60 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
ORIGIN OF THE SLOPES. 
The origin, age, and general phenomena of the broad slopes have been 
elsewhere considered in detail.* 
They are generally regarded by geologists as wholly sub-aerial in 
origin, and as deposited without oceanic aid in the distribution of the 
detritus. A prolonged study of these slopes leads to a different view. 
While recognizing the present sub-aerial distribution, it is found to be 
below the level of higher and older slopes which were evidently formed 
under different conditions. These higher portions are above the reach 
of the streams and floods of the present. They supply the detrital mate- 
rials, in a large degree, to the present streams. We thus have slopes of 
different ages. The older and higher may be said to be the initial slopes, 
and the materials washed out from them and reassorted, distributed, and 
laid down at lower levels may be called derivative, and the slopes so 
formed derivative slopes. 
The initial slopes are those which give the characteristic topography 
and the scenic effects of the region. The derivative slopes are lower and 
are generally out of view and confined to the arroyos and washes cut in 
the older initial slopes. While these derivative accumulations are evi- 
dently sub-aerial in their distribution, the higher initial slopes have the 
appearance of being shaped by oceanic action during a period of slow 
subsidence of the land. 
The general leveling distributing action of oceanic tides and currents 
appears to be recorded by the regularity of the surface of the slopes and 
their general uniformity of elevation, at their upper margins, around the 
Valley of Tucson. 
The study of the topography of the valley, and especially of the higher 
parts of the slopes, reveals the fact that the deposits are older than the 
existing river and other drainage channels. The slopes were evidently 
first formed and have since been extensively eroded and cut away by 
the river and flood-waters, and to such a degree that in many places por- 
tions only of the original slope remain; but such portions show that they 
were once parts of a continuous sloping surface or inclined plane. It 
would appear that such deposits antedating the drainage must have been 
built up during a period of subsidence succeeded by elevation. The 
streams flowing from the mountains to-day are destroying and not build- 
ing up such slopes. They cut through and erode the old slopes, in some 
instances cutting across the slope, as, for example, on the San Pedro River, 
where a long slope is bisected by the river, leaving a ridge of detrital 
slope-deposits on each side. So also the Rillito north of Tucson cuts 

*The flanking detrital slopes of the mountains of the southwest portion of the 
U. S.; Meeting of the American Association for the Advancement of Science, New 
York, December, 1906. Published in Science, N. S., vol. xxv, No. 651, p. 974. 
