OCR Text |
Show COMPARISON OF OROGRAPHIC FORMS. 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 conspicuous. 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 individualized, 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 provincesâ€"the Park system, the Plateau system, and the Basin system. The uplits of the plateaus approach in the forms of their displacements 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- |