BRODIE— PURBECK INSECT BEDS. 477 



This is the lowest of the Middle Purbecks ; and the Lower are — ■ 



LOWER PURBECKS. 



26. Hard gray marly limestone, having a conchoidal fracture ; identical ft. in. 



in appearance with the upper Insect heds of Ridgway. Cypris 



Purbeckensis} common 3 6 



27. Dirt-bed 3 



28. Laminated clay and soft and hard marls 1 6 



29. Hard marl with conchoidal fracture , 8 



Bottom of quarry. 



In my earher examination of the Vale of Wardour, some of the 

 quarries where the above appear were not worked, being only partial 

 excavations for stone ; and several others then open are now entirely 

 closed. I had then never observed the " Cinder " anywhere in situ, 

 which forms so good a line of demarcation in Dorsetshire, occurring 

 towards the lower part of the Middle Purbecks. A violent storm 

 had fortunately exposed it at the time of Mr. Fisher's visit, in a lane 

 leading from Chicks-grove to TefFont, and hence the connexion of 

 the Purbecks in Wiltshire was determined with greater accuracy. It 

 therefore appears evident that the Isopod and Insect limestones (pro- 

 perly so called) overlie the " Cinder." At the eastern end of the Vale, 

 where they first emerge from beneath the Green-sand, they seem to be 

 much less expanded than they are towards TefPont, a little further to 

 the west, and although ArchcBoniscus is tolerably abundant through- 

 out, the Insects are scarcer, and not so well preserved. There is no 

 doubt a certain general agreement in mineralogical character between 

 the Purbecks in Wilts and Dorset ; but in the former case, where the 

 whole series is so much reduced in bulk, it is extremely difficult to 

 make a very correct or close comparison. The forthcoming Memoirs 

 of the Geological Survey will no doubt give an accurate account of 

 them, while a closer investigation of the Mollusks and Cyprides and 

 other fossils by Professor E. Forbes will form a complete and valuable 

 history of the Purbeck formation in Great Britain. 



Dorsetshire. 



The Section at Durdlestone Bay, near Swanage, is now so well 

 known that it is unnecessary to repeat it here. I shall, therefore, 

 merely refer for details to Professor Forbes' s paper above-mentioned, 

 and to the Bev. J. H. Austen's ' Guide to the Geology of the Isle of 

 Purbeck*,' in which a full account is given of the strata and their 

 fossil contents f. The richest depositary of Insect remains at Durdle- 

 stone Bay, on the coast, is in the lower Purbecks, while in Wiltshire 

 (with one exception) they are confined to the middle di^asion. 

 During a late visit to Swanage, I discovered a small wing in a band 



* Published as a small pamphlet, 8vo, by Shipp, Printer and Publisher, Bland ■ 

 ford, and Penney, Swanage. 



t Of course the important and earlier papers by Dr. Buckland, Mr. Webster, 

 and Dr. Fitton, on the Dorsetshire Purbecks, published in tlie Geological Trans- 

 actions, must not be forgotten, for nothing was previonsly known of the compli- 

 cated coast sections in Durdlestone Bay and other places, nor of the interesting 

 organic remains which thek investigations brought to hght. 



