With a better knowledge of the physical geography of the globe at successive epochs we may more hopefully attack the great problems of mountain growth, continental development, and earth dynamics. But our first work is to map the lithologic individuals, while our associates, the paleontologists, distinguish the faunal units of stratigraphy.

BAILEY WILLIS.

THE DISCRIMINATION OF TIME-VALUES IN

GEOLOGY.

THE imperfection of the present systems of classification and correlation of sedimentary rocks concerns more directly the interpretation of the facts than the facts themselves.

The work of the geologists of the United States in mapping and recording the stratigraphic sequence of formations was never more exact and comprehensive. The paleontologist was never more particular in his records of the faunal contents of each formation and fossiliferous zone, and his comparisons were never more full and precise. But the extension of knowledge over vast territory has brought to light hundreds and thousands of outcrops of the same formations, showing a diversity of faunal composition which cannot be translated entirely into difference in geologic age.

So long as surveys were confined to local areas separated by spaces across which the continuity of formations could not be traced, it was practicable to use a system of nomenclature and classification in which lithologic formations and their stratigraphic succession were chiefly considered. When, however, the intervals between local areas were filled up and it was necessary to correlate geological sections in which the formational divisions are in part or wholly dissimilar, the duality of the lithologic and biologic facts become apparent. These two sets of facts are entirely different in nature and in origin, and for their scientific discrimination duality of nomenclature is essential.

The confusion of these two kinds of evidence was natural, and has been perpetuated by the common practice of adopting the lithologic formation as the unit of classification, making the time divisions to apply strictly to the formations instead of to the faunas and floras, by which alone the chronologic epochs in which they were formed can be discriminated. This confusion

it seen in the discussion of classification and nomenclature in the Tenth Annual Report of the United States Geological Survey, and in the legends of the folio maps. For instance, take the Sewanee Folio, Tennessee: The legend is as follows, viz.:

According to the rules in the Tenth Annual Report, the first series of names are "structural divisions

units of cartog

defined

raphy, and shall be designated formations” (p. 64).

The second series of names are "time divisions primarily by paleontology and secondarily by structure, and they shall be called periods" (p. 65).

Although everybody understands what is meant by the classification in the legend, the principle described in the rules is wrong in that the legend on the map refers to a classification of rocks and the real fact in the case is that in the Sewanee quadrangle the Walden, Lookout, Bangor and Fort Payne formations together constitute the Carboniferous system, and the map makes no record of periods of time but only of formations of rocks. The Devonian system of that quadrangle consists of the one Chattanooga formation; and the Rockwood, Chickamauga, and Knox formations are the only representatives of the Silurian system recognized on the sheet.

The European nomenclature avoids this confusion by recognizing a set of stratigraphic names and their categories; with a corresponding set of categories for the chronologic classification—the names of the divisions being the same in both the stratigraphic and chronologic scales. Instead of referring all stratigraphic divisions to one category (the formation), different

categories are used for formations of different relative size; making the list of names to be group, system, series, stage, as adopted by the international Congress. Each of these stratigraphic divisions has its corresponding chronologic category, viz., era, period, epoch, age. The European has no difficulty in expressing on the map, or in discussing, either the time or the structural relations of the formation. On the map, the Walden, Lookout, Bangor, and Fort Payne would be to him four series, together constituting the Carboniferous system. By placing the Chattanooga in the category of series he at once would indicate that he does not regard the formation as necessarily representing the whole Devonian system. On the other hand, when he speaks of the Carboniferous period he is not discussing any local set of formations but the total period of time in which lived a definite set of plants and animals, only a few of which are discovered in any one local formation. There can be no question that the systems of the European geology can be recognized in this. country only by the fossils-but that does not change them from formational aggregates into time divisions.

The implication in the Tenth Annual Report that the divisions which are discriminated by fossils must be chronologic, and not structural, suggests the way in which our usage may be improved; but the fallacy of the principle is seen by noticing that the smaller formations (the series and ètages of the international nomenclature), are to be discriminated by their fossil contents as well as the larger ones (the systems). If discrimination ,"primarily by fossils" were to be the test as to whether the division were structural or chronologic then formations would become chronologic divisions in every newly surveyed area in which actual lithologic continuity could not be traced to some standard outcrop.

These two sets of facts (structural and paleontological) both have to do with the classification of formations on a time basis; and those who are accustomed to frame their conceptions of geological time on the basis of one set of facts, find difficulty in even conceiving that there is any other basis.

TIME VALUES OF FORMATIONS.

The regular sequence of stratified sediments forms a natural geological column, which, in any particular section of the earth's crust, is so conspicuously subdivided by lithologic differences in kind of sediments that the divisions form the most satisfactory kind of natural time division for geological classification.

These natural, lithologic divisions of the crust of the earth are technically called formations in the nomenclature of the United States Geological Survey. And in any standard section, such as that of New York state, the order, composition and thickness of the several formations is exactly known; and for that section, too, the fossils of each separate formation are known accurately and in large numbers. Geologists have been accustomed to use such a local column of known geological formations as a standard time scale; and as examination has extended to sections of the crust in other parts of the continent, the classification and correlation of the other columns have been made to correspond, by correlation, with such a standard column of formations. Two methods have been used in establishing the correlation (1) by tracing continuity in the lithologic formation; and (2) by recognition of identity of the fossil species contained in the formations. Both of these methods have rested on an assumed interpretation of the facts, the correctness of which may be questioned quite independently of the established fact of continuity or of identity.

The assumptions on which these interpretations rest are that correlations of time relations can be established in the first case by continuity of lithologic formation, and in the second case by identity of fossils. In regard to the first case, it would be incorrect to say that the assumption is entirely false, for in some cases, and to a limited extent, lithologic continuity of a formation is undoubtedly synonymous with sameness of the period of the sedimentation represented by the formation. But the facts are abundant, and well known to all field geologists, to prove that formational continuity is not co-ordinate with lithologic uniformity; and since our standard definition of a formation