89 



aqueo-igneous pastiness, or even complete aqueo- 

 igneous fusion. Finally, this softening determines a 

 line of yielding to horizontal pressure; consequently, 

 as the interior of the earth contracts and the shell is 

 crushed up, the upswelling takes place in these lines. 

 The enormous folding of the strata which must occur 

 in the formation of mountains by lateral thrust 

 would, of necessity, produce fractures at right angles 

 to the direction of the thrust, and the walls of such 

 fractures would often slip under the influence of 

 gravity, or else might be pushed one over the other 

 by the sheer force of the horizontal thrust, thus form- 

 ing the different kinds of faults ; the common kind 

 in which the hanging wall has gone down along 

 the foot wall, and the uncommon kind generally 

 found only in strongly folded strata, in which the 

 hanging wall has been pushed upwards. 



Such breaks and fractures as those mentioned by 

 Le Conte must form lines of weakness and of 

 shattered strata ready to be removed by the first 

 denudant that may act upon them ; while on account 

 of the sheer force of the horizontal thrust, few or no 

 synclinal curves could come up to the surface un- 

 broken, and therefore unmodified, and form valleys 

 solely due to the trough of a synclinal curve. 



It may be said that protrusions of irruptive rocks 

 are capable of forming " intercolline spaces " (Lyell), 

 or valleys between protruded hills. Such hills also, 



