354 GEOLOGY OF THE ISLE OF PURBECK. 



slates and coarser limestones, or Purbeck stone, 

 as they are termed, are also shelly conglomerates, 

 the prevailing shells being small species of the 

 genera Cyclas, Cyrena, &c* 



The stone now quarried is in the upper part of 

 the series of shales, clays, and limestones. Mr. 

 Webster states, that in the thickness of 125 feet, 

 there are more than fifty layers of useful stone ; 

 these include a remarkable deposit, 12 feet thick, 

 termed "Cinder" by the workmen, and which 

 consists of a regular oyster-bed. The lower 150 

 feet are made up of shales, clays, and shelly 

 limestones, too friable to be of any value in an 

 economical point of view. The total thickness, 

 from the uppermost bed to the Portland marine 

 limestone, is about 275 feet.f Fibrous carbonate 

 of lime, in layers two inches thick, is often found 

 adhering to the bands of marble, as in the in- 

 stance observed in Sandown Bay (p. 136). 



Fossils of the Purbeck strata. — From what 

 has been stated, the reader will be aware that the 

 fossils of the Purbeck strata are identical in their 

 general character, with those of the upper divi- 



* Some of the most ancient pavement in Ely Cathedral consists of slabs of 

 the Purbeck bivalve limestone. 



* Mr Webster, in Geol. Trans, vol.ii. new series. Mr. Webster mentions 

 having found some nodular concretions of chert, containing silicified fresh- 

 water shells in the state of chalcedony ; I have not obtained any specimens of 

 • Mis kind. 



