146 
IOWA ACADEMY OP SCIENCE 
6. Nine distinct hypotheses have accounted for the larger orographic fea- 
tures of the desert region of western United States. They are: (a) Gentle 
folding, as advanced by King; (b) simple, normal faulting on a grand scale, as 
urged by Gilbert and others; (c) complex block-faulting as suggested by Lauter- 
back; (d) sharp, asymmetric folding, as postulated by Spurr; (e) great erosion 
under former conditions of a moist climate, as also urged by Spurr; (f) block- 
faulting accompanied by ' great dissection through stream action as suggested 
by Davis; (g) general desert-leveling, locally interrupted by deformation and 
dislocation; (h) thrust-faulting, mainly during the period of aridity; (i) gen- 
eral desert-leveling, chiefly through wind erosion, with the grander geographic 
features accentuated by faulting, either normal or reversed, and directed some- 
what by the local character of the ancient tectonics. 
