EROSIONAL ORIGIN OF GREAT BASIN RANGES 35 
and soft, which alternate frequently, are evenly beveled, indicating 
that the country at the time of planation must have been only 
slightly above the sea-level. (2) The principal orogenic deforma- 
tion and faulting appears to have taken place in early or mid- 
Tertiary times, and prior to the period of the general planning-off 
of the country, as represented by the Mesa de Maya level. (3) The 
numerous mountain ranges of New Mexico, outside of the Rockies, 
are subequal in height, a fact, when taking into account the period 
of the principal deformation and faulting, the general alternation 
of hard and soft belts of rock, and the extent of the subsequent 
denudation, showing that the present cycle of erosion must have 
started with the country already more or less of a plain. (4) The 
present bilateral symmetry of most of the desert ranges, even in the 
cases of the so-called tilled block-mountains, as the Jemez, Sandia, 
Franklin, Magdalena, and Caballos ranges, for examples, is suggestive 
of long-continued attack by the elements upon the hard mountain 
rock. In each of the mountains mentioned the major fault-line, 
if it really exists, is as far from the crest of the mountain-ridge as is 
the foot of the backslope. (5) Plateau plains that lie high above the 
present ‘general plains-surface, but still far below the Mesa de Maya 
level, are beveled rock-surfaces, protected usually by lava flows. (6) 
With all of the present ranges of the so-called block-type bordered on 
either side by soft beds of great thickness, and the very resistant 
mountain strata, in monoclinal attitude, once extending such rela- 
tively long distances beyond the present mountain crests, it does 
not seem likely that general lowering of the surface of the country 
could have gone on so evenly without something of a plains-surface 
to begin with. (7) The postulation of a general mountainous sur- 
face at the commencement of the present geographic cycle as rep- 
resented by the Mesa de Maya planation surface finds many 
incongruities which need not be dwelled upon at this time. 
How utterly inadequate is any purely tectonic explanation of the 
larger relief features of the Western desert region, as they today exist, 
becomes readily apparent so soon as the real nature and the physio- 
graphic significance of certain of these characteristics are taken into 
account. In the consideration of erosional effects in the desert there 
are some important pecularities which are commonly overlooked. 
