rEALE.] GEOLOGICAL FORMATIONS JUEA-TEIAS. 179 



been drawn lithologically, the massive red sandstone being consid- 

 ered the top of the Triassic. The conflictiug evidence presented by the 

 organic remains found in the Jurassic and Triassic beds of the West has 

 been referred to in previous reports (1874 and 1875). 



Area A. — Hardly more than 60 square miles of the area are covered 

 with the red sandstones of the Trias. A massive red sandstone, becorri- 

 iDg lighter colored toward the top, cross stratified at many places, is the 

 prevailing characteristic rock. Its thickness is from 500 to 1,000 feet. 

 The beds become lanimated below, and gypsiferous, passing gradually 

 into the Upper Carboniferous. The variegated sandstone shales that 

 lie above, have been colored on the map to represent the Jurassic. The 

 general character is the same as previously described — soft greenish and 

 gray argillaceous and arenaceous shales and marls near the top, passing 

 into the Lower Dakota sandstone, and dull, reddish laminated sandstones 

 and shales at the base. The limestones that occur farther north near 

 the base appear to be absent here. The area occupied is about 80 square 

 miles. The Rio San Miguel has an outcrop of Jurassic at the bottom 

 of its caiLon walls almost its entire length. A few of its tributaries also 

 cut deep enough to expose the Jura. The mesa northeast of Paradox 

 Valley is capped with the reddish shales that lie at the base of the for- 

 mation. This is also the case with the region included in the bend of 

 the Dolores north of Island Mesa. The streams tributary to the Do- 

 lores here cut into the Eed Beds beneath. All about Island Mesa, Ju- 

 rassic rocks arc exposed, and the debris filling Gypsum Valley is from 

 rocks of the same age. The structure of this region was described in 

 Chapter II, and a study of the map and sections will make the descrip- 

 tion more intelligible. 



Area B. — In Area B the Trias and Jura are found only along Grand 

 River, the former principally on the south side, and the latter on the 

 north. The lithological line separating them is much better defined 

 than it is in Area A. The sandstones forming the upper i)art of the Red 

 Beds are very massive, and the shaly beds just above contain thin layers 

 of limestone. Below the massive sandstones come blood red shales, fol- 

 lowed by massive sandstones, generally of a deep-red color, although 

 in many places the color fades to almost white. The thickness exposed 

 is nearly 500 feet, which represents only a portion of the formation. 



In the Red and Variegated Beds the Grand cuts a cafion of varying 

 height, but at no place does it exceed 1,000 feet. 



The general section of the beds referred to the Jurassic is about as 

 follows : 



Base. Feet. 



1. White bands of sandstone with greenish marls and a few thin 



layers of cherty limestone at the base 80 



2. Variegated green and red sandstone shales and marls ) ^^^ 



3. White sandstone ] ^^^ 



Total 200 



The variegated character of the shales and marls is much more marked 

 at some localities than at others. At Station 10, near Horseshoe Bend 

 of the Grand, they are rather brilliant in color, and gypsum seems to enter 

 largely into their composition. From Alkali Creek to the Bend the Ju- 

 rassic beds are seen as remnants capping isolated areas, and on the 

 Dome Plateau the lower jjortion of the same formation covers a consid- 

 erable area south of the hogbacks that border the Grand River Valley 

 at this place. A reference to the map of Area B will give the best idea 

 of the extent of the formation. 



