156 FAULTS. 



that the operation of water has, in many cases, tended largely 

 to modify the nature of these caverns, by eroding the sides 

 and the bed, and altering their form and configuration. The 

 fracture and dislocation of rocks at the surface have often 

 produced the phenomenon of a natural bridge, as exemplified 

 in the accompanying figure (95), which depicts that of Ico- 

 nonzo, in South America, which is produced by the fissurmg 

 of the strata, and the fall of two masses of rock, one above 

 the other, extending across the chasm. 



FAULTS The fissures and dislocations which interrupt 



the continuity of a bed are termed faults. The accompany- 

 ing figure (96) represents an example of this kind, where 

 strata which once were continuous, either by the subsidence 

 of the strata on one side of the fault, or their elevation on 

 the other, have been dislocated and displaced 



FIG. 96. 



These interruptions are a source of considerable difficulty 

 in coal-mines, where they often suddenly interrupt the miner 

 in his course, and deprive him of the treasure which he has 

 found. They, however, offer corresponding advantages, since 

 they counteract the tendency of the coal-seams to plunge to 

 depths at which it would be impossible to reach them ; and 

 when filled with solid materials, they form embankments 

 which keep out the water, that would otherwise flood the 

 mine, and prevent the possibility of working it ; they also 

 act as barriers, and stop the progress of flames. 



DECEPTIVE APPEABANCE or FAULTS. These disturb- 

 ances, however, frequently prove a source of loss and disap- 

 pointment to inexperienced persons, who, observing different 

 appearances of coal at the surface, conclude that they repre- 

 sent so many separate seams ; whereas, in reality, a single 



