146 THE GEOLOGY OF THE PURBECK HILLS. 



are nearly horizontal, dipping about 8 a little west of north ; 

 but half way along the section they suddenly bend upwards 

 in a magnificent curve, forming a quarter of a circle at the 

 great fault, which here interrupts their continuity. This 

 fault is indicated in Plate I., Fig. 1., but this section, being 

 taken some distance from the cliffs, does not show the struc- 

 ture of the fault, which, however, is exhibited in Plate III., 

 Fig. 3. So many excellent photographs of these cliffs and 

 the fault have been published in various works that we have 

 not thought it necessary to reproduce one ; but the best is 

 probably that in Dr. Rowe's " Zones of the White Chalk of 

 the Dorset Coast," p. 35. On the southern side of the fault 

 the beds are vertical, but they gradually decline from this 

 angle to their junction with the Punfield beds. Thence are 

 found in succession the Upper Greensand, Gault (which, 

 however, cannot be identified distinctly) and the Lower 

 Greensand, which are found in some 220 yards, each dipping 

 N. at a gradually diminishing angle, until succeeded by 

 Wealden strata which, at half a mile from Punfield Cove, 

 appear to have escaped the disturbing effect of the fault, 

 and resume the nearly horizontal level of the chalk north of 

 of the rupture towards the Foreland. The total thickness 

 of the chalk in this section has been estimated at 1,300 ft. 

 It gradually decreases throughout the range of Purbeck Hills 

 westward, being 1,200 ft. at Lulworth, beyond the Purbeck 

 area, and about 500 ft. at Blackdown, where the chalk vanishes 

 to seaward. The thicknesses of the three great divisions of 

 the chalk in this section are given by Dr. Strahan as upper 

 chalk, 1,049 ft*. ; middle chalk, 102 ft., and lower chalk, 

 149 ft. The chalk rock, Melbourn rock, and chloritic marl, 

 which severally divide these sections, can each be traced in 

 their usual positions, and the zones into which each division is 

 subdivided have been exhaustively worked out and described 

 by Dr. Rowe hi his paper quoted above. Reference to the 

 three diagrams in Plate I. will show that the geological 

 structure here described is continuous throughout the range 

 of the Purbeck Hills. 



