OF THE EARTH. 



139 



are parallel to each other. They are unconformable when 

 their planes are not parallel. Thus, in Fig. 66, all the 



Fig. 66. 



horizontal strata a are conformable to each other, and so 

 are also the inclined strata b; but the strata b have the 

 strata a lying unconformably upon them. All the strata 

 b were deposited in a horizontal position, and there was 

 no upheaval till the last of them was completed. Then 

 the state of quiescence was broken, and the strata were 

 lifted up as you see; and when the upheaving force 

 ceased to act, the upper horizontal strata began to be 

 deposited. Periods of immense length were required 

 for all these successive processes, for the upheaval was 

 probably very slow, as the depositions certainly were. 



223. Faults. Whenever in a fracture of strata there 

 occurs a dislocation of them, leaving those on one side 

 of the fissure higher up than those on the other, it is 

 called a fault, a name which was first given to such a 



state of things by miners. 

 In Fig. 67 you have a rep- 

 resentation of a fault. The 

 strata marked with corre- 

 sponding letters on each 

 side of the fissure w r ere 

 continuous till they were 

 all deposited, and then, by 

 some force, either the stra- 

 ta on the right hand were depressed, or those on the left 

 were elevated. Faults vary much in the degree of dis- 

 location, from a few feet up to many thousands. Some 

 of the grand changes in the earth's crust have been pro- 

 duced by faults. 



224. Denudation. This term is applied to the wear- 



