Page:The Journal of geology (IA journalofgeology11893univ).pdf/299

The formations, as we find them classified in this time-scale, are arbitrarily limited and classified, but back of this arbitrary classification, certain grand events in the history of the earth are indistinctly seen. The primary units of the classification are called systems. Beginning at the base of the fossil-bearing series resting upon either Archæan or rocks of uncertain age there are first, the (1) Cambrian system of Sedgwick, restricted and also expanded as the result of later investigation. Second, the Silurian system of Murchison, divided into two, the lower Silurian which, to avoid confusion, and to give definiteness to the nomenclature has been named (2) Ordovician, by Lapworth, and the upper Silurian, for which we will retain, thus restricted, the name (3) Silurian. The fourth system, (4) Devonian, was proposed by Murchison and Sedgwick. The (5) Carboniferous system follows, which was early defined in Geology, but it is not clear who first proposed the name early applied to the coal-bearing rocks. Above this is the (6) Triassic system of Bronn, followed by the (7) Jurassic system of Brongniart. To the next system the name (8) Cretaceous was applied by Fitton. The next system still retains the name (9) Tertiary, of Cuvier and Brongniart, and is terminated by the (10) Quaternary system, whose name was introduced by Morlot. Tertiary and Quaternary were applied on the plan of Lehmann's classification which, in other respects in the course of events, has dropped out of the nomenclature.

Without explaining how the series of stratified rocks come to be divided into these particular ten systems, it may be said that their retention is due mainly to the relatively sharp boundaries