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 ae 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 
