JURASSIC. 557 



in the Vermilion Cliff sandstones below, often so rich and complex that it excites constant 

 admijation. Dr. Newberry speaks of it with enthusiasm as presented in the Triassic sandstones 

 of New Mexico. But it is far less wonderful than the cross-bedding which the Jurassic presents 

 at every exposure. * * * The Jurassic sandstone was deposited over an area which can 

 not fall much short of 35,000 square miles, and the average thickness exceeds 1 ,000 feet. The 

 imagination is utterly baffled in the endeavor to conceive how a mass so vast and at the same 

 time so homogeneous and intricately cross-bedded throughout its entire extent could have 

 been accumulated. 



The cross stratification of the Jurassic sandstones of southern Utah has been 

 discussed by Huntington ^"^ as an evidence of wind action. Button continues : 



Overlying the white sandstone is a series of beds which may be called shales with some 

 reservation, and here we find for the first time an abundance of distinctive fossils. They are 

 clearly of Jurassic genera and species and enable us to correlate the horizon with confidence. 

 They belong to a well-marked formation, which is represented not onlj'- throughout the greater 

 part of the plateau province, but also in Colorado, Wyoming, and northern New Mexico. From 

 many large areas, indeed, it has been denuded, but throughout Utah it is never wanting from 

 those exposures where its presence could be looked for. 



The constancy of lithological character wliicli is so conspicuous in older Mesozoic members 

 does not prevail in this one, for it is liighly variable not only in the mass but also in the consti- 

 tution of the beds. In some exposures it is more than 1,000 feet thick; in others, it is less 

 than 200. Where its volume is greatest it is more arenaceous, and where the volume is less 

 the beds are shaly, marly, and calcareous. Usually several seanas of Hmestone occur, ami in 

 these the fossils are found, often abundantly. One notable feature is the small amount of 

 cement in the arenaceous layers, which are, therefore, very poorly consohdated, and the rock 

 weathers and wastes away with extreme facility. Gypsum and selenite occur abundantly in 

 these beds and especially noticeable is the latter mineral, wliich is seen sparkhng and glittering 

 in the sunUght in the badlands to which the decay of the strata gives rise. 



A section of the Triassic and overlying Jurassic observed by Walcott in the 

 Kanab region is quoted by Cross in an article on the red beds. (See I-K 12-13, 

 Chapter XII, p. 514.) Stanton " comments on Cross's suggestion that the La Plata 

 sandstone of southwestern Colorado may be correlated with the White Cliff sand- 

 stone of Utah and says that in that case the La Plata is well down in the Jurassic, 

 because the White Cliff sandstone underlies marine Jurassic beds that are of the 

 same age as the Twin Creek, Ellis, and Sundance formations. 



In western Colorado at the east end of the Uinta uplift Gale ^^'^ has found 

 marine Jurassic fossils in a white sandstone which he correlates with the White 

 Cliff and the La Plata. 



In southwestern Colorado the Jurassic is represented by the La Plata sand- 

 stone and probably by the McElmo formation, although the age of the latter is 

 not definitely known to be Jurassic; it may be Lower Cretaceous. The La Plata 

 and McElmo are equivalent to the Gunnison formation farther north. Cross '*' 

 gives the following classification and descriptions : 



"■ Comment on manuscript. 



