Studies for Students. 



THE ELEMENTS OF THE GEOLOGICAL TIME-SCALE. 



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 Archaean or rocks of uncertain age there 

 are first, the (i) Cambrian system of Sedgwick, restricted and 

 also expanded as the result of later investigation. Second, the Sil- 

 urian system of Murchison, divided into two, the lower Silurian 

 which, to avoid confusion, and to give definiteness to the nomen- 

 clature 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 fol- 

 lows, 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 re- 

 tains the name (9) Tertiary, of Cuvier and Brongniart, and is 

 terminated by the (10) Quaternary system, whose name was intro- 

 duced 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 



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