16 C. D. WALCOTT ALGONKIAN FORMATIONS OF MONTANA 



cover a wide area to the west and south and are apparently interbedded 

 in the quartzite series exposed on the line of the Great Northern railroad 

 to the westward, on the ridge cut through at Haskell pass. Up to date 

 this series may be referred to the Algonkian, although this is probably 

 the same as Dawson's Cambrian to the north, in British Columbia. 



These limestones appear to be a portion of the Blackfoot Limestone 

 series of the Mission range. 



Unconformity between Algonkian and Cambrian 



I have described the unconformity existing between the strata of the 

 Belt terrane and the Cambrian in the Big Belt mountains and in the 

 vicinity of Helena. It was found that in one instance upward of 3,000 

 feet of Algonkian beds had been removed by pre-Cambrian erosion, and 

 that it was exceptional to find the same Algonkian strata in contact with 

 the Cambrian in localities a few miles distant from each other.* We 

 find similar conditions as the contact of the two systems of strata is 

 traced to the west and northwest. At Lewis and Clark pass the Cambrian 

 rests on a series of reddish brown sandstones, 1,015 feet in thickness, 

 above the Helena calcareous beds. At Helena, 40 miles distant, there 

 are 70 feet of similar sandstones between the Cambrian and the Helena 

 limestone. At the Dearborn section, 10 miles north of Lewis and Clark 

 pass, the Cambrian rests on siliceous and calcareous strata that appear to 

 belong to a portion of- the Algonkian section 1,000 feet or more beneath 

 the horizon in contact with the Cambrian at Lewis and Clark pass. In 

 the vicinity of Scapegoat mountain, 35 miles northwest, the Cambrian is 

 superjacent to a series of gray sandstones and shaly beds, 1,700 feet thick, 

 that do not appear to be represented in the Dearborn section. 



No contacts with the Cambrian have been observed west of the ridges 

 between the north fork of the Flathead river and the Swan range, 

 although from the presence of Cambrian fossiliferous limestones west of 

 the Mission range such contacts may be found. 



Eighty miles north of the Scapegoat area, in the vicinity of Cut Bank 

 pass, at the head of Nyack creek, it is difficult to locate the line of 

 demarcation between the fossiliferous limestones and the Algonkian 

 strata, 2,000 feet below. Further stud}*" is needed in this area. 



One hundred miles farther north the section appears to be conformable 

 from the Ordovician down through the Middle Cambrian and the Lower 

 Cambrian of the Bow Biver series, and not to reach down to the Algon- 

 kian as it occurs in Montana, the Bow Biver series being the sediment 



* Loc. cit., pp. 210-215. 



