396 



ISLE OF PURBECK. 



THE ISLE OF PURBECK. On the western shore of the Isle 

 of Purbeck, the chalk is raised to a vertical position, and the 

 wealden beds beneath are clearly displayed. (Fig. 275.) 



FIG. 275. Plan of the Stratification of the Coves on the South-western Coast of the 

 Isle of Purbeck. a, Chalk; fc, Chalk-marl; c, Firestone; d, Gault; e. Wealden: 

 /, Purbeck ; g, Portland Oolite. 



The limestone which underlies the chalk is well known as 

 Purbeck marble. It is composed of small freshwater shells, 

 Paktdinar, intermixed with the minute shelly coverings of 

 Cyprides. This limestone was so extensively employed 

 during the middle ages for ecclesiastical edifices, that there 

 is scarcely a cathedral or church of importance in which the 

 Purbeck marble was not used for decoration. This section, 

 from Durlstone Head to Ballard Downs (fig. 277), exhibits 

 the relative position of the strata from the chalk to the 

 Portland oolite. 



ISLE OF PORTLAND. The Island of Portland is a bold 

 promontory off Weymouth, sloping gradually towards the 

 land : it is about four miles and a half in length, and two in 

 breadth, and is united to the mainland by the Chesil beach. 



The base of the island consists of Kimmeridge clay, 



^HZH^) Freshwater, calcareous slate. 

 Dirt-bed and ancient forest 



Lowest fresh-water beds of the 

 j lower Purbeck. 

 ^ :rx . __. \ Portland stone (oolitic series). 



FIG. 276. Section of the Isle 'of Portland, Dorset. 



and the oolitic limestone, which supplies the celebrated 



