38 



Mr. Webster on the Purheck and Portland Beds. 



at Tillywhim Quarry^ in the south-east corner of the Isle of Purbeckj they 

 may be observed reposing on an oohtic limestone which 1 consider as identical 

 with part of the series of beds in the Isle of Portland. Here however, and 

 all along the south side of the Isle of Purbeck, from Durlstone Head to 

 St. Adhelm's Head, the strata are horizontal ; and many quarries are opened 

 in them for the stone, which is called the Purbeck Portland, and which is in 

 general harder than that in the Isle of Portland. 



On the remainder of the south coast from St. Adhelm's Head to Gad Clifif, 

 near Warbarrow Bay, still lower beds are seen, consisting of clay and dark- 

 coloured compact limestone, which have obtained the name of Kimmeridge 

 beds from a village in that district. 'V 



The Isle of Portland, as is well known, is situated near to Weymouth, and 

 is connected with the land by a bank of shingles, or water-worn pebbles, 

 about 10 miles in length, called the Chesil Bank. Its beds dip also to the 

 north ; and on that side, under the great oolitic mass (of which the upper part 

 of the island consists), the Kimmeridge beds are exposed. 



Having thus exhibited a general view of the situation of the stmta which I 

 propose to give some account of, I shall begin by stating some particulars re- 

 specting the Purbeck limestone series. 



In Durlstone Bay I obtained from the quarrymen the thickness of the se- 

 veral beds, and the names by which they distinguished them from each other: 

 but I should observe that these local appellations are not used by London ar- 

 chitects and builders, the whole together passing here under the name of Pur^- 

 beck stone only. 



The following is a list of the strata of limestone, which are mostly separated 

 from each other by a thin bed of a shivery clay or shale. 



Thickness. 

 Nos. Ft. In. 



1. Marble, not worked at present . uncertain 



2. Marble rag 8 



Shiver, marl, &c 20 



3. Single leaper, good stone ... 1 3 

 Shiver, marl, &c 2 



4. Step bed, good stone .... 7 



5. Grey rag, good stone .... 7 

 Shiver, marl, &c 10 



6. Toad's eye, a very hard but-* 



A y } 4 



goon stone J 



Shiver 1 



7. Good bed, bad stone .... 1 

 Shiver, &c 10 









Thickness. 



Nos 









Ft. 



In. 



8. 



Pitching- stone bed. 



good -^ 











stone .... 



_ . . 







 



5 



9. 



Tomb-stone bed, good stone 



6 



10. 



Yellow bed, bad, networked 



d 







4 



11. 



White roach, not very good 



> 



1 









White earth, a marie 











9 



12. 



Leaper, excellent 





1 



1 







13. 

 14. 



White bed, excellent 

 Soft bed, excellent 







1 

 





 7 



15. 



Hard bed, excellent 











6 



16. 



Mock hard bed . 











7 





Earth .... 







1 











17. 



Pitcher bed . . . 







fi 



