THE GEOLOGY OF THE PURBECK HILLS. 151 



by Dr. Strahan, and since more minutely by Dr. Rowe 

 in his " Zones of the White Chalk of the Dorset Coast," 

 have proved that the curved beds north of the fracture, 

 and the vertical beds below, belong all to the Belemni- 

 tella mucronata zone of the Upper White Chalk, which is 

 the highest zone of the series. It is assumed by Dr. Strahaii 

 that the vertical beds are the lower series of this zone and 

 the curved beds above the fracture the upper beds of the 

 same series, and on this supposition my three first diagrams 

 are based. If this be so, the line of fracture must have occurred 

 as indicated by the line A B C in Fig. 2, Plate II. It would 

 have commenced along a bedding plane dividing the zone 

 into an upper and lower portion, of which the former subse- 

 quently formed the curved beds above the fault and the 

 lower became the vertical beds below it. These latter must 

 have been cut across by the fracture as at B, the rupture 

 then continuing upwards across the elevated beds of the 

 monocline as at A, which beds have since been denuded off. 

 A movement then took placo from N. to S. along the fracture, 

 the upper beds being thrust bodily, in a sheet 20 miles wide, 

 first southward for at least 400ft. (the present height of the 

 Downs), then upwards over the upturned edges of the station- 

 ary lower beds, and thence up and above what is now the top 

 of the cliff. This means that the whole block DD (Plate II., 

 Fig. 2) was forced up and over the others, whilst its place 

 was taken by the strata which formerly had occupied the 

 space to the right of the dotted line EF. The result is shown 

 in Plate III., Fig. 1, of which the shaded portion shows the 

 existing condition of the beds, the extruded portion and the 

 rock debris, after being thrust up and over the present strata, 

 have since been denuded off, leaving the contour of Ballard 

 down as it now exists. The bedding plane, along which this 

 movement took place, as well as the junction between the 

 stationary and displaced beds of the zone, are now hidden 

 beneath the cliff, but their probable arrangement is shown 

 in the lines drawn in the diagram. Thus, could a section be 

 cut at a point where the curving beds resume the horizontal 



