140 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
limestones constitute the upper member of the sedimentary series. 
The surface remains fairly uniform, so far as we could judge, and it is 
for the most part covered by basalt flows (whose appearance indicates 
that they should be classed with the recent rather than with the ancient 
basaltic outpourings recognized elsewhere in the Plateau region). 
These relations suggest a peneplain surface bevelling the underlying 
inclined sediments and covered with sheets of lava. Robinson ('07, 
p. 109-129) has recently presented evidence from other localities 
pointing to the same conclusion. The displacement of the lava along , 
the Oak Creek Fault indicates a relatively recent date for the fault, 
and it is probably correlated in time with the more recent move- 
ments along the Hiu'ricane Ledge in the vicinity of the Virgin River, 
described below. 
From Cedar Spring, (fig. A, C. S.) northwest of San Francisco 
Mountain, a broad valley or wash, known as Hull Wash (fig. A, 
H. W.) runs northeast to the Little Colorado. The northwestern 
side of this wash is bounded by a rather i)rominent line of cliff's. For 
long distances the clift'-line is remarkably straight, and small valleys 
often debouch from the face of the clifl^s into the broad wash through 
sharply cut V-shaped openings, rather than by gradually widening 
valley mouths. It is believed that the cliff's have been develoj^ed along 
a fault line trending northeast-southwest. Baker's Butte (fig. A, 
B. B.), a volcano six miles northeast of Cedar Spring, was ap])arently 
formed across the line of displacement. 
A short distance west of Lockett's Tank (fig. A, L.) there is a north- 
south line of clift's several miles in extent, due either to a faulting 
movement that dropped the area west of the fault line, or to erosion 
on a faulted mass. The face of the cliff is of lava, as is also the floor 
of the valley just west. But limestone appears a short distance back 
of the cliff face. No time was available for careful study, but the 
features seemed to indicate that a broad north and south valley, 
possibly related to an early fault line, had been floored with lava, 
while a recent uplift along the eastern side of the valley had develo])ed 
the west-facing scarp. A narrow gorge passes from the broad valley 
eastward through the scarp. Lava flowed through this gorge, and for 
several miles along its continuation eastward across the plateau. 
INIuch of the lava has been removed from the eastern part of this chan- 
nel,, but toward its western end the lava is still in place in the bottom 
of the gorge. Lockett's Tank is a pool developed where the run-off 
