102 EXPLORING EXPEDITION FROM SANTA Ffi 



series forming the table-lands of the Colorado Plateau. This interesting region requires 

 for its full illustration the detailed description to be hereafter given. 



Its general character is, however, as follows: 



The Nacimiento Mountain here bursts up through the table-lands in an unbroken 

 wall forty miles in length, with a nearly north and south trend. On its western flank 

 the various stratified rocks composing the Colorado Plateau lie inclined at a high angle 

 with the horizon. Of these the Carboniferous series and the overlying Triassic forma- 

 tion stand nearly vertical. Upon these the Lower and Middle Cretaceous groups rest 

 with a gentler inclination, while the Upper Cretaceous sandstones, and perhaps Tertiary 

 strata, are represented in detached buttes and mesillas, the remnants of continuous 

 sheets mostly removed by erosion. A few miles westward the disturbing influence of 

 the elevation of the Nacimiento range ceases to be visible, and the different stratified 

 rocks, which I have enumerated, hold a nearly horizontal position, which they main- 

 tain thence westward, with little variation, to and beyond the Colorado. Through the 

 table-lands which they here form, Canon Largo is excavated, gradually deepening till it 

 joins the valley of the San Juan, where its walls have an altitude of 700 to 800 feet. 

 At the eastern terminus of Canon Largo, and along the base of the Nacimiento Mount- 

 ain, are scattered the insulated buttes to which I have before referred as representing 

 the summit of the series of conformable strata composing the Colorado Plateau ; a group 

 of purple, white, and gray marls, interstratified with soft, yellow calcareous sandstones, 

 the latter predominating as we descend, and forming what has been described in the 

 preceding pages as the Upper Cretaceous group, containing, near its base, the fossils 

 collected on the Rio de la Plata, Ammonites placenta, Baculitcs anceps, &c.; a horizon 

 probably corresponding to No. IV of Meek and Hayden's Nebraska section. From the 

 perfect conformability throughout this group of rocks, their general lithological simi- 

 larity from summit to base, where they contain Cretaceous fossils; in short, from 

 absence of proof to the contrary, I have been driven to include them all provisionally 

 in the Cretaceous formation. It is by no means certain, however, that this classifica- 

 tion is rigidly correct, for it is not impossible that future investigations will prove that 

 the upper part of this group should be regarded as Eocene Tertiary. That no Miocene 

 beds occur here, is, I think, certain, as all the information obtained of the geology of 

 the Rocky Mountain region goes to show that, between the time of the deposition of 

 the Upper Cretaceous strata and the Miocene epoch, important changes occurred in 

 the physical geography of the central portion of our continent changes of level by 

 which the conformability prevailing throughout the sedimentary strata, from the Silu- 

 rian up, was finally interrupted. The only reasons we yet have for suspecting that 

 the upper portion of the series on the San Juan is Tertiary, are the inordinate thick- 

 ness (if all Cretaceous) 3,000 to 4,000 feet of the rocks which are more recent than 

 the Triassic formation; and the more suggestive fact that, in Texas, the Eocene beds 

 exhibit a marked lithological similarity to those forming the summit of the series at the 

 head of Canon Largo, and follow the Cretaceous strata in perfect conformability. We 

 can only say, therefore, that, with the evidence now before us, while it is possible, and 

 perhaps probable, that the uppermost members of the Colorado series are Eocene Ter- 

 tiary, we have as yet no proof that such is the case. 



