494 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



evidence for believing to have been the case, but no cataclasmic interposi- 

 tion of barriers is necessary to account for the individual course of develop- 

 ment which the fauna seems to have pursued in the Rocky Mountain region 

 and in what is now the Mississippi Valley. 



The approximate uniformity of the Madison limestone is evidence of a 

 protracted period of quiet and uniform physical conditions, while the rapid 

 alternation of sediments in the typical Mississippian would seem to indicate 

 frequent and important changes in environment which find equal expression 

 in the varied faunas that have resulted. The wide separation of the two 

 regions geographically would readily permit of their independent develop- 

 ment, both faunally and physiographically. 



Still, the conclusion is not warranted that the Madison limestone repre- 

 sents the entire period of Mississippian deposition. In Montana, at "Old 

 Baldy," near Virginia City, Meek has reported fossils of the Chester group, 

 with a probability of the lower formations being present; and from near 

 Fort Hall, between Ross Fork and Lincoln Valley, in Idaho, the same 

 authority has identified many species characteristic of the oolitic beds of 

 the St. Louis group at Spergen Hill, Indiana, 1 



These localities are both comparatively near the Yellowstone National 

 Park region, and it is hard to believe that if the Madison limestone was 

 contemporaneous in part with either of these periods the fauna should not 

 clearly indicate it. Indeed, Meek, commenting upon certain collections 

 made at various localities in Montana not far from the Yellowstone Park, 

 and from a horizon which I believe to be equivalent to the Madison lime- 

 stone (see ante, p. 487), makes the following statement: "At the same time 

 that I would refer the beds from which these fossils were obtained to the 

 Carboniferous, it should be remarked that we have every reason to believe 

 that they belong to a lower horizon in the series than those from which 

 nearly all the collections from 'Old Baldy' Mountain were obtained; also, 

 than the fossiliferous beds on the divide between Ross Fork and Lincoln 

 Valley, Montana." (Loc. cit, p. 433.) 



We may therefore conclude that the Madison limestone does not proba- 

 bly represent the period of the Genevieve group, but, while showing distinct 

 affinities with the Kinderhook, may have persisted through the period of 

 the Osage group as well. 



'Hayden, Sixth Arm. Kept. 0. S. Geol. Surv. Territories, etc., for 1872, 1873, pp. 433, 434. 



