670 THE POPULAR SCIENCE MONTHLY. 



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 plica- 

 tion 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. 



2. Arched Mountain Ranges. 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 Archaean 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 dis- 

 location. 



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 struct- 

 ure, we shall further examine this question of the method and the 

 origin of the diverse manifestations of mountain-building. 



