulrich, Williams, and Grabau.^ 



Correlation by inorganic contents — similar mineral or lithological fea- 

 tures — has proven of value in limited and closely connected exposures, but 

 for areas separated by any considerable interval it has been too frequently 

 shown of no value to carry any significance of contemporaneity, though 

 the value of such evidence as suggesting similar conditions of deposition is 

 unquestioned. 



Correlations of beds by the size of material, content, and depth of 

 weathering have been applied to the solution of problems in climatic changes 

 and terrace formation and destruction; also to the age of glacial deposit. 

 The arguments suggested by the authors cited may be extended to the inter- 

 pretation of more ancient deposits in favorable cases. For a discussion of 

 the value of diastrophic changes in correlation the student is referred to the 

 citation from Ulrich given above. 



VI. CLIMATOLOGY OF THE PAST. 



The climatological changes of the past most obviously recorded are those 

 of the great extremes, as in periods of local or regional glaciation where 



^ Ulrich, E. O., Revision of the Paleozoic Systems, pp. 295 and 575. 

 ^ Ulrich, E. 0., Revision of Paleozoic Systems. 



Williams, H. S., Correlation Problems Suggested by the Eastport Quadrangle, Maine, 

 Bull. Geol. Soc. Amer., vol. 24, p. 337, 1913. 



Grabau, A., Principles of Stratigraphy, chap. 32. 

 4 



