424 PROCEEDINGS OF THE ACADEMY OP 



higher position in the Cretaceous of the Old World. The most important ex- 

 ception to this rule we have met with is Inoeeramus umbonatus, which is evi- 

 dently closely allied to /. involutus of Sowerby, from the Upper or White Chalk. 

 There still remains, however, some doubts iu regard to the position of this 

 fossil in the Nebraska formations, no specimens of it having yet been col- 

 lected in situ by any person familiar with the order of succession of the Nebraska 

 rocks. 



Upper Cretaceous Series of Nebraska. 



Fort Pierre Group. In the foregoing review of the Lower Series of Ne- 

 braska CreUceons rocks, it has been shown that the subdivisions often shade 

 into each other lithologically, and that at least the upper two members, are so 

 intimately related by the range and affinities of their organic remains as to 

 indicate that they are merely subordinate formations of one great series, or 

 primary division of the Cretaceous system. There seems to be no horizon at 

 which we can, upoa pahtontological principles, draw a sharp line separating 

 the beds below from those above, until we ascend to the top of the Niobrara 

 Division. In crossing this line, however, we not only generally observe a 

 marked lithological change, but, so far as our present knowledge extends, we 

 meet with the remains of an entirely different group of animals. It is true, 

 nearly or quite all the genera seen in the beds below, are still observed, along 

 with many others in the strata above this horizon ; but the species yet found 

 in these upper formations, in Nebraska at least, are all distinct from those 

 hitherto found in the lower series. 



We are aware that further investigations may possibly show closer relations 

 between the organic remains of the upper and lower series of these rocks in 

 this region than have been hitherto discovered. Yet, when it is borne in mind 

 that our conclusions are based upon a careful study and comparison of exten- 

 sive collections from numerous localities, scattered over a great area of country, 

 it will be readily understood how very improbable it is that future explorations 

 will materially modify these results. 



At the base of the Fort Pierre group, the inferior member of the upper 

 series of Nebraska Cretaceous rocks, there is, at some localities along the 

 Missouri below the Great Bend, a local bed ten to thirty feet in thickness, com- 

 posed of very dark unctuous clay, containing great numbers of small scales of 

 fishes, much iron pyrites and carbonaceous matter, with crystals, veins and 

 seams of sulphate of lime. This bed usually occupies depressions in the pre- 

 viously eroded upper surface of the formation beneath. 



With the exception of the local deposit just mentioned, the Fort Pierre Group 

 consists of a vast accumulation of fine gray and dark colored clays in mode- 

 rately distinct layers, but never presents a laminated or slaty structure like 

 the Fort Ben r on Group. When wet, these clays are soft and plastic, but in 

 drying they often crack and crumble so as to obliterate the marks of deposi- 

 tion in vertical exposures. 



This formation composes all the hills on both sides of the Missouri at Fort 

 Pierre, and much of the country between there and the Bad Lands. It also 

 forms the bluffs along the river below Fort Pierre to the Great Bend, just be- 

 low which the Niobrara Division rises above the level of the Missouri. From 

 the Great Bend down to the mouth of Niobrara River, the country is made up 

 of these two formations, which rise with the general inclination of all the rocks 

 in this region, so that the Fort Pierre Group finally runs out in the form of out- 

 liers on the tops of the hills below the mouth of Niobrara River. Above Fort 

 Pierre, it gradually sinks beneath the Fox Hills Group in the region of Chey- 

 enne and Moreau Rivers, but continues to be seen in the bluffs of the Missouri 

 and other streams for some distance beyond. 



On the Upper Missouri, this formation comes to the surface again, and forms 

 much of the country in the region of Milk and Muscle tehell Rivers. It is also 

 known to extend far up into the British possessions, some of its fossils having 



[Dee. 



