CANNON BALL MARINE MEMBER OF LANCE FORMATION 541 



undifferentiated lower part of the formation, and the absence of 

 marine fossils in this member is in contrast to the Cannonball 

 marine member. The Ludlow lignitic and the Cannonball marine 

 members are considered to be contemporaneous in age. 



RELATIONSHIPS IN OTHER AREAS 



The Cannonball sea presumably advanced into western North 

 and South Dakota from the east or northeast, and by inference the 

 Cannonball member continued with undiminished or with increased 

 thickness to the north and northeast. Practically the whole region 

 east of Missouri River is deeply covered with a mantle of the drift 

 of the last Wisconsin ice invasion, and the underlying formations 

 are exposed only in isolated localities. In this region the Lance for- 

 mation has been mapped about 50 miles eastward from Bismark but 

 is presumably unrecognizable farther northeast.' It seems a very 

 reasonable supposition that the lower fresh-water part of the Lance 

 may not extend very far to the east and that where it is absent 

 marine sedimentation was continuous throughout the time when 

 the fresh-water Lance was being deposited elsewhere. 



Beyond the limits of Cannonball marine sedimentation on the 

 west the Fort Union formation rests directly on the fresh-water 

 beds of the Lance, and, except along the badlands of Little 

 Missouri, no definite line of demarkation between the two has been 

 drawn. In general the Lance is distinguished lithologically by a 

 greater proportion of somber-colored shale and sandstone, the 

 large number of the bog iron ore nodules and layers, by irregularity 

 of deposition, and by a paucity of lignite. The Fort Union, on 

 the other hand, contains a larger proportion of yellow and white 

 sandstone, quartzite, and thin limestone, and almost everywhere 

 contains a large number of lignite beds, some of which are very 

 thick and persistent. The rocks of this formation are fine-grained 

 and regularly bedded and give rise to angular topographic forms. 

 In a large area in eastern Montana, mapped by the U.S. Geo- 

 logical Survey in 1910, it was found that the color change from 

 somber below to yellow above does not take place everywhere at 



'A. G. Leonard, "The Geological Map of North Dakota," Qiiar. Jour. Univ. 

 N.Dak., IV (1913), 4. 



