180 DEPARTMENT OF THE INTERIOR 



2 GEORGE V,, A. 1912 



entire period. It may have been succeeded by an emergence of the land 

 area for a brief period, but the probability is that the interruption to the 

 downward movement, if it occurred, was slight. Next, the widespread pre- 

 Cambrian subsidence preceding the formation of the Flathead quartzite 

 took place, and the Cambrian sea covered large areas that had hitherto 

 been above the sea level. There is a marked difference in the character 

 of the beds of the two groups. Little, if any, induration is seen in the 

 Flathead formation, while the Belt beds are so altered in most cases as to 

 resemble the metamorphic rocks which underlie them, and from the break- 

 ing down of which they were derived. Notwithstanding the metamorphism, 

 there is no mistaking their sedimentary character.'! 



In 1896 Feale summarized his net conclusion regarding the correlation in 

 the following words : 



' It is possible that further investigation may result in the reference 

 of this formation to the lower part of the Cambrian. At present, however, 

 it is referred provisionally to the Algonkian.'* 



"Weed, Iddings, and Pirsson in several publications issued between the years 

 1894 and 1899, refer the terrane to the Algonkian, though Weed and Pirsson, 

 after close study of the Castle Mountain (Montana) mining district, wrote: 



' Both the character of the sediments and their position beneath the 

 beds of Middle Cambrian age indicate their similarity to the Bow River 

 beds of the Canadian geologists, in which Lower Cambrian fossils are 

 found. It has, however, been decided to class the beds as Algonkian.'§ 



In 1898 Walcott made a general study of the terrane as exposed" in the 

 Big Belt and Little Belt mountains and in the Helena district. He writes : — 

 ' The results of my investigation were the discovery of a great strati- 

 graphic unconformity between the Cambrian and the Belt formations; 

 that the Belt terrane was divisible into several formations, and that fossils 

 occurred in the Greyson shales nearly 7,000 feet beneath the highest beds 

 of the Belt terrane.'^ 



Walcott's columnar section of the terrane is that given in Col. 9, of 

 Table VIII. 



In 1902 Willis stated in the following words his correlation of the Lewis 

 series as exposed in the Clarke and Lewis ranges : 



' The oldest formation of the series, the Altyn limestone, is assigned 

 to the Algonkian period on the basis of fossils discovered by Weller in its 

 characteristic occurrence at the foot of Appekunny mountain near Altyn, 

 Montana. These fossils are fragments of very thin shells of crustaceans 

 [chiefly Beltina danail The fossiliferous strata of the Belt 



t A. C. Peale, Bull. U.S. Geol. Survey, No. 110, 1893, p. 19. 



* Three Forks Folio, U.S. Geol. Surv., 1896, p. 2, 



§ Bull. U.S. Geol. Surv., No. 139, 1896, p. 139. 



JC. D. Walcott, Bull. Geol. Soc. America, Vol. 10, 1899. p. 204. 



