64 ORIGIN OF GREAT FAULTS. [Oh. V. 



as would have been produced if the broken ends of the rock had been 

 rubbed along the plane of the fault.* In the Tynedale and Craven faults, 

 in the north of England, the vertical displacement is still greater, and the 

 fracture has extended in a horizontal direction for a distance of thirty 

 miles or more. Some geologists consider it necessary to imagine that the 

 upward or downward movement in these oases 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 those polished surfaces called "slicken- 

 sides," that the striae are not always parallel, but often curved and ir- 

 regular. 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 indicate 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, formed at the depth of several miles, 



■ Fig. 88. 





A B 



c 



A 



v 2 



izzmmzz^^^^^i 



whether by the gradual contraction in bulk of a melted mass passing into 

 a solid or crystalline state, or the shrinking of argillaceous strata, baked 

 by a moderate heat, or by the subtraction of matter by volcanic action, or 

 any other cause. Now, if this region be convulsed by earthquakes, the 

 fissures f g, 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 subterranean cavity ; but 

 it is far more probable that the sinking will be effected at successive 

 periods during different earthquakes, the mass always continuing to slide 

 in the same direction along the planes of the fissures /<?, and the edges 

 of the falling mass being continually more broken and triturated at each 

 convulsion. If, as is not improbable, the circumstances which have caused 

 the failure of support continue in operation, it may happen that when the 

 mass B has filled the cavity first formed, its foundations will again give 

 way under it, so that it will fall again in the same direction. But, if the 

 direction should change, the fact could not be discovered by observing 

 the slickensides, because the last scoring would efface the lines of previous 

 friction. In the present state of our ignorance of the causes of subsidence, 

 an hypothesis which can explain the great amount of displacement in 



* Phillips, Geology, Lardner's Cyclop, p. 41. 



