THE NATURAL BOUNDARIES. 
11 
with the Paria subdivision, invoking its presence whenever it can be util- 
ized and dismissing it from the discussion whenever it is de trop. 
If we were content to discuss merely the existing topography, the 
northern boundary would soon be chosen at the base of the Vermilion 
Cliffs, where the splendid succession of terraces ends and the broad expanse 
of the desert begins. But the geologist, looking beyond the visible present 
into the past, seeks for history and the process of evolution. The history 
of the Grand Canon district is a remarkable one, and its few remaining 
records are in great part disclosed to us in the terraces which lead up to the 
High Plateaus. These terraces may be regarded as the appanage of either 
district — as the common ground where the threads of their respective his- 
tories are interwoven. Neither district can be comprehended without some 
knowledge of the terraces, and since they have been lightly touched upon 
in the monograph of the High Plateaus, it is deemed necessary to give a 
fuller account of them here. 
The southern boundary of the district is a continuation of the western 
boundary. The grand escarpment which overlooks the sierra country to 
the west stretches southward across the Colorado, preserving identical 
features as far as it has been thoroughly studied; that is, as far as thirty or 
forty miles south of the river. We do not know as yet whether this con- 
tinuity of character and relations in the terminal escarpment persists far 
beyond that point, but, from the superficial knowledge we possess, it may 
be inferred that it does. As far as we know, and we have accounts cover- 
ing the entire southern border, the great mural termination of the Carbonif- 
erous platform which constitutes the surface of the district slowly changes 
its trend south of the river, and at length follows an east-southeasterly 
course through eastern Arizona. There, as at the Grand Wash, the surface 
of the country descends at once from the horizontal platform into a lower 
country, apparently identical in its topographic and geologic features with 
the Great Basin and with the terrible desert along the lower courses of the 
Colorado. But whether the passage is across a great fault, as it is at the 
western side of the district, still remains to be investigated. 
The four plateaus thus far named, all lie upon the northern side of the 
Colorado. They are for the most part divided by distinct lines and only 
