234 GEOLOGY OF THE HIGH PLATEAUS. 



Southward from the northern center of eruption of the Sevier Plateau 

 the floods are piled up in grand succession sheet upon sheet. No narrow 

 streams or rivers of lava were here, but great deluges, which welled up and 

 rolled majestically over vast Phlegra?an fields, and, spreading out in broad 

 lakes, left after their congelation an even stratification, which may be read 

 miles away from distant summits. Standing upon the verge of the Awapa 

 Plateau and looking across Grass Valley, these old floods are seen lying 

 calmly and evenly with an outward resemblance to dark stratified rocks cut 

 by ravines and terraced off into trappean ledges. Ten or fifteen miles 

 southward they have commingled by intercalation with the coulees from 

 the middle eruptive focus of the plateau. 



The eruptions from this middle locality were inferior in magnitude to 

 those from the northern vents, though absolutely they were by no means 

 small. Its lavas differ somewhat in character from those derived from the 

 northern vent. Trachytes are present in considerable volume, and here as 

 elsewhere alternate with dark doleritic lavas. They succeeded the ande- 

 sites in the order of eruption. Here we find also the same inclination of 

 the pseudo-strata which is observed in the Blue Mountain, the layers dip- 

 ping away from the central mass in opposite directions. 



Around this eruptive locus we find also those great beds of conglom- 

 erate which are so conspicuous throughout the entire district and especially 

 in its southern portions. A mighty wall of > this material is pi'esented 

 towards Sevier Valley, just north of the middle vent, and extends for about 

 8 miles in that direction, where it thins out; but before being quite lost by 

 attenuation is cut off by erosion. It is well stratified and weathers into an 

 abrupt cliff. Here, as elsewhere, it was formed in an ancient valley, lying 

 between the two vents, and has the alluvial-cone structure. The great 

 Sevier fault has cut the formation, and its continuation is seen upon the 

 eastern slopes of Sevier Valley, 3,000 feet below. Upon the southern side 

 of the vent the conglomerate is seen in still greater mass. In truth, its 

 magnitude here becomes astonishing. 



Upon the Grass Valley side of this central eruptive locality is seen 

 what is undoubtedly a remnant of a very ancient volcanic cone, afterwards 



