15S 
IOWA ACADEMY OF SCIENCE 
cari; (4) old lava-sheets which cover soft shales and sandstones, of which the 
Mesa de Maya and Mesa del Acoma are conspicuous examples; and (5) surface- 
wash locally hardened through the evaporation of lime-salts near the surface 
of the ground, well represented by the Galisteo Ceja south of Santa Fe. 
A notable peculiarity of the fault-structures of the Great Basin region in par- 
ticular, and of the arid region in general, is that they are, with some minor 
exceptions, very old, much older than is demanded to control the surface 
relief. When the so-called Basin-Range structure is critically examined, the 
frontal scarps of the tilted mountain blocks are found to have no relation what- 
ever to the profounder fault-lines. No one seems as yet to have discovered such 
lines at the base of the present mountains. Whenever the fault-lines which seem 
to bound the range-blocks are met with, they are invariably several miles away, 
out on the plains. Notwithstanding the fact that many desert ranges are tiled 
mounain blocks, they are symmetrically developed. This would not be true 
were they recently uplifted or tilted. 
Under conditions of a normally moist climate the existence of flat-topped hills 
is now commonly ascribed to circumdenudation effects on an upraised peneplain. 
All remnants of the old graded surface are on the same general level. Through- 
out the arid region the plateau-plains which rise above the surface of the gen- 
eral plains-surface also appear to be the direct result of circumdenudation; 
but, as will be seen later, of a somewhat different kind. In marked contrast 
to the humid-land effects the remnantal plains of the desert, whether their 
surfaces be on stratum-planes, beveled tables of flexed strata, lava-sheets, or 
cemented regolith, are of different elevations, even in the same district. In 
New Mexico, for example, these plains attain all altitudes above the general 
plains-surface, from a few feet, in the instance of the Malagro malpais in the 
Hueco bolson northeast of El Paso, to the broad Mesa de Maya, which is 3,500 
feet above the general plains-surface, and 9,000 feet above the level of the 
sea. The Sierra del Datil, in western New Mexico, has a migniflcent northward- 
facing escarpment 1,000 feet high; and in sight is the Acoma Mesa 500 feet above 
the plains. 
That the arid plateau-plains, small as some of them are, in reality are rem- 
nants of former plains-surfaces of great extent cannot be questioned. Whether 
these old remnantal areas are preserved by lava-flows, indurated strata, or 
cement-beds, it is certain that their surfaces represent essentially the horizon 
of the general plains-surface of the region when it stood at a higher level than 
at present, while since that time general erosion has proceded vigorously until 
the present lower general plains-surface has been reached. The fact that the 
surfaces of the plateau-plains have a vertical range of more than 4,000 feet 
above the present general plains-surface seems to afford incontestible proof of 
the continuity of erosional activities through a very considerable period of time. 
There has been general-leveling wholly independent of normal base-leveling. 
The total amount of erosion accomplished is roughly measurable by the dis- 
tance between the general plains-surface and the crests of the present desert 
ranges that, significantly enough, present remarkable uniformity in height. 
The elevation of the main mountain ranges is about a mile above the general 
plains-surface; below the latter, the canyons of the four through-flowing rivers 
extend more than half of this distance. 
