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PROCEEDINGS OF THE ACADEMY OF 



is extensively developed in New Mexico ; though it appears there to be generally 

 inseparably blended with the Niobrara Division, the two forming together the 

 middle division of his section of the Cretaceous of New Mexico, which attains a 

 thickness of from twelve to fifteen hundred feet. Dr. Shumard has also identi- 

 fied it in Texas, where it is apparently quite thin.* 



All the facts show that this rock thins out, both in the south, and at the 

 north, in an easterly direction, its greatest thickness being at Fort Benton, and 

 along the mountains south of there, and in New Mexico ; while on the Missouri, 

 between James and Big Sioux Rivers, it is only about one hundred feet in thick- 

 cess, and Dr. Shumard gives fifty feet as its thickness in his Texas section. 



This formation contains a number of interesting organic remains, some of 

 which are known to have a wide geographical range, and, as may be seen by 

 the foregoing section, also pass up into the succeeding rock above. We also 

 have reason to believe that several of them likewise occur further south, in the 

 formation below, thus apparently linking together, as already suggested, these 

 three rocks as subordinate members of one great series. At any rate, the fos- 

 sils described by Dr. Shumard from the " Marly Clay, or Red River Group" of 

 his Texas section, which we think he has correctly placed on a parallel with 

 our Dakota Group (=No. 1 of former sections), are both individually, and as a 

 group, apparently very closely allied to forms occurring in the formation under 

 consideration, in Nebraska. For instance, his Inoceramus eapulus is scarcely 

 distinguishable, as he has suggested, from our /. umbonatus ; and we think it 

 probable his Ammonites Graysonensis is not distinct from A. percarinatus, Hall & 

 Meek. Again, his Scaphites vermiculus is allied to our S. larvceformis. 



Niobrara Division. The typical localities of this rock are along the Mis- 

 souri, near the mouth of Niobrara river, where it forms perpendicular cliffs 

 from ninety to one hundred feet in height. In this region it consists mainly of 

 lead gray richly calcareous marl, which, where long exposed, assumes a light 

 buff or whitish color, and presents much the appearance of true chalk. Below, 

 it passes into more compact beds of soft bluish gray limestone. It is first seen 

 in descending the Missouri, a short distance below the Great Bend, where it 

 rises by a gentle dip from beneath the succeeding formation (the Fort Pierre 

 Group). Further down the river it is seen to rise higher and higher, and gra- 

 dually assumes the character of a surface rock, not far below the mouth of Nio- 

 brara river. When much exposed to the action of the weather, here and on Little 

 Blue river, near the northern boundary of Kansas, it becomes a rather hard 

 whitish limestone. 



This formation can be traced by exposures in north-eastern Kansas, near 

 Little Blue and Smoky Hill rivers, through Arkansas into Texas and New 

 Mexico. In 1857 we pointed out that it is represented by the upper beds of 

 Mr. Marcou's Pyramid Mountain Section; and Dr. Shumard has placed the 

 " Washita Limestone," and " Indurated Gray Marl" of his Texas section, on a 

 parallel with it and two of the same beds in Mr. Marcou's section. 



At the Black Hills this rock sometimes presents its normal appearance, but 

 generally there, as well as along the Rocky Mountains further west, it is scarce- 

 ly distinguishable lithologically from the formation below. The fossils hither- 

 to found in it in Nebraska, are Ostrea congesta, Inoceramus problematicus, I. avi- 

 culoides, and a small Baculite, together with large scales of fishes. All excepting 

 the fish scales being identical with species found in the strata beneath. In- 

 oeeramus problematicus, or at least a form scarcely distinguishable from that 

 species, and Ostrea congesta, occur in it almost every where that it has been met 

 with. 



In Texas, Dr. Shumard found in the bed he places on a parallel with this 

 formation, Ilolaster simplex, Epiaster elegans, Cidaris hemigranosus, Gryphcea 

 Pilcheri (common variety and G. Tucumcarii) G. sinuata, Marcou (notSowerby), 



* Transactions St. Louis Academy Sciences, vol. 1, p. 583. 



[Dec. 



