DAKOTA AND NEBRASKA. 11 



identical with the one so abundant at the mouth of the Judith River, and near Fort 

 Clark, on the Missouri River, holding a similar geological position. Other fossils are 

 found, and other proofs are known which show that these beds are intermediate 

 between the true marine formations of the Cretaceous and the strictly fresh water of 

 the Tertiary. It is not our purpose in this brief introduction to present a detailed 

 account of the geological features of this great region, but we wish simply to indicate 

 that it is possible to trace the history of the growth of the continent step by step 

 from the purely marine waters of the Cretaceous ocean, and the period when the 

 mountain ranges were elevated in well-defined lines above the waters, causing the 

 ocean to recede to the eastward on the one side and to the westward on the other ; 

 and that this elevating force continued to act throughout the Tertiary period as well 

 as the Post-tertiary; and that it probably continues even up to the present time. 

 At any rate the Rocky Mountains formed immense water-sheds, which gave birth to 

 innumerable fresh-water streams, which fed those great Tertiary lakes along the 

 eastern slope. As we have before remarked, the waters were brackish at first, but 

 very soon, by the superabundance of fresh water, though with access to salt water, at 

 first the lakes became entirely fresh, and the sediments reveal no remains but those of 

 purely fresh-water or terrestrial animals or plants. That there have been periods of 

 repose as well as subsidence, there can be no doubt, and this seems to be shown by 

 the great system of terraces along all the streams of this country ; but the general 

 upward tendency has been continued without any marked interruption even up to 

 the present time. The observations that have been made thus far point to the con- 

 clusion that during the Tertiary period there were at least four, and possibly five, 

 fresh-water lakes in the west, and two of them were certainly of great extent. 



1st. The Bad Lands of the Judith, the beds of which are included in the Section, 

 in the Fort Union or Lignite group. This basin occupies an area about forty miles in 

 length, and from fifteen to twenty in breadth. The Missouri River cuts a channel 

 through it, separating it in two nearly equal portions. It is located near the mouth 

 of the Judith River, near lat. 48° and long. 110°. This basin has never been care- 

 fully explored, and there is still so much doubt as to its age and exact position in 

 the geological scale that it would be unsafe to venture an opinion in regard to it 

 until more facts can be secured. We know that the strata are of both estuary and 

 fresh-water character, and that they are composed of indurated sands, clays and 

 impure lignites, the whole presenting a dark brown or sombre color, not unlike the 

 lignite beds below Fort Union. Large numbers of shells and a few plants were 

 preserved in its sediments, but it is chiefly remarkable for its peculiar Saurian fauna, 

 calling to the mind of the palaeontologist that of the Wealden of England.* 



* Trans. Am. Philos. Soc. Vol. 11 p. 139. 



