FAULTS. 
61 
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. 
i 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 , 
F 
