September 24, 1891^ 



NATURE 



505 



physical geology indicate that the Glacial epoch in America 

 was widely differentiated and of long duration. How many 

 distinct periods it embraced we do not as yet know. 



Prof. Cope said an abundant tropical fauna is found in the 

 " Equus beds," which, if they be of interglacial age, indicates at 

 this time a very warm climate. This fauna is succeeded by a 

 truly boreal fauna. In this is contained material for a chrono- 

 logical subdivision of Pleistocene deposits. 



Third Day.— The President announced as the subject for 

 discussion, the correlation of geological formations. 



Mr. Gilbert opened this discussion by presenting a general 

 classification of methods of correlation. 



Strata are locally classified by superposition in chronologic 

 sequences. Geologic correlation is the chronology of beds not 

 in visible sequence. For convenience in discussion, methods of 

 correlation are classed in ten groups, of which six are physical 

 and four biotic. 



Physical Methods of Correlation. 

 (i) Through visible continuity. The outcrop of a bed is 

 traced from point to point, and the different parts are thus 

 correlated one with another. 



(2) Strata are correlated on account of lithologic similarity. 

 This method, once widely prevalent, is used where the distances 

 are small. 



(3) Correlation by the similarity of lithologic sequence has 

 great and important use where the localities compared fall 

 within the same geologic province, but is not safely used in 

 passing from province to province. 



(4) Physical breaks, or unconformities, have a limited use, 

 especially in conjunction with other methods. The practice of 

 employing them in the case of localities wide apart is viewed 

 with suspicion. 



(5) Deposits are also correlated with their simultaneous rela- 

 tions to some physical event — for example, a beach with the 

 lake beds it encircles ; a base level plane with a contiguous 

 subaqueous deposit ; and alluvial, littoral, and subaqueous 

 deposits standing in proper topographic relation. In the 

 Pleistocene, glacial deposits are widely correlated with reference 

 to a climatic episode assumed to arise from some general cause. 



(6) Deposits are correlated through comparison of changes 

 they have experienced from geologic processes supposed to be 

 continuous. Newer and older drift deposits in different regions 

 are correlated according to the relative extent of weathering 

 and erosion ; induration and metamorphism afford presumptive 

 evidence of age, but yield to evidence of other character. Meta- 

 morphism holds prominent place in the correlation of pre- 

 Cambrian rocks where most methods are inapplicable. 



These physical methods are qualified by the geographic dis- 

 tribution of geologic processes of change and of geologic 

 climates, 



Biotic Methods of Correlation. 



(7) A newly- discovered fauna or flora is compared with a 

 standard series of faunas and floras by means of the species it 

 holds in common with them severally. 



(8) It is also compared by means of representative forms, or 

 through genera and families. 



(7a) and (8fl) These comparisons are strengthened if two or 

 more faunas in sequence are found to be systematically related to 

 the faunas of a standard series. 



(9) Two faunas or floras otherwise related are compared in 

 age through their relation to the present life of their localities. 

 This method was applied by Lyell to Tertiary rocks. 



(10) Faunas are correlated by means of their relation to 

 climatic episodes taken in connection with station. For ex- 

 ample, boreal shells found in latitudes below their present range 

 are referred to glacial time. 



In general the limitations to accurate correlation by biotic 

 methods arise from the facts of geographic distribution. Cor- 

 relations at short range are better than those at long range. 



Biotic correlation by means of fossils of different kinds may 

 have different value. In general, 'he value of a species for the 

 purposes of correlation is inversely as its range in time, and 

 directly as its range in space. The value of a biotic group 

 depends (i) on the range of its species in time and space ; (2) 

 on the extent to which its representatives are preserved. 



