UTAH PENEPLAINS 
137 
the southern extremity of the Cordillera, so that their identifica¬ 
tions are seemingly fixed beyond peradventure. 
East Side. 
General surface 
Maya 
Raton 
Corazon 
Tucumcari 
Crest. 
West Side. 
Laramian 
Comanchan 
Jurassic 
Present 
Miocene 
Stream-valleys 
Obliterated 
Obliterated 
Summital 
Central monad’s 
Plains-surface 
Markagunt 
Aquarius 
Paunsagunt 
Rafael 
The broad quaquaversal uplift through which the Colorado 
River cuts its Grand Canyon was long known commonly as the 
Colorado Dome or Colorado Plateau. Since, however, the Rocky 
Cordillera in the state of Colorado was recently determined to be 
really a low arch or flattened dome the same title was widely 
fixed to this uplift also. In order to avoid confusion in nomencla¬ 
ture it was thought advantageous to designate the Arizona dome 
by another name. Navajo-Dome seemed to be most appropriate. 
The Arizona dome area is distinctively the Navajo country. 
Its arid surface is peopled by the well-known Indain tribe of this 
name. Navajo Mountain is a large, isolated, rounded elevation 
forming the boss on the recumbant shield. 
Over the central part of the Navajo Dome now spreads as a 
surface cover a thick plate of very resistant limestone of Carbonic 
age, the surface of which is 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 layers, their more indurated layers forming inward¬ 
facing escarpments, or cliff-lines. 
Above the clean smooth limestone rise occasional buttes and 
plateau-plains of limited area preserved by remnants of ancient 
lava-flows, the substructure of which are the red shales and softer 
beds which once reposed unbrokenly upon the limestone through¬ 
out the region. Recent volcanic cones now also dot the plains- 
surface. Around the San Francisco mountains, the chief peak of 
which rears itself a full mile, are grouped 400 to 500 smaller 
volcanic vents and their characteristic ash-cones. But these sur¬ 
mount the Navajo surface and came into existence long after it 
was denuded of its shale mantle. 
Save at the Grand Canyon the great limestone surface plate is 
