IV. — Observations on the Purbeck and Portland Beds. 

 By Thomas WEBSTER, Esq., Secretary to the Geological Society. 



[Read November 19th, 1824.] 



1 HE following communication is the result of a short visit which I paid to the 

 Isles of Purbeck and Portland some years ago, subsequently to the publication 

 of my Letters to Sir H. Englefield. 



In that work I traced out the great features of this very interesting district; 

 but a detailed account of the beds is yet a desideratum. To supply in the 

 mean time some materials for such a purpose is the object of the present 

 paper. I shall confine myself here to the beds which furnish the well-known 

 Purbeck stone, which is extensively employed in London for the side pave- 

 ment of the streets ; and to the beds in the Isle of Portland from which the 

 principal material for our public buildings has been procured. 



The beds in the northern part of the Isle of Purbeck are considerably in- 

 clined ; they dip to the north, coming out to the day from under a series of 

 clays and sandstones which are the equivalents of the Hastings beds described 

 in a former paper. The situation of all these, and their relations to the chalk, 

 are well seen in Swanwich and Durlstone Bays, in the east end of the Isle of 

 Purbeck, and in Warbarrow Bay on the west. Their general direction being 

 from east to west, a very instructive section of the limestones usually known 

 by the name of the Purbeck Beds may be advantageously studied in the latter 

 of these places. 



A line of elevated ground, consisting of these calcareous strata, extends 

 from the town of Swanwich to Warbarrow Bay ; and the northern part, as far 

 as the village of Worth, is covered with quarries which have long been worked 

 for the London and other markets, as well as for the supply of the neighbour- 

 hood. Quarries have lately been opened also in the most western parts. 



In Durlstone Bay, the number and disposition of the beds may be well 

 seen ; on the northern part of the bay they are very regular, but about the 

 middle of the bay they are bent and distorted in a very singular manner* : 



* Several dravrings which I made from these contortions have been already engraved, and pub- 

 lished with the letters above mentioned. 



