22 C. D. WALCOTT ALGONKIAN FORMATIONS OF MONTANA 



"The only fossils obtained from this formation were collected by Dr G. M. 

 Dawson at the summit of the Vermilion pass in 1884, and consist of a couple 

 of trilobitic impressions, one of which has been identified by Prof. C. D. Wal- 

 cott as Olenellus gilberti, a characteristic Lower Cambrian fossil." 



My object in noticing the Bow River and Castle Mountain sections is 

 on account of the occurrence of strata that appear to belong to the Castle 

 Mountain group on the Continental divide at the head of I^yack creek, 

 Montana, latitude 48° 30'. These beds appear to correspond to the Cam- 

 brian and superjacent groups, as they occur in the Dearborn River section, 

 latitude 47° 15'. On the north fork of Dearborn river the Cambrian 

 sandstone, shales, and limestones rest unconformably on the Algon- 

 kian and have a thickness of 2,205 feet. These are overlain by 1,385 feet 

 of limestones referred to the Ordovician and Silurian and 3,255 feet of 

 Carboniferous limestone, a total of 6,845 feet of strata, mainly calcareous, 

 above the Algonkian. It is quite probable that it is the northward exten- 

 sion of the lower portion of this series, that, beneath the Carboniferous, 

 forms the Castle Mountain group at the head of Nyack creek, and also, 

 still farther north, in the section of Mr McConnell. On Nyack creek the 

 calcareous strata are more impure and massive than on Dearborn river, 

 60 miles south, and they are still more so in Mr McConnelPs section, 130 

 miles to the north. 



Another feature is introduced in the Bow River series: If the speci- 

 mens of Olenellus reported were from the Bow River series, then we 

 have here the Lower Cambrian strata that are absent in the Montana 

 sections, as there the Middle Cambrian strata rest unconformable on the 

 Algonkian. 



Doctor Dawson has described a second or western series of "Cambrian" 

 rocks, which he correlates with the eastern section of Mr McConnell. 



•ian \ 



Western Section Eastern Section 



Adams lake 25,000. . . ( Castle mountain (lower part) . 4,000 



' ^ Nisconlith 15,000... ) 



Of the eastern section he said:* 



"... Our typical and most carefully surveyed section is that in the 

 Rocky mountains proper or Laramide range, on the line of the Bow River pass. 

 This has been studied by Mr R. G. McConnell, and it is the only section for 

 which some direct paleontological evidence exists.t The base of the Cambrian 

 is, however, not seen in this section. In the Gold ranges, where the Cambrian 

 is frequently found resting on the Archean, the Nisconlith, its lowest recognized 



* Bull. Geol. Soc. Am., 1901, vol. 12, p. 65. 



t For details of the Bow River Tass section, see Annual Report Geological Survey of 

 Canada, vol. ii (N. S.), part D. 



