Rev. 0. Fisher — On Faulting, Jointing., and Cleavage. 209 



have now contracted, the matter of which it consists is stable, except 

 in so far as gravity may affect it. It seems however certain that 

 faulting, without some amount of deformation, cannot completely fill 

 the void spaces. But since in nature the movements will go on while 

 the mass is solidifying, this very condition implies that it still 

 possesses a certain degree of plasticity. 



On account of the fixity of the matter along the bottom CD, the 

 faults must die out when they reach that depth. But a total throw, 

 through the height oi A B above a b, is available for the throw at 

 the surface. We may feel sure that the result proposed is never 

 fully accomplished, for, if it were, it would imply that the surface 

 became strictly horizontal under the action of gravity, wliich would 

 mean that the material was liquid ; which it is not. But neverthe- 

 less horizontality of surface is the result to be appi'oximated to. 

 Crossed faults will effect this most readily ; for let there be two 

 faults affecting two areas _P and Q (Fig. 8), with an area of length 

 X intervening ; and suppose for simplicity that they have equal 

 hades, ^, and that P and Q at the surface -^ig s 



are thrown equally through a with 

 reference to the fixed bottom. Then, 

 as shown in (4) of Part I., these will 

 be left level ; while the area X will 

 occupy a trough, and be depressed 

 through a with respect to either of 

 them. A horizontal extension of the 

 surface equal to 2a cot 6 will be gained, and a volume, ax + a-cot 6, 

 will be available towards filling the gaps. 



In the further case (Fig. 9) of a certain thickness of sedimentary 

 matter inclosed between two beds, of which the upper as well as 

 the lower does not contract, the top laj'er of the upper half will be 

 under similar conditions to the bottom layer of the lower half. 

 And if we take, in the uncompressed 

 matter, a horizontal layer anywhere be- T'ig. 9. 



tween the top and bottom, this will be- 

 come broken up by crossed faults, into 

 portions dovetailed into one another. 

 The throws of the faults will on the 

 whole diminish, both upwards and down- 

 wards, as we recede from this, until 

 they die out when they reach the two 

 beds which do not contract. 



Examples of what are perhaps such faults on a minute scale may 

 be seen in Mr. Teall's slate, ^ only it must be recollected that their 

 hades may not be what they appear to be, because the plane of 

 cleavage is probably not perpendicular to the planes of faulting. 



In connection with this subject of faulting, we cannot regard the 



rocks either as decidedly rigid or as decidedly plastic. They must 



partake of the quality of rigidity, inasmuch as the forces acting 



upon them produce fractures along certain surfaces ; and they must 



1 Geol. Mag. Dec. III. Vol. I. PI. I. 



DECADE III. — VOL. I. — NO. V. 14 



