RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 315 



which has been determined by the previous study of formations which 

 have been directly identified. Therefore, while direct identification is 

 coniined to the geological limits of separate formations, relative identi- 

 fication may extend throughout a large region by the overlapping of 

 formations. Such a recognition of the taxonoraic position of formations 

 is usually the direct result of empirical study of a given series within a 

 limited region, but it is often the result of those philosophical studies 

 which have been made in connection with the construction of the great 

 geological scale, especially in cases where a wide time-hiatus exists 

 between formations.* The latter, however, approaches correlation in 

 character. 



Identification, except in the relative cases justmentioned, is always the 

 result of empirical studies; but correlation is more philosophically deter- 

 mined. The latter not only implies the existence of closely similar 

 biological characteristics in systems or series of formations in different 

 and widely separated parts of the world, but in each case it has refer- 

 ence to a similarity of relation to preceding and succeeding series of 

 formations respectively. That is, correlation relates to the serial recog- 

 nition of the various divisions of the geological scale in more or less 

 widely separated parts of the world, and is based wholly upon biology, 

 those divisions coinciding with stages in the evolutional progress of 

 development of organic forms which has occurred during geological 

 time. 



In the practical study of the structural geology of a region the prin- 

 cipal use of correlation is to apply universally acceptable names to the 

 different groups of formation. That is, the structural geology of great 

 regions, or even of the greater part if not the whole of continents, may 

 be minutely and comprehensively studied and all the stratified rocks 

 accurately classified in chronological order by means of direct and rela- 

 tive identification of formations and without necessary reference to 

 their correlation with those of any other part of the world except as a 

 means of detecting such cases of wide time-hiatus between formations 

 as are not otherwise clearly revealed. 



Because fossil remains constitute the principal criteria in the practi- 

 cal identification of formations it is desirable in this connection to con- 

 sider the relative value of the different kinds for this purpose, although 

 the subject has been briefly discussed and frequently referred to in 

 the preceding essays. 



Fossils being the remains of animals and plants most of the princi- 

 pal kinds of which lived under different physical conditions, some of 

 them have a more direct relation than others to the formations in 

 which they are found. Therefore they differ materially as to their 



What I have here designated as relative identification has often been by. authors 

 included under the head of correlation, I also have done so in Bulletin I'. S. Geo- 

 logical Survey, No. 82, ]>i» 17-25, bui the more clearly to state the principles involved 

 I herein restrict the use of that term as indicated in preceding paragraphs. 



