60 



FAULTS. — DIKES. 



The nature of the rock, and the thickness and di- 

 rection of the strata, prove that they are the same 

 formation, and that the intervening mass has been 

 removed by some local causes. When strata ter- 

 minate abruptly, they are said to end in an escarp- 

 ment, as at b c. When rocks present nearly perpen- 

 dicular faces, they are said to form mural or wall- 

 like precipices. 



Strata are said to be conformable when their gen- 

 eral planes are parallel, as a a, figure 13 : and uncon- 

 formable when a series of upper strata rest on a 



Fig. 13. 



lower formation, without any conformity to the 

 position of the latter, as b b* 



What is called a fault is such a dislocation of the 

 strata that not only their continuity is destroyed, 

 but the series of beds, on one or both sides of the 

 fractures, are forced out of their original positions. 

 This is applicable particularly to minerals, which 

 are generally found in veins, such as lead, copper, 

 and sometimes coal. In such cases the bed may 

 be formed again, either above or below its original 

 level. 



In the following figure the dark strata represent 

 a bed of coal cut off by the vein running vertically. 

 The matter filling the vein is called a dike. If we 

 suppose this to have been projected from beneath in 

 a liquid form, it will account for the dislocation of the 

 bed of coal. In such cases we are to look for the 



* Comstock's Geology. 



