TO JUNCTION OF GRAND AND GREEN RIVERS. 63 



eastern slope reaches as far as the Little Colorado. Up to this point the more recent 

 formations have been removed from the Carboniferous, and this occupies a broad belt 

 of country, indicated in the geological map, accompanying the report of Lieuten- 

 ant Ives. As we go northward this Carboniferous belt rapidly narrows, and on the 

 flanks of the Wasatch Mountains it exhibits simply a line of outcrop, as on the opposite 

 ranges of the Rocky Mountains. Along the Little Colorado the Carboniferous lime- 

 stones dip beneath the overlying rocks, and are lost to view in all the interval between 

 that point and the Rocky Mountains. On the east side of the Little Colorado, its 

 valley is bounded by a mesa composed of the Triassic marls and red sandstones, which 

 form a distinct step in the table-lands, reaching eastward for many miles, with a still 

 greater extension north and south. Above this, on the east and north, the Lower Creta- 

 ceous rocks form a third plateau, along its margin cut into detached mesas and messil- 

 las by valleys of excavation Farther east and north these mesas combine, and the 

 Cretaceous rocks in an unbroken sheet underlie the whole country. 



The broad eroded valleys of the San Juan, of Grand and Green Rivers, and the 

 Upper Colorado generally, exhibit precisely the same structure as that of the valley 

 of the Little Colorado. The Carboniferous limestones are exposed in the canons of 

 these streams, while the broader valleys through which they flow are excavated in the 

 red Triassic rocks ; large surfaces of which are exposed on the banks of all these 

 streams. The process of erosion has there, as on the Little Colorado, everywhere left 

 the most surprising monuments of its action; domes, castles, walls, spires, which by 

 their vivid colors and fantastic outlines attract the attention and excite the wonder of 

 every explorer who beholds them. Above the Triassic rocks the Lower Cretaceous 

 sandstones — massive beds of comparatively resistent material— cap mesas which stretch 

 away to a great distance both east and west of the Colorado. They form the floor of 

 the great sage-plain lying between the Sierra La Plata and Sierra Abajo. South of 

 the Sierra La Plata they are covered by the Middle and Upper Cretaceous beds form- 

 ing the Mesa Verde — the third great step in the table-lands above the Carboniferous — 

 which on the east connects with the plateau bordering Canon Largo and extending to 

 the base of the Nachniento Mountain. High table-lands are visible west of the 

 Colorado, where it is formed by the junction of Grand and Green Rivers, evidently 

 corresponding to the Cretaceous mesas just mentioned, but whether they correspond 

 precisely to it in geological structure can only be determined by further explorations. 



The country bordering the Upper Grand and Green Rivers, as described by Dr. 

 Schiel (Pacific R. R. Rep., vol. 2), by Mr. Egloffstein and Colonel Henderson, who 

 have kindly communicated to me their observations, has essentially the same structure 

 with that bordering the Little Colorado and the San Juan. The Triassic rocks are 

 freely exposed in the valleys of erosion, while table-lands floored with Cretaceous 

 strata occupy the greater portion of the intervals between the mountain chains. This 

 margin of the Colorado basin is very irregular, and its outline has not been accurately 



traced. 



Between the Little Colorado and the San Juan the country is almost exclusively 

 occupied by Cretaceous rocks. The Lower Cretaceous sandstones are here largely 

 developed, containing numerous impressions of leaves of land-plants and heavy beds 



