REPORT OF THE CHIEF ASTROXOMER 187 



SESSIONAL PAPER No. 25a 



as it occurs in Montana, the Bow River series being the sediment deposited, 

 in part, at least, in the erosion interval between the Algonkian and the 

 Middle Camibrian.'t 



In another place he writes: — 



' Absence of Lower Cambrian rocks and fauna is accounted for by 

 the fact that that portion of the continent now covered by the Belt and 

 associated middle and upper Cambrian rocks was a land surface during 

 lower Cambrian time.':}: 



The detailed work of Weed and Pirsson resulted in the definite conclusion 

 that during- the deposition of the Belt terrane, there was a land area covering; 

 the region north of Neihart in the Little Belt mountain district. -While the 

 Belt beds were being laid down the pre-Belt rocks were reduced to a nearly 

 level plain. In Flathead time there was a submergence of the old peneplained 

 surface, with a resulting overlap of the sandstone and shale upon the pre-Belt 

 formations.* Similarly, during most or all of the Belt terrane period there 

 seems to have been land in the southern and eastern parts of the Livingston 

 folio area in Montana (see folio) ; in the area covered by the Yellowstone 

 National Park*- ; in the Absaroka quadrangle (see folio) ; in the Black Hills 

 area of South Dakota and Wyomingff ; in the Bighorn mountains and vicinity.§ 



There thus seems to be little doubt that the Belt terrane sediments were 

 in part supplied by the erosion of a large land area covering South Dakota, 

 Wyoming, and eastern Montana. In part they were supplied from the moun- 

 tainous pre-Cambrian land of western Idaho, Washington, and Oregon. In 

 other words, the rocks of the Belt terrane were laid down in the relation of a 

 typical geosynclinal prism elongated in a meridional sense between the two 

 land areas. The unconformity postulated by Walcott and his colleagues has 

 been deduced from a study of the eastern shore-zone of the ancient gulf or 

 sea. 



The irregularities of such a coast line, coupled, it may be, with minor oscil- 

 lations of level, would necessarily involve maximum variations of thickness 

 in the different sedimentary lenses of the geosynclinal. The lenses must thin 

 to nothing either at the actual shore-line, on the rims of off-shore depressions, 

 or at the outer edge of the coastal shelf which was swept by waves and currents 

 during a long period of stationary sealevel. The resulting irregularities of 

 deposition are homologous to those observed in the section of a river delta 

 which has grown out into sea or lake; those irregularities are necessarily most 

 pronounced near the shore. The failure of individual members of the Belt 

 terrane to appear beneath the Flathead sandstones cannot, therefore, be directly 



f Bull. Geol. Soc. America, Vol. 17, 1906, p. 16. 

 t Bull. Geol. Soc. America, Vol. 10, 1899, p. 210. 



* U.S. Geol. Surv., Little Belt Mountains folio, 1899, and Fort Benton folio, 1899. 

 ** See Yellowstone Park folio and U.S. Geol. Surv., Monograph 32, part 2, 1899. 

 ft See the various United States government publications on the Black Hills. 

 § See the Hartville, Aladdin, and Sundance folios (1903-5) of the U.S. Geol. Sur- 

 vey: also N. H. Darton's Geologv of the Bighorn Mountains, Prof. Paper, No. 51, 1906. 



