GREAT FAULTS. 333 



If no horizontal or undisturbed strata be visible in any part of tho 

 dislocated tract, either in superposition or in juxtaposition, the limit 

 of least antiquity vanishes, and we are in danger of imagining too 

 modern a date for the convulsion ; if the newer members of the dis- 

 located group of strata be concealed, there is danger of ascribing too 

 high an antiquity to the convulsion. 



Eolation of Igneous Rocks to Convulsions. The almost universal 

 coincidence of convulsive dislocation of the strata with eruptions of 

 plutonic rocks seems enough to prove their common dependence 

 upon one pervading cause of internal movement. In the same man- 

 ner as the modern earthquake precedes the eruption of lava, so the 

 ancient convulsion or fault preceded the injection of plutonic rocks. 

 Also precisely as in the present day the earthquake shakes countries 

 far removed from volcanic centres, so in more ancient periods many 

 tracts were dislocated, but the fissures thus formed were not filled 

 with melted rocks, at least near the surface. As far as at first appears, 

 the common dependence of the two orders of effect upon one cause is 

 merely to the amount that the mechanical transference of melted rocks 

 has been effected by the same internal pressure which dislocated the 

 strata ; whatever occasioned the pressure was also the cause of the 

 fluidity of the rocks when the pressure was reduced or removed. 



Various mechanical modes besides contraction may be conceived 

 by which such pressure may have been occasioned, and various con- 

 ditions assumed for the production of melted rocks, and these may 

 be wholly distinct from one another; but the exhibition of these 

 rocks along the lines of convulsion can only be ascribed to the same 

 mechanical cause which produced the convulsion. 



Fractures and Dislocations of Strata. 



Faults. Those dislocations known as "faults" which break the 

 continuity of the beds along certain planes or fissures, and elevate or 

 depress one side, often plainly declare themselves to be the result of 

 single movements. Inspection of the phenomena in this country 

 will leave no room for doubt that the dislocated strata were put 

 into their present relations, not by a repetition of small and gra- 

 dual movements, but by one movement, which may have been slow 

 or rapid. 



The extent of dislocation to which the name of fault accurately 

 applies is extremely various, the difference of level thus occasioned 

 being sometimes a few inches, in many cases 100 feet, in others 

 more than icoo yards. This marks out in very clear characters the 

 degree of force exerted in each case. Those dislocations which 

 make the greatest difference of level range through the greatest 

 lengths of country. The ninety-fathom dyke so named from the 

 observed extent of its dislocation ranges from the eastern sea across 

 the whole breadth of Northumberland ; and certain dislocations in 

 Yorkshire have ranges of ten, twenty, and thirty miles in one nearly 

 straight line. 



