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secondary formation of the Lehmann classification, were named Tertiary to indicate their geological importance and their relative position in the geological scale. These naturalists did not, however, perfect the geological classification which their biological studies suggested.

William Smith in England ("Tabular view," 1790, and in unpublished maps and sections of the first and second decades of this century) emphasized the value of fossils as means of identifying strata in different regions, and others had some part in the elaboration of the principle involved, but Lyell more than any one else perfected the scheme of classification of geological formations on the basis of their fossil contents. We find him saying, in the second edition of his Elements of Geology, published in 1841, "When engaged in 1828, in preparing my work on the Principles of Geology, I conceived the idea of classing the whole series of Tertiary strata into four groups, and endeavoring to find characters for each, expressive of their different degrees of affinity to the living fauna" (p. 280). A mathematical comparison was made between the proportionate numbers of recent and of extinct species to the several divisions of the Tertiary rocks of England. The result is given in the following table (copied from his "Elements," 2d Ed., Vol. I, p. 284).

In the nomenclature here proposed Eocene is derived from the Greek ηως, dawn, and καίνος, recent; Miocene from μείον καίνος, less recent; Pliocene from πλειον καίνος, more recent, and the definite meaning of the nomenclature and the classification is to signify that the strata called Eocene contain the first traces of the fauna now living, the Miocene strata a small proportion of the living species, the Pliocene and Post-Pliocene more and still more of the living types, and that the whole of the Tertiary