RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 325 



reference to such characterization it is the adequacy of those types to 

 sharply define the limits of stages or to clearly identify substages in 

 widely separated parts of the world that I deny. This denial, of course, 

 implies what has before been stated, that the geological scale now in 

 general use is not an absolute standard for the correlation of the strati- 

 fied rocks of the whole earth, but I repeat what was said iu connection 

 with that statement, that this scale is unquestionably the best that it 

 has been possible to devise, and make the additional statement thai it 

 needs adjustment rather than material change.* 



The foregoing discussions having required frequent reference to cer- 

 tain erroneous views which have prevailed upon the subject of corre- 

 lation that subject has necessarily been somewhat antagonistically 

 presented. That is, its scope has been to a large extent negatively 

 rather than positively indicated. 



It has been shown that the presence in widely separated parts of the 

 world of all the systems of the geological scale, as well as of some of 

 their larger divisions, has been demonstrated by the labors of a multi- 

 tude of geologists and that the fact of correlation is therefore not to 

 be called in question. The principal questions which have been raised 

 concern the scope of correlation or the limitation of the assemblages 

 of strata the relation of which to respective divisions of the scale is 

 more or less obvious. These questions are of practical application in 

 the study of the structural geology of any part of the world other 

 than that in which the geological scale wns established, but they are of 

 such a character that they must be conventionally rather than arbi- 

 trarily determined. 



For example, iu discussing the questions which have arisen concern- 

 ing the earlier and later limits of the systems of the geological scale in 

 North America the difference of opinion as to those limits have been 

 wider and more various with regard to the later systeais than to the 

 earlier. This is because of the greater number and variety of the kinds 

 of fossil remains to be considered in such discussions of the later sys- 

 tems, their difference from the earlier in this respect being plainly 

 indicated by the table on Plate xiv, and by the accompanying statement s 

 relating to it. It is therefore evident that in reaching a conclusion as 

 to the limitation of any of these systems, or of any of their subdivisions, 

 it is necessary to take into consideration all available facts, physical 

 as well as well biological. It is equally evident that it is the duty 

 of every American geologist to hold in abeyance any final decision as 

 to the correlation of the groui>s of strata which he may study with 

 divisions of the European scale until all such facts have been duly and 



"Although the views concerning correlation which arc enunciated in this essay 

 are opposed to those which were generally held by the early geologists, sonic of 

 tnose pioneers held views which are much in accord with those herein advocated. 

 Sec, for example, de la Heche, Henry T. : .Sketch of a (Jlassilication of the European 

 Kocks. Am. Jour. ScL, 1st ser. Vol. xvill, pp. liG-37, 1830. 



