48 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



t 

 filled with beautiful crystallized suljihuret of iron. It is pretty well 

 exposed below the mouth of Iowa Creek, where the Missouri cuts the 

 bluffs, and here we see all the rocks in their order : 



4. Yellow marl, a recent deposit. 



3. Niobrara group, layers of white and yellow chalky lime, passing 

 down into gray marly rock. 



2. Black plastic clay, with hard layers, containing inoceramus, a spe- 

 cies of Ostrea, like 0. congesta, remains of fishes, many crystals of sul- 

 phuret of iron, selenite, &c. 



1. Dakota group, sulphuret of iron, fragments of wood, impressions 

 €f leaves, willow, laurel, i&c. 



Near the mouth of the Niobrara River the black shaly clays of the 

 Fort Pierre group begin to make their appearance on the hills over the 

 Niobrara division, so that within the limits of Nebraska proper we have 

 four out of five of the important divisions of the Cretaceous rocks of 

 the West. 



Near the mouth of Iowa Creek there seems to be a bed of impure 

 lignite in the Fort Benton group, or in the transition between the Da- 

 kota and Fort Benton groups. This bed, which has been worked to a 

 considerable extent, and the coal used by blacksmiths in this vicinity 

 with some success, does not seem to be the same as that seen along the 

 Indian reserve, which is undoubtedly in the sandstone of the Dakota 

 group. 



I am inclined to the opinion that this bed of lignite near Ponka City 

 is a local bed or at least restricted in its geographical extent, and is the 

 result of an accumulation of drift-wood in an estuary of the Cretaceous 

 sea. 



I am informed that it is seen over on the Elkhorn Eiver, about 35 

 miles west of this point. 



Mr. Clark tells me that he dug twelve or fifteen feet below this bed 

 and struck another seam of coal much better than the one cropping out. 

 The lower bed must be the one in the Dakota group. Lithologically it 

 is impossible to draw a line of demarkation between these formations 

 here. Number 1 passes so imperceptibly into number 2, and number 2 

 into number 3, that there is no break, and yet their principal character- 

 istics are very distinct. The first is a sandstone 5 second, a black plas- 

 tic clay; third, a chalky limestone; and yet I cannot tell the exact 

 point where one commences and the other ends. 



The impressions of leaves have ceased to appear before the close of 

 tbe Dakota group. The sandstones of the Dakota group occupy the 

 whole country along the Platte from the mouth of the Elkhorn to a 

 point some twenty miles beyond the entrance of the Loup Fork. The 

 intermediate counties between the Missouri and Platte have very few 

 i^xposures of rock of any kind, so that quarries in this region, even 

 though the rock is of inferior quality, are much prized. 



The Tertiary beds which make their appearance along the greater 

 portion of the Niobrara, and really occupy a very large portion of West- 

 ern Nebraska, do not furnish much good building-stone. In order that 

 the general geology of all this region may be better understood, I will 

 give a general section of the Cretaceous and Tertiary rocks of the Mis- 

 souri Valley, which was first published by Mr. Meek and myself in the 

 proceedings of the Academy of Sciences at Philadelphia. I have made 

 such changes as the present state of our knowledge of this region requires, 

 which are not of great importance. 



The accompanying profile, also, along the Missouri River, from Fort 

 Benton to the southern line of Nebraska, will show the basin-like char- . 



