398 C. D. Walcott — The Taconic System of Emmons. 



writers for the system of strata characterized by the " First 

 Fauna," and the upper the Champlain of Emmons, the Lower 

 Silurian of Murchison, or the Ordovician of some more recent 

 authors. 



Classification op North American Cambrian Rocks. 



In the classification of the fossiliferous sedimentary rocks of 

 all countries it becomes more and more evident that the great 

 systems — Cambrian, Silurian, Devonian, etc. — must rest on the 

 broad zoologic characters of their included faunas and not on 

 stratigraphic breaks between the systems, and that geologists 

 will need to recognize the fact so well stated by Lapworth, that 

 " we have no reliable chronological scale in geology but such as 

 is afforded by the relative magnitude of zoological change — in 

 other words, that the geological duration and importance of 

 any system is in strict proportion to the comparative magnitude 

 and distinctness of its collective fauna."* In pursuance of the 

 above principle I have separated the Cambrian System in 

 North America from the Lower Silurian. In the magnitude 

 of sedimentation and extent of the fauna it ranks with the 

 other great geologic systems, and we cannot unite it with the 

 Lower Silurian except from reasons that, if followed out, will 

 unite all the systems from the Cambrian to the Quaternary. 



In arranging the different strata composing the Cambrian 

 System three primary divisions are distinguished by the pre- 

 dominence in each of a fauna that, in assemblage of genera and 

 species, may be separated from others whenever two or more 

 of them occur in the same stratigraphic section. This extends 

 to the identification of the relative geologic horizon by the 

 fauna when its vertical or geographic connection with other 

 faunas is not preserved. The three divisions of the table have 

 been recognized to a greater or less extent in all the sections 

 of Cambrian strata studied in North America, and all the ob- 

 served Cambrian faunas come within their limits. 



The second column in the table gives local names that have 

 been applied within certain geologic provinces, where the 

 fauna and the sedimentation indicate a greater uniformity of 

 conditions than existed throughout the larger areas outlined by 

 the first three divisions. The right-hand column gives the 

 names of local subdivisions where the conditions of sedimenta- 

 tion and of life were still more restricted. 



The table is a correlation of the various sections described 

 in the introduction to U. S. Geological Survey Bulletin No. SO, 

 and hence is tentative. It is the expression of my present 

 knowledge and opinion. All who use it in geologic work 



* Geol. Mag., vol. vi, p. 3, 1879. 



