Geology. 



427 



while the White River basin is more argillaceous, appears to be destitute 

 of lignite, and is well known to be one of the most remarkable reposito- 

 ries of extinct mammalian remains on the face of the globe. In addi- 

 tion to this, not one of the species of Mollusca in our collection from the 

 Lignite formations, is identical with any of those described by Dr. Evans 

 and Dr. Shumard from the "White River basin. 



Formations immediately beneath the Tertiary in this district. — It would 

 seem that the change of physical conditions which closed the Cretaceous 

 epoch and ushered in the Tertiary, in this part of the world at least, was 

 gradual, not violent. We find that even while the Cretaceous conditions 

 still existed, (during the deposition of No. 5 of the series*) the approach- 

 ing close of that state of things, and the coming of the Tertiary era, 

 were foreshadowed by the introduction of Fasciolaria, Pleurotoma, and 

 Belemnitella, with many shells of other genera, quite as near in their 

 specific affinities to Tertiary as to Cretaceous forms ; while the sea was 

 gradually becoming more shallow, as is shown by the increase of Gastero- 

 poda. We even know from the presence of a few remains of Lycopodia- 

 ceous plants, and an occasional unbroken leaf of some Exogenous tree, that 

 there was dry land at this time somewhere not very far away. Gradually, 

 as we ascend in the series, the strictly marine animals disappear, and we 

 meet with Ostrea, Corbula, and Cerithium, mingled in the same bed with 

 Melania, Paludina, Physa, Cyrena, <fcc, all of tertiary types ; while a 

 little higher in the series we find at some places only the remains of land 

 and fresh-water Mollusca. 



From the above facts, especially the presence of Pleurotoma, Fasciola- 

 ria, and Belemnitella, in this upper member of the Cretaceous system of 

 this country, we cannot think it represents any part of the Green Sand of 

 English geologists. Numerous well marked Cretaceous forms show it can- 

 not be Tertiary, consequently we think it must represent some portion of 

 the true chalk. We are by no means inclined, however, to adopt the 

 views of M. Alcide D'Orbigny, who regards all the Cretaceous formations 

 of the United States and Western Territories as referable to a later epoch 

 than the Green Sand, as the next succeeding formation below that of which 

 we have just been speaking, (No. 4 of the series), is characterized by nu- 

 merous fossils of unquestionable Green Sand type. We think confusion 

 has been created in tracing out the parallelism between American and 

 European Cretaceous formations, by fossils from different positions in this 

 country having been mingled together and described as if they occurred 

 in the same bed. 



Formations at the base of the Cretaceous Series of this district. — As 

 previously stated, near the mouth of Milk river, Cretaceous strata which 

 are not seen for a long distance below this on the Missouri, again rise to 

 view. They consist of the upper two members of the series (No. 5 and 

 No. 4) which, in consequence of their inclination to the east, are found to 

 rise higher and higher as we ascend the river, so that nearly all the hills 

 close to the Missouri, between Milk and Muscleshell rivers, consist of these 

 formations. Some four or five miles below the mouth of Muscleshell 



* For a section of the rocks of this country, see a paper by James Hall and F. B. 

 Meek in the Memoirs Am. Acad. Arts and Sci., vol. 5, New Series. Likewise a paper 

 by F. B. Meek and F. V. Hay den in Proceed. Acad. Nat. Sci. Philad., March, 1856. 



