STRATIGRAPHIC TAXONOMY 079 



is only a matter of refinement in classification that the Swedish geolo- 

 gists with their imperfect section could not anticipate. 



That the Dictyonema zone is post-Ozarkian and not greatly older than 

 the Tetragraptus fauna is strongly suggested by the close alliance of 

 Bryograptus, Clonograptus, and Staurograptics, species of which genera 

 often accompany Dictyonema flahelliforme, to Goniograptus and the 

 DichograptidcB generally that are found in the Tetragraptus zone. In- 

 deed, Bryograptus and Clonograptus are common to the two zones. Die- 

 tyonema fldbelliforme, besides, is too much like subsequent species of the 

 genus to suggest a much greater age than Canadian. Considering the 

 extraordinary rapidity of the evolution of the graptolites, especially the 

 Dichograptidce, during the middle and earlier stages of the Canadian, it 

 seems unreasonable to assume that a long geological time was required to 

 overcome the really small diiferences between the older species of Clono- 

 graptus and Bryograptus on the one side and the later species of these 

 and closely allied genera in the Tetragraptus zone on the other. That 

 both zones belong to the same broad stratigraphic division is strongly 

 indicated by the field relations of the beds holding the respective faunas. 

 No break, either stratigraphic or lithologic, has so far been discovered 

 between them, and, in my opinion, there is none of great consequence — 

 certainly none of the importance it would have to be if the D. flabelli- 

 forme zone were upper Cambrian. Both are found in precisely the same 

 kind of sediments, so that without the fossils it has seemed impossible to 

 discriminate between them. While thin conglomerates are common they 

 are clearly intraformational, being found interbedded with successive 

 beds containing the same fauna. Under the circumstances my reference 

 of the Dictyonema zone, and with it most — perhaps all — of Matthew's 

 Bretonian, to the Canadian, requires no apology. 



The true relations of the Olenus fauna with respect to the Ozarkian 

 and the Canadian are not easily determined. The problem is exceed- 

 ingly complicated, many factors entering that can not be touched here. 

 Tf tlio Bretonian — the lower half of wliicli contains the Olenus fauna, 

 in part associated with Dictyonema flahelUforme, while its upper mem- 

 hoY contains the Tetragraptus fauna — is, as thought l^v ^ratthew, a prac- 

 tically continuous deposit, I see no good reason why the whole of the 

 groii]) or formation should not be referred to the Canadian. The faunas 

 living on the exterior platform of the continent, being adjacent to the 

 permanent oceanic deeps, may reasonably be expected to exhibit greater 

 persistence than the epicontinental faunas — a probability that may ac- 

 count for the Cambrian and Ozarkian aspect of the Olenus fauna. 



