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



the hade o fthe shear (as it were a refraction) — a phenomenon known 

 to occur under such circumstances, and if it should happen that the 

 co-hade of the surface of shear when it crosses the harder bed should 

 become greater than its angle of repose, that bed will be faulted or 

 jointed instead of being cleaved. Instances of both these effects are 

 figured by Phillips in his report,' and may be frequently seen 

 accompanied by slickenside where a hard band crosses a coarse slate. 



The downward acceleration being g — - will be greater where -r 



is less, i.e. where Ti is greater. This justifies the assumption already 

 made that the downward velocity will be greatest at the crest. 



We have hitherto considered only the simple case of a single crest, 

 and of one corresponding place of greatest sinking. But it is pro- 

 bable that, in an elevated and contorted tract, when settlement took 

 place, it would not be uniform. Although the absolute movement 

 might be everywhere downwards, nevertheless the movement of 

 different regions relatively to the adjoining ones might be in some 

 places downwards, and in others upwards. Where it was upwards, 

 the conditions just now described would be reversed, and the surfaces 

 of shear would slope in the opposite direction, that is, in an anticlinal 

 manner towards the axis of greatest upward relative movement. 



At some place intermediate between the synclinal and anticlinal 

 cleavage, the relative movement would be nil, and in the neigh- 

 bouring region the surfaces of shear would correspond nearly to the 

 .value of 6, which would satisfy our equation, 



1 (P— TF) sin20=;tt: 



where fx expresses the force per unit area which would suffice to 

 cause the rock to shear, or flow. This force being small in argil- 

 laceous rocks, sin 20 need not be so large as to make the angle 

 90 — 6 that is, the co-hade, greater than the angle of repose. If it 

 should be so, there will be no cleavage developed in that region, 

 but faults will supply its place, if there is relative movement at all. 

 Such an arrangement of the cleavage surfaces accords with natural 

 appearances. 



In crossing the cleavage planes in the direction towards which 

 they dip, the downward settlement increases at every step we take : 

 and when the whole comes to be summed up for a distance of many 

 miles, we must arrive at a surprising total, which must itself be 

 measured by miles. Is such an amount of sinking conceivable ? 

 Let us inquire. 



It seems impossible to account for the intense movements of 

 various kinds, to which the earth's crust has been subjected, except 

 on the hypothesis, that it rests on a yielding substratum. If this be 

 true, the substratum must necessai-ily be more dense than the crust. 

 This hypothesis will explain the simultaneous relative up-and-down 

 movements, as well as the amount of sinking, required by the theory 

 of cleavage now proposed. 



Assuming that, when the contortion or disruption of the strata, 

 1 Loc. cit. p. 384. 



