TO JUNCTION OF GRAND AND GREEN RIVERS.' 25 



During my former transit of the plains I fornied the opinion that the western 

 boundary of this Tertiary belt was passed near the crossing of the Cimarron; and it 



is still possible that Tertiary strata will not be found west of' that point which are con- 

 tinuous with those forming the table-lauds bordering the Arkansas, but during the past 

 season 1 noticed similar beds recurring at various points quite up to the bases of the 

 Rocky Mountains, and 1 think we have evidence that at one time Tertiary rocks 

 occupied the surface of a large territory in this region, from which they have been 

 entirely removed, or so nearly so as to he represented by isolated and often widely 

 separated patches. 



As will be seen by reference to the sections exposed at various points along the 

 western portion of the Santa fY road, these Tertiary rocks are entirely unconformable 

 to those upon which they rest : are of very much later date, and were deposited, not 

 only Sllbsequenl to the period when the entire series of Cretaceous strata had been 

 laid down, hut after they had been much disturbed, and elevated to such a point, that 

 valleys, eroded by surface action, had been cut down to and through the base of that, 

 series. \w different localities the Tertiary strata rest up on tin; Middle or Lower 

 Cretaceous rocks, or even on the underlying Triassic formation. 



The vicinity of the Raton Mountains has, in former times, been the theater of 

 violent and wide-spread volcanic action. At that time numerous mountain masses and 

 subordinate huttes of trap were thrown up, and floods of lava poured out, covering an 

 extensive area in their vicinity. During this period of violence the Cretaceous rocks 

 were locally much disturbed and metamorphosed, and the lowest members of that 

 series elevated to, and perhaps far above, the surface of the ocean. At some time sub- 

 sequent to the period of greatest volcanic activity, and yet apparently before the tires 

 in this great furnace wore • entirely extinguished, the Tertiary strata began to be 

 deposited in the depressions, and over the irregularities which then existed on the 

 surface. Unfortunately, in all the localities where 1 examined these strata, they seemed 

 to be destitute of fossils. We are, therefore, as yet without the light which they 

 would throw upon the conditions of their deposition, and the conclusions to which I 

 have arrived in reference to the precise age of these beds are to a certain extent con- 

 jectural, and liable to be modified by future discoveries, yet there seems to be good 

 r< ason lor supposing that they are what I have called them, Tertiary, and that they 

 are continuous with the Tertiary basins of Northwestern Kansas and Nebraska. The 

 reasons for this conclusion are, first, that these deposits are considerably more recent 

 than any portion of the Cretaceous series re-presented in the region in which they 



exisi ; second, that in their lithologfcal characters and tin 1 circumstances of their depo- 

 sition they are the exact counterparts of the fresh-water Tertiary strata of the basins 

 to which 1 have referred, andwhich are also only locally and rarely fossiliferous ; third, 

 that the Tertiary strata of the Upper Platte extend southward toward the Arkansas, 

 where their only possible representatives are those under consideration : fourth, 

 Tertiary fossils are said to have been found in these beds on the banks of the Arkansas, 

 above the crossing. 



In lithological characters the Tertiary rocks of the Arkansas basin are considerably 

 unlike any marine deposit with which I am familiar, but the greater portion of them 

 ■1 s F 



