230 



STRUCTURAL GEOLOGY. 



it is sometimes mashed together, sometimes thrown into 

 arches and hollows. Such hendings of the crust produce 

 enormous fractures parallel to the axis of the bending, 

 and parallel to mountain-ranges, since mountain-ranges 

 are produced in this way. Sometimes there is a system 

 at right angles to the main system, or in the direction of 

 the cross-valleys of mountains. 



The characteristics, therefore, of great fissures are 

 1. Their occurrence in systems, usually parallel to the 

 axis of elevation. 2. Their length, often extending for 

 hundreds of miles. 3. Their depth, sometimes breaking 

 through miles of thickness of rock. When filled at the 

 moment of formation with fused matter from below, they 



FIG. 136. Fault in Southwest Virginia : a, gilurian : d, carboniferous. (After Lesley.) 



form dikes ; and all great dikes and igneous overflows 

 have been through such fissures. But if not filled at once 

 with fused matter, but sloivly afterward with mineral 

 matter, they form the great fissure- veins. Whether they 

 are filled at once with fused matter, or afterward slowly 

 with mineral matter, or remain empty, the walls do not 

 usually remain in their original position, but nearly always 

 slip one on the other up or down. Such a displacement 

 of the crust on the two sides of a fissure is called a fault. 

 We have already treated of dikes ; we shall hereafter take 

 up mineral veins. We must now speak briefly of faults. 



Faults. 



As already explained, these are displacements of fissure- 

 walls. They take place on an immense scale. Lesley 



