26 HISTORICAL GEOLOGY 



basins by transferred matter" with resultant encroachment of 

 the sea over the land ''essentially contemporaneous the world 

 over," which in turn implies "a homologous series of deposits the 

 world over." Thus the times of great diastrophism (recognized 

 by great unconformities and overlapping deposits) should form the 

 basis of correlating at least the larger groups, or even systems, of 

 strata in the earth's crust. 



II. Paleontological (biological) criteria. The signifi- 

 cance of fossils in the determination of geological chronology has 

 already been discussed, but it should here be repeated for emphasis 

 that " order of superposition" of the strata, studied in connection 

 with their fossil content, furnishes the general standard for building 

 up a geological chronology, and affords the best basis for the cor- 

 relation of formations. In fact, for correlation of formations in 

 distant portions of a continent, or different continents, paleonto- 

 logical criteria alone are satisfactory. 



1. Identity of species. This is an extremely important method 

 of correlation, especially when species with wide geographic dis- 

 tribution and short geologic range are employed. It is not wise 

 to depend upon a single species for the correlation of far distant 

 formations, because then the time necessary for the migration of 

 the species must be considered. This seldom gives trouble be- 

 cause the geologist usually deals with a number of rapid-moving 

 species. In a restricted area, where formations are to be correlated, 

 the same organisms may have continued for a long time, but nearly 

 always some peculiar species furnishes the clew. 



2. Aggregations of forms. When groups of strata in different 

 areas carry similar aggregations of similar forms, the groups of 

 strata may be safely correlated. Even though a small percentage 

 of the species vary, the method still holds because such varia- 

 tions are to be expected on account of migratory and geographic 

 conditions. 



3. Stage in the evolution of organisms. Since there has been a 

 gradual development of life with increasing complexity throughout 

 geologic time, the stage of development or evolution shown by the 

 fossils in a group of strata will serve as a basis for general correla- 

 tion at least. Each era, or even period, shows a characteristic 

 stage of evolution of forms. 



4. Percentage of living species. This applies only to rock for- 

 mations of later geologic time, because the older rocks contain no 



