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E. J. Hehert — Reversed Faults in Slates. 



sawn through diagoually as indicated ; apply equal and opposite 

 pressures P, P, Q, Q, at right angles to each of the sides, at their 

 middle points, as indicated by the arrows in Fig. 1. Then if P be 



p 



Q — ^ 



^A 



4— Q 



/mm^ 





T 



P 



Rg. 1. 



Fig. 2. 



Fig. 3. 



equal to Q, there is no tendency to a displacement ; whereas if P 

 be greater than Q, the square has a tendency to assume the form of 

 Fig. 2, and if P be less than Q, of Fig. 3. Now suppose this 

 square, instead of being of wood, and lying on a table, forms a part 

 of the earth's crust, and stands vertically, the grain of the wood 

 being replaced by the strata of which the crust is made up ; then if 

 the vertical pressures upon the sides of this square of rock exceed 

 the horizontal, the strata of which it is made up will tend to 

 assume the form indicated by Fig. 2, i.e. a direct fault will probably 

 take place ; whereas if the horizontal pressures exceed the vertical, 

 the strata will tend to take the form of Fig. 3, i.e. a reversed fault 

 will probably ensue. 



These tendencies, however, will not of necessity result in faults, 

 since any displacement may be prevented by the more or less rigid 

 connexion of the square of rock, under consideration, with the 

 adjoining strata, of which it forms a part. Friction also, as well as 

 a multitude of other modifying causes, will influence the result. 

 Now since it is probable, where strata are tolerably free from con- 

 tortion, as in the English Coal-measures, that vertical pressures 

 have exceeded horizontal pressures, (arguing as above) we should 

 here exj)ect to meet with direct faults ; but since it is certain in 

 highly contorted and cleaved strata, as in the English and Welsh 

 slates, that horizontal pressures have at one time far exceeded the 

 vertical pressures, we should here similarly expect to meet chiefly 

 with reversed faults ; and in each case our surmises would prove 

 connect. 



It should, however, be remarked, that the above are considerations 

 of the most simple case, and in practice we should meet with many 

 variations which would modify the result. First and foremost 

 amongst these should be mentioned that the hade of the faults will 

 seldom happen to be 45° (as in the figures), a lower hade tending to 

 neutralize the motive power of the vertical pressures, and a higher, 

 that of the horizontal pressures, while if the fissure be vertical, the 

 horizontal pressures will have no motive power at all. Still further 



