238 GEOLOGY OF THE HIGH PLATEAUS. 



group, overlaid by an enormous mass of volcanic conglomerate. Between 

 the two are thin layers of those fine-grained marls and sandstones which 

 have been derived from the decay of ancient lavas, and which were evi- 

 dently deposited in water. Of the age of these intermediate beds it is pos- 

 sible to say but little. They are apparently conformable to the Bitter Creek 

 below, but the conformity is no proof of continuity of deposition. They 

 contain no fossils. The finer marly and arenaceous deposits are often of 

 an exquisite apple-green color, and in some of the exposures the color is 

 most charmingly delicate. The larger masses are from strong gray to 

 white, when the grain is fine, and brown when it is coarse. Small decayed 

 gi'anules of volcanic sand, hornblendes, mica, and a green mineral, which may 

 be epidote or " viridite," are intimately commingled. Veins of chalcedony 

 and agate often cut the beds, and the fragments strew the soils and bad- 

 land at the foot of the clifis. 



The fault which uplifts the plateau has not been affected in any notice- 

 able manner by its passage from the volcanic to the sedimentary region. 

 It cut through a country which had apparently been long in repose ; where 

 time had been gradually smoothing down the inequalities which had been 

 produced by volcanic activity. When this new disturbance set in it seems 

 to have laid out its line of operations regardless of existing inequalities, 

 splitting whatever it found in its way. In the sovithern part of the Sevier 

 Plateau it has sheared the old volcanic pile, and passing southward among 

 the sedimentaries and conglomerates it treated them in the same fashion. 



The termination of the Sevier Plateau southward is effected by cliffs 

 of conglomerate fringed with buttes. The conglomerate attenuates in that 

 direction, and when its thickness has diminished to about 600 feet it is cut off 

 by the undermining of the sedimentaries upon which it rests. At the end of 

 the plateau the Sevier fault has diminished its throws to less than a thousand 

 feet, and farther southward the throw reaches a minimum of about 600 feet, 

 and thenceforward it increases again. This has produced a very slight sag, 

 in which lies the Panquitch Hayfield, a broad valley-plain having an abso- 

 lute altitude of a little less than 7,000 feet. 



