ol EXPLORING EXPEDITION PBOM SANTA Fft 



my command The divide between the valleys of the Red Fork and its tributary, the 



( )cat(', as well as the western hank of the ( )cat<\ is formed of the Middle Cretaceous 



limestones and marl, enumerated in the preceding section. 



The structure of the picturesque table-lands from which these streams issue 1 had 

 no opportunity of determining, as they lie several miles north ol' the road, but I could 

 sec that they are in part covered by a thick layer of trap, to which their nearly hori- 

 zontal surfaces and precipitous sides are doubtless due. 



VALLEY OF THE CANADIAN TO LAS VEGAS. 



As far westward as Burgwin's Spring, patches of Tertiary tufa appear in 

 numerous localities, usually restingupon trap, which is the prevailing surface-rock, and 



which caps the picturesque mesas bordering the road on either side. Here, as farther 



east, numerous trap buttes stud the prairies, and there are several lines of trap hills 

 which run off toward the south as diverging spurs from the Raton Mountains. Near 



Dragoon Spring, by a long and painful ascent we reached the summit <>1 a trap mesa, 

 apparently the continuation of the table-lands bordering the Upper Canadian. The 

 underlying sedimentary rocks are here concealed by the debris of their trappean cover- 

 ing. A few miles farther westward, however, the Lower Cretaceous sandstones again 

 come into view, and it is probable that they here form the surface over which the vol- 

 canic flood was poured. The trap of all thisregion exhibits great similarity ol char- 

 acter; its weathered surfaces are black, while within it is dark brown or gray. It is 

 generally more or less vesicular, sometimes quite scoriaceous. 



About Port Union the only rocks visible are a mass of vesicular trap similar to 

 that just described, apparently only a narrow and local lava-stream, and the Lower 

 Cretaceous sandstones, which are very fully exposed. 'This latter group has here a 

 thickness of at least .".00 feet, perhaps considerably more, as its line of junction with 

 the underlying rocks is nowhere reached. It forms the hills at the base of which the 

 fort is located, and a large part of Qallinas Mountain. Probably still older strata are 

 thrown up at the latter locality, but I had no opportunity to look for them. Just 



below Fort Union the Cretaceous sandstones, dipping to tin; south and east, away 



from the mountains, pass beneath the surface, and are succeeded by the Middle Creta- 

 ceous limestones so fully exposed on the banks of the Canadian. These rocks occupy all 



the interval between this point and Las Vegas. On my former visitto Las Vegas I was 

 unable to discover any fossils in the blue limestone which crops out there, though 1 

 regarded it as equivalent to the Middle Cretaceous beds elsewhere examined. This 

 conjecture I was able more recently fully to confirm by finding in it Tnoceramus prob- 



lematicus and other fossils of that horizon. 



At Las Vegas the road leaves the prairies, and thence to Santa Vv is constantly 

 involved in the foot-hills of the Rocky Mountains. As the geology of this portion of 

 our route is somewhat complicated, and was studied principally on excursions made at 

 different times from Santa l-Y, it will perhaps more; properly form part of a distinct 



chapter, where the structure of a limited geographical district is considered by itself. 

 and where the light derived from this one of its parts may serve to illuminate the 

 others. This division is the more natural, as in this vicinity we passed out of the 



