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beds, in which are found, in some localities, in seams varying from a few 

 inches to several feet in thickness, made up almost entirely of different 

 species of Ostrea, and mingled with them are a few well-known brackish- 

 water forms. All the forms of Mollusoa akin to those known in this 

 group flourish in the brackish waters of our day. 



Passing upwards, we gradually come to a great thickness of strata in 

 which fresh- water Mollusks predominate. A careful study of the entire 

 Upper Lignitic group from the bottom of the first division to the summit 

 of the third, extended over the entire area of the West, would doubtless 

 show no abrupt physical break in the series of molluscous life, but 

 the marine and brackish- water forms gradually drop out of existence as 

 the fresh waters slowly predominate, until only fresh- water forms are 

 found. From the first appearance of the vegetable life, many forms 

 appear which have a very extended vertical as well as horizontal range 

 geographically. Now the question would arise as to where the line should 

 be drawn between the Cretaceous and Tertiary formations. The strati- 

 grapher may have much to do with this ; for, if done at all, it must be 

 drawn mostly on his testimony. That there may be faults or breaks in 

 the series is quite possible, inasmuch as there must have been continued 

 alternations or oscillations of land and water throughout 8,000 or 10,000 

 feet of strata, and, therefore, would even a slight un conform ability be 

 sufficient to decide the line of demarkation 1 It may be said that there 

 are brackish- water strata in formations of all ages; but it must be recol- 

 lected that the brackish- water strata in the Lignitic group are not restricted 

 to a limited locality, but are as extensive as the formation itself. They 

 extend from British America to New Mexico, and it is only in groups 

 of Tertiary age that we have such widely-extended fresh-water depos- 

 its. I must insist, therefore, that my series of transition, or beds 

 of passage, be acknowledged as having a real existence until some 

 evidence is presented to the contrary. In all portions of the divisions 

 mentioned above, we find, 1st, a gradual dropping-out of the strictly 

 marine ; 2d, the same gradual disappearance of the brackish- water forms. 

 The third or fresh- water division will doubtless be regarded by all geol- 

 ogists as of Tertiary age. 



In reply to the statement that the term "Lignitic" is not applicable to 

 any one group, from the fact that coal is found in all the formations 

 from the Jurassic to the summit of the Tertiary, I will say that I am well 

 aware of that fact. In the Annual Report of the Survey for 1870, page 

 59, I mention the occurrence of several thin seams of lignite as occur- 

 ring in the Green Eiver group, on Henry's Fork. Great quantities ot 

 fresh-water shells were also found in close proximity. In the Washa- 

 kie group, which extends along the railroad from near Separation to 

 Bitter Creek, forming the divide of the continent, there are several beds 

 of impure lignite. Many fresh- water shells occur in this group, similar or 

 identical with those found at the base of the Green Eiver group near the 

 mouth of Henry's Fork. I have no doubt, therefore, that the Washa- 

 kie group extends on westward across the anticlinal, re-appearing at 

 Green River near the mouth of Henry's Fork from beneath the Green 

 Eiver group. 



In the summer of 1870, 1 traced with considerable care the vast thick- 

 ness of strata which are revealed from the junction of Henry's Fork 

 northward along the west bank of Green Eiver, from the Cretaceous 

 through the Lignitic group, and the western extension of the Washakie 

 group up to the Green Eiver shales, and in the report of that year a 

 full account is given. Up to this time I do not find occasion to modify, 

 to any great extent, the opinions therein expressed. 

 No. 5 12 



