PLIOCENE F0EMAT10J5IS. , 563 



easterly-dipping part of the anticlinal having been covered over first by 

 the' Eocene deposits, and then subsequently by trachytes. 



Pliocene Foematioks. — To the north, the eroded edges of the Green 

 River series are overlaid by the . soft friable beds of the valley Tertiary of 

 the Humboldt Pliocene, which here, as elsewhere, are approximately hori- 

 zontal. They extend to Dixie Creek, where they are, for the most part, 

 covered by Quaternary detrital matter, and at Dixie Pass entirely con- 

 cealed by accumulations of both trachytes and rhyolites. From the pass 

 to the neighborhood of Cave Creek, the w^estern foot-hills are skirted by a 

 continuous band of Pliocene Tertiary, consisting of remarkably fine calca- 

 reous clays and sands. So much of the earlier rock of the range immedi- 

 ately underlying these Pliocene beds is of limestone, that it is not surprising 

 to find so general a calcareous character in the Pliocene strata. In the 

 region of Pinon Pass, they are mostly calcareous, where the beds, for a 

 thickness of from 80 to 100 feet, are of white compacted lime-rock, in 

 which is mingled about 10 per cent, of siliceous sand. These Tertiary lime 

 formations are very chalky in texture and appearance, and are evidently 

 made up of exceedingly fine material; in places, they are also more or less 

 impregnated with alkaline salts, as if the lake in which they formed had 

 been saline. Carbonates and chlorides predominate, but there are, at least 

 in two cases, a little sulphate of soda and lime. 



An analysis of this white lime-rock, from the Pine Valley Pliocene, 

 by Mr. B. E. Brewster, yielded : 



Silica 12.07 



Alumina 1.28 



Ferric oxide 0.57 



Lime 45.29 



Magnesia -. 1.86 



Soda and potassa ■ 0.90 



Carbonic acid 36.23 



Water 2.65 



100.85 



