490 
Van Rise—Earth Movements . 
thickened the strata along a relatively weak belt. After a time 
the increased thickness of material is sufficient to present a 
greater total resistance to deformation than the equivalent thin¬ 
ner layers adjacent. The stresses now deform them, and they 
are plicated and thickened. These areas now resist deformation, 
and the stresses deform the adjacent thinner belts, which in 
turn are thickened and plicated, and so on. In this manner 
various zones of plications and consequent thickening and eleva¬ 
tion may be produced in a plateau area. 
The second cause suggested for elevation, deep-seated flowage, 
is dependent upon the hypothesis that the rigidity of the super¬ 
ficial rocks is sufficiently great so that under the horizontal 
stresses in each region there arise a series of arches, which in 
a measure support themselves. Consequently the stresses are 
less than normal below the arches, and greater than normal upon 
the flanks, because of the transmitted thrusts. These stress- 
differences would result in deep-seated flowage from the flanks 
of the arches toward their central parts. This is a modification 
of Willis’s 1 theory of competent structure. The explanation dif¬ 
fers from his in that the broad arches are supposed to be only 
partly supported by the strength of the superficial rocks. If, on 
account of increased temperature, or because of this and decreased 
pressure, the deep-seated rocks are less rigid than in the super¬ 
ficial spherical shells, it would only be necessary for the arches 
to be partly supported by the strength of the rocks. The deep- 
seated materials, as soon as subject to stress-differences greater 
than their rigidity, would ever closely follow the arches and help 
to support them. 
The character of the plateau areas accords with this explana¬ 
tion. As already noted, they are regions of alternating mount¬ 
ain chains or systems, and intervening unfolded or gently folded 
areas. The mountain ranges are the areas of thickening and 
greatest elevation. The intervening unfolded areas may be those 
of the arches and partial self-support. The sides of the mountain 
zones receive the downward thrusts of the intervening arches. 
These forces and the downward pressures of the mountain masses 
1 Mechanics of Appalachian structure by Bailey Willis: Thirteenth 
Ann. Kept. U. S. Geol. Survey, Pt. II, 1893, p. 250. 
