DESCRIPTION OF FORMATIONS 467 



of the Zuni plateau referred to the Permian by Dutton, as will be brought 

 out in the discussion of correlation. 



Dolores formation. — General characterization. — The Dolores formation 

 embraces the Triassic portion of the Red beds of southwestern Colorado* 

 several hundred feet in thickness. It is limited, both above and below, 

 by planes of unconformity. In most places it rests on the Cutler for- 

 mation and is overlain by the La Plata sandstone. It consists of sand- 

 stones, shales, and fine-grained conglomerates, all more or less calcareous. 

 There are two divisions of the formation : the lower embraces variable 

 sandstones, conglomerates, and shales, partly greenish or gray in color 

 and persistently fossiliferous at several horizons ; the upper portion is a 

 very fine and evenly-grained sandstone and shale of strong red color. 



Lower fossiliferous portion. — The lower portion of the Dolores forma- 

 tion consists of an alternation of reddish sandstones, more or less shaly, 

 with conglomerate, consisting chiefly of very small limestone pebbles. 

 These conglomerates characterize several bands within the lower 300 or 

 400 feet of the Dolores, and, owing to their fossil content and notable 

 lithologic characteristics, these beds are the most diagnostic of the whole 

 formation. The conglomerate beds are very variable in number. In 

 some places a ledge 20 feet thick may be seen to consist chiefly of con- 

 glomerate with numerous sandy partings and common cross-bedding. 

 A few yards distant the same strata may be composed chiefly of sand- 

 stone with a number of thin layers of conglomerate. 



Some of the conglomerate bands are more persistent than others, but 

 it is probable that no stratum of the conglomerate is continuous for any 

 great distance. The limestone of the pebbles in the conglomerate is 

 usually very fine grained, and does not appear to have been derived from 

 the older limestones of the Hermosa formation, since no fossil-bearing 

 pebbles have been found. The pebbles are commonly very small, and 

 sometimes they are of such minute, shot-like appearance as to suggest 

 that they are pisolitic. Pebbles several inches in diameter are sometimes 

 found, and these are usually unsymmetrical. It is believed the con- 

 glomerate is, at least partially, derived from the breaking up of limestone 

 deposits of the Dolores sedimentation. 



The conglomerates are commonly associated with thin bedded gray 

 sandstones or greenish-gray sandy shales. A complex of such alternat- 

 ing strata 50 to 75 feet in thickness can be traced for long distances on 

 the flanks of the San Juan mountains. Carbonaceous material is com- 

 mon in the shaly beds, but determinable leaves have not been observed. 

 The lowest stratum of the Dolores is generally more or less conglom- 

 eratic, and in some localities is a harder stratum, forming distinct ledge 

 outcrops. The thickness of the lower division of the Dolores seems to 



