BRIDGEE EPOCH. 11 



liarities. It is nearly identical with that of the Suessonian of Western 

 Europe, which is at the base of the Eocene series. The fullest account of 

 it is that which I have given in the Report of Captain Wheeler of Explora- 

 tions and Surveys West of the 100th Meridian, vol. IV. 



THE BRIDGER. 



This is one of the more important of the groups among those that, in Western 

 Iforth America, are referred to the Tertiary i^eriod, especially as regards the vertebrate 

 remains that have been obtained from its strata. It is most fully and characteristicaUy 

 developed in the region known as the Green River basia, north of the Uinta Mount- 

 ains, only the southeastern portion of the formation, so far as it is now known, extend- 

 ing into Northwestern Colorado. In its typical localities it is found resting conform- 

 ably upon the Green Eiver Group, into which it passes without a distinct plane of 

 demarkation among the strata. 



Irs molluscan fossil remains correspond closely with those of the Green River 

 Group, some of the species being common to both, all indicating a purely fresh con- 

 dition of the waters in which the strata of both groups were deposited. At the typical 

 localities the group is composed in great part of soft, variegated, b^d-laud sandstones, 

 a peculiar greenish color often predominating over the others, which are reddish, pur- 

 ple, bluish, and gray. Limestone strata, marly and clayey beds, and cherty layers are 

 not uncommon, and grits and gravelly layers sometimes occur. 



To the above general remarks of Dr. C. A. White I add, that the ma- 

 terial of this formation consists of indurated clays more or less arenaceous, 

 which display various degrees of hardness. The harder beds are, however, 

 thin, and the intervening strata yield readily to meteoric influences. They 

 are frequently quite arenaceous, and rather thin beds of conglomerate are 

 not uncommon. The colors that predominate are greenish-gi-ay and 

 brownish-green, with frequent ash-colored beds. The peculiar condition 

 of hardness of most of the strata render it one of the formations which 

 most generally present the bad-land scenery ; it permits the erosive action 

 of the elements without general breaking down, great numbers of frag- 

 ments of the strata remaining in spaces between the lines of erosive action. 

 The result is the extraordinary scenery of Black's Fork, Church Buttes, 

 and Mammoth Buttes, of which mention will be made in the section of this 

 volume especially treating of the Bridger foimation. 



The distribution of the Bridger formation is limited, and is, so far as I 

 am aware, restricted to three areas, whose mutual connection is as yet un- 

 certain. Its principal mass is in the Bridger basin, which extends from the 



