CONTACT BETWEEN UPPER AND POST-CAMBRIAN 729 



siliferous sections of the Upper Cambrian in the Rocky Mountain geo- 

 syncline to the south. But the future student should not be surprised 

 if he finds the Devonian resting on the semi-Ozarkic faunas which imme- 

 diately overlie the Lynx formation in the Mount Eobson region, and this 

 might even happen in the summit of Mount Eobson itself. 



The problems involved are very interesting, and the rocks may prove 

 even more generous to the student who shall make more than a rapid 

 reconnaissance in this fossiliferous region. 



Contact between the Middle and Upper Cambrian (§7) 



Walcott draws the line between the Middle and Upper Cambrian at 

 the base of the "unfossiliferous'' Lynx limestones and above the Titkana 

 limestones, in which he found a fauna comparable with that of the Ste- 

 phen formation. The discovery of many other fossil horizons in the 

 1,600 feet which separate the Stephen faunal horizon of the Titkana 

 from the L3aix, together with abundant fossils in a dozen or more hori- 

 zons in the Lynx formation, has shown the author the faunal correctness 

 of the separation, and the writer has already described^^ the lithological 

 and stratigraphical grounds which confirm the presence at this point in 

 the section of a break similar in character and magnitude to the break 

 between the Middle and Upper Cambrian in the Canadian Pacific Rail- 

 way section 200 miles to the south. In each section the upper layers of 

 the Middle Cambrian exhibit the downwarping which has been ascribed 

 by the writer to peculiar conditions of subaerial desiccation, and the 

 lower layers of the Upper Cambrian exhibit mud-crack, ripple-mark, 

 cast-of-salt crystal, and other shallow-water sedimentation phenomena. 



Contact between the Lower and Middle Cambrian (§8) 



(§ 8a..) Walcott places the boundary between the Lower and Middle 

 Cambrian between the "Hota" and the Chetang, the former described 

 as carrying the "new Lower Cambrian subfauna" with OJenellus, the 

 latter as carrying ATbertella. 



The lowest fossil horizon secured by Walcott in the Chetang is 550 feet 

 above the base of the formation in Chetang cliff, and evidence is pre- 

 sented, in the discussion of the "Hota," Mahto, and "Tah" formations 



^^ Shallow-water deposition in the Cambrian of the Canadian Cordillera. Ottawa 

 Nat., vol. 29, 1915, pp. 87-88. 



Notes on the sti'atigraphy of the Rocky Mountains, British Columbia and Alberta. 

 Summary Rept. Geol. Surv. Canada for 1915, p. 99, 1916. 



Downwarping along joint planes at the close of the Niagaran and Acadian. Journ. 

 Geol., vol. 25, 1917. pp. 145-149. 



XLVIII— Bull. Geol. Soc. Am., Vol. 34, 1922 



