COMPAEISON OF OBOGKAPHIC FOEMS. 51 



is very striking. If we compare these uplifts with the Park Ranges and 

 with the Uintas, the similarity of the structural profiles is very conspic- 

 uous. But in the plateaus there is greater simplicity, less subordinate 

 flexing (indeed almost none at all), and an absence of convexity in the 

 section lines. 



Crossing the abrupt boundary which separates the plateaus from the 

 Great Basin, we are at once among mountains of a very different order. 

 The Basin Ranges are many in number and inferior in magnitude to those 

 of Colorado, though of no mean dimensions. They are strongly individ- 

 ualized, each being separated from its neighbors by broad expanses of plains 

 as lifeless and expressionless as Sahara. It is as difficult to find a type-form 

 representing the construction of these ranges as for those of Colorado. Yet 

 there are common features of almost universal prevalence among them and 

 at the same time thoroughly distinctive of the group. There is on one side 

 of the range, sometimes a single great fault, or more frequently a repetition 

 of faults throwing in the same direction, while upon the other side the 

 strata slope down to the neighboring plains and there smooth out again. 

 There is much variety in the details of the dislocations, and so complicated 

 do they become in certain localities, that they sometimes mask the general 

 plan until we carefully unravel it. The strata also are almost invariably 

 tilted to high degrees of inclination, thus contrasting strongly with the low 

 and almost insensible slopes of the plateaus. Hence on one side of the 

 range the slope of the profile is along the dip of the strata, on the other 

 side it is across their upturned edges. 



We may now compare the orographic forms prevailing' in the three 

 great provinces — the Park system, the Plateau system, and the Basin sys- 

 tem. The uplits of the plateaus approach in the forms of their displace- 

 ments more nearly to those of the Park Ranges than to those of the Basin, 

 but are much simpler, much less complicated by subordinate fracture and 

 flexing, and have undergone a much smaller amount of vertical movement. 

 There is, however, one very striking contrast between the Plateaus and 

 the Park Ranges. In the latter, erosion has played a most important part in 

 their history and development. The mountain platforms have undergone 

 an amount of degradation which never fails to revive astonishment when- 



