Perhaps none of the various formations which contribute such important elements to the geologic history of the Rocky Mountain area have been the subject of so much discussion and difference of opinion as the series which has come recently to be known as the Laramie and allied formations. This series of strata, first known as the Lignite series, included the coal-bearing strata of the Upper Cretaceous with the lower Tertiary or Fort Union strata and was all regarded as Tertiary in age. Later the Laramie was differentiated by King as the uppermost division of the conformable Cretaceous series, and the Fort Union group ultimately associated with the lower Tertiary, although still regarded by some as belonging to the Laramie series. The Laramie as thus characterized was supposed to be very sharply circumscribed, but soon it was pointed out that certain of the lower coal-bearing members appeared to be in . . .