194 bulletin: museum of comparative zoology. 



slope, a promising specimen for the study of the structure of such 

 cones. One of the northernmost of the eruptions in this field (Y, Fig- 

 ure 1) seems to be also one of the youngest ; it lay several miles south 

 of the rough trail that we followed eastward from Hull spring to the 

 Flagstaff-Tuba road, and as seen at that distance it seemed to form a 

 ragged black mesa, two or three hundred feet high. The various rela- 

 tions of lava flows to small valleys were beautifully exhibited in the same 

 district. In one case a flow banked against the scalloped escarpment of 

 the upper Aubrey, and a little waterway has since been eroded along 

 the line of junction, with yellow-gray limestone on one side and black 

 lavas on the other ; this was well seen on the canyon road, a few miles 

 north of Hull spring. Further east, we followed valleys eroded to 

 a depth of two or three hundred feet in the Aubrey layers, and then 

 floored with streams of lava from some neighboring cone. In one nar- 

 row valley of this kind, the wash of waste from the walls seems to have 

 prevented the formation of new waterways along the lava margin ; here 

 the present stream bed lies for a distance on one side of the valley, then 

 trenches obliquely across the lava and follows its other side for a 

 stretch. In a broader valley, the lava surface was untrenched, and new 

 waterways ran persistently along its margins, this being evidently the 

 incipient stage in the formation of a lava mesa. At Lockett's tank 

 (probably several miles north of Black tank of the topographic map) a 

 narrow canyon of moderate depth in the Aubrey limestone has been 

 filled nearly to its brim with a slender but heavy lava flood, but at 

 present the stream has re-excavated part of its valley, consuming the 

 terminal part of the lava flow for half a mile or more, although leaving 

 scraps of lava here and there, frozen to the walls ; and at the head of 

 the new valley is an abrupt fall from the surface of the lava that still 

 remains (Figure 13). Here the wet-weather floods have scoured a 

 basin, in which water remains long after the supplying storm has 

 cleared away ; cattle tracks converge on the dry upland from all sides 

 towards this tank. 



A narrow dike was noted on the west-facing slope of the Triassic 

 escarpment, just south of the valley of the Moencopie and not far from 

 Tuba ; the dike formed a sharp ridge, quite unlike the more tabular 

 forms normally associated with the escarpment. 



The great lava cascades that descend from the Uinkaret into the Toro- 

 weap and even into the canyon itself are among the most magnificent 

 geological phenomena of the region. Our camp by Oak spring, south of 

 Mt. Trumbull, was at the scarped edge of one of the younger lava flows 



