ON THE GEOLOGY 



OF THE 



TERTIARY FORMATIONS OF DAKOTA AND NEBRASKA. 



By Prof. F. V. IIayden. 



The vast extent of our country west of tlie Mississippi seems to liave been tlie 

 arena on which were enacted, during the Mesozoic and Cenozoic times, some of the 

 most important events in the geological history of the American continent. There 

 are indications that in this region are still to be wrought out some of the most 

 important problems in geological science. It seems even quite probable that the 

 chasm that has always existed between the two great periods, the Cretaceous and 

 Tertiary, will be bridged over by means of some transition beds, or beds of passage 

 belonging to the lignite series, which will illustrate the continuity of growth of the 

 Western Continent as has been shown in no other portion of the world. Much more 

 study is required before we can arrive at any positive conclusions on this subject. A 

 very large portion of the West has never yet been explored by the geologist, and all 

 the work that has been done up to this time has been of necessity very superficial. 

 The facilities which will soon be aiForded for travel through those wild regions, on 

 the completion of the Pacific Railroads, must give a very great impulse to explora- 

 tions, and it is to be hoped that not many years will elapse before a sufficient number 

 of facts may be gathered together from every portion of the West, to enable the 

 geologist to work out the general plan of its geological structure with some degree of 

 completeness. 



From the observations which have already been made, we believe that at the 

 close of the Cretaceous period the ocean rolled uninterruj^tedly across the area now 

 occupied by the Rocky Mountain ranges. Whether some portions of the mountain 

 peaks did not project above the ocean waters during that period it is impossible now 

 to determine, but the evidence seems to be quite clear that the greater part of the 

 country, at least, was beneath the ocean level during that period. Near the close of 

 the Cretaceous era the surface had reached an elevation so great as to form long 

 lines of separation between the waters of the Atlantic on the east and those of the 

 Pacific on the west ; and then this great water shed began to rise above the 

 surrounding country. Then also began the existence of tlie first of that series of 



