152 GEOLOGY OF THE HIGH PLATEAUS. 



flexure. It forms one of the terraces which lie west and north of the San 

 Rafael Swell, but north of that area it dips beneath later formations, and is 

 buried thousands of feet beneath the Cretaceous- Eocene deposits. A hun- 

 dred miles north of the San Rafael it is turned up again upon the southern 

 slopes of the Uintas with the same characteristics which it shows elsewhere. 

 The line of outcrop with the intervals of concealment thus traced is nearly 

 500 miles. Wherever exposed along this belt the lithological characters 

 are preserved without material change. But, on the other hand, if we trace 

 the sandstone across this general line of strike and follow it southeastward 

 into northeastern Arizona and New Mexico, its thickness slowly diminishes, 

 its features lose force and individuality, and it seems to blend gradually 

 with the Vermilion Cliff sandstones below. It is not certainly known at 

 present whether the whole formation thins out in this direction or whether it 

 forms a part of the beds which have been assigned by Newberry to the 

 Upper Trias. Most probably it thins out altogether. A little way beyond 

 the Glen Canon in New Mexico the fossiliferous Upper Jurassic shales are 

 seen to rest directly upon sandstones which are believed to be Triassic, and 

 the Jurassic white sandstone of the High Plateaus is nowhere seen. A 

 little farther on the Jurassic shales also disappear, and the Cretaceous 

 touches the Trias. Thus the Jurassic sandstone appears to have been a 

 littoral or off-shore 'formation thrown down along the coast of the Mesozoic 

 mainland, which occupied the region now forming the Great Basin. Some 

 doubt still attaches to the origin of those portions which flank the Uintas, 

 but our ideas of a geography so ancient are very vague and our knowledge 

 veiy fragmentary. 



The lithological characters of the Jurassic white sandstone render it a 

 very conspicuous formation. Through a thickness of more than a thousand 

 feet, sometimes of nearly two thousand feet, it is one solid stratum, with- 

 out a single heterogeneous layer or shaly parting. A few horizontal cracks 

 are seen here and there, but inspection shows that they are merely the 

 seams where two systems of cross-bedding are cemented together. In gen- 

 eral, it is one indivisible stratum. This massive character has had its effect 

 upon the cliff-forms that have been sculptured out of it. These forms are 

 bold headlands and gigantic domes, usually without any minor details, but 



