460 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



it would, when viewed in connection with its associates, and all the 

 other known facts, furnish a strong argument in favor of the formation 

 feeing at least as old as the Cretaceous. There are good reasons, how- 

 ever, for believing these depressed specimens, as well as the convex 

 ones, are all upper valves of the same shell, only modified in convexity 

 by accidental circumstances of station, as their slight obliquity as seen, 

 for instance, in a look at the interior of both, is found to be in the same 

 direction, instead of the reverse, as would be the case if they were oppo- 

 site valves of the same shell ; while among thousands of specimens no 

 example of a depressed and a convex valve united has been seen, nor 

 have any been found that would come near fitting together. 



On the other hand, the Corbiculas are decidedly Tertiary in their 

 specific afBnities, as well as in their subgeneric ; G. fracta^ for instance, 

 and G. crassatelliformis, from the Hallville mines, being very closely 

 allied to Paris Basin Tertiary forms, the first-mentioned species being 

 the type of a sub-genus, so far as known, peculiar to the Tertiary else- 

 where. The same may also be said of G. cytheriformis, which also seems 

 to belong to a group ( Veloritina) peculiar to the Tertiary in Europe. 



But the most suri)rising fact to me, supposing this to be a Cretaceous 

 formation, is, that we found directly associated with the reptilian remains 

 at Black Butte, a shell I cannot distinguish from Viviparus trocM/ormiSj 

 originally described from the Lignitic formation at Fort Clark, on the 

 Upper Missouri, a formation that has always been regarded as Tertiary 

 by all who have studied its fossils, both animal and vegetable. The 

 specimen mentioned does not show the aperture, nor all of the body 

 volution ; but, as far as can be seen, it agrees so exactly with that very 

 peculiar species in size, the form and proportions of its volutions, the 

 slopes of its spires, its surface markings, the nature of its suture, and, 

 in fact, in every respect so far as can be seen, that I have scarcely any 

 doubt of its identity with the same. 



The occurrence of this last-mentioned species here, along with a Cre- 

 taceous type of reptilian, and a GorMcula apparently identical with G. 

 cytheriformis of the Judith Kiver brackish-water beds, together with the 

 presence of Corbulas very closely allied to Judith Eiver species, at lower 

 horizons in this series, and the occurrence of some vertebrates of Cre- 

 taceous aftinities at the Judith Eiver localities, would certainly strongly 

 favor the conclusion, not only that this Judith formation, the age of 

 which has so long been in doubt, is also Cretaceous; but that even the 

 higher fresh- water lignite formation at Fort Clark and other Upper 

 Missouri localities may also be Upper Cretaceous instead of Lower Ter- 

 tiary. 



That the Judith Eiver beds may be Cretaceous, I am, in the light of 

 all now known of the geology of this great internal region of the con- 

 tinent, rather inclined to believe. But it would take very strong evi- 

 dence to convince me that the higher fresh-water lignite series of the 

 Upper Missouri is more ancient than the Lower Eocene. That they are 

 not is certainly strongly indicated, not only by the modern affinities of 

 their moliuscan remains, but also by the state of the preservation of the 

 latter. Indeed, these shells [Plcmorhis, Viviparus, Goniohasis, Fliysa, 

 &c.) are found loose, as they fall from the incoherent sand in great 

 numbers, so entirely free from adhering matrix, either internal or exter- 

 nal, and so little changed, that any one not familiar with the existing 

 species of the country would naturally think them merely dead shells 

 of the same, picked up along the shores of the streams, l^e entire 

 flora of this Upper Missouri lignite group has also always been consid- 



