368 Prof. J. F. Blake — Recent Papers on Faults, 



that end ? Nothing that I can see, except to confuse the reader to 

 such an extent that he may find the acceptance of the theory whole, 

 the best way to get out of it. 



Next we start on quite a different tack to learn the " rationale of 

 the mechanical action." Good bye, contraction! farewell, gaps ! We 

 now are to have a surface of shear and two forces as right angles — 

 perhaps we shall get on better with these. Nothing like bringing in 

 a 6 ! Meanwhile there is a dallying with plasticity. We are told 

 that the assumption that the pressure varies as the area on which it 

 acts introduces the idea of plasticity ! Is a steam boiler then plastic ? 

 or a table on which a book of uniform thickness rests ? However, 

 it does not seem to matter; for we are immediately told what will 

 happen "if it is rigid." Nevertheless, rocks cannot be "rigid" in 

 a mathematical sense if they are to shear — for the definition of rigid 

 is that they will not shear. Starting now with our W P and jn, 

 we get an equation, and that is about all we do get. What light it 

 throws upon the subject is not clear, but there is one peculiar feature 

 about it. The /x or resistance to shearing stress along a plane is 

 made independent of the pressure perpendicular to that plane. Now 

 is this so ? Has it been proved ? Friction, which comes into play 

 when the rock splits, depends on the pressure ; why not the resist- 

 ance to shearing? If there are any experiments to prove this, of 

 which I have never heard, it would be more instructive to quote 

 them than Tresca's, which seem to have little to do with faults. But 

 if this is not so, the whole of the mathematics fall to the ground. 

 Without critically examined experiments to prove it, I should 

 never believe that a normal pressure made no difference to shearing. 

 On the next page there is an attempt to unite these supposed forces 

 with the contraction spoken of before, but it leads to "joints" and 

 not to faults, as might be expected, and jul to be dropped and k taken 

 instead; as we know nothing about either, it does not much matter. 

 We are led, however, to the remarkable conclusion that if a rock is 

 cracked, the force which tends to crack it, is greater than that which 

 tends to keep it from cracking ! only it is put rather more scientific- 

 ally (!) "The tension P . . increases during contraction. 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 increasing until it becomes equal to k." But it appears 

 this tension P depends also upon the reaction of the fixed bottom. 

 There is a sort of three-cornered duel, and as P beats both his adver- 

 saries, they must be equal between themselves ! It would appear, 

 however, froin Fig. 7 that they are not equal, because at the bottom 

 this contraction has been resisted ; but at the top it has caused a 

 separation. After the parenthesis, we get back to our equation, and 

 it is made to show that faulting, if allowed, would always be ready 

 to occur at 45°. There are, it appears, two conditions for faulting : 

 one is, there must be room to move, and this is to be brought about 

 by cracks. "Their foi'mation " is "explained" by an equation! 

 which we may suppose produces a suitable crack, though the only 

 one mentioned is a vertical one ; our fault, however, is one at 45°. 



