20 C. D. WALCOTT ALGONKIAN FORMATIONS OF MONTANA 



largely reddish brown in color, with interbedded belts of grayish red and 

 gray sandstones and sandy shales. The section is here interrupted by a 

 band of compact, impure, siliceous and arenaceous gray limestone 198 

 feet in thickness. The occurrence of this band of limestone and a thin 

 deposit of calcareous Cryptozoan 500 feet below indicates local conditions 

 favorable for the deposition of limestone. 



Below the siliceous limestone there is a great series of reddish gray and 

 reddish sandstones 3,089 feet in thickness. This, with the arenaceous 

 beds above the limestone, gives a total thickness of 8,280 feet, which is 

 interrupted only by the one band of siliceous limestone 198 feet in thick- 

 ness. This series appears to be a great thickening of the Empire, Spo- 

 kane, and Greyson formations of the Belt Mountains section.* 



The next formation below (Blackfoot) is formed of shaly limestones 

 (see plate 3) alternating with thin layers of gray limestone, 2,500 feet 

 of the thickness of which is largely limestone, with some interbedded and 

 incorporated siliceous and arenaceous material. The lower 1,850 feet is 

 largely a highly siliceous limestone with bands of almost purely siliceous 

 material. This limestone series has abundant remains of Cryptozoan. 

 On the North fork of Blackfoot river the central portions of the lime- 

 stones are much like those of the Newland limestones of the Belt Moun- 

 tains section, while in the Mission range, where they are somewhat meta- 

 morphosed, they present the bold cliffs and hard, massive, flinty layers 

 characteristic of the cliffs of Altyn limestone of the Lewis Range section. 



The Blackfoot series appears to be identical in stratigraphic position 

 and character with the Newland limestone of the Belt Mountains section, 

 and the Altyn limestone of the Lewis Range section, and the Wallace 

 calcareous series of the Cceur d'Alene section of Idaho. 



Below the Blackfoot there is, west of the Mission range, a series 8,255 

 feet in thickness of purple, greenish and gray, siliceous, and arenaceous 

 beds, which completes the section measured by me in the season of 1905. 

 This series, named Ravalli, probably represents that portion of the Cceur 

 d'Alene section between the Wallace and the Prichard series, and it may 

 be a part of the upper portion of the Prichard. 



The Cceur d'Alene series does not appear to extend upward to the 

 horizon of the Helena limestone, as there is only 1,000 feet of the reddish 

 beds above the Wallace series which is correlated with the Newland Lime- 

 stone horizon. 



The strata below the Wallace and above the Prichard are correlated 

 with the Ravalli series; and the Prichard, which is composed of dark, 



*Mr M. Collen, of White Sulphur Springs, Belt mountains, Montana, wrote me in the 

 spring of 1905 that he had found a marked unconformity between the Greyson shales 

 and the Newland limestone on Birch creek. 



