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University of California Publications. [Geology 



positions on the slope below the lava cap (pi. 11, fig. 1). On 

 the edge of the mesa, blocks half a mile or more in extent may 

 be seen in the first stages of movement. In the lower part of 

 the valley near the union of Virgin Creek and Beet Creek a 

 long series of lava-capped hills reaches for a distance of at 

 least two miles from the mesa down to the present stream-bed 

 (see pi. 5). The strata in these hills are frequently sharply 

 inclined, usually with the dip toward the mesa (pi. 11, fig. 2). 

 The lava capping consists of material identical with that in the 

 basalt cap of the mesa. 



The separation of large blocks from the valley wall is 

 evidently clue in a considerable measure simply to the cutting of 

 small streams, the basalt cap having protected the underlying 

 mass until the wall was cut down to a very steep slope. The 

 breaking away of blocks of large size is evidently assisted greatly 

 by seepage developed through breaks in the lava cap. The 

 presence of such channels of seepage on the mesas both north 

 and south of Virgin Valley is evidently indicated by a series of 

 peculiar lakes scattered over the table-lands. The lake basins, 

 in some cases at least a mile in diameter, are situated on the 

 level lava tables (fig. 4). They are usually approximately 

 circular, with steep marginal walls formed by the basalt. Though 

 the lava cap has disappeared over the area of the lake basins, 

 there is no lateral outlet for wash. The only supposition on 

 which we can account for the presence of these depressions 

 seems to be that they have been formed by the sinking of the 

 lake floor. This would most probably be caused by lines of 

 seepage causing readjustment of the ash strata below, partly 

 through the condensing effect of water on the beds of loose ash. 

 Seepage of this character would also tend to separate large 

 blocks from the main valley wall. Depressed areas, developed 

 some distance away from the margin of the mesa, when reached 

 by recession of the valley wall would presumably tend to move 

 as large slides. 



In the movement of the numerous blocks which have been 

 detached as slides from the mesa walls it is not improbable that 

 earthquake shocks have played a part of some importance. In 

 the fault movement which caused the four hundred foot displace- 



