X. 



but more frequently it is inclined. The angle which the fault 

 plane makes with the vertical in this latter case, is the angle 

 of hade. The overlying portion of the strata, along an 

 oblique fault plane, constitutes the hanging wall of the fault. 

 The underlying portion constitutes the foot wall. If the 

 hanging wall of the fault has slipped down, tension is indi- 

 cated, for the strata now occupy more horizontal space 

 than before, as can be easily tested by an experiment, with 

 blocks to represent the strata. In such a case, the faulting 

 was caused by the action of gravity, which pulled down the 

 hanging wall. Therefore such a fault is called a "gravity 

 fault" and in as much as most faults are gravity faults, 

 thev are commonly called "normal faults" (fig. vi., 1). II 

 however, the hanging wall slips up, a compression is indicated, 

 which shortens the beds, so that they occupy less horizontal 

 space than before. A thrust force is required for the produc- 

 tion of such faults, and they are therefore called "thrust 

 faults." Being of less frequent occurrence than the other 

 class, they are also called "reversed faults" (fig. vi., 2, 3). 

 It is the latter kind which occurs in this vicinity. 



Related to the disturbances which produced faults and 

 folds, are those which produced joint cracks, i. e. those 

 prominent fissures which traverse all the rocks of this 

 region (see Plates III., XXVI. and XXVII.). One expla- 

 nation of these is, that they originated through the action 

 of earthquake shocks, which traversed the rocks, and pro- 

 duced a series of earth-waves, which in unconsolidated 

 material would produce little effect, but in solid rock would 

 produce these fissures at regular intervals (Crosby). The 

 other well accepted explanation which accounts for these 

 joints is, that by unequal elevation, the beds have become 

 twisted and have been subjected to a torsion strain, and 

 that this has produced the parallel and intersecting joints 

 (Daubree). Both causes undoubtedly co-operate in the 

 formation of these joints, as is well illustrated, when a sheet 

 of glass is twisted and then a shock sent through it by a 



