202 C. I). WALCOTT PRE-CAMBRIAN FOSSILIFKROUS FORMATIONS 



Dr J. S. Newberry crossed the Belt mountains in 1883 and noted the 

 Cambrian (?) slates, which he correlated with the strata cut by Prickly 

 Pear can3^on on the west side of the Missouri river, also with the forma- 

 tion beneath the " Potsdam " in the Wasatch mountains near Salt Lake 

 City and in the canyon of the Colorado.* 



In 1886 Professor AV. M. Davis described the Belt Mountain rocks as 

 "a vast series of lower Cambrian barren slates, at least 10,000 to 15,000 

 feet thick at many points." His sections show the fossiliferous Cambrian 

 beds conformable to the Belt Mountain slates.f Subsequently Dr A. C. 

 Peale described the " Belt formation " as it occurs near Three Forks, 

 Montana,! the same formation that he had called the East Gallatin 

 group in 1884, § when a thickness of 2,300 feet was carefully measured 

 and the series considered to be of Cambrian age. Dr Peale in 1893 

 tentatively referred the Belt formation to the Algonkian on account of 

 the absence of any fossil remains, the metamorphosed condition of the 

 Belt rocks, and the existence of the unconformity between the Cam- 

 brian Flathead quartzite and the Belt beds below. This unconformity 

 he considered as one caused by subsidence. He states : || 



"There iS no doubt that after the Belt formation was deposited there was an 

 orographic movement by which the Archean area of nearly the entire region rep- 

 resented on our map south of the Gallatin and Three Forks was submerged just 

 prior to the beginning of the Cambrian, before the Flathead quartzite was depos- 

 ited. AVhether this movement occurred immediately after the laying down of the 

 Belt beds or after an interval is of course the question to be decided, and the de- 

 cision can not be positively reached with the meager data now at hand. I am in- 

 clined to think that the subsidence of the Archean continent (or possibly islands) 

 began with the first accumulation of the sediments that formed the lower portion 

 of these beds and was coincident with their deposition throughout the 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 preced- 

 ing the formation of the Flathead quartzite took place, and the Cambrian sea cov- 

 ered large areas that had hitherto been above the sealevel. There is a marked 

 diff'erence in the character of the beds of the two groups. Little, if any, indura- 

 tion is seen in the Flathead formation, while the Belt beds are so altered in most 

 cases as to resemble closely the metamorphic ci'ystalline rocks which underlie 

 them, and from the breaking down of which they were derived. Notwithstanding 

 the metamorphism, there is no mistaking their sedimentary character." 



In 1896 Dr Peale, in describing the Belt formation, says : 



" It is possible that further investigation may result in the reference of this for- 



*Ann. Report N. Y. Academy of Science, vol. Ill, 1884, p. 249. 



t Tenth Census, vol. XV, Mining Industries, 1886, pp. 697, 700, 7()-2. 



t Bull. U. S. Geol. Survey, no. 110, 1893, pp. 16-20. 



§ Sixth Ann. Report U. S. Geol. Survey, 1885, p. 50. 



D Bull. U. S. Geol. Survey, no. 110, 1893, p. 19. 



