434 GEOLOGICAL SURVEY OF THE TERRITORIES. 



all cases, originated at some single locality, and thence distributed itself 

 over the world wherever we now find its remains, and admitting that a 

 part of these little shells, at the two localities, are really exactly iden- 

 tical, specifically, it may be asked, were these species created some- 

 where in the bottom of the old Carboniferous sea, at or near the Indiana 

 locality, from which they migrated 1,200 to 1,500 miles to the northwest- 

 ward ; or, was the reverse the direction of their distribution I Such ques- 

 tions are, of course, far more easily asked than answered ; but if we do 

 not admit that there were distinct centers of distribution for species, it 

 would seeui more probable that these forms originated at some interme- 

 diate ijoint, (where the strata, containing their remains, are now beneath 

 thousands of feet of more modern rocks,) and migrated thence southeast- 

 ward and northwestward to the two remotely separated localities. 



Professor Edward Forbes maintained that, to find the same group of 

 fossils at two distantly separated localities, as in this case, so far from 

 proving that the rocks are exactly contemporaneous, as often supposed, 

 demonstrates exactly the reverse ; or, in other words, that time enough 

 must have intervened between the deposition of the strata at the two 

 localities, for the migration of the species of fossils through all the 

 intermediate distance. However this may be, there is little reason to 

 doubt that in such cases the rocks occupy very nearly the same relative 

 horizons in the series of their respective, districts, whatever may be the 

 diftereuces between their actual ages. Hence, I regard the bed from 

 which these little fossils were obtained in Montana as representing the 

 Saint Louis limestone of the Lower Carboniferous series of the Missis- 

 sippi Valley, to which horizon the Spergeu Hill beds are known to 

 belong. It is also evident that the physical conditions affecting animal 

 life must have been very similar at these localities during the deposi- 

 tion of the strata in which these fossils occur, although, lithologicall}', 

 the rocks are quite different, that in Montana being a bluish-gray, some- 

 what crumbling semi-crystalline limestone; while that at Spergen Hill 

 is a light-colored oolite mass. 



" Old i?flW7/."— The collections from " Old Baldy," near Virginia City, 

 Montana, present a group of forms that could only, in our present state 

 of knowledge, be referred to the Lower Carboniferous series. The 

 presence among them, however, of such coal-measure types as Athyris 

 .suhtilita, Pknrofomaria sphcerulafa, Astartella Netcherryi, &c., with the 

 aflinities of other species, point to a high position in this lower series; 

 while the occurrence in the same association of Pentremites Godoni and 



.P. symmeiricus, or very closely allied representative forms, together 

 with the aflinities of other types, show that their position is that of the 



' Chester beds (or possibly of the Saint Louis limestone) of the Mississippi 

 Valley. 



A single species, however, Stropliomena analoga, Phillips, from the 

 same locality, would indicate that lower members of the series probably 

 also exist there, as this species, if I remember correctly, is not known 

 to occur above the horizon of the Burlington beds, in the Carboniferous 

 series of the Mississippi Valley. There is thickness enough, however, 

 of Carboniferous strata, at these distant northwestern localities, for this 

 whole system of rocks to be de\ eloped there. 



The collections from this and some of the other localities in Montana 

 contain the first specimens of the genus Pentremites I have ever seen 

 from any localities west of Missouri and Iowa. 



JURASSIC AGE. 



Lower Canon of Yelloicstone. — The collections from the Lower Canon 

 of the Yellowstone, the lower beds at Spring Canon, and those from the 



