THE CRUST OF THE EARTH. 



79. In undulating and mountainous countries it is found, 

 however, that, instead of being horizontal, they are variously 



Fig. 26. 



inclined (fig. 26), and sometimes bent into, or even beyond, the 



vertical direction (fig. 27). 



"When it is considered that at the time of their deposition the 



strata must have 

 been horizontal, it 

 will be evident that 

 the position shown 

 in these figures could 

 only have been given 

 to them by a force 

 acting from below, 

 by which they were 

 heaved upwards, 



and by which the crust was broken, the igneous rocks having 



forced themselves through it. 



80. In such cases hills or mountains of greater or less elevation 

 are formed, at the summits of which the igneous rocks which 

 have been protruded through the stratified crust are at the 

 surface, and the edges of the strata thus broken are ranged along 

 the flanks, the lowest or most ancient being nearest the summit, 

 and the others succeeding each, other in their proper order in 

 descending towards the adjacent valley or plain. 



81. It will be evident that the stratum whose edge is highest 

 on the mountain is that which lies the lowest on the plain, and 

 that whose edge is lowest on the mountain is that which is 

 highest or nearest the surface on the plain. 



82. In the cases here exhibited the igneous rocks have split the 

 stratified crust and broken quite through it. This, however, is 

 not always the case. It often happens that the uplifting force 

 loses its energy before the superincumbent strata are cracked, in 

 which case they would cover the elevation preserving the 

 order of their superposition, but would be, as before, inclined 

 in accordance with the declivity of the hill produced by the 

 uplifting force. 



72 



