336 GEOLOGICAL SURVEY OF THE TERRITORIES. 



The deposits formed by the lake basins of the Upper Missouri region 

 are shales, marls, and earthy limestones, with immense quantities of lig- 

 nite, but with almost no traces of volcanic products. The number of fossil 

 plants and animals is much greater there than farther west; and we have 

 in these deposits proof that during unnumbered ages this portion of the 

 continent exhibited a diversified and beautiful surface, which sustained 

 a luxuriant growth of vegetation and an amount of animal life far in 

 excess of what it has done in modern times. This condition of things ex- 

 isted long enough for hundreds and even thousands of feet of sediments 

 to accumulate in the bottoms of extensive fresh-water lakes. These 

 lakes were gradually and slowly diminished in area by the filling up of 

 their basins and by the wearing away of the barriers over which passed 

 their gently flowing, draining streams. Since the deposition of the 

 fresh- water tertiaries which occupy the places of the old lakes, great 

 changes have taken place in the topography of this region by the up- 

 heaval of portions of the Eocky Mountain ranges. In some localities 

 these lake deposits are found turned up on edge and resting on the 

 flanks of the mountains which border the plains on the west. It is cer- 

 tain, however, that much of the Eocky Mountain belt existed anterior 

 to this date. We have in these, and many other facts that might be 

 cited, proofs of the truth ot the assertion I have elsewhere made that 

 these great mountain chains, though existing at least in embryo from 

 the earliest paleozoic ages, have since then been subject to many and 

 varied modifications ; that they have been, in fact, hinges upon which 

 the great plates of the continent have turned ; lines of weakness where 

 the changes of level experienced by the continent have been most sen- 

 sibly felt. 



It is a somewhat remarkable fact that the collections of fossil plants 

 made by Dr. Hayden from different localities differ so much among 

 themselves. In every newly, discovered plant-bed he has obtained more 

 or less species of which we before had no knowledge, and it is even true 

 that between some of his collections there are no connecting links. It is 

 also true that much of the material he has collected has not yet received 

 the study it needs. From these facts it will be seen that mucb yet re- 

 mains to be done before the great interval of time during which this series 

 of fresh- water tertiaries accumulated can be divided into definite periods, 

 and before we can venture to affirm that such an epoch had a flora of 

 such or such a botanical character, and, therefore, this or that average 

 annual temperature. Some interesting facts came out, however, at once 

 in the examination of these materials; to these I will briefly refer. 



In the beginning of the cretaceous age, ISorth America, as we learn, 

 presented a broad land surface, having a climate similar to. the present, 

 and covered with forests consisting, for the most part, of trees belonging 

 to the same genera with those that now flourish upon it. In the progress 

 of the cretaceous age, the greater part of the continent west of the Mis- 

 sissippi sank beneath the oceanj and the deposits made during the later 

 portions of the cretaceous age contain a vegetation more tropical in 

 character than that which had preceded it. It seems probable that at 

 this time the lands which existed as such west of the Mississippi were 

 islands of limited extent, washed by the Gulf Stream, which apparently 

 had then a course north and west from the Gulf of Mexico to the Arctic 

 Sea, 



The earlier tertiary epochs were, however, marked by an emergence of 

 the continent and a gradual approach to previous and present conditions. 

 This is indicated by the fact that the oldest tertiary deposits (eocene?) 

 contain a flora less like the present than is that of the miocene or middle 



