LITERATURE OF THE SUBJECT 203 



mation to the lower part of the Cambrian. At present, however, it is referred 

 provisionally to the Algonkian." * 



When summing up the results of the study of the Montana Algonkian 

 rocks, Professor C. R. Van Hise says : f 



" There is also in this region, as shown by the work of Davis and Peale, a great 

 series of unaltered strata which are probably Algonkian. This series is a down- 

 ward succession of barren slates below the fossiliferous Cambrian, and, if Algonkian, 

 is the uppermost division and equivalent to the upper Algonkian of the Wasatch. 

 Peale's results indicate that while there is no actual unconformity, there is a change 

 of physical conditions, a subsidence, and perhaps a real time break between the 

 Cambrian and Algonkian. Nowhere yet have the unaltered barren slates and the 

 more crystalline series of clastic origin been found in contact. Between the slates 

 and the Archean gneisses is a great unconformity, and there is little doubt, when 

 the unaltered series is carried over to the vertical limestones, quartzites, and quartz- 

 schists that it will be found to rest upon them unconformably. There is, then, in 

 this region probably two series of Algonkian rocks — one almost completely unal- 

 tered, the other thoroughly crystalline, and both of great thickness." 



Brief reference is made to the Belt beds by Messrs Iddings and Weed 

 in 1894,1 where they are provisionally assigned to the Algonkian. In 

 1896 Messrs Weed and Pirsson described with considerable detail the 

 Belt rocks as they occur in the Castle Mountain mining district of Mon- 

 tana, noting the various formations of which the terrane is there com- 

 posed and showing the areal distribution on a geological map of the 

 district. In this work the authors referred them to the Algonkian. § 



In the Little Belt Mountain folio of the Geologic Atlas of the United 

 States, now in press, Mr W. H. Weed separates the basal quartzite of the 

 Belt formation as the Neihart quartzite, the upper portions of the series 

 being mapped as the Belt shale formations of the Algonkian ; the various 

 formations composing this group are described in the text. 



Author's investigations. — During the summer of 1895 I drove from 

 Neihart to Townsend with Mr Weed, crossing the Little Belt and Big 

 Belt mountains. The observations thus made led me to visit the region 

 again in 1898 and make a general study of the pre-Cambrian rocks of the 

 Big and Little Belt mountains and of the exposures of the Prickly Pear 

 Valley area northwest and southeast of Helena. A visit was also made 

 to the section on the Gallatin river, and special attention was given to 

 the study of the relations of the Flathead Cambrian sandstones to the 

 subjacent Belt formations. In part of the latter work I was accompanied 



* Geologic Atlas of the U. S., Three Forks folio, 1896, p. 2 of text, 

 t Bull. U. S. Geol. Survey, no. 86, 1892, p. 286. 

 J^Geologic Atlas of the U. S., Livingston folio, 1894, p. 2 of text. 

 § Bull. U. S. Geol. Survey, no. 139, 1896, pp. 26 and 32 et seq. 



