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Show 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 hundred miles north of the San Eafael 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 very 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, without 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 general, 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 |