FAULTS. 15 



one another; that is to say, supposing the hade of a fault to be in 

 an easterly direction, then if the beds are upraised on the east, 

 instead, as would usually be the case, of being thrown down 

 on that side, this would be a reversed fault. Nevertheless it is not 

 always easy or possible to determine in a small section if an 

 apparently reversed fault is really so, for the reversed hade may be 

 merely an irregularity in the hade of an ordinary fault. (See Fig. 2.) 



The walls of a fault are sometimes grooved or striated by the 

 friction attending the disturbance which produced it : these features 

 are known as ' slickensides.'' In limestones the groovings are often 

 obscured by a stalactitic coating of carbonate of lime. ^ 



One of the largest and most important faults in England and 

 Wales is the Pennine Fault, which commences in the south of 

 Scotland, and runs southwards to near Brough in Westmoreland. 

 On the eastern side Carboniferous rocks, and even Cambrian 

 (Ordovician) strata are brought against the Permian rocks on the 

 west, with a maximum throw estimated at from 6000 to 7000 feet. 

 The fault continues by Kirkby Stephen to near Kirkby Lonsdale, 

 where it was termed the Craven fault by Sedgwick. He observes, 

 " It crosses my native valley of Dent about half a mile below the 

 village, dislocating and setting on edge all the lower limestone 

 beds. These ' edge-beds,' well known to all the quarrymen of the 

 neighbourhood, greatly affect the external features of the country 

 through which they pass." ^ (See Fig. 25.) 



The Tynedale fault, known also as the ' Ninety-fathom dyke,' in 

 the Newcastle Coal-field, is so termed because the same beds are 

 sometimes ninety fathoms lower on the northern than on the 

 southern side of the fault ; the amount of the downthrow is at other 

 times much greater. On the east side of the Vale of Clwyd the 

 Silurian rocks are faulted against the Bunter Sandstone with a 

 throw of not less than 1200 feet. (See Fig. 3.) 



The Radstock slide-fault in the Somersetshire Coal district is 

 a curious example of beds faulted in a nearly horizontal position, 

 for it has thrust the upper portion of the Radstock series over the 

 lower half, back from the direction of the Mendips northward, for 

 a distance of from 50 to 350 yards. It is a reversed fault. 



Sometimes in the neighbourhood of a fault the beds exhibit an 

 attenuation. In contrast to this attenuation are what are termed 

 ^wants'" in strata, which consist of interrupted (broken, not nodular) 

 bands of hard rock in a clayey or shaly formation. In the former 

 case the facts might lead to the supposition that the disturbance 

 took place before the beds were thoroughly consolidated, but the 

 experiments made by Mr. L. C. Miall tend to show that most rocks 

 are both elastic and plastic, when subjected to long-continued 

 pressures or strains of low intensity. The 'wants,' on the other 

 hand, would indicate a more sudden strain, the hard bands indi- 

 cating the tension by separation, whilst the clayey beds, as Mr. R. 



^ G. V. Du Noyer, Geologist, iii. 38. 



2 Q J. viii. 36 ; T.G.S. (2j, iv. 60, 69 ; Phillips, Ibid. iii. 5. 



