78 



LYELL'S ELEMENTS OF GEOLOGY. 



Grooved surfaces of Faults. 



The very different levels at which the separated parts of the 

 same strata are found on the different sides of the fissure, in 

 some faults, is truly astonishing. One of the most celebrated in 

 England is that called the " ninety-fathom dike," in the coal-field 

 of Newcastle. This name has been given to it, because the same 

 beds are ninety fathoms lower on the northern than they are on 

 the southern side. The fissure has been filled by a body of sand, 

 which is now in the state of sandstone, and is called the dike, 

 which is sometimes very narrow, but in other places more than 

 twenty yards wide.* The walls of the fissure are scored by 

 grooves, such as would have been produced if the broken ends 

 of the rock had been rubbed along the plane of the fault.f In 

 the Tynedale and Craven faults, in the north of England, the 

 vertical displacement is still greater, and the horizontal extent of 

 the movement is from twenty to forty miles. Some geologists 

 consider it necessary to imagine that the upward or downward 

 movement in these cases was accomplished at a single stroke, 

 and not by a series of sudden but interrupted movements. This 

 idea appears to have been derived from a notion that the grooved 

 walls have merely been rubbed in one direction. But this is so 

 far from being a constant phenomenon in faults, that it has often 

 been objected to the received theory respecting slickensides, that 

 the strisB are not always parallel, but often curved and irregular. 

 It has, moreover, been remarked, that not only the walls of the 

 fissure or fault, but its earthy contents, sometimes present the 

 same polished and striated faces. Now these facts seem to indi- 

 cate partial changes in the direction of the movement, and some 

 slidings subsequent to the first filling up of the fissure. Suppose 

 the mass of rock A, B, C, to overlie an extensive chasm d e, 



the subtraction of matter by volcanic action, or any other cause. 

 Now, if this region be convulsed by earthquakes, the fissures f 

 and others at right angles to them, may sever the mass B 

 from A and from C, so that it may move freely, and begin to 

 sink into the chasm. A fracture may be conceived so clean and 

 perfect as to allow it to subside at once to the bottom of the sub- 



A 



Fig. 79. 



B 



C 



formed at the depth 

 of several miles, 

 whether by the gra- 

 dual contraction in 

 bulk of a mass of 

 strata, baked by a 

 moderate heat, or by 



♦ Conybeare and Phillips, Outlines, &c. p. 376. 

 t Phillips, Geology, Lardner's Cyclop, p. 41. 



