92 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



were peculiarly favorable for the production and existence of animal 

 life. Although the rivers cut deep channels through the different forma- 

 tions, we do not meet with the Fox Hills Group along the Missouri until we 

 reach nearly up to the niouth of Cannon Ball Eiver, yet fifty miles or 



Fig." 3. 



Concretions in Missourri River, near Chain de Roche Creek. 



more before reaching that point it has overlapped the Fort Pierre 

 Group. In traveling across the plain country westward from Fort 

 Pierre we find it occupying the entire area. Very soon after passing 

 west of the Big Cheyenne Eiver the traveler will readily recognize 

 its presence by the more cheerful appearance that it gives to the surface, 

 as well as by the greatly increased growth of vegetation. The water is 

 pure and good, and springs become quite common in the hills. In this 

 group also there is*a remarkable zone of fossils, exending across the 

 country in either direction from the Missouri Biver. Kear the mouth 

 of the Cannon Ball Biver, the surface is covered with rounded concre- 

 tions of rusty-brown arenaceous limestone, crowded with beautiful mo- 

 luscous fossils. This belt is quite narrow and extends eastward toward 

 the Coteau cle Prairie, and westward between the Big Cheyenne and 

 Grand Rivers, along the north side of the Black Hills. I have thus 

 given the typical features which those groups assume on the Missouri 

 Biver. As we recede from this region southward there are many 

 modifications, especially lithologicaliy ; yet to one familiar with them 

 they never lose all their characters, so that they cannot be detected. 

 Like the human face, neither time nor distance can so change it that 

 it does not retain some trace of its original features. In my explora- 

 tions I have traced these groups over hundreds of miles in every direc- 

 tion, and I have no doubt that they extend from the Arctic Circle to 

 the Isthmus of Darien; and that at some future period they will be so 

 carefully studied at different points that they may be connected into 

 one harmonious group. All the facts that I have been able to gather 

 up to this time tend toward the unity and simplicity of the geology of 

 the entire Bocky Mountain system. 



If the reader has the patience and interest to follow me through this 

 report, he will find frequent allusions to all these groups in the descrip- 

 tion of the geology of various localities. Some of these groups come 



