168 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Ou the northeast side of the valley is another bluff, which faces 

 the southwest. This also is capped with Dakota sandstones, which dip 

 lOci-lSo to the northeast. The angle soon decreases and the beds 

 form the floor of the Basin Plateau. 



At Station 10 the sandstones are nearly 1,400 feet above the valley, 

 and below them the Jurassic shales and Triassic red sandstones outcrop. 

 It is probable that a portion of the Upper Carboniferous also sho^vs. 

 Following the ridge to the northwest it is noticed that the dip toward 

 the northeast becomes less, and the country is more mesa-like. At a 

 point a little east of north from Island Mesa, at the westward bend of 

 the Dolores, on the north side (see a in Fig. 1, Plate IX), there are evi- 

 dences of an anticlinal i?old, the axis of which is ou a line with the edge 

 of the bluffs on which Station 10 is located. At the station there is no 

 fold. At several points on the opposite ridge, however, there are ob- 

 scure evidences of a fold, but the beds in the centre of the valley have 

 been completely eroded away. Northeast of Island Mesa the Jurassic 

 rocks show the folding as indicated at b in the section. (Fig. 1, Plate 

 IX.) Whether the lower part of the valley is a synclinal or not is diffi- 

 cult to determine. I am inclined, however, to think not, but that the 

 condition is folding and faulting, as represented in the section. The 

 eastern fold dies out to the southeast, although the fault continues. This 

 also dies out, or rather changes to a fold, for at the head of the valley 

 the sandstones of the Dakota group are seen curving around to con- 

 nect with those of the southwestern ridge. There is considerable ob- 

 scurity here, resulting from the soft character of the strata, owing 

 largely to the presence of gypsum. 



BASm PLATEAU. 



From the northeastern edge of Gypsum Yalley the Dakota sand- 

 stones dip gently to the northeast, and become almost horizontal in the 

 centre of Basin Plateau, and beyond rise in a low ridge througli which 

 Basin Creek cuts a deep canon. The centre of Basin Plateau is floored 

 with the lower portion of the Colorado Cretaceous, remnants of the 

 strata that form mesas farther south. Toward the northwest the Da- 

 kota sandstones form the surface, extending eastward to the southwest 

 edge of Paradox Valley. The ridge through which Basin Creek cuts its 

 way out of the plateau begins in the southeast as a very gentle fold. Its 

 culmination appears to be at the canon of Basin Creek. (See sections, 

 place No. IX.) The red beds are exposed in the centre. The western 

 side of the anticlinal diminishes in inclination as we trace it toward the 

 northwest, where it forms the wall of Paradox Valley. The eastern 

 side of the fold, as we trace it, is seen curving to the eastward, forming 

 the rim of a dish like depression. On the east side of this is a line of 

 faulting which fades out into a fold to the southeast. The axis of this 

 fold is parallel to that next west, but the fold is much gentler, and soon 

 dies out in San Miguel Plateau. The sections in Plate No. IX will give 

 a clearer idea of this folding. 



PAEADOX VALLEY. 



About four miles above the mouth of the San Miguel Eiver the Eio 

 Dolores is seen cutting its way from a broad valley through a bluff" wall 

 some 1,600 feet in height. The valley is about three miles in width, and 

 the river comes into it from a caiion on the west which is almost as deep 

 as the one by which it makes its escape. This valley, which stretches 



