54 GEOLOGICAL SURVEY OF THE TEREITORIES. 



NIOBRARA DIVISION. 



In many respects tbis is the most interesting and most valuable group 

 of the Cretaceous rocks in the West. Its principal character is a gray or 

 light-yellow chalky limestone ; much of it is so pure as to make good 

 chalk for commercial purposes. 



It would also be useful, doubtless, as a fertilizer. In ascending the 

 Missouri it is first seen in thin outliers on Pilgrim's Hill, a portion of 

 the Omaha reserve. It then grows gradually thicker as we ascend, and 

 south of Dakota City, in the hills, it becomes ten to twenty feet thick. 

 At Ponka City, Saint Helena, and mouth of the Niobrara it is exposed 

 fifty to two hundred feet in thickness, exhibiting a great variety of 

 color and texture. 



All along the Missouri this rock is much used for the construction of 

 buildings with success. The fact that so large an area of country exists 

 below the first appearance of this formation destitute of any rock for 

 lime must render this group of much economical importance to the 

 settlers. Its soft, yielding nature gives rise to long ranges of precipitous 

 bluffs along the river. 



It is easily cut into innumerable ravines by the temporary streams, 

 and these bluffs often present the appearance of a series of cones. 



This formation extends up the river to the foot of the Great Bend, 

 ■where it passes beneath the water-level. The fossils in this group are 

 few in the number of species, but the individuals are abundant. Layers 

 of considerable thickness are mostly composed of the shells of Inocera- 

 mus prohlematicus and Ostrea congesta. 



Fish-remains of great perfection and beauty also are found. Only a 

 fei/v good specimens have ever been taken from the rock; but the 

 myriads of fragments, as bones, scales, and fins, show that they existed 

 in great abundance in the Cretaceous seas. The connection of this group 

 ■with the Fort Benton group below is quite plain, there being no line of 

 demarkation in most localities. At Saint Helena, however, the trans- 

 ition is abrupt, passing directly from the black plastic clays of the one 

 to the yellow chalk ot the other. 



This fact seems to me to show clearly that the grouping of these 

 formations in the manner already done is correct. 



Between the Dakota group and the group above there are transition 

 rocks at different places which obliterate any abrupt break, while at 

 other localities the break is evident. 



All our investigations show more and more clearly that in the Creta- 

 ceous series of the West there are three divisions paleontologically and 

 five groups lithologically. 



The Niobrara division undoubtedly extends all along the mountain 

 elevations; but it seems to possess an intermediate character between 

 Nos. 2 and 3, as seen on the Missouri Uiver, so that it is difficult to 

 decide to which the rocks belong, the Ostrea congesta being common to 

 both. This formation, like the Dakota group, extends across the 

 country, in the form of a belt or zone, southeast and northwest. 



It is found extending north high up the Big Sioux, Vermillion, and 

 James Eivers, in Dakota Territory, and southward into Kansas and 

 New Mexico. 



FORT PIERRE GROUP. 



This formation is most largely developed from the Great Bend to a 

 13oint 200 miles above Fort Pierre. It begins to make its appearance on 



