DESCRIPTION OF FORMATIONS ,463 



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 abun- 

 dant 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 calcareous sandstones. Greenish and 

 grayish tints are locally found in the nodular limestones and a mottling 

 with red is common. Some of the nodular limestones appear to be intra- 

 formational conglomerates. 



The sandstones frequently grade into arkose grits and these into con- 

 glomerate. 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 mas- 

 sive, being in some places almost homogeneous from top to bottom, while 

 more frequently divided by several thin shale or sandstone layers. Cross, 

 bedding is almost universal. Sporadic pebbles are present in all grits, 

 and with their increase the stratum becomes a conglomerate. 



The sandstones are mainly quartzose, the grits contain much feldspar, 

 mica, and small pebbles like the larger ones of the conglomerates. The 

 latter contain pebbles of granite, gneiss, and various schists, of quartzite 

 and limestone, of greenstone and porphyry, and many of red, pink, 

 smoky, or white quartz, part of which may come from veins. 



The pebbles are in general larger near the San Juan mountains. 

 Boulders a foot in diameter are occasionally present, but most pebbles 

 are only a few inches in diameter. The relative abundance of different 

 rocks among the pebbles varies according to locality. The greenstone 

 schists, metadiorite, and green porphyry are most abundant in the Uhcom - 

 pahgre valley exposures ; granites and quartzites are always prominent. 



Taking the formation as a whole, the grits and conglomerates com- 

 prise about one-third or less of its total thickness in the quadrangles sur- 

 veyed, and they are distributed throughout the section. It may be 

 assumed that as distance from the source of the pebbles increases, the 

 formation becomes more and more a series of fine-grained sandstones and 

 shales, with subordinate grits and conglomerates. 



Typical section. — No opportunity to measure a complete section of the 

 Cutler beds has been found, but the various partial sections exposed in 



LXII— Bull. Geol. Soc. Am., Vol. 16, 1904 



