THE GEOLOGY OF THE PURBECK HILLS. 155 



have resumed the horizontal somewhere further north than 

 in Fig. 2, Plate II., continuing between two of the bedding 

 planes for an unknown distance. When the movements 

 were completed the effect shown in Fig. 5 would have been 

 produced, the two triangular fragments (Fig. 4, B and D D) 

 having been forced up and over the present summit of the 

 downs, and subsequently denuded off as in the former case. 

 With them would have been carried upwards the Tertiary 

 Beds, as in Fig. 5, which also have since been denuded. 

 The structure as now displayed in the cliffs would thus have 

 been produced, as shown in the shaded portion of Fig. 5. 

 But in the unexposed strata below sea-level its condition 

 would be essentially different from that of the former method, 

 as a comparison of the two diagrams, Figs. 3 and 5, will 

 illustrate. Instead of the lower beds rapidly resuming the 

 horizontal, as in Fig. 3, they would be cut through by a 

 continuation of the fracture down so far as Fig. 5, C ; and 

 between them and the upper portion, as at E, we 

 should expect to find a fragment of Tertiary strata which 

 had been sheared off by the original fracture plane, as in 

 Fig. 4. The possibility of this is admitted by Dr. Strahan 

 (op. cit., p. 217). 



Until further evidence be produced we must admit that 

 the two hypotheses described may either of them prove 

 to be correct, the balance of probabilities remaining with 

 the former, the chief objection to its acceptance being the 

 enormous thickness of the Belemnitella zone which it assumes. 

 The three points upon which information is required will then 

 be : (1) The nature of the vertical beds as to whether they 

 are inferior or identical with those of the curving strata above 

 the fracture, (2) a correct estimate of the greatest possible 

 thickness of the Belemnitella zone in this section, and (3) 

 a proof of the presence of Tertiary strata below and to the 

 north of the lowest exposed portion of the fault in Ballard 

 Cliff. More thorough and minute examination of the strata 

 exposed may throw further light upon the first two points ; 

 but as to whether the problem is of sufficient interest and 



