20 BOTANICAL GAZETTE [JULY 
the main of marine-rock sheets with a general inclination eastward, due to broad 
regional tilting, in which the plains and mountains have shared together. 
But the present surface grade of the Plains is not that of the original tilting. 
The surface has undergone a series of transformations. These have all been 
accomplished by the eastward-flowing streams from the mountains. In a first 
stage the mountain streams, traversing the Plains, cut into the smooth structural 
slope, and produced a topography of parallel broad valleys and ridges. In 
a second stage they ceased to cut, depositing instead, and refilling the valleys 
Fic. 1.—Gully on the east side of the Mesa; alternation between thicket and gra 
formation. : 
they had excavated, even burying the intervening ridges, to a smooth uppé 
surface. The original surface was a product of deformation, the second of @ 
destructive process of stream erosion, the third a product of stream deposit and 
construction, involving the spreading of a waste sheet to great distances and 4 
uniform level, and to a depth over the greater valleys often of several hundred feet 
In the final and present stage, virtually the same streams have returned to the 
earlier destructive habit, and erosion has in large part carried away the high 
level plain of stream construction. About midway of the long slope, in the 
north-south irregular belt, large uneroded fragments of the smooth constructional : 
plain remain. As we have seen above, these fragments constitute the High Plains: _ 
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