FAULTS. 



77 



The plane of separation between the elevated and depressed por- 

 tions of the strata is sometimes vertical, but generally sloping a little. 



The direction of inclination of 

 the plane of a fault is termed its 

 hade. In this case a peculiar 

 general relation is observed be- 

 tween the inclination of this plane 

 and the effect of the dislocation. 

 In fig. 29, for instance, the plane 

 of separation, z z, slopes under the 

 depressed, and over the elevated 

 portions of the disrupted strata, 

 making the alternate outer angles 

 2 z &, z z V acute. In several 

 hundred examples of such disloca- 

 tions which have come under notice an exception to this rule is 

 rarely found. The direction of the hade is almost invariably towards 

 the downthrow. A similar law is found to prevail very generally in 

 the crossing of nearly vertical mineral veins; for instance, in fig. 

 30, a a are two portions of a metallic vein, dislocated by another 



Dlsloeativn of a JVin. 



Fig. 29. 



Fig. 30- 



Fig. 31. 



po 



em, b b. In this case the relation of the line b b to the lines a a, 

 s the same as that of z z to the lines b &', &c. The contrary 

 ppearances, had they occurred, would have been as represented in 

 g. 31, and such occur in the mining district of Cornwall; they 

 re termed upthrow or reversed faults. When faults are parallel to 

 h other, and the throw is always in the same direction, the 

 rata descend like steps, and the faults are called step faults. When 

 ,ults cross each other they produce the phenomena termed trough 

 ults or cross faults. 



The line in which a fault extends is always sinuous, and owing to 

 isplacement faults always include many pockets in which minerals 

 ay accumulate. 



The line of dislocation is generally distinguished by a fissure 

 hich is filled by fragments of the neighbouring rocks or by basalt, 

 d then is called a dyke, or by various sparry and metallic minerals, 

 d is then called a mineral vein. The faulted surfaces which have 

 been compressed against each other are hardened, striated, and often 

 lished, when they are termed slickensides. 



