5o6 



NA rURE 



[September 24, 1891 



Prof. E. W. Claypole considered that the different methods 

 of geologic correlation differed very greatly in their value. It 

 is improbable that the plant or mammalian record will ever equal 

 in its perfection that of the marine invertebrate fauna. The 

 marine fauna is to the geologist what a primary triangulation 

 is to the geodesist. It marks out the main divisions, which are 

 subsequently further subdivided through the aid of other fossils, 

 such as plants and vertebrates. 



Prof. C. R. Van Hise spoke of the methods of correlation 

 employed for pre-Cambrian rocks, which occur in widely 

 separated areas and are devoid of fossils. Physical data, only 

 are available for correlating these formations. Experience has 

 shown that, among all physical methods, unconformity is by far 

 the most important. Other physical criteria, such as the degree 

 of induration, metamorphism, and relation to eruptives, are 

 valuable for the subdivision of single areas, but cannot be safely 

 used in identifying synchronous formations in widely-separated 

 areas. The idea that lithological character is any direct proof 

 of geological age has retarded the scientific subdivision of 

 pre-Cambrian rocks. The researches of Pumpelly and others 

 in the eastern United States have demonstrated that Silurian, 

 Devonian, and even Carboniferous deposits might become, 

 under certain physical conditions, as highly crystalline as much 

 more ancient rocks of the West. For this reason it has been 

 found necessary to abandon such terms as Huronian and 

 Keweenawan. Evidences of life are not lacking in pre-Cambrian 

 rocks, and it is to be hoped that the palaeontologist will succeed 

 in differentiating several separate formations below the Cambrian, 

 as the Cambrian itself was differentiated from the base of the 

 Silurian. 



Fourth Day.— Prof. E. W. Hilgard laid stress upon the 

 importance of the abundance or scarcity of species in the 

 correlation of strata. He thinks some quantitative estimation 

 of the species should be made. He is of the opinion, also, that, 

 as compared with marine fauna, plants have but little value for 

 purposes of correlation owing to their local distribution, their 

 accidental proximity to water, transportation, and preservation. 

 Plants can be so used only after large areas are worked over. 



Prof, Lester F. Ward continued the discussion. He de- 

 veloped two of the more general principles of correlation by 

 means of fossil plants, as follows : — 



(i) That the great types of vegetation are characteristic of the 

 great epochs in geology. 



This principle is applicable in comparing deposits of widely 

 different age when the stratigraphy is indecisive. For example, 

 even a small fragment of a Carboniferous plant proves conclu- 

 sively that the rocks in which it occurs are palaeozoic, or a single 

 dicotyledonous leaf proves that they must be as late as the 

 Cretaceous. 



(2) That for deposits not thus widely different in age, as, 

 for example, within the same geologic system or series, ample 

 material is necessary to fix their position by means of fossil 

 plants. 



Neglecting this principle has led to the greater part of the 

 mistakes of palaeobotanists, and has done most to bring palaeo- 

 botany into disrepute. Geologists have expected too much of 

 them, and they, in turn, have done violence to the truth in 

 attempting to satisfy extravagant demands. On the other hand, 

 where the material is ample, fossil plants have often corrected 

 the mistakes of stratigraphical geologists, and solved problems 

 concerning geologic age, which seemed impossible of settlement 

 by any other class of evidence. 



Prof. Henry S. Williams laid stress upon the relations of 

 species to the conditions of deposition. The abundance of a 

 species varies with environment, and a study of correlation 

 should embrace a study of these conditions. Sandstones 

 deposited near shore may have a fauna different from that of a 

 limestone deposited off shore at the same time, and a change of 

 fauna may be induced by a change of the conditions of 

 .deposition. The age of beds should be determined by com- 

 paring species of the same genera rather than by comparing 

 those of different genera. There are centres of abundance which 

 exhibit great variability in their characters ; outside of these 

 centres the species exhibit varieties which may be called extra- 

 limital, and which are not typical though they have often been 

 published as types. 



_ Prof. Charles Barrois said that there was no general basis, either 

 biologic or lithologic, for the correlation of the pre-Cambrian rocks 

 of Europe with those of North America : even 



' Europe 

 NO. 



