Page:Popular Science Monthly Volume 39.djvu/688

670 Highlands, only the bases of the ancient mountains remain. Eroded, perhaps repeatedly, almost to the sea-level, these plicated areas have been again elevated and are now deeply incised with valleys, fiords, and lochs. In the western mountain belt of North and South America a long and eventful history of extended plication and upheavals in many separated geologic epochs is more or less clearly revealed; but the latest accidents befalling this belt during the Quaternary era will call for special description under the fourth structural type, with which the earlier revolutions of this most prolonged mountain chain will be reviewed.

—Far less frequent than the foregoing, and indeed known only in parts of the Cordilleran belt of the western United States, is the arched structure, which may be best described in its most typical example, the Uinta range of northeastern Utah. According to Powell's report on the geology of these mountains, a great thickness of many rock formations has been here raised in an arch about one hundred and fifty miles long from east to west and thirty to forty miles wide. The strata range in age from the Archæan and Cambrian to the Cretaceous and Tertiary, and they appear to have reposed horizontally, as laid down in the sea, until the end of the Cretaceous period. The upheaval took place during the Tertiary era, mostly in its earlier portion, and the whole extent of the upward arching was about five and a half miles. Erosion, however, has gone forward during the growth of the arch, so that the highest peaks of the range have an altitude of only about two and a half miles, or thirteen thousand feet. Upon each side of the Uinta arch and about its ends the stratification is steeply inclined and occasionally cut by faults; but higher up the inclination diminishes and the strata extend across the top as a flattened dome, without folding or dislocation.

How were the mountain-building forces applied to form this arch? Its short extent in proportion to its width and the absence of plication make it difficult or impossible to refer it to lateral pressure, which has been regarded as the manner of application of the energy forming the great folded ranges. All the features of the Uinta range, instead, point to upward pressure as the form of mountain-building energy to which its elevation was due. It is very important, however, to note that the process of the Uinta elevation was so gradual and slow that the rivers which flowed across the area before its upheaval were not turned aside, being able to cut down their channels, which in the heart of the mountains are precipitous, narrow canons, as fast as the elevation progressed. After the consideration of the remaining types of mountain structure, we shall further examine this question of the method and the origin of the diverse manifestations of mountain-building.