TKIASSIC. 511 



the Uinta and Wahsatch region the lower 600 or 800 feet are a bed of solid but very fine 

 graiaed, slightly argillaceous limestone, and the upper 800 feet are made of fine calcareous 

 argillites. As a whole the series is a lime and clay deposit. 



Later observations have shown that part of the supposed Triassic is Permian, 

 some part is Triassic, there are pre-Jurassic unconformities, and the Triassic and 

 Permian deposits which were considered to be marine are in fact largely continental. 

 Cross has recently reviewed the literature and field evidence of this red-beds prob- 

 lem. His statements regarding the probable correlation and distribution of the 

 Permian have been quoted at some length (pp. 480-484), and extracts from those 

 relating to the Triassic follow. The San Juan district of southwestern Colorado 

 is the type locality of the Dolores formation (Triassic) . Of this region Cross "^^ 

 writes : 



The Dolores formation embraces the Triassic portion of the red beds of southwestern 

 Colorado, several hundred feet ia thickness. It is limited both above and below by planes of 

 unconformity. In most places it rests on the Cutler formation [Permian?] and is overlain 

 by the La Plata sandstone [Jurassic]. It consists of sandstones, shales, and fine-grained con- 

 glomerates, 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 fossilif erous at several horizons ; the upper portion is a very fine and evenly grained 

 sandstone and shale of strong red color. 



The lower portion of the Dolores formation 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 conglomerate with numerous 

 sandy partings and common cross-bedding. A few yards distant the same strata may be com- 

 posed chiefly of sandstone with a number of thin layers of conglomerate. 



The conglomerates are commonly associated with thin-bedded gray sandstones or greenish- 

 gray sandy shales. A complex of such alternating strata 50 to 75 feet in thickness can be 

 traced for long distances on the flanks of the San Juan Mountains. Carbonaceous material 

 is common in the shaly beds, but determinable leaves have not been observed. The lowest 

 stratum of the Dolores is generally more or less conglomeratic and in some locaUties is a harder 

 stratum, forming distinct ledge outcrops. The thickness of the lower division of the Dolores 

 seems to vary considerably, increasing from the mountains west and southwest toward the 

 plateau country. 



The upper member of the Dolores is commonly a very even fine-grained reddish sandstone, 

 free from conglomerate but variably shaly in different places. The shales are, however, 

 always very sandy and there are seldom pronounced division planes of great lateral extent. 

 The bands of parallel massive sandstone are often 20 feet or more in thickness. The material 

 is mainly quartz sand, with a calcareous cement. In color this upper sandstone is usually 

 bright brick-red or vermilion, shading sometimes into purplish above and a dull darker red 

 below. In texture this red sandstone is very much like the overlying La Plata sandstone, and 

 where the latter is highly colored the two formations seem sometimes inseparable, but the 

 La Plata is commonly orange or yellow in color when it departs from the normal gray or white. 



The upper member of the Dolores is absent in the Ouray and parts of the Telluride quad- 

 rangles but presents an increasing thickness southward through the Rico and La Plata quad- 

 rangles. Nearly 500 feet of quite uniform sandstones were observed on the eastern flanks 

 of the La Platas. The variation in thickness and the disappearance of the red sandstones 

 to the north is due chiefly to the post-Dolores erosion. 



