CONTACT BETWEEN LOWER AND MIDDLE CAMBRIAN 731 



lated to the second group have yielded true Middle Cambrian fossils^ the 

 qnartzites in the second group of sections have all been tentatively re- 

 ferred to the Middle Cambrian.^^ The succession in the Mount Robson 

 region is abrupt, however, and more nearly resembles that in the first 

 group of sections named. The underlying quartzites in all the sections 

 of this group have so far proven unf ossilif erous ; but, since Olenelliis has 

 been collected from the upper part of the quartzific series in the closely 

 related southern Wasatch Eange (Big Cottonwood) section and in the 

 Oquirrh and Pioche sections, the quartzites in these, as well as those in 

 the first group of sections, and in the Mount Eobson region, are referred 

 to the Lower Cambrian. 



(§ 8d.) In the Canadian Pacific Railway section it has not been so 

 much a question of where to draw the line in an unfossiliferous interval 

 as of where to draw the line in a very fossiliferous series where scattered 

 fragments of Olenellus have been identified from horizons well above the 

 basal elastics. Such occurrences have been ascribed by the writer to 

 recurrence, and the discovery of the presence .at the top of these elastics 

 of a diastrophic break of considerable magnitude^* seems to confirm this 

 delineation of the Lower-Middle Cambrian boundary. Walcott does not 

 accept recurrence as an explanation and minimizes the stratigraphic im- 

 portance of the unconformity at the base of the Mount Whyte, placing^-^ 

 the boundary at the top of the Mount Whyte, well above any possible 

 occurrence of Olenellus. A recent textbook^^ even makes the statement : 

 "It should be made clear that the genus Olenellus became extinct before 

 the Middle Cambrian strata were deposited,^' and calls it one of the prin- 

 ciples laid down as a basis for the subdivision of the Cambrian system. 

 But we are coming to realize that the sudden introduction of new forms 

 is of far more importance than the ultimate extinction of the old, and 

 that in the delimitation of stratigraphic boundaries we must substitute 

 the proper valuation of a debatable series of diastrophic and organic 

 phenomena for a simple yes or no. 



(§ 8e.) In the Mount Robson region it is more nearly possible to use 

 the shorter method; but even here the drawing of the Lower-Middle 

 Cambrian boundary involves placing it somewhere in an unf ossilif erou- 

 1,200-foot quartzite series (Mahto) which separates known Lower Cam- 

 brian limestones ("Tah" = Mural) below from known Middle Cambrian 

 limestones ("Hota^' = Adolphus) above. We have given our reasons for 



^2 Burling : Museum Bull., Geol. Survey Canada, no. 2, 1914, p. 110. 

 i-* Summ. Kept. Geol. Surv. Canada for 1915, p. 100, 1916. 

 15 Smithsonian Misc. Coll., vol. 67, no. 3, 1917, pp. 61-67. 

 1^ Miller : Historical Geology, 1916, p. 57. 



