STRATIGRAPHIC EVIDENCE 327 



basin of Colorado, it appears, according to Dr. C. A. White, that the 

 invertebrate fauna in the upper portion, while it contains some of the 

 species of the lower portion, is on the whole distinct. This is taken to 

 indicate that the full section of Fox Hills is nowhere present in the Da- 

 kotas, and that the incomplete development of the formation here is to be 

 ascribed to removal of the upper part by erosion rather than to diminished 

 rate of sedimentation or to stratigraphic overlap. 



In Worthless Creek Valley, South Dakota, the pre-Lance unconformity 

 is angular as well as erosional, the Fox Hills dipping north at an angle 

 of 4 degrees, whereas the overlying Lance is horizontal. On the Moreau 

 River, near Govert post-office, South Dakota, the Lance is horizontal, 

 while the underlying beds dip northwest at an angle of 10 degrees. x4t a 

 number of points in the eastern part of Custer County, Montana, the 

 Lance rests on a distinctly eroded surface of the Fox Hills. Likewise, 

 according to Barnum Brown, the same condition obtains on Hell Creek, 

 in Dawson County, Montana. 



Throughout much of Montana and Wyoming the conditions are the 

 same as those above described, namely: The Lance is" found resting on 

 Fox Hills of different thicknesses, often with eroded surface, and in some 

 cases/as at Forsytli, Columbus, etcetera, the Fox Hills is entirely absent 

 and the Lance rests on Pierre and not always on its uppermost member. 



It has been suggested that in those cases where the Lance rests directly 

 011 Pierre the lower sandstones of the Lance may be the fresh-water 

 phase of the Fox Hills. Proof of this contention would be the finding 

 of an area in which there is evidence either of a transition laterally from 

 the purely marine conditions of the Fox Hills through brackish water to 

 the suggested fresh-water facies of the "Lance," or a barrier separating 

 two such areas of deposition on which neither facies was laid down. If 

 the country within which the Lance is found resting on Pierre was one 

 in which the stratigraphic relations were obscure on account of few or 

 poor exposures, such . transition or barrier might possibly have escaped 

 detection; but on the contrary it is a region in which exposures are 

 numerous and ample, and moreover is one in which investigation in 

 recent years has been intensive, but no such condition has been observed. 



In the cases thus far considered the discordance between the Lance 

 and underlying beds is not always evident, nor is it always conspicuous 

 when present, and hence it is held by some geologists to be of no more 

 importance than unconformities acknowledged to be present at various 

 horizons in the Lance. The essential difference lies in the fact that the 

 unconformities within the Lance are obviously local and can be traced 

 only for short distances, whereas the evidence in support of the pre-Lance 



