18 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
covered with lavas. Where the peneplain was underlaid by weak Per- 
mian beds, its surface has now been worn away to a lower level in the 
Uinkaret block, and in the Kanab block also somewhat farther north. 
Truly, the fact that different horizons are here covered by the lavas 
might be interpreted as meaning that the western lavas were poured 
out before any fault had occurred and when the land surface was at a 
Permian horizon ; that the eastern lavas were poured out after the land 
surface had there been worn down to an Aubrey horizon; and that a 
recent fault, passing between the two lava-covered areas, had raised the 
eastern lava field to the same level with the western. But this conclu- 
sion is extremely improbable because it is encumbered with the necessity 
of attributing to the lavas a highly specialized recognition of two areas 
that were afterwards to be divided by a fault, and of two different levels 
that were afterwards to be brought into accord by faulting. 
Ficure 6. 
Sketch section across Wonsits plain. 
There is still a distance of some ten or fifteen miles from the lava beds 
to the more open part of Toroweap valley, through which the fault has 
not been traced. When this district is viewed from the top of Mt. 
Trumbull, there seems to be a west-facing bluff of Aubrey or super- 
Aubrey beds extending north from the east side of the valley past certain 
lavas in the valley head toward the low bluff of super-Aubrey beds, 
above-mentioned as overlooking Wonsits plain. This district is not 
easily examined in summer on account of the scarcity of springs. It 
might be studied earlier in the season when certain water pockets can 
be more safely depended upon, and I do not doubt that the fault might 
then be almost continuously traced into the Toroweap. : 
Tue Torowgear Fautt at THE GRanD Canyon. — As one descends 
into the broad Toroweap valley by the trail that leads down the slope 
of a grand lava cascade three miles southeast of Oak spring, it is easy to 
see that the eastern wall of the valley rises higher than the western, 
although both are rimmed with upper Aubrey limestone; and that the 
red lower Aubrey beds are abundantly exposed along the lower half of 
the eastern wall for fifteen or more miles north of the canyon, while 