I 143. VOL. 44] 



imerica ; even the terms applied 



to these rocks were liable to be mi>understood. Certainly the 

 divisions used in France cannot be correlated with those now 

 used in the United Slates, General correlation cannot, as yet, 

 be based upon nonconformities ; autopsy is the only basis upon 

 which a comparison can be instituted. He pointed out certain 

 parallelisms between the histories of the crystalline schists of 

 America, as illustrated by Mr. Pumpelly, and the gneissic rocks of 

 Brest, where the Cambrian slates are altered to gneisses of 

 Archaean aspect, while the alternating fossiliferous quartzites are 

 changed to crystalline quartz. Geologists must see the beds 

 together in order to reach a common understanding of the 

 crystalline rocks. 



Prof. E. D. Cope discussed the question from a general point 

 of view with especial reference to the value of vertebrates for 

 purposes of correlation, particularly for inter- continental correla- 

 tion. He pointed out that there is a marked difference in the 

 present verebrate faunas of continents, and that the variation of 

 such forms must be sought in vertical rather than in horizontal 

 ranges. Such study shows that we have had invasions of a 

 given region by a fauna from without ; for example, a South 

 American fauna invaded North America at one time and then 

 retreated, while a North American fauna once invaded South 

 America, and traces of it still remain in that country. He is in- 

 clined to believe that certain vertebrate forms did not spread 

 over the earth from a single place of origin, but that they 

 originated at different places upon the earth. We have parallel- 

 ism in separate places, but the parallelism is defective in the 

 Laramie. 



Mr. G. K. Gilbert was of the opinion that many methods of cor- 

 relation must be used. He doubted the trustworthiness of the 

 correlation of non- fossiliferous rocks by comparative change, even 

 locally. He thought the abundance and scarcity of fossil forms 

 comparable with lithologic differences, and considered the simple 

 occurrence of a species as valuable for purposes of correlation as 

 its abundance. 



Fifth Day. — Subject for discussion : map-colouring and 

 cartography. 



Major J. W. Powell exhibited charts illustrating the colour 

 system used by the U.S. Geological Survey, explained the 

 methods of using the colours, and gave the reasons for them. 

 The colours assigned to rocks of different ages are as follows :— 



Period. Period colour. 



I. Neocene Orange 



2 Eocene Yellow 



3. Cretaceous Yellow-green 



4. Jura- Trias Blue-green .. 



5. Carboniferous Blue 



6. Devonian Violet 



7. Silurian Purple 



8. Cambrian Pink 



9. Algonkian Red 



Mark. 



,. N. 



. E. 



. K. 



■ J- 



. C. 



. D. 



. S. 



,. c. 



. A. 



The colours are used to designate geologic periods, patterns 

 of these colours designate formations ; minor divisions are 

 usually relegated to the text. The number of patterns for 

 designating formations can be indefinitely enlarged, but follow a 

 definite system, 



Mr. Joseph Willcox showed that in the scheme described by 

 Major Powell the colours were not evenly distributed through 

 the chromatic scale. 



Prof. C. R. Van Hise pointed out that Archaean rocks are 

 shown by a brown underprint, and that metamorphic rocks of 

 known age are given the colour of the corresponding unaltered 

 rocks. 



Major Powell explained that it was not attempted to select 

 colours equally distributed through the chromatic scale, but to 

 use those that may be most readily recognized. 



Mr. H. M. Cadell asked why black and gray were not used. 



Major Powell replied that blue was used in place of the dark 

 shades for the Carboniferous ; that dark colours are misleading 

 in regard to the occurrence of coal, which occurs in the Creta- 

 ceous and Tertiary as well as in the Carboniferous. 



Mr. Christie found the black colour very inconvenient, because 

 it often made the details of the map covered by such colours 

 illegible. 



Mr. H. M. Cadell said that the maps of the Geological Sur- 

 vey of Great Britain were coloured by hand, and that the system 

 used by the U. S. Geological Survey could not for this reason 

 be economically employed. 



