Chap. VIII. DEPRESSION OF THE PLATEAU. 559 
VIII. — Although this great longitudinal basin may be 
called a plateau or table-land, from its predominating 
elevation above the level of the sea, yet there are 
considerable variations in its height, and three large 
declivities — not to mention similar smaller outlets — open 
through its borders, and form transitions from its inner 
and more elevated to the outer and lower land : the slope 
of the Rio Grande, that of the Colorado and Gila, and 
that of the Columbia. 
Between the central part of the Rio Grande valley and 
the central part of the Gila valley, the plateau is less high 
than to the north and south of this line. The Laguna de 
Guzman, as Mr. Schuchart, the companion of Colonel 
Gray, communicated to me, lies even lower than the 
surface of the Rio Grande, near El Paso. The Laguna 
de Santa Maria has the same level. Into this latter lake, 
as I have already mentioned, the Rio Mimbres pours 
its waters coming from the north, whilst from the south 
the Rio de Santa Maria, flowing down the central pla- 
teau of Chihuahua with a rapid fall, empties itself into 
the same lake. A line drawn from these two lakes 
toward the Dry Lagoon on Cook's Road, which I have 
before described, forms a north-western continuation of 
this depression of the table-land ; and from this latter point 
the central part of the Gila can be reached without having 
to cross any considerable height. 
From the mouth of the Rio Grande, therefore, up this 
river, crossing from its central course over to the central 
Gila, and down this river as far as the mouth of the 
Colorado, we can follow a depression of the continent from 
one ocean to the other; which depression, quite apart from 
the above explained general relations, separates the system 
of the Rocky Mountains, together with the whole of the 
