Dr. Ricketts — On Fissures, Faults, Contortions, etc. 205 



have fallen downwards and filled up what would otherwise be ex- 

 tensive chasms in the earth's crust, have increased to some extent 

 the superficies of its surface. 



The displacement or downthrow which occurs on the formation of 

 a fault is simply a falling or slipping downwards of sepai'ated rock- 

 masses along planes of fracture ; it is evident that by such move- 

 ment the strata cannot become bent or contorted, nor can the 

 original direction of the dip be altered, for ihe mass is prevented 

 from tilting over in consequence of the impulse being checked by 

 the opposite wall of the fissure. 



Slickensides are frequently met with in connexion with faults, 

 and indicate the direction of some movement which has taken place. 

 They may sometimes have been caused by the friction induced on 

 the occurrence of a fault, but in such a case the striations could only 

 have a direction downwards along the hade of the fault. Such is 

 the direction of the marks in the principal or north and south faults, 

 of this district (the neighbourhood of Liverpool) ; but as the strata 

 have been subsequently exposed to compression at right-angles to 

 these faults, it is to the latter circumstance their presence must be 

 attributed, for the action of the second movement would obliterate 

 the effects of the first. In those faults having an east and west 

 direction, or nearly approaching it, the striations are sometimes 

 nearly horizontal ; in others, and more frequently, more or less 

 oblique, and seem to indicate that in the one case there has been a 

 lateral movement, in the other that, in addition to this, there has 

 been at the same time an amount of upward pressure. As the lateral 

 pressure by which these Triassic rocks have been acted on has been 

 in an east and west direction, the tendency would be, in the 

 instances referred to, to compress into a less lateral compass the 

 slightly consolidated beds of the Upper Bunter Sandstone, and, by 

 causing a re-arrangement of the particles of which they consist, or 

 by inducing foldings in the beds, occasion some amount of elevation 

 of the superincumbent strata, and the movement thus produced 

 would cause that friction along the sides of faults which has rendered 

 their surfaces polished and striated. 



The opinion of Sir James Hall, illustrated by him in 1813, that 

 contortions and flexures of stratified rocks have been the result of 

 lateral compression, forcing the strata more closely together laterally, 

 but elongating and making them to rise upwards in a direction 

 perpendicular to the pressure, is generally accepted. The principle 

 is even admitted by those who attribute these foldings to the effect 

 of upheaval alone ; but " that would stretch instead of crumpling 

 the surface," and therefore contortions under such circumstances 

 would be impossible. 



The researches of Sedgwick, and of Phillips, of Sharpe, Sorby 

 and Tyndall, have successively demonstrated that cleavage differs 

 essentially from stratification, being the result of a change in the 

 structure of sedimentary rocks ; due to mechanical causes ; and that 

 it has been induced by pressure of the mass in a direction perpen- 

 dicular to the planes of cleavage. 



