DAKOTA AND NEBRASKA. 



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The geographical extent of this wonderful basin is not even yet clearly known, 

 inasmuch as species identical with some found on the Niobrara River have been 

 discovered in remote j^ortions of the west. Fossils have been sent to Dr. Leidy from 

 Texas and from California, apparently identical and closely allied with species found 

 in the Bad Lands, and enveloped in a matrix which appears identical in composition 

 and color. We have evidence of its existence in Colorado, in Middle Park, and on the 

 Arkansas River, various parts of Utah, and in 1860 I saw isolated patches of the Plio- 

 cene beds in the valley of the Gallatin's Fork of the Missouri River. We may there- 

 fore look for the Pliocene beds in almost any portion of the Rocky Mountain range. 

 By reference to the geological map accompanying this memoir it will be seen that the 

 formation is now known to cover an area of at least 100,000 square miles, but as there 

 are isolated patches extending far across the Missouri northward into Minnesota, we 

 may reasonably infer that it once occupied a much larger area, and that these isolated 

 portions, like Medicine Hill, Bijoux Hill, &c., are monuments left from erosion to 

 point out minutely the former limits of the lake. We think therefore that some time 

 during the Tertiary period this great lake must have covered an area of at least 

 150,000 square miles. 



That the Rocky Mountain range had reached an advanced stage in its upward 

 movements before the accumulation of the sediments composing this formation, 

 seems to be indicated by the evidence already secured. While the lignite beds seem 

 to have suffered an inclination equal with the older fossiliferous beds, by the disturb- 

 ing influences that elevated the mountain ranges, the White River beds seldom 

 incline more than 5° or 10°. All along the foot of the Black Hills and the main 

 range the white marls and sands fill up the valleys and jut against the sides of the 

 older rocks, always reposing unconformably upon them. Sometimes these beds are 

 elevated several hundred feet, still remaining horizontal. We can thus see that, 



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