Rev. 0. Fisher — On Faulting, Jointing^ and Cleavage. 267 



That is to say, the inclination of the fault to the vertical {i.e. 

 90° — 0, which we may call the co-hade) must be greater than the 

 angle of repose, or else a fault cannot be formed, because the friction 

 will be too great to allow of the surfaces moving past each other. 

 It will be shown in Part IV. that this condition has an important 

 consequence. 



It must be recollected that Wis, the vertical compressing stress, 

 and not the weight of the cover. Failure of support from beneath 

 would therefore cause W to vanish. 



A reversed fault would probably be the earliest consequence 

 of horizontal compi-ession affecting a tract of country. As the com- 

 pression still further increased, and crumpling of the rocks super- 

 vened, the fault itself would be overturned, and jaerhaps so broken 

 up as to be no longer recognizable as a fault, and certainlj'^ have lost 

 its original hade. As the compression died away, in its later phases, 

 faulting might again be sufficient to satisfy its energy. We might 

 then find a highly disturbed tract affected by faults cutting up the 

 distorted strata with a considerable degree of symmetry. For 

 instance, in Dr. Stapff's section of Mont St. Gothard,^ several 

 faults are inserted. No doubt they must be to a- certain extent 

 imaginary, the only available data being the section as seen in the 

 tunnel, and the outcrops of the strata. But probably we may rely 

 upon the correctness of the section to a considerable extent. These 

 faults do not reduplicate the nearly vertical strata, so that they are 

 direct faults as regards verticality. They will, therefore, be reversed 

 faults as regards horizontality, and have been produced by horizontal 

 compression. The steepest has a co-hade of about 22°, and the least 

 steep of about 50°. The existence of these various hades may be 



JF 

 thus explained. Suppose a given value of the ratio — . Then the 



relation (3) gives a limiting value of 0, and sliding along a fault 

 plane will not be possible for any larger value of 0, i.e. for any 

 steeper fault. Hence, if a fault has been already formed, and, as the 

 pressure dies away, another fault is needed, sliding along the old 

 fault is no longer possible, and a new one must be started which is 

 less steep, to relieve the now lesser pressure. 



Upon a large slate (27 x 21^ inches) from Cumberland, in the 

 Woodwardian Museum, are two crossed faults, which reduplicate the 

 horizontal strata, and together show a horizontal compression of 

 about 2 inches. The hades of these faults are on an average about 

 25° and 40° respectively. There is a slight deviation from recti- 

 linearity in their traces on the slate. They have evidently been 

 formed along the lesser angles given by the relation, 



and correspond to the surfaces of least friction given by (2). 



These two miniature faults are perpendicular to the cleavage, and ap- 

 pear to have been formed while the sediment was still slightly ductile, 



^ Studien iiber die "Warmevertheilung im Gotthard. Bern 1877. 