Prof. K. von Zittel spoke in reference to the biotic methods, 

 and gave his opinion of the relative value of plants and animals 

 for purposes of correlation. He regarded plants as relatively 



NO. I 143, VOL. 44] 



unimportant. Among animals, those which are marine, lacus- 

 trine, and land animals may be distinguished. Of these classes 

 marine invertebrates are most valuable for purposes of correla- 

 tion. The vertebrates change rapidly, but are frequently alto- 

 gether wanting. For instance, no vertebrates occur in the 

 Alpine beds corresponding in age to those which contain the 

 mammalian fauna of the Paris basin. In certain lacustrine 

 deposits invertebrates may be absent, and in such cases the 

 vertebrate fauna is the surest guide. 



Baron de Geer emphasized the importance of a numerical 

 comparison between different species. The actual counting of 

 individuals in a given formation is of great value. 



Prof. Marsh expressed his agreement in general with the con- 

 clusions communicated by Prof, von Zittel, but would give 

 special weight to vertebrate fossils. In the Mesozoic and 

 Tertiary beds of the Rocky Mountains he had found that the 

 vertebrates offer the surest guide for correlation. This is in 

 part because invertebrates are either wanting or are lacustrine. 

 Prof Marsh in 1877 named a sequence of horizons after the 

 most characteristic vertebrate genus in each which is confined 

 exclusively to it. He presented an outline of such classification 

 brought down to date, with a section to illustrate vertebrate life 

 in America. 



Mr. C. D. Walcott spoke of the value of plants for purposes 

 of geologic correlation. 



Prof. T. McK. Hughes spoke of the present and growing 

 tendency towards a natural classification. The evidence is com- 

 plex, and includes a considerable variety of diverse relations. 

 He pointed out exceptions to the normal conclusions deduced 

 from superposition, lithological character, and similarity of 

 sequence. We must have a system of criteria so varied that if 

 one or more fails others can be employed. All classes of evi- 

 dence are useful, both positive, negative, and circumstantial. 



Major J. \V. Powell spoke of the necessity of specialization 

 on the part of geologists engaged in the work of correla'ion. 

 The evidence derived from physical and biotic facts might 

 apparently disagree. But that a satisfactory result may be 

 reached, these two classes of evidence must be brought into 

 harmony. He cited an example from his own experience, of 

 how an identification of synchronous formations might be made 

 over a wide area through a union of physical and biotic 

 methods. 



Mr. W. J. McGee remarked that in the coastal plain of the 

 United States physical correlation alone is employed. The 

 bases accord with those outlined by Mr. Gilbert, with certain 

 minor modifications and an important addition, as follows ; — 



For local discrimination 

 and correlation 



For correlation through- 

 out the province 



For correlation with con- 

 tiguous provinces 



For general correlation... 



Visible continuity ; 

 Lithologic similarity ; 

 Similarity of sequence. 

 Physical breaks viewed as in- 

 dices of geography and topo- 

 graphy. 

 'Relation to physical events, 

 including continental move- 

 ments, 

 transportation of materials, 

 land sculpture, &c. 

 Homogeny or identity of origin. 



By correlation upon these bases the physical history of a con- 

 siderable fraction of the continent may be so definitely ascer- 

 tained as to permit fairly accurate mapping of the geography, 

 and even the topography of each episode in continent growth. 

 After these episodes are clearly defined, and the fossils found in 

 the formations are studied, it will be possible definitely to as- 

 certain the geographic distribution of organisms during each 

 episode ; then palaeontology may be placed on a new and 

 higher plane. 



Prof, W. M, Davis showed that it was possible to decipher 

 geological history not only through the records of deposition, 

 but also by processes of degradation. As an example of this 

 method he explained a topographical section from the city of 

 New York westward. In this we have evidence of the existence 

 of an ancient peneplain, or base-level lowland of Cretaceous age. 

 This surface was subsequently elevated (more toward the west 

 than toward the east) at the end of Cretaceous, or at the begin- 

 ning of Tertiary time. It has since been dissected by the 

 excavation of more recent valleys. The Hudson Valley lowland 

 was cited as an example of this recent dissection. 



