NATURAL SCIENCES OP PHILADELPHIA. 123 



and on the Moreau, the entire bed is exposed, and attains a thickness of about 

 thirty feet. At these latter localities it is seen to repose directly upon No. 5, or 

 the most recent member of the Cretaceous system of the north-west, the upper 

 part of which it so nearly resembles in iis lithological characters that ihe line 

 of demarkation betweeu the two can often be only ascertained by the organic 

 remains characterizing: each. This fact, together with the general resemblance 

 of many of the fossils found in the upper part of No. 5 of our general section to 

 Tertiary types, would have caused us to doubt the propriety of referring this 

 part of that bed to the Cretaceous epoch, were it not for the presence of Scaj)h- 

 ites Conradi, and other well marked Cretaceous forms. 



From the estuary character of the fossils found in the bed Q of the last sec- 

 tion, {Ci/rena, Oslrca, Melania, &c.,) and the absence of Cretaceous forms, we 

 think there can be little room to question the propriety of referring it to the 

 Tertiary system. The question, however, in regard to its exact position in that 

 system may yet be regarded as a little doubtful, but the weight of evidence is 

 on the side of the conclusion tliat it is Miocene. This conclusion is also borne , 

 out or strengthened by the general relations at other localities between this 

 bed and those above, in which Miocene fossils occur. 



The preceding section, although not representing all the beds of the great 

 Lignite basin, (there being some higher beds seen at other localities,) gives a 

 correct idea of the composition and general structure of these deposits,* and 

 shows how remarkably they differ in their lithological characters from those 

 of the White River basin, in which fine white argillaceous material so largely 

 predominates, and no lignite or remains of plants have been found. 



Of the age of the fresh-water and Estuary deposits seen near the mouth of 

 Judiih River, we have refrained from the expression of any very decided opinion, 

 in consequence of the fact that they appear to be mingled by upheavals with a 

 series of sandstones, clays, lignite, &c., which we think belong to the same 

 epoch as No. 1 of our general section. Amongst some of the collections inves- 

 tigated by us from these Judith River estuary and fresh-water beds, and pub- 

 lished in one of our former papers, we found species of Faludina, Corbula, &c., 

 so closely related to shells we had from the Great Lignite basin near Fort Clark, 

 that we were for a time in some doubt about the propriety of considering them 

 distinct. In this upper member of the Judith River formations, great numbers 

 of a small oyster occur along with the fresh-water shells of which we have just 

 spoken. This oyster so nearly resembles a species in our possession from the low- 

 est bed of the last section, obtained at a locality near Grand River, that we have 

 not yet seen any characters by which they can be distinguished. We are also 

 inclined to reg.ird these shells as identical with a sjjecies described by Drs. 

 Evans and Shumard from Grand River, under the name of 0. subtriyonalis. 

 There is also, in the fresh-water beds at the Judith localities, a Unio, which we 

 think the same as a shell found in the Miocene lignite formations on the Yellow 

 Stone. In addition to these facts, a species of Trionyx described by Dr. Leidy, 

 from about the middle bed of the Judith fresh-water deposits, is regarded by 

 that gentleman as identical with specimens from the bed Q, near Long Lake 

 below Fort Clark. 



Now as this bed Q, of the previous section at the Moreau, Grand River, and 

 Long Lake localities, beyond all doubt, rests directly upon formation No. 5 of 

 the general section, and forms a part of the Great Lignite series, we would not 

 for a moment hesitate in regarding the deposits of which we have spoken, near 

 the Judith, as Tertiary, were it not for the fact that the saurian and fish remains 

 occurring in these beds, as may be seen by reference to Prof. Leidy's remai-ks 

 respecting them, are allied to Wealden and older types. 



In the midst of evidence of such a conflicting nature, it is of course unsafe to 

 express any very positive opinion respecting the age of these formations. At 



*It should be borne in mind, however, that the beds of lignite represented in this 

 section vary much in extent, as well . s in purity, at different localilies; some of those 

 seen on the Yellow Stone being as much as seven feet in thickness. At Fort Berihold. 

 on the Missouri, a two foot bed is pure enough to be used as fuel. 



1857.] 



