176 
STRUCTURAL GEOLOGY 
shaped fold as the latter plunges beneath the post-Paleozoic rocks 
of the Great Plains region. 
It is against the south slope of the sharp Siouan anticline that 
the belted Paleozoic terranes of northeastern Iowa are upturned, 
and apparently cut off. Bearing in mind the geographic position 
of the recognized anticline, an arch between the crest of which 
and the base of the limbs there is a stratigraphic interval of more 
than 5000 feet, it is obvious that the Paleozoic belts originally did 
not really terminate against it in southern Minnesota, but rather 
extended over it, or across it before elevation, and were continuous 
with the Canadian belts. Such being the case it is equally obvious 
that the Iowa belts should not terminate against the remnants of the 
arch, but should swerve sharply and pass westward, in a direction 
parallel to the axis, but beneath the Cretacic cover. This is found 
actually to be in accord with recently observed facts. Keyes. 
Tectonic Setting of Utah's High Plateaus. To visualize pro¬ 
perly the larger structural relationships of the high Plateaus of 
Utah the latter have to be projected as features of the Colorado 
Dome of Arizona. Over a vast central tract of this dome now 
spreads as a surface cover a thick plate of very resistant lime¬ 
stone of Carbonic age, the medial area of which has been swept 
clean of the great mass of shales and soft sandstones which once 
mantled it. Around the flanks of the dome these weak, or non- 
resistant, beds appear in successive belts, their more indurated 
layers forming the coping of inward facing escarpments, or cliff¬ 
lines. 
Above the smooth clean limestone rise occasional buttes and 
plateau plains of limited areas preserved by remnants of ancient 
lava-flows the substructure of which are the red shales and soft 
beds which elsewhere reposed unbrokenly upon the limestone 
throughout the region. Recent volcanic cones also now dot the 
dome surface. Around the San Francisco Mountains, the chief 
peak of which rears itself a full mile above the plain, are grouped 
400 to 500 smaller volcanic vents and their characteristic ash- 
cones. But these surmount the dome surface, having come into 
existence long after the removal of the shale mantle. 
Save at the Grand Canyon the great limestone surface plate 
is untrenched by important drainage ways. KeyEs. 
