July i8, 1895] 



NA TURE 



2SI 



necessary elements of their superior chronological value, which 

 have just been mentioned. The following summary of facts 

 relating to the marine invertebrates show their [irincipal claims 

 to the highest estimate of value in characterising the divisions of 

 the geological scale, and in determining tlie geological age of the 

 strata in which their remains are found. 



The marine invertelirates emljrace five of the six suit-kingdoms 

 or branches of the animal kingdom. 



They have coexisted in every stage of geological time, while 

 the known time-range of other animals, as well as of land plants, 

 has been very much less. 



The ]}reservation of their remains having been a natural con- 

 sequence of the character of their habitat, they are faunally 

 more complete than are those of any land animals, and for the 

 same reason they are florally more complete than are remains of 

 land plants. 



They all lived under the same or closely similar conditions, and 

 those conditions were more nearly uniform throughout all geo- 

 logical time than were those imder which any other forms of life 

 existed. Their remains have, therefore, produced a more nearly 

 uniform chronological record. 



Their relations to one another were wholly congruous, while 

 the relations of all of them to all non-marine fainias and land 

 floras was more or less incongruous, and in many cases ex- 

 tremely so. 



The formations containing their remains are for the whole world 

 and the whole of the geological scale far in excess of those which 

 contain the remains of any oilier forms of life, especially the 

 remains of land plants and land animals. 



Correlative Geology and its Critkria. 



The term " correlative geology" is not in common use, but it is 

 adopted as a present convenience in discussing the correlation of 

 assemblages of strata as divisions or subdivisions of the geological 

 scale as it is developed in separate regions, and the identification 

 of formations within one and the same district or region. .\s 

 liere used, the term correlation refers to geological systems or 

 >ther comjirehensive series of stratified rocks which occur in 

 different and more or less widely separated parts of the world, 

 Itetween which parts there is no phy.sical continuity of strata, 

 ir none that it is possililc to discover. Correlation applies to 

 ^rneral geolog)', identification to local or regional investigations. 

 The latter may be discussed under two heads, direct and re- 

 lative. Direct identification applies to formations the characteristics 

 'if which at one or more localities have been ascertained, and as 

 these are naturally of limited geographical extent, the application 

 !^ similarly restricted. 



.\lthough fossils in all cases ccmslitute not only much themo.sl, 

 liut usually the only, trustworthy criteria f)r such indentification 

 of formations as is indis|iensalile in the .study of structural geology, 

 the various kinds difler materially as to their relative value. This 

 value, however, has no necessary relation to that which they may 

 possess as indicators of geological time, or of the C(jrrelation of 

 the strata containing them with tho.se of other parts of the world. 

 The two values are distinct, although one kind of fossil remains 

 may often possess both. 



\V'hile fossil remains unqueslionalily afford the most trustworthy 

 and often the only means of cither direct or indirect identification 

 of formations, in the absence of these means the geologist often 

 reaches conclusions in this resjiect by methods of reasoning that 

 it would be difficult even for himself to foruudale, and these con- 

 clusions are valualjle in proi>ortion to his acquirements and 

 experience. .Vmong these le.ss clearly definable methods is that 

 which takes cognisance of homogeny ; that is, of a method in 

 connection with which certain inherent lilhological and strati- 

 graphical characteristics, which are ].iossesseil by a formation or 

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

 pre accepted as evidence that it had a common origin with a for- 

 mation or series presenting .similar characteristics in another part 

 'f the same region. Such a conclusion neces.sarily implies that 

 •riginally there was physical continuity of .similar strata between 

 -iich localities, ani.1 that il has either been destroyed ctr t>!)scured. 

 This methoti of identifying formations is one of minor im]iort- 

 nce as compared with that which is based upon fossil remains, 

 ul unfortunately it has, especially within the Last few years, been 

 idopted by certain geologists in charge of important works, almost 

 ' 1 the entire exclusion of palasonlological considerations. .Although 

 11 cannot lie 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 investi- 

 gation have occurred by its adoption to the exclusion of palaeonto- 

 logical evidence. 



It has been the custom of a large proportion of geologists to 

 regard the geological scale as it has Iteen established in Europe 

 as the absolute standard for the whole earth. A necessary con- 

 sequence of this view is their assumption that the systems which 

 physically constitute that scale, and at least most of the divisions 

 of those systems, may not only be recognised, but as clearly 

 defined in all parts of the earth as they are in Europe, if in 

 those parts contemjioraneous deposits were made and still remain 

 intact. 



In view of known facts and principles, the idea held by the 

 early geologists, as w'ell as by some of those now living, that 

 identity of fossil types proves synchronism or exact contenqiora- 

 neity of origin of any two or more series of strata containing 

 them, is quite untenable. The facts which have been presented 

 also suggest that the term *'^homotaxy" must be used with some 

 degree iif latitude as to its application to the subdivisions of 

 systems, because the order of sequence in the occurrence of the 

 types which characterise them, res]5ectively, in one part of the 

 world is in another part sometimes jjartially reversed or partially 

 interchanged. That is, the taxonomy of those divisions, as 

 biologically indicated, is not the same for all parts of the world. 



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 labours of a multitude of 

 geologists, so the fact of correlation is not called in question. The 

 principal questions which are here 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 was established ; 

 but they are of such a character that they must be conventionally 

 rather than arbitrarily determined. 



For example, in discussing the questions which have arisen 

 concerning 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 systems 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 systems. 

 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 avail- 

 able facts, jihysical as well as biological. It is equally evident 

 that il is the duty of every American geologist to hold in 

 abeyance any final decision as to the correlation of the groups of 

 strata which he may study with the divisions of the Eurftpean 

 scale luitil all such facts have been duly and justly considered. 

 In short, the idea of absoluteness in such cases is as much out 

 of place as is the assertion or recognition of personal authority. 



Although these remarks refer directly to North American 

 geology and geologists, they are equally applicable to other parts 

 of the world when reference is made to the scale as represented 

 by the European rocks. 



Notwithstanding the great excellence of the scale now in 

 general use, and the fact that so little change has been made in 

 it since it was first devised by the early geologists, the future 

 progress of geological science will demand modifications the 

 necessity for which will be especially urgent when the true 

 character of correlation for all the principal ]iarts of the earth 

 has been ascertained. Hitherto correlation has been investigated 

 v\ith the single jHirpose i^f adjusting the series of formations 

 which occur in each of the various parts of the world to the 

 scale now in use ; but although its general applicability to that 

 purpose is not to be questioned, the ultimate result of the study 

 of correlation will be to modify this scale and adjiLst it to the 

 systematic geology of the whole earth. That is, the scheme of 

 stratigra[)hic classification, which has been the main factor in 

 adjusting the elements of systematic getilogy, must in turn he- 

 itself adju.sted to the great .sysiim uhi.li it will have been the 

 principal agent in producing. 



Critf.ria of Past Aijllois Conditions. 



.\mong the more conspicuous facts in geology are some of those 



which relate to the manner of origin as well as to the original and 



present condition of the sedimentary formations. These subjects 



have already been discussed, and among those discussions are 



NO. 1342, VOL. 52] 



