242 



STRUCTURAL OEOLOOY. 



these figures. Fig. 148 shows an example in the Alps, 

 taken from Heim. 



There is another evidence that mountains are formed 

 wholly by horizontal crushing, viz., the phenomenon of 

 xl'ffy cleavage. We have already seen (page 195) that slaty 



-y i x... ) 



Fit,. 148. Section across central Alps : j, Jurassic ; t, triassic ; , schist. 



cleavage always shows a crushing together horizontally, 

 and an extension vertically, of the whole mass. Now, 

 cleavage is always associated with folded strata and with 

 mountain-ranges. 



Mountains are often spoken of as due to "upheaval." 

 There is no objection to the use of this term, if it be re- 

 membered that the upheaval is not usually due to a force 

 acting from beloiu upward, but to a horizontal force crush- 

 ing together and swelling upward by thickening the whole 

 squeezed mass. 



We have, in Fig. 143, B, given the ideal structure of a 

 mountain-range if there had been no erosion. But, of 

 course, as soon as a mountain begins to rise, rain-water 

 begins to cut it away, and in all mountains the amount 

 cut away is immense, in many far greater than what is 

 left. This fact is represented in the preceding figures 

 (144-148). In all these figures, however, except the last, 

 the range is composed wholly of stratified rock ; but in 

 most great mountain -ranges we have an axis of crystalline 



