ORIGIN OF TOrOGEAPniC FORMS. 109 



are of continental extent, producing plateaus, while ethers have been very 

 limited in extent, throwing up narrow ridges or blocks. They have 

 uplifted the strata at various angles, so high in some cases as to throw them 

 beyond the vertical, infolding the strata and even breaking them by faults. 



Incidental to the uplifts are flexures and faults. The flexures may be 

 classed as anticlinal folds, where they are bent downward on either side, 

 and monoclinal flexures, where local strata first bend downward and then 

 by a reverse curve resume horizontally. In a fault the rock is divided by 

 a fracture and one part is moved up past the other. 



It is through uplift that continuous mountain ranges, ridges, and 

 inclined plateaus have originated — not, however, in the shapes that appear 

 to-day, for most of them during ami since their rise have been carved by 

 erosion out of all resemblance to the forms which uplift alone would have 

 given them. 



The ridges and valleys of the Appalachian region are the results of 

 uplifts, with numerous sharp folds and faults, which raised at various angles 

 an alternation of hard and soft beds, from which erosion lias since carved 

 the existing alternations of ridge and valley. 



Other movements of uplift, resulting- from the intrusion among the 

 strata of great lenses of volcanic rock, have usually resulted in the forma- 

 tion of elliptic mountains or groups of mountains. As these movements 

 have occurred at different periods in geologic history, some have been 

 affected more, others less, by erosion. Certain mountains of this volcanic 

 type present to-day an aspect little affected by erosion, while others have 

 been greatly modified by its agency. 



Sierra la Sal, in eastern Utah, is an example of this class. Here the 

 stratified beds above the volcanic rock which were bent upward by the 

 uplift w r ere probably broken over the top, ami have been removed by 

 erosion until now they only surround the base of the group, dipping away 

 from it steeply, forming hogbacks. 



In New Mexico there are seen numerous volcanic "necks" rising 

 abruptly from the plateau. These necks are intrusions of volcanic rocks, 

 which were forced up while molten into the stratified rocks. The latter 

 have since been eroded away, leaving the harder necks as isolated, precip- 

 itous mountains. 



