THE GEOLOGY OF THE PURBECK HILLS. 153 



had occurred before the movement commenced. It is possible 

 that denudation had proceeded to a very considerable extent, 

 and that the amount of strata covering the present 

 level of Ballard Down was not much greater, when the fracture 

 occurred, than it is now. This would help to explain why 

 the edges of the vertical strata were so easily sheared off by 

 the moving mass, which would then be taking the line of 

 least resistance. But had the vast mass of chalk strata 

 remained intact (as represented in the diagram), it would offer 

 a far greater resistance to the thrust movement than seems 

 necessary or probable. The answer to this will depend upon 

 how long after the upheaval of the anticline the thrust move- 

 ment commenced. 



II. AN ALTERNATIVE THEORY OF THE FAULT 

 FORMATION. 



It will be observed that the above explanation of the process 

 by which the present relation of the strata at the fault came 

 about, depends upon the supposition that the beds above and 

 be'ow the fracture unite under the present cliffs. The 

 vertical beds would then resume the horizontal and continue 

 northward, as indicated in Plate III., Fig. 3. As has been 

 stated, in this case the order of beds from above downwards 

 through the fault must be continuous, and no bed be missing 

 unless one or two had been faulted out and thrust up ; but 

 if so they would re-appear and resume their place at some 

 point towards C in the diagram. But further examination 

 may possibly show that the vertical beds, which belong equally 

 with the upper beds to the same zone, may actually be the 

 same beds, or a portion of them, as those above. In this 

 case an entirely different method of formation of the fault 

 will have to be assumed, and what this process would have 

 been is indicated in Figs. 4 and 5, Plate III. As to whether the 

 vertical beds are lower beds of the same zone, or a repeat of 

 the same beds as those above the fault, much will depend 



