56 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
The basalt is now also being largely utilized for broken stone for the 
streets and pavements of Tucson and for concrete constructions. 
In addition to the amygdaloidal structure of portions of the massive 
basalt, there is a remarkable development of laminated or schistose 
structure, apparently the result of pressure and flow or of shearing stress. 
The northern slope of Tumamoc exhibits a succession of transverse sub- 
ordinate or low ridges some six in number, as viewed from the Hospital 
road approximately parallel in direction, but largely of the vertically 
laminated portions. One of the more striking examples of such lamina- 
tions is found on Sentinel Peak. The laminated structure is in curves 
nearly vertical at the cropping and then inclining downward till they 
become nearly horizontal. The layering is very distinct in the gray 
basalt of the lower summit of the peaks. The trend or direction of the 
layers is about north 70° east, the result apparently of the local direction 
of the flowing mass. 
THE MOUNTAIN SLOPES. 
The mountain ranges about Tucson, and generally in the Southwest, 
are separated by broad valleys, commonly regarded as plains or mesas, 
but in reality sloping surfaces stretching outward and downward at a 
gentle inclination from the rugged ridges of the mountains. The topog- 
raphy changes suddenly from rocky precipitous acclivities to gently 
inclined surfaces of gravel and loosely aggregated material—the detrital 
accumulations washed out from the ridges by streams and floods. These 
deposits consist of boulders and fragments of the rocks, broken up and 
partially rounded, together with gravel and sand and argillaceous layers 
all rudely stratified. These materials are coarser and heavier near the 
mountains than at some distance away, where they become more water- 
worn and are finer, but large boulders and coarse gravels are distributed 
to great distances, even miles away from their sources, and form thick 
deposits evidently of great age. 
The great horizontal extent of these slopes, together with their regu- 
larity and evenness of outline, bounding the vision like the horizon line 
at sea, form, with the exception of the mountains, the most striking of 
the scenic features of the Southwest. They constitute the greater por- 
tion of the surface area, estimated together with the alluvions at not less 
than nine-tenths of the whole. 
The angle of inclination is greatest near the ridges, but for the middle 
and lower parts of the slope, from 80 to too feet per mile is a common 
gradient. 
Although the outlines of these long slopes form such conspicuous 
features of the landscape in the Southwest, it is difficult to illustrate them 
by photographs, as they are so extensive and panoramic, but an attempt 
has been made, with the results shown in plates 50 and 51. 
