Joseph Le Conte proposed Psychozoic, on the same principle, for the latest geological period in which man has appeared.* Chronological Succession included in Lyell's System.-Lyell proposed to make, on this basis, a geological time-scale, and he applied the term Period to each of the several divisions of the scale. Thus we find in his Geology,† second edition, published in 1841, a recognition of the time element in classification, without, however, the adoption of the biological nomenclature. He gives a table showing the order of superposition, or chronological succession, of the principal European groups of fossiliferous rocks." Under the heading 66 "Periods and Groups" we find the following: I. Post-pliocene Period : II. Tertiary Period: III. Secondary Period: { A. Recent. B. Post-pliocene. C. Newer Pliocene. E. Miocene. F. Eocene. G. Cretaceous group. H. Wealden group. I. Oölite, or Jura Limestone group. K. Lias group. L. Trias, or New Red Sandstone group. M. Magnesian Limestone group. O. Old Red Sandstone, or De IV. Primary Fossiliferous (P. Period: 19. vonian group. Silurian group. Later Lyell adopted the biological nomenclature, and was prominent among geologists in developing and elaborating the idea of the successive appearance of new types of organisms coördinate with the progress of geological time. Dana's Elaboration of a Geological Time-scale.-Dana was the first to classify and teach the facts of geology from a purely * See Le Conte, "Elements of Geology," first edition, New York, 1878. + Lyell, Elements of Geology," second edition, London, 1841, vol. II. p. 178. historical point of view. In 1856* he wrote: "Geology is not simply the science of rocks, for rocks are but incidents in the earth's history, and may or may not have been the same in distant places. It has a more exalted end—even the study of the progress of life from its earliest dawn to the appearance of man; and instead of saying that fossils are of use to determine rocks, we should rather say that the rocks are of use for the display of the succession of fossils. . . . From the progress of life geological time derives its division into ages, as has been so beautifully exhibited by Agassiz." Referring to the nomenclature he used in the classification of American geological history he speaks of having adopted for the subdivisions of the Paleozoic the names given by the New York geologists; but, he adds, "I have varied from the ordinary use of the terms only in applying them to the periods and epochs when the rocks were formed, so as to recognize thereby the historical bearing of geological facts." The nomenclature proposed by Dana in 1856 is given in the following table: 1. Niagara Period. 66 Hudson River Shale (Hudson River shale and Blue limestone of Ohio in parts of the West). Ist epoch. Oneida conglomerate, etc. 2. Onondaga Period.. Ist epoch. Galt limestone, etc. 3. Lower Helderberg Period, etc. II. Devonian Age. 1. Oriskany Period... Ist epoch. Oriskany sand stone, etc. *American Journal of Science, vol. XXII. pp. 305 and 335. 2. Upper Helderberg 1st epoch. Schoharie grit, etc. 3. Hamilton Period.... Ist epoch. Marcellus shales, etc. 4. Chemung Period.... Ist epoch. Portage, etc. This classification was further elaborated in his manual, the first edition of which appeared in 1863,* and it has become the standard classification for American geology. Here we find the larger divisions, called times: I, Archean; II, Palæozoic; III, Mesozoic; and IV, Cenozoic times. The Palæozoic time is classified into ages, viz.: The age of Invertebrates, the Cambrian and Silurian; the age of Fishes, the Devonian; the age of Coal Plants, the Carboniferous. The Mesozoic is called the age of Reptiles. The Cenozoic time includes the age of mammals and the age of man.† Each of the ages is subdivided into periods and epochs, in which the stratigraphical groups and formations form the basis, and the particular faunas and floras of each constitute the data of determination for the time-divisions. The following chart shows the modifications in the nomenclature through which the classification now in use has grown out of the classifications of earlier authors: James D. Dana, "Manual of Geology; treating of the principles of the science, with special reference to American Geological History," Ist edition, 1862; 2d edition, 1874; 3d edition, 1880; 4th edition, 1895. In the article of 1856 the following periods were named (i.e., Triassic, Jurassic, Cretaceous, Tertiary, and Post-tertiary), but divisions into epochs were in this paper proposed only for the latter. The divisions of the Posttertiary were the Glacial Epoch, the Laurentian Epoch, and the Terrace Epoch. Quaternary has been substituted, in the manual for Post-tertiary, and Champlain epoch for Laurentian. In the last edition (1895) Era has taken the place of Age in the former editions, a Cambrian Era has been recognized in addition to Lower Silurian, and Carbonic Era has been substituted for Carboniferous Age; the name Carboniferous being applied to the formations included under the terms Coalmeasures and Millstone grit of the early classifications. The distinctions upon which the above divisions are based are primarily stratigraphical, and we have still to seek a timeclassification on a purely biological basis for the whole geological series. Biological Classification of Oppel.-One of the earliest attempts at systematic classification upon a purely biological basis was made by Dr. Oppel in classifying the Jurassic formations on the basis of the successive Ammonites characterizing the beds.* Oppel divided the lower part of the Jurassic system (the Lias) into 14 zones or beds, characterized successively from below upwards by their dominant fossil forms, chiefly ammonites. Thus the successive zones were those of: 1, Ammonites planorbis; 2, A. angulatus; 3, A. Bucklandi; 4, Pentacrinus tuberculatus; 5, A. obtusus; 6, A. oxynotus; 7, A. raricostatus; 8, A. armatus; 9, A. Jamesoni; 10, A. ibex; 11, A. Davæi; 12, A. margaritatus; 13, A. spinatus; 14, Posidonomya Bronnii. Later classifications, elaborations or revisions of Oppel's system, have been made by Wright, in 1860; Judd, 1875; Tate and Blake, 1876, etc. This method of classification recognized the principle of temporary continuance of species and of associated faunas and it has been applied with greater or less success all through the geological scale of formations for the definition of the lesser divisions. As early as 1838 the importance of the biological evidence in determining the time-scale was clearly enunciated by Murchison, who wrote in the introduction to the Silurian System, "that the zoological contents of rocks, when coupled with their order of superposition, are the only safe criteria of their age." + Geological Terranes and Time-periods Contrasted.-The making of the geological time-scale has now been traced far enough to clearly demonstrate the fact that the ordinary classification of geological formations, as found in our text-books, includes two distinct series of facts: (1) geological terranes, arranged stratigraphically and classified by their positions relative to * A. Oppel, "Die Juraformation, Englands, Frankreichs und des südwestlichen Deutschlands" (1856-1858). "The Silurian System." p. 9. |