RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 319 



those which underlie, and overlie it and which have been biologically 

 identified, or the relative age of a formation of volcanic origin intercalated 

 between known fossiliferous formations may 1 >e thus determined. Again, 

 there maybe a great series of unfossiliferous stratified rocks, such as is 

 the pre-Cambrian, the divisions of which are identifiable by means of 

 lithological criteria aided by the general evidence of homogeny. 



In the case of single unfossiliferous sedimentary formations such as 

 have just been referred to, their relative identity is determinable by the 

 underlying and overlying fossiliferous formations. The same is true in 

 case of the formations of volcanic origin which have also been referred 

 to, the claim formerly made that the geological age of this kind of rocks 

 can be determined by the mineral composition having been generally 

 abandoned. The geological age of the pre-Cambrian formations also 

 have not been determined by any inherent evidence, but only by their 

 ascertained stratigraphic relation to the fossiliferous Cambrian rocks. 

 That is, their geological age or relative identification could be ascer- 

 tained only by reference to biological criteria. 



While fossil remains unquestionably afford the most trustworthy and 

 often the only means of either direct or indirect identification of forma- 

 tions, in the absence of these means the geologist often reaches conclu- 

 sions in this respect by methods of reasoning that it would be difficult 

 even for himself to formulate, and these conclusions are valuable in 

 proportion to his acquirements and experience. Among these less clearly 

 definable methods is that which takes cognizance of homogeny; that 

 is, of a method in connection with which certain inherent lithological 

 and stratigraphical characteristics, which are possessed by a formation 

 or series of strata in one part of a given region under investigation, are 

 accepted as evidence that it had a common origin with a formation or 

 series presenting similar characteristics in another part of the same 

 region. Such a conclusion necessarily implies that originally there was 

 physical continuity of similar strata between such localities, and that it 

 has either been destroyed or obscured. 



This method of identifying formations is one of minor importance as 

 compared with that which is based upon fossil remains, but unfortu- 

 nately it has, especially within the last few years, been adopted by cer- 

 tain geologists in charge of important works almost to the entire exclu- 

 sion of paleontological considerations. Although it can not be denied 

 that in the hands of an experienced and broad minded investigator this 

 method of identifying formations is of great value, the fact remains that 

 some of the most grievous mistakes that have ever thrown discredit 

 upon geological investigation have occurred by its adoption to the 

 exclusion of paleontological evidence. 



The term correlation, as restricted by the preceding definition and 

 distinguished from identification, is applied to a comparison of the 

 stages and substages of the geological scale as they are respectively 

 recognizable in distinctly separate regions. Such regions as are referred 



