78 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Tumamoc Hill, the reversal of dip toward its center, and the direction 
of the flow toward the north and northeast make it probable that the 
center is hidden under the body of the flow itself below the top of Tumamoc 
Hill. One of the noticeable features of these flows is their viscosity, thus 
differing from the ordinary liquid basaltic extrusions. This is the reason 
for the peculiar character of the final eruption, which was a swelling or 
piling out of viscous lava, and a consequent sealing of the conduit by 
the erupted material. This possibility is shown in section C—D by dotting 
in a theoretical plug. 
The description of the small intrusion marked B’, and described micro- 
scopically under the name of amygdaloidal basalt, has been delayed 
because there are few data to locate it in the history of the region. This 
was intruded into the andesite underlying basalt No. 3, and probably 
did not reach the surface. It will be found in the southeastern corner 
of the map. The only evidence bearing on the time of its intrusion is that 
both B, and B, are bent up around it, while the tuff does not show any 
change in dip. It is therefore probable that the intrusion occurred some 
time between the intrusion of B, and the deposition of the tuff. 
The second jaulting.—B, was extruded over what formed at that time 
a part of the Tucson Mountain slope, dipping at a gentle angle to the north- 
east. The flow extended as far east as the summit of Sentinel Hill (as- 
suming that the upper basalt covering the summit of the peak is a part 
.of this flow.) Subsequently the basalt and the underlying tuff were eroded 
from the area between the two hills. If the alternate hypothesis is cor- 
rect, that this patch of basalt marks a separate center of eruption, the 
lava overflowing to the north over the tuff, for which there is evidence 
somewhat similar to that suggesting a center under Tumamoc Hill, but 
little erosion need be postulated between the eruption of B; and the 
faulting which gave the hills their present form. 
The existence of faulting is discovered by indirect evidence, as I was 
not able to locate the walls of the break at any point. Some of the evi- 
dence bearing on this is the elevation of both the conglomerate and the 
tuff above the level at which they lie to the west; the sharp cliffs that 
separate the basalt and tuff from the underlying andesite on the south 
and east sides of the hill; the cliffs and truncated formations facing the 
Santa Cruz River on the southeast, etc. The fault around the small 
center hill is indicated by the perpendicular cliffs of basalt underlying 
the tuff on the south and west sides of the hill, and by finding a remnant 
of tuff and underlying andesite-conglomerate in the arroyo bottom, from 
which position it has been faulted up as shown in section E-F. 
Some idea of the time that has elapsed since the faulting can be ob- 
tained by an examination of the fringe of talus conglomerate cemented 
by caliche which surrounds the hill. The torrential precipitation aided 
by steep slope and gravity has no trouble in handling the volcanic blocks, 
