482 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



of Soda Butte Creek, northeast of Abiathar Peak, Absaroka Range, is unique 

 in the collection, and may fitly be called a gastropod limestone. The rock 

 consists of comminuted organic remains cemented by a shaly limestone, and 

 carrying certain small gastropods in great abundance. Two of the types 

 represented are described below, under the names of Platystoma niinutiim 

 and Loxonema delicatum, but the others, of which there are perhaps three or 

 four genera and some half dozen species, are too poor for either identification 

 or description. They are small, and nearly all badly eroded, and often 

 concealed by a matrix from which they are not easily separated. It has 

 therefore proved difficult or impossible to acquire data even for certain 

 generic references ; but Loxonema, Pleurotomaria, Murchisonia, Platystoma, 

 and perhaps other genera are probably present. The associated brachiopods 

 show this fauna to be of Middle Devonian age. 



It can be affirmed beyond question that all the localities here discussed 

 occur below the Carboniferous. Indeed, there is a decided faunal break 

 between them and the great series of beds regarded as representing the 

 base of that formation, so much so that the two groups have not a single 

 species, and scarcely even a single genus, in common. At the same time, 

 I believe that the lower series is neither wholly nor in part Silurian, but 

 that it was in fact laid down in Lower or Middle Devonian time, repre- 

 senting the Hamilton, or perhaps the Upper Helderberg, of the New York 

 system. Many of the generic identifications strongly suggest a Devonian 

 facies, or, at worst, are ambiguous, while the specific references, though 

 often doubtful, all point to the Devonian rather than the Silurian age of 

 these strata, Finally, the fauna seems to be rather closely related to that 

 of the White Pine district, etc., above referred to, of which Meek says 

 (loc. cit,, p. 6): "Hence we can not doubt that these beds belong to the 

 Devonian, and probably to about the horizon of the Hamilton group of 

 the New York series." 



