154 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



D. PERMO-CARBONIFEROUS OF SOUTHWESTERN COLORADO. 



The upper Paleozoic series of southwestern Colorado is generally con- 

 sidered as comprising the 



Cutler — Permo-Carboniferous. 

 Rico 1 



Hermosa ^Pennsylvanian. 

 Molas J 



The Moenkopie of northwestern Arizona has been shown to be similar 

 or identical in stratigraphic position with the Cutler of southwestern Colo- 

 rado. The most complete description of the red-bed series in the latter 

 region was given by Cross and Howe.^ 



"The Cutler formation embraces somewhat more than the lower half of the 

 Red Beds section of southwestern Colorado. Its strata are invariably red in 

 color and include sandstone, arkose grit, conglomerate, shale, and limestone. 

 The maximum observed thickness is about i,6oo feet. 



"The formation seems conformable with the underlying Pennsylvanian beds, 

 but above it occurs a stratigraphic break with at least local unconformity. The 

 base of the formation is indicated by the Pennsylvanian fossils of the Hermosa or 

 Rico formations and in a broad way by the color line. No fossils have been found 

 in the Cutler beds. 



''Details of lithologic character. — Great variability in lithologic constitution, 

 both vertical and lateral, is one of the most striking features of the Cutler forma- 

 tion. The sandstones are sometimes fine-grained and massive, but bedding is 

 ordinarily distinct and few homogeneous beds exceed lo or 15 feet in thickness. 

 All strata are calcareous, and the finer grained sandstones grade into calcareous 

 shales and impure marls or into sandy limestones. These rocks are naturally 

 more or less friable and crumbling. 



"The finer-grained strata are of the strongest red color, which is due to a 

 ferritic pigment, and they are also commonly characterized by abundant bronze or 

 rusty mica, which renders them fissile. Clay beds are rare, as is massive limestone. 

 Commonly the more calcareous strata are nodular or gnarly and grade into calca- 

 reous sandstones. Greenish and grayish tints are locally found in the nodular lime- 

 stones and a mottling with red is common. Some of the nodular limestones 

 appear to be intraformational conglomerates. 



"The sandstones frequently grade into arkose grits and these into conglomerate. 

 With increasing coarseness of grain the red changes to pink, and locally beds of 

 coarse grit are gray or almost white. In other cases the finer matrix of grits and 

 conglomerates is dark red. The cement of the strata is calcite, and most of the 

 conglomerate and arkose beds are comparatively resistant to weathering and form 

 prominent ledge outcrops on all steep slopes. 



"The grit beds often reach 35 feet in thickness. They are variably massive, 

 being in some places almost homogeneous from top to bottom, while more fre- 

 quently divided by several thin shale or sandstone layers. Cross-bedding is 

 almost universal. Sporadic pebbles are present in all grits, and with their in- 

 crease the stratum becomes a conglomerate. 



^ Cross, Whitman, and Ernest Howe, Red Beds of Southwestern Colorado and Their Cor- 

 relation, Bull. Geol. Soc. Amer., vol. 16, p. 461, 1905. 



