PURBECK BEDS. 



343 



Beds. Prof. Blake has, however, suggested a different reading of 

 the section as follows ^ : — 



Portlandian 60 feet. 



Bolonian 660 ,, 



The Bolonian is a division recognized in the Boulonnais, and 

 usually regarded as the uppermost stage of the Kimeridgian 

 (Kimeridge Clay). 



The fissures in the beds at Portland and Tisbury contain stalactitic deposits, 

 called "congealed water," and beautiful yellow crystals of calcite called "sugar 

 candy stone." Fuller's earth has been found in the Portland Stone at Brill. 

 The Portland Stone forms bold hills, usually devoted to sheep-walks. The soil is 

 a ' poor stone-brash.' Water is heki up or thrown out at the base of the Sands by 

 the Kimeridge Clay. 



PURBECK BEDS. 



This name was given from the great development of the beds in 

 the ' Isle,' or more properly peninsula, of Purbeck. It probably 

 originated from the extensive use of the Purbeck stone at Swanage, 

 and the name has been applied in a geological sense since 181 6, 

 when it was used by T. Webster. 



The formation is essentially of freshwater origin, but it contains 

 a few estuarine or marine beds, which serve to link it with the 

 Oolitic group. It consists of an alternating series of limestones, 

 clays, and marls. The various sections at Durlston Bay, 

 Worbarrow Bay, Lulworth Cove, and Ridgway Hill, show much 



Fig. 53. — Section on the East Side of Lulworth Cove, Dorsetshire.^ 

 (H. W. Bristow.) 



_/- „.^y. 



? fj- 



A. Wealden Beds. 



a to r. Purbeck Beds. (See p. 344.) 



C. Portland Beds. 



variation in thickness, both in the mass and in the divisions. The 

 greatest thickness measured by Mr. Bristow at Durlston Bay is 

 upwards of 360 feet, at Mewps Bay the beds are less by about 100 

 feet, and at Ridgway Hill, measured by the Rev. O. Fisher ^ and 

 Mr. Bristow, the beds show further diminution. 



^ P. Geol. Assoc, vii. 358. 



* Damon, Geol. Weymouth, 1884, p. 120. 



^ Trans. Camb. Phil. Soc. ix. 574. 



