FAULTS. 



Fig. 14. 



bed above the lowermost strata. In other words, if 

 the dike makes an acute angle with the upper sur- 

 face of the coal-vein, the strata are elevated on that 

 side ; while, if the angle is obtuse, they are thrown 

 down. 



The extent of vertical displacement occasioned 

 by faults varies from a few inches or feet to thou- 

 sands of feet. It is not uncommon in coal-fields 

 to find the strata raised on one side of the dike to 

 the extent of 5 or 600 feet. There are scarcely 

 any phenomena in geology which prove more con- 

 clusively the application of a powerful internal 

 force than the displacements under consideration. 

 Sometimes they extend from 20 to 40 miles in 

 length; and all the rocks which have been disturb- 

 ed by any fault have experienced on one side the 

 same movement and to the same extent, excepting 

 only those portions which have been subjected to 

 violent pressure. It has been remarked, that min- 

 eral veins are not otherwise different from faults 

 than by reason of the fissures which these have 

 opened in the rocks being filled with spongy and 

 metallic matters. This filling of a fissure consti- 

 tutes a mineral vein; a similar fissure, filled by ba- 

 saltic or other rocks, would be called a rock dike^ 



