Rev. 0. Fisher — On Faulting, Jointing and Cleavage, 211 



experiments), the material will not separate, but after motion has 

 commenced will behave like a Tiscous substance. It must, however, 

 be remembered that our jli is still a resistance to shearing stress, and 

 not the coefScient of viscosity. 



The tension P is evidently a force which increases in intensity 

 during contraction. To determine how large it may become, we 

 observe that P depends upon the horizontal cohesion of the rock, 

 and upon the reaction of the fixed bottom. Let k be the cohesion 

 per unit area of a vertical section. Then, on account of the great 

 energy of molecular forces, we may expect that P is capable of in- 

 creasing until it becomes equal to k. The tension arising from the 

 contraction is resisted by the stress exerted by the bottom, and the 

 whole tension along a length x will be proportional to the length ; 

 and will amount to P. 



.', P=\x (suppose). 



Now in order that the force may be sufficient to overcome the 

 cohesion, we must have P = k. 



,\\x =z k; and x =: — 

 X 



If we knew the values of k and X, this would give an idea of the dis- 

 tance between two vertical cracks, which would be 2—, because mid- 



A, 



way between two cracks the action of the bottom would be nil, and 

 there would be no tension. 



We observe that /c is a tensile, whereas ^ is a shearing force. 



It seems probable that in hard rocks both k and X would be large, 

 and in soft rocks both small. Since then the distance between the 

 cracks depends on the ratio of these two quantities, the blocks 

 between the joints might be of about the same size in soft rocks as 

 in hard. 



We have seen that the shearing force on ADP along AB is 

 J (jf^_j_p) sill 2 e. 



And if this were just sufficient to cause separation, 

 i {W-\-P) sin2 9 = fjL. 



There are two values of 2 which Fig. 11. 



satisfy this condition, as indicated in 

 Fig. (10). Let them he JT J/ and 

 K L, Fig. (11). If we bisect 

 these angles by m and I, then 

 K m and KOI are the two angles 

 given by the above equation. 



It appears that if P were to in- 

 crease until it was larere enouoli to 

 produce separation, this would occur 

 with the least value of P capable 

 of doing it, and with the largest 

 value of sin 2 0. Hence, so far as 

 the mere magnitude of the force is 

 concerned, separation would take place along A B when inclined 



