148 THE GEOLOGY OF THE PTJRBECK HILLS. 



hills, beneath the southern cliffs of Purbeck. Another im- 

 portant feature common to each of these chalk anticlines is 

 that their northern side's are steeper and more abrupt than 

 the southern. These considerations lead to the conclusion 

 that this series of anticlines, including that of the Purbeck 

 area, were all formed about the same time, and by the same 

 series of earth movements. These movements were tangential 

 and compressive. Their direction must have been from 

 N. to S., or N.N.E. to S.S.W., thus producing a greater steep- 

 ness of fold on the side against which the pressure was applied. 

 In each case the massive chalk strata presented the greatest re- 

 sistance to the movements, and hence were raised up in earth 

 waves, protecting in each case, except that of the Purbeck 

 Hills, the softer southern strata from extreme disturbance. 

 That the disturbance penetrated southward of the chalk in 

 the Isle of Purbeck may be accounted for by several considera- 

 tions : (1) That the earth wave had here become deeper, 

 and so passed partly beneath the chalk, its superficial 

 force having been absorbed by the more northern beds ; 

 (2) that the chalk strata here offered less resistance than 

 elsewhere ; and (3) by the chalk fracturing in the line of the 

 great fault. Each of these facts would encourage the main 

 earth movement to upheave the beds beyond the chalk, and 

 hence cause the original anticline of the Isle of Purbeck to 

 attain vaster dimensions than its northern contemporaries. 

 The original outline of the anticline is shown in Plate I., 

 Fig. 1. It formerly extended for at least three miles out to 

 sea, but has been denuded back to what is probably the center 

 of its axis at the present coast line. It is evident, therefore, 

 that the Purbeck Hills here form, as stated, the northern 

 monoclinal edge of this great anticline, so differing from the 

 otherwise similar chalk hills to northward. Geologically 

 they are continuous with the chalk axis of the Isle of Wight, 

 and form a westerly extension of the southern lip of the 

 Hampshire basin. The total length of this anticline from 

 Brixton to its western termination in Weymouth Bay is 

 about fifty miles. 



