94 GEOLOGICAL SURVEY OF THE TERRITORIES. 



" The passage from the brackish to the fresh- water beds in the oldest 

 member of the tertiary of this region seems not to be marked by any 

 material alteration in the nature of the sediments. Nor have we, so 

 far as is yet known, any reasons for believing that any climatic or other 

 important physical changes beyond the slow rising of the land, and the 

 consequent recession of the salt and brackish water, took place during 

 the deposition of the whole of the oldest member of the tertiary here, 

 since we find a considerable proportion of the species of fresh- water 

 mollusca ranging through this whole lower member. The principal dif- 

 ference between the fossils of its upper and lower beds consists in the 

 gradual disappearance of strictly brackish- water types as we ascend from 

 the inferior strata. The entire series of Nebraska tertiary rocks con- 

 sists of three or four groups, three of which, at least, (and probably 

 four,) evidently belong to separate and distinct epochs. They usually 

 occur in isolated basins, but have, with one exception, all been seen in 

 such connection as to leave no doubts in regard to their order of super- 

 position." 



The most important thought evolved from the study of this Fort 

 Union Group is the fact, which we now believe is well established, that 

 it contains the history of the growth, step by step, of a most important 

 period of our continent. The area which it occupies is not yet known, 

 but every year it is extended north, south, and west. It is also charac- 

 terized by numerous beds of coal, or lignite as it was formerly called, 

 and, so far as the Upper Missouri is concerned, most of the coal is true 

 lignite. It is quite probable that the coal-making period began in the 

 latter portion of the cretaceous era, and extended up into the tertiary. 

 The observations of geologists in New Mexico and Utah point to the 

 conclusion that large deposits of excellent coal occur in the upper cre- 

 taceous series. The field fo» minute study in this direction is immense, 

 and we must await the results of future explorations before we can de- 

 cide positively. Another interesting feature connected with this group 

 is the splendid series of fossil plants which it has yielded, showing the 

 existence during the early tertiary period, on these now treeless plains, of 

 forests of almost subtropical character and luxuriance. Dr. Newberry, the 

 celebrated geologist and botanist, has already described more than fifty 

 sj>ecies of plants from this group, which were collected on the Yellow- 

 stone and Missouri Eivers, many of them indicating forests of huge 

 growth. Among them are not less than eight species of Populus, (pop- 

 lars,) four species of Platanus, a sycamore, and a species of fan palm, 

 the leaves of which must have had a spread of nearly twelve feet. The 

 very interesting remarks of Dr. Newberry in this connection will be read 

 with pleasure and instruction by every student of geology : 



Tiiese fossils are generally well preserved in a calcareo-argillaceons rock of a light- 

 drab color, upon which the leaves are delineated with a distinctness that renders them 

 pleasant objects of study, as well as attractive specimens for the cabinet. They are 

 usually detached with their petioles in such numbers and forms as indicate maturity, 

 and a common cause of fall, such as an annual frost. The mollusks associated with 

 them show that they were deposited in the sediment which accumulated at the bottom 

 of some fresh- water stream or lake, and they are generally spread out so smoothly and 

 so entire, that it is evident no violence, not even the action of a rapid current, could 

 have been attendant upon their deposition. The sediment which inclosed them was 

 usually very fine ; a fact also indicative of a tranquil state of the water in which they 

 were suspended. 



The explorations of Dr. Hayden prove that this miocene lignite formation occupies 

 the beds of extensive lakes, which filled deep basins on the surface of the continent 

 when it had but recently emerged from the cretaceous sea. As has been remarked 

 elsewhere, the lower members of the series contain a few estuary shells, showing the 

 access of salt water at the period of their formation ; but during the deposition of by far 

 the greater portion of these beds, the water of the ocean was entirely excluded from 



