Near the rapidly growing city of Great Falls, Montana, the Missouri river emerges from the Belt mountains and begins its long eastward course through the great plains. The rapidly flowing stream soon commences to cut through the nearly horizontal strata of the plains, and near Great Falls plunges over a series of sandstone ledges in a succession of cataracts collectively known as the “Great falls of the Missouri.” Below the falls the sandstones gradually pass beneath the dark carbonaceous shales so well exposed at Fort Benton, from which place they take their name. These sandstones, with their interbedded shales, now known as the Great Falls formation, have long been known to all geologists visiting the region, but until recently failed to reveal any fossil remains and were referred to the Dakota epoch, whenever mentioned, on account of their inferior position to the well . . .