GEOLOGICAL SURVEY OF THE TEREITOEIES. 349 



of the Paris basin has also some rich beds of lignite. Does not this read 

 like a true epitome of the descriptions given of ourEocene"? 



This brings forward again what I consider the last unanswered ques- 

 tion in relation to the distribution of the American Eocene. Its base is 

 everywhere ascertained as immediately resting upon the Upper Creta- 

 ceous J the lower sandstone is recognized as either a massive homoge- 

 neous compound or as interlaid at different places by beds of lignite 

 or of shale. The fossil flora, with some difference, has the same charac- 

 ters in the strata connected with these lignite-beds, at all the stations. 

 The group is therefore satisfactorily limited so far, but where does it 

 pass to a higher division of the Tertiary or to the Miocene ? I have 

 already remarked that I consider the conglomerate formation seen at 

 Evanston and other localities as the upper beds of the Eocene. But I 

 have not myself found any positive proof of this assertion, and as these 

 conglomerates have been referred to diflereut groups according to the 

 strata which they appear to cover, the assertion is contestable. The 

 observations, however, of Dr. Hayden, who, after years of careful field 

 explorations, has become the true interpreter of the geology of the 

 Eocky Mountains, will supply this last evidence. In beginning his 

 description of the Green Eivcr Group, and in marking its superposi- 

 tion to the Eocene sandstone, he says :* 



This iutertsting valley (Henry's Fork) is filled with beds which show a perfect con- 

 formity. The first bed is a yellow-brown, rather fine-grained sandstone, dipping 75° 

 a little west of north. Then comes a series of yellow and light-gray arenaceous or 

 marly clays, with beds of yellow-brown and light-gray sandstones projecting somewhat 

 above the surface. Alternating with these layers of sandstone are quite thick beds of 

 pudding-stone and conglomerate composed of round pebbles of all the older forma- 

 tions. These conglomerate beds are intercalated among the sandstone through 300 or 

 400 feet in thickness, and are probably of Upper Eocene a-ge. Above them are at least 

 500 feet of sandstone which have a diminished dip 20° to 30°, and then pass up into 

 the calcareous layers of the Middle Tertiary of Green River group. 



The relative position of the conglomerate as underlying the Green 

 River Group is thus positively ascertained. Comparing this with what 

 has been described and marked in the sections of Evanston, Cheyenne, 

 Gehruug's in Colorado, the lignite basin of the Arkansas Valley near 

 Caiion City, the Santa Ee marls, the Gallisteo group, &c., such remark- 

 able analogy is seen in the composition and geological distribution of 

 these conglomerates that the unity and contemporaneity of the formation 

 becomes evident. The upper part of the section of Evanston is a coun- 

 terpart of that of the conglomerate-beds, described above by Dr. Hay- 

 den. It is, indeed, reduced in thickness, as also in the size of the 

 materials entered into its compound. But, as remarked already, this 

 recluction is everywhere relative to the distance of the older rocks which 

 have furnished the materials. And it ought to be so ; for the formation 

 is a kind of drift, spread over a wide area by water or by glacial agency, 

 and of course the coarse and heaviest materials are found nearer their 

 point of origin. It has been, but it cannot be, considered as a recent 

 drift. I have seen no trace of recent glacial agency on this side of the 

 Eocky Mountains, where, indeed, moraines, or heaps of materials trans- 

 ported by glaciers, would be, I think, a kind of anomaly. The glaciers 

 have, like the peat-bogs, a development relative to atmospheric 

 humidity. It is not the rain which increases the density of the snow, 

 transforms it into nev6 and then into ice, but the fogs. Therefore, the 

 eastern slopes of the Eocky Mountains have snow in their high, deep 



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* F. V. Hayden's Geological Report, 1670, p. 69. 



