232 



The Realm of Nature 



CHAP. 



strata or by volcanic action from below. Figure 47 repre- 

 sents a section across the chain of the Alps from north 

 to south, the dotted lines indicating the anticlinal arch. 



Mont Blanc 



FIG. 47. Section of the Alps, a, Tertiary rocks ; b, secondary and primary 

 rocks ; c, central core of schistose and igneous rocks. 



Erosion by solar energy probably accompanies the whole 

 process of ridging up a mountain range, and after the 

 elevation is complete the aspect of its scenery, the form of 

 its slopes and valleys, are increasingly due to this cause. 

 Streams flowing down opposite sides of the slope of the 

 long mountain ridges hollow transverse valleys, and so cut 

 the ridge into peaks. Two transverse valleys meeting in a 

 col or pass allow of easy access between the longitudinal 

 valleys which lie between the ridges. Anticlines are much 

 more rapidly eroded than horizontal strata, even when the 

 surface may have the same slope, for the direction of the 

 joint planes and the dip of the rocks favour the formation 

 of landslips. An anticlinal mountain may be viewed as 

 geologically unstable, like a pile of inverted saucers. In 

 many cases the low mountains of the Scottish Highlands, 

 which in remote ages excelled the Alps in height, are now 

 carved out by erosion ( 329) from synclinal strata a form 

 of structure which gives great stability, like a pile of saucers 

 set one within another right side up. 



304. Theories of Mountain Origin. The theory most 

 generally held is that horizontal strata subjected to great 

 thrusting stresses have wrinkled up along a line of weakness 

 in the Earth's crust, by which the whole crumpling is con- 

 fined to a narrow area, the actual lifting power being 

 derived from the contraction of the heated interior of the 

 Earth. Mr. Mellard Reade has brought forward another 



