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lands, Frankreichs und des südwestlichen Deutschlands, 1856-1858). 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 zoölogical contents of rocks, when coupled with their order of superposition, are the only safe criteria of their age." (The Silurian System, p. 9).

The making of the geological time-scale has no progressed to the stage when it is pretty clearly seen 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 each other and by their lithological characters; and (2) chronological time-periods, which may be locally marked by the stratigraphical division planes, but which depend, fundamentally, upon biological evidence for their interpretation and classification. Gilbert^[1] has concisely expressed the important fact of the purely local nature of the division-planes separating the formations stratigraphically into stages, series, systems or groups in