Pro/. J". F. Blake — Recent Papers on Faults. 369 



Does the fault stop at a joint? or what has this joint to do with it? 

 Then we have horizontal joints ! does any one know them in strati- 

 fied rocks ? What Prof. Tait would say to P being called a force 

 and =z\x and then to the "force \" being spoken of, I cannot think, 

 — probably he would never read so far, nor indeed should I except 

 for criticism, as I cannot see what is to be got out of it all. 



Next there is a second condition for faulting, that is, "the hade of 

 the fault must not be less than the angle of repose. (By hade here 

 is meant the inclination to the horizon and not to the vertical, as is 

 usual.) This marvellous proposition with respect to the case in 

 which vertical pressure is alone su]3posed to act — the horizontal force 

 having been spirited away — requires nine lines of mathematics ! Who 

 can doubt that if the upper mass is in "repose," it will not move? 

 or, imagine that a force which is great enougli to tear a rock will 

 Bot move it when torn ? How can tearing be shown but by motion? 

 Then we have the following paragraph : [the remarks in brackets 

 are Mr. Blake's. — Edit.] " If the angle of repose is less than 45° 

 [as it is for all known substances with approximately flat surfaces], 

 the hade of the fault surface will be 45° [which is very rarely the 

 case] ; but, if the angle of repose is greater than 45° [which it never 

 is, except the surfaces are hooked], the hade will be the angle of 

 repose, provided it lie within I m [hence vertical faults are im- 

 possible ?] 



Here we end the first half of the paper. Can we extract any 

 ideas from it as to the modus operandi of faulting ? No doubt the 

 attem^^t will be unsuccessful, but this is what I gather. A mass of 

 rock contracts : vertical contraction makes it sink, horizontal makes 

 it crack; the total result may be an oblique fault, whose inclination 

 to the horizon will be greater as the forces in operation are less. 

 The position of the vertical cracks may be determined as follows. 

 At the bottom, i.e. where the cracks end, the contracting force is 

 resisted by the stress exerted by the bottom, which will be propor- 

 tional to half the distance between the cracks ; at the top it will be 

 resisted by the cohesion of the rock ; therefore the cohesion must 

 equal the total bottom stress ; or half the distance between the 

 cracks equals the ratio between the cohesion per unit area and the 

 coefficient of bottom-stress. The fault will first be started by the 

 increase of the horizontal conti'acting force, and the easiest to make 

 is one at 45°. There would be no room for the motion, however, 

 and we must start again. The same force will pull across any crack, 

 and will do this easiest when not resisted by the vertical force, 

 hence it will make a vertical crack (!) When these cracks are made, 

 the horizontal force will be exerted in making horizontal cracks [but 

 this horizontal fox'ce is not the same as the other horizontal force, 

 that one " might be a pressure or a tension," and " the tension 

 arising from the contraction will amount to" it ; but this one is a 

 "contractile force" and "not a compressing force"!] The only 

 force then left to make faults is the vertical one, and this will make 

 one at 45°, terminated by the vertical cracks, which somehow have 

 turned into gaps. 



DECADE III. — YOL. I. NO. VIII. 24 



