154 THE GEOLOGY OF THE PURBECK HILLS. 



upon the thickness assigned to the zone in question. Dr. 

 Rowe gives 250ft. as the thickness exposed, which is simply 

 the height of the cliff where the beds above the fault resume 

 the horizontal. But to this must be added (in the first 

 hypothesis) the thickness of the vertical strata belonging to 

 that zone. This Dr. Rowe gives as having been identified 

 by him for 39ft. (Plate VIII., " Zones of the Chalk," p. 36.) 

 Dr. Barrois, however, is quoted by Dr. Strahan (Memoirs of 

 Geo. Survey, p. 166) as estimating the average thickness of 

 the Belemnitella zone in this district as 98ft. to 164ft. Accept- 

 ing Dr. Rowe's estimate we have a thickness of at least 

 289ft. at Ballard Head as the total for the zone, to which 

 must be added an unknown thickness below the exposure. 

 This certainly seems an extreme estimate, especially in the 

 light of Dr. Barrois' more moderate figure, which, however, 

 is for the district of S. Dorset (see note, p. 150), and lends 

 more colour to the supposition that the vertical beds may 

 be identical with the lower beds of the upper portion. I 

 proceed then to describe ' the manner of formation of the 

 fault on this second hypothesis, which, from this point of 

 view, seems the more probable. 



In Plate III., Fig. 4, is shown a restoration of the mono- 

 clinal edge of the anticline as it must have existed before the 

 start of the thrust movement. It is seen there that the 

 horizontal or northern beds must then have been below the 

 level of the present cliffs, and that the Tertiary Beds covered 

 them, as in Plate II., Fig. 1, but in the inferior position, as 

 indicated in the diagram, Plate III., Fig. 4. The line of 

 fracture in this case will have been as at ABC (Fig. 4), 

 and at its northern or lower extremity would have cut down 

 through the upper beds diagonally through a portion of 

 the Tertiaries, thus forming a wedge-shaped mass in the 

 northern chalk beds as at D D. As the thrust movement 

 continued this mass must have been forced bodily upwards 

 and southwards, carrying with it and shearing off a triangular 

 mass of the vertical beds, as at B, in the direction 

 of the letter A. The lower end of the fracture C would 



