308 W. T. LEE MORRISON A CRETACEOUS FORMATION 



son, Colorado, and are desiguated lt> tlic same name. However, in west- 

 ern Colorado the Gunnison fonuiitioii. the upper part of which has been 

 generally correlated with the Morrison, consists of two memliors (8, page 

 .31), the upper of which is lithologically like the Morrison and contains 

 the same kinds of dinosaurs, while the lower one consists of eveidy bedded 

 sandstone and some fresh-water limestone. The upper, or dinosaur-bear- 

 ing, member is regarded as the direct equivalent of the typical Morrison 

 east of the mountains mikI of the Mcb^hno formation of areas farther to 

 the west and south. The lowci- nicmher has been considered equivalent 

 to the La Plata sandstone of sonthwestern (Colorado and to the White 

 Cliff sandstone of eastern l.'tah (19), which underlies marine Jurassic. 

 This may be correct, but it is not definitely known that the Cunnison 

 includes rocks of such diverse age, nor is it beyond question that the La 

 Plata and the White Cliff sandstones are time equivalents. 



In northeastern New j\[exico the typical bone-bearing Morrison rests 

 on a massive sandstone, the Exeter (10, page 45), which is not present 

 in all places. No fossils have ever been found in this sandstone and little 

 progress has been made toward determining its age. When T first de- 

 scribed it, I suggested that it niiglit be as old as Triassic, l)ut was prob- 

 ably younger. Move recent investigations indicate that it is ])ossibly a 

 time equivalent of the La Plata sandstone. East of the mountains it 

 has not been found north of Xew Mexico, but extends westward to Las 

 Vegas (15, page 37), and occurs on the western slope of the mountains 

 south of Lamy. New Mexico (16, page CA\)). its extension farther to 

 the northwest, in a region where little is known of the geology, is prob- 

 lematical, and the La Phita sandstone of southwest Colorado has not been 

 traced eastward beyond Piedra Valley, in southern Colorado (!•, page 

 44) ; hence its correlation Avith the Exeter rests on lithology, structure, 

 and stratigraphic position. 



CHARACTER OF THE MORRISON 



The Morrison formation, including at least the upper part of the Gun- 

 nison, the McElmo, and other beds of equivalent age, extends from Mon- 

 tana to New Mexico and from the Black Hills and eastern New Mexico 

 westward to Utah, an area about 600 miles long and 300 miles wide. It 

 has been examined in lumdreds of ])laces and its character found to be 

 so constant and its thickness so regular tliat it must originally have ex- 

 tended with practical continuity over this great area. It consists gen- 

 erally of shale of various colors and tine-grained sandstone, with a small 

 amount of limestone ; but in some places, especially in western Colorado, 

 it contains some conglomerate. It is too well known to require extended 



