MOUNTAINS 



333 



unsymmetrical (fig. 4). In the Alps and mountain- 

 ranges of similar character the flexures of the strata 



Fig. 3. Symmetrical Flexures of Swiss Jura 

 ( Mountains of Elu vat .on ). 



are frequently reversed the beds being doubled 

 back so that older strata are inverted and over- 

 turned upon younger beds. The accompanying 



section exhibits the principal features in what is 

 known as the Alpine type of mountain structure 



Fig. 4. Unsymmetrical Flexures of Swiss Jura 

 (Mountains of levation). 



(fig. 5). A glance at the diagram will show that 

 the greater features of the surface coincide approxi- 

 mately with the larger flexures of the strata, but 



Fig. & Section across a Mountain-chain ( Elevation Mountains ) : a a, Isoclinal Folds ( showing inversion 



of strata) ; 6 6, Symmetrical Folds. 



that these features have been greatly modified 

 by denudation. In such mountain-regions the 

 highly flexed and contorted strata are frequently 

 dislocated ; but for the sake of clearness such 

 dislocations are not represented in the diagram. 

 Occasionally we find that the prominent features 

 in a mountain-region have been determined by pro- 

 found dislocations of the rocks, as is well shown in 

 the parallel ranges of the Great Basin, of western 

 Arizona, and of northern Mexico. That region has 

 lieen divided into a series of long narrow blocks by 

 a system of parallel dislocations the prominent 

 mountain-ridges corresponding to the blocks on 

 the high or upcast side of the fractures. Although 

 the direction and general form of those mountains 

 are thus the result of earth-movements, the 

 evidence of subsequent erosion and denudation is 

 everywhere conspicuous. 



( c ) Mountain* of Circumdenudation. In countries 

 composed of undulating and highly-folded strata 

 which have lieen for long ages exposed to the 

 action of eroding agents the ultimate form assumed 

 by the ground i- din-i-tly dependent on the character 

 of the rocks and the mode of their arrangement 

 (fig. 6). Plateaus in course of time come to be 





Fig. 6. Escarpments fe.e) and Hills of Circum- 

 denudation ( h, A ). 



deeply trenched in different directions and eventu- 

 ally lose their plateau character. The remaining 

 portions of high ground then form groups of moun- 

 tains and hills. In regions of horizontal or approxi- 

 mately horizontal strata the mountains assume the 

 form of pyramids or flat-topped mesa* and butte*, 

 excellent examples of which are seen in the 

 western territories of North America, and in the 

 much-denuded basaltic plateaus of Iceland and the 

 Faroe Islands (fig. 2). In regions of folded and 

 contorted strata, composed of diverse kinds of 

 rock, the orographic features are more variable. 

 A highly-denuded plateau of folded strata seen 

 from a height presents the appearance of a tumbled 

 and billowy sea the Scottish Highlands and the 

 high grounds of Scandinavia being examples of 

 mountains of circumdenudation which have been 

 carved out of elevated plateaus of denudation 

 (fig. 7). The origin of a plateau formed of such 

 folded and contorted strata requires a word of 



explanation. Mountains of elevation are in the 

 course of time denuded and degraded, and should 

 the land of which they form a part remain long 

 enough above the sea, the whole surface must 



Fig. 7. Plateau of Denudation, A B ; showing 

 Mountains of Circumdenudation (m,m). 



eventually be reduced to the condition of a low 

 gently undulating plain. Should elevation now 

 ensue, this plain becomes a plateau the surface 

 of which by-and-by is trenched and furrowed by 

 running water, &c., as is the case with the ancient 

 plateaus of Scotland and Scandinavia. 



Age of Mountains. As might have been expected, 

 mountains are of all geological dates, and the age 

 of a large number has been determined. But this 

 has reference chiefly to those that owe their origin 

 to accumulation (volcanic cones) or to com- 

 pression and folding of strata during great earth- 

 movements (mountains of elevation). Many of 

 the latter are the result, not of one, but of several 

 successive periods of uplift. It can often be shown 

 that between those periods of movement the 

 mountains have been subjected to long-continued 

 erosion, and partially or even wholly submerged, 

 while newer accumulations of sediment were gradu- 

 ally piled up over their denuded surfaces. The 

 Pyrenees, the Alps, the Jura, the Himalayas, the 

 Andes, and many other conspicuous ranges have 

 been formed by successive upthrusts, separated by 

 longer or shorter periods of degradation and sedi- 

 mentation. Some mountains of elevation, which 

 originated in very early geological times, appear to 

 have been denuded down to their very roots 

 reduced to the condition of low-lying plains. Such 

 plains have subsequently been pushed up bodily 

 and converted into plateaus', which in the course of 

 time have been profoundly modified by denudation, 

 so as now to present the appearance of a rolling 

 mountainous country the mountains being moun- 

 tains of circumdenudation (fig. 7). The greatest 

 and loftiest mountains of elevation have all received 

 their latest uplift in comparatively recent geological 

 times. Amongst such young ranges we find that 

 the larger orographic features coincide more or 

 less closely with the greater convolutions, folds, 

 and fractures of the strata. In ranges belonging 

 to much older dates denudation has profoundly 



