The unconformable relation of the basal Cambrian formation (Flat‐head) with the underlying Algonkian (Beltian). sediments was known before 1899. However, a number of stratigraphers have been inclined to doubt the existence of the unconformity in western Montana. The reason for the doubt is that the unconformity is rarely sharply angular. The discordance is so slight that in most areas where the two systems of rocks are exposed the Flathead sandstone lies in apparent conformity upon the Beltian sediments of varying composition. In fact, four seasons of field work, supplemented by a thorough examination of the literature, have revealed to the writer only eight localities in which the Flathead has been observed to rest with any marked angular discordance upon the slightly beveled surfaces of the Beltian strata.

Failure to recognize this unconformable relation has led some geologists to the assumption that the Beltian rocks are of Lower Cambrian age . . .

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