74 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
smaller hills the most easterly of the group is Sentinel Hill and the third, 
which is a ridge between the two, is unnamed. ‘Topographic analysis 
leads to the following divisions, which will be described in order. 
(1) The Tucson Mountain Slope.—This extensive slope reaches down 
from the Tucson Mountains several miles to the west. It comprises 
more than half of the height of the range above the Santa Cruz River, 
to which it descends. 
(2) The Tumamoc Hill Slope-—The Tucson Mountain slope passes 
under the superposed Tumamoc Hill slope. The two meet on a line 
marked approximately by the main gully southwest of Tumamoc Hill. 
The lower limit of this slope is well defined on three sides by sharp con- 
tact with the Tucson Mountain slope, and on the east by the Santa Cruz 
alluvial flat. The upper limit, however, practically defies definite limi- 
tation, culminating in irregular talus slopes reaching up to the top of the 
hill in places, while again the lava rock extends down to the Tucson 
Mountain slope or the alluvium. 
(3) The rock surjace——This can not always be separated from the 
débris-built slope, as there is more or less volcanic rubbish, even on the 
tops of the hills. In all the hills it has a moderate dip toward the north, 
but faces the south with a much steeper declivity, developing small cliffs. 
The northern rock surface is diversified in a minor way by small hum- 
mocks 5 or 1o feet high, formed by a turning-up of the edges of the 
cooled surface of the lava by a second movement. Talus patches lie 
below these cliffs. 
(4) The alluval plain.—This is the product of the Santa Cruz “ River,” 
a small intermittent stream (over 100 miles in length, however), which 
until a few years ago was in the depositional stage, but, aided by an 
abandoned ditch, succeeded in starting a new cut which it is now extend- 
ing southward. 
GEOLOGY. 
The Tumamoc Hills are the product of the final (probably Quaternary) 
stage of the extensive Tertiary volcanic activity of this portion of Ari- 
zona. The interesting features are presented within an area of about 2 
square miles in the immediate vicinity of the above-mentioned hills. 
Originally most of the surface was mantled with a covering of volcanic 
blocks, averaging roughly between 6 inches and 3 feet in diameter, and 
under those conditions the first impression would have been that the 
hills were formed by explosive action. The industry of the Mexican 
teamsters who have transported most of the surface covering to Tucson, 
where it is in demand for foundation purposes, has developed unusual 
facilities for observation, the results of which suggest that many of the 
déebris-choked lava’ hills have a more interesting and complicated struc- 
ture than is suggested by their exterior. With the accompanying topo- 
graphical and geological maps (plates 26 and 27) before the eye, it seems 
