DESCRIPTION OF FORMATIONS 461 



exists for including within the Dolores formation the red sandstones, 

 shales, conglomerates, etcetera, occurring below the well marked hori- 

 zon of the "Saurian conglomerate." The contingency provided for in 

 the original definition of the Dolores (page 451) has arisen and the 

 subjacent portion of the Red beds, as far as the uppermost Rico or 

 Hermosa horizon, must be distinguished as a separate formation. It 

 seems appropriate to name this stratigraphic unit from the district 

 where its true relations to the Dolores are most clearly exhibited. Since 

 the terms Ouray and Uncompahgre have already been applied to other 

 formations, we have named these lower Red beds after Caller creek, an 

 eastern tributary of the Uncompahgre, entering it some 4 miles below 

 Ouray, at the old settlement of Portland. The bright red sandstones 

 and shales of the Cutler formation are very prominent beneath the Tri- 

 assic conglomerate on the north side of Cutler creek and the angular 

 unconformity is there very plain, though less pronounced than at the 

 localities described. Near the mouth of the creek several of the heavy 

 conglomerate bands are well exposed. 



The name Cutler formation has already been used in the Silverton 

 and Needle Mountains folios, now in press. 



As a groundwork for the remarks on correlation, which appear in the 

 second part of this paper, it seems desirable to review the |characters 

 exhibited by the Cutler, Dolores, La Plata, and McElmo formations, as 

 observed in the San Juan region. 



The last two do not strictly belong to the " Red beds," but assume 

 reddish colors in some places to be referred to, and in the discussion of 

 correlation it will be necessary to note the probable equivalents of the 

 La Plata and McElmo formations in certain districts. 



General characterization. — The Cutler formation embraces somewhat 

 more than the lower half of the Red beds section of southwestern Colo- 

 rado. Its strata are variably red in color and include sandstone, arkose 

 grit, conglomerate, shale, and limestone. The maximum observed thick- 

 ness is about 1,600 feet. 



The formation seems conformable with the underlying Pennsylvanian 

 beds, but above it occurs a stratigraphic break with at least local uncon- 

 formity. The base of the formation is indicated by the Pennsylvanian 

 fossils of the Hermosa or Rico formations and in a broad way b} r the 

 color line. No fossils have been found in the Cutler beds. 



Details of lithologic character. — Great variability in lithologic consti- 

 tution, both vertical and lateral, is one of the most striking features of 

 the Cutler formation. The sandstones are sometimes fine grained and 

 massive, but bedding is ordinarily distinct and few homogeneous beds 

 exceed 10 or 15 feet in thickness. All strata are calcareous, and the finer 



