North of the Grand Cafion of the Colorado. 463 
we have designated as table mountains. 
The expiring energies of these eruptive agencies have left 
great numbers of cinder cones standing in lines along the fis- 
sures. Many of these have well defined craters, and they 
everywhere form conspicuous features on the landscape. 
Cafions and Valleys. 
No sharp line of division can be drawn between cafions and 
valleys. For convenience, we designate intervening depressions 
caused by erosion, cafion valleys, but all of these excavated 
pons and troughs will be included under the general head of 
valleys. 
This is a region almost everywhere of naked rock. The 
cafion walls, and cliffs, present vertical sections of strata of 
great magnitude, and the nakedness of the upper surface of the 
rocks, together with the exposure in the escarpments, make it 
possible to examine the geological structure of the countr 
with great thoroughness; and conclusions may be reached with 
a degree of certainty elsewhere rarely attainable. Under these 
circumstances, it has been ible to understand the causes 
which have combined to determine the vast system of drain- 
age, and to discover the relation in the direction of the valleys 
to the dip of the folds. I propose to classify the valleys o 
this country in the following manner: ; 
Order first: transverse valleys, having a direction at right 
angles to the strike. AiO 
Order second: longitudinal valleys, having a direction the 
same as the strike. : 
Of the first order three varieties are noticed : 
a, monoclinal, those which pass through a fold ; i 
b, acclinal valleys, that run in the direction of the dip; 
e, contraclinal valleys, that run against the dip of the beds. 
