walcott. ] CORRELATION. 423 



be a greater difference in the species, and the general or the spe- 

 cific grouping of the genera will have to be taken as the data for com- 

 parison. In separate provinces the species may be the same in some 

 instances; but it is by the comparison of representative species and 

 genera that the correlations are to be made. In provinces separated 

 by great distances, or upon two continents, the general assemblage of 

 the characters of the two faunas are to be compared. It may be 

 accepted as a general rule that the more detailed the comparisons are 

 the more restricted is the area in which specific results are of value. 

 The results, however, may be of the highest value in strengthening a 

 correlation based upon representative types. The exception to this is 

 the occurrence of similar life zones in widely separated regions. 



(h) Zoologic provinces have been more or less limited in area from 

 the beginniug of Paleozoic time. This is shown by the character and 

 distribution of the earlier Cambrian fauna. The provinces were as dis- 

 tinctly marked then as at any time during the deposition of the Pale- 

 « ozoic. 



(i) Many of the earlier zoologists and paleontologists prior to the 

 period of Darwin studied the strata and the contained fauna with the 

 view that each group of species had a greater or less distribution and 

 were then suddenly exterminated, a newly created fauna taking its 

 place. This enabled them to correlate their formations with great 

 accuracy, and to differentiate them upon the evidence offered by the 

 occurence of a very few species. 



(j) The environment or condition under which the faunas lived and 

 were deposited is also to be noted. Whether they were littoral or 

 pelagic, whether they lived in cold or warm water, upon a sandy or a 

 muddy bottom, in salt or in fresh water, are all considerations which 

 must be taken into account in discussing their value as factors in cor- 

 relation. 



(]{) The presence of recurrent faunas in strata lithologically similar 

 may give rise to imperfect and erroneous correlations. This is not 

 liable to occur except in a limited geologic basin, where the conditions 

 of sedimentation were uniform over large areas, and the environment 

 • of the fauna such that it continued, under favorable conditions, in some 

 portions of the basin, and occupied other portions intermittently, as 

 the varying phases of sedimentation were favorable or inimical to its 

 distribution. 



(I) The relative age, serial relation, or homotaxis of sedimentary 

 formations is best determined by the marine faunas, as the conditions 

 under which they lived and were deposited were much more uniform 

 than those surrounding the terrestrial fauna and flora. 



The evidence furnished by paleobotany is fragmentary, and its se- 

 quence is so interrupted by nondeposition and nonpreservation that 

 no comparisons of great value have been established by its aid. What 

 is true of paleobotany is also true, to a less extent, of terrestrial faunas. 



