364 F. H. KNOWLTON 



beds' is very applicable, as the strata are made up chiefly of gray 

 to dark clays and muds with intercalated lenticular sandstone 

 members. There is rapid horizontal alteration in character of 

 material so that a section measured at any one locality does not 

 compare in detail with one measured a short distance away. Car- 

 bonaceous zones occur at frequent vertical intervals and the lowest 

 few feet of the formation is almost invariably a lignitic zone. 



"As a result of field study by Pishel, Barnett, and the writer, 

 it seems certain that the line between the Fox Hills sandstone and 

 the Lance formation is marked by an unconformity, but the import 

 of that unconformity is of course a matter for the paleontologist 

 rather than for the stratigrapher to decide. However, the evi- 

 dence gathered by the stratigrapher may possibly have some weight 

 in arriving at a conclusion, and that evidence is here presented. 



"The maximum thickness of the Fox Hills sandstone is in the 

 neighborhood of 200 feet, but it was found in the field that this 

 measurement is entirely too great for certain localities. On 

 Worthless Creek, in T16N, R20E, Black Hills Meridian, exposures 

 are especially good and it was here that the most striking 

 example of unconformity between the Fox Hills and Lance forma- 

 tion was observed. On the west side of Worthless Creek Valley, 

 near the line between sections 25 and 26, it was noted that the 

 'somber beds' of the Lance formation transgressed across the Fox 

 Hills sandstone and that the upper part of the latter formation 

 down to the banded shale member was absent. The unconformity 

 at this locality is angular as well as erosional, for the banded shale 

 dips to the north at a 4-degree angle, whereas the 'somber beds' 

 are horizontal. Within a horizontal distance of 500 feet the 

 ' somber beds ' fill a channel eroded in the banded shale of the Fox 

 Hills, so that the total vertical amount of combined transgression 

 and erosion is more than 40 feet. On the opposite side of the valley 

 the total thickness of undoubted Fox Hills is even less, for it 

 appears that the lignitic zone of the ' somber beds ' rests on Pierre 

 shale. In any event there is surely less than 25 feet of the Fox 

 Hills present at this place. In view of the fact that the Fox Hills 

 sandstone is normally at least 150 feet thick it would seem that 



