560 



Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



eventually found in a band of shale, No. 58 of Mr Austen's section, in the brackish portion 

 of these Turtle beds. 



We now have arrived at one of the most striking deposits among these strata — the oyster 

 bed — called locally the " Cinder." At Durlstone bay it is as much as twelve feet thick, and 

 consists of three portions, in the upper of which are scarcely any shells, except the Ostrea 

 distorta ; in the middle portion is a band not above a foot thick, which contains the 

 Hemicidaris Purbeckensis and its spines, with Serpula? mingled with the oysters. The lower 

 portion affords another species of oyster, broader than the O. distorta, and a Cardium. The 

 Cinder is a rock which, although it crumbles in the weather, nevertheless, from its great 

 thickness and the enormous masses of its blocks, has had more effect upon the contour of the 

 country than any other of the beds of the Purbecks, and may often be found crowning the 

 escarpments of the hills. 



Leaving the Cinder bed we descend to beds which are again of freshwater origin, and are 

 lithologically characterized by the presence of much silica, which forms occasionally thick beds 

 of chert. There is also much vegetable matter, mixed with muddy deposit, full of broken 

 shells, forming what may be termed dirt beds, these being interstratified with Cypris-shales and 

 indurated marls. The shells of this portion of the series are Cyclas, Physa Bristowii 

 (Forbes), Planorbis, Valvata, Neritina, Melania Chilina ; but they are not found so well 

 preserved at Durlstone bay as at 'Kingston in Purbeck, at Osmington, and Ridgway. Stems 

 and seed-vessels of the Chara also abound at the latter localities. The thickness of these 

 middle beds is about thirty feet*. 



As we proceed westward from Durlstone bay the " middle Purbecks" lose their pre- 

 eminence in the system. At Worbarrow they retain something of it, but at Lulworth they 

 are much diminished in thickness, and still more so at Osmington and Ridgway. They are 

 not of sufficient importance at either of these places to be generally quarried, though at 

 Osmington the broken-shell-limestone, on account of its durability, is sometimes used for founda- 

 tions. The Cyrenae in this bed are often in a much better state of preservation there than 

 they are ever found at Swanage ; and at Upwey, close to Ridgway, very good specimens may 

 be obtained. The Archaeoniscus has never been found in the middle Purbecks in the Dorset- 

 shire district anywhere but at Swanage ; but I am persuaded that the chief deposit of these 

 crustaceans in the Vale of Wardour, as well as of the insects figured in Mr Brodie's " Fossil 

 Insects," are from these estuary beds of the middle Purbecksf. 



The marine bands forming the " Pecten beds" at Ridgway -hill consist of a series of sandy 

 beds, composed of the casts of Mytilus Spatula, Thracia Fisheri (Forbes), Modiola Fittoniana, 

 Perna, Pecten, Avicula Dorsetensis (Forbes), &c. ; and towards the bottom two feet of ripple- 

 marked sandstone, splitting into slabs, and used for fencing and rough paving. The ripple 



» It is amongst these dirt beds, No. 93 of Mr Austen's Sec- 

 tion, that Mr W. Brodie has found the Macellodus Brodeii, 

 and that interesting mammalian fossil the Spalacotherium 

 Tricuspidens, described by Professor Owen in the Geological 

 Journal for November, 1854. 



■)- Since this paper was read the November number of the 



Geological Journal has appeared, in which Mr P. Brodie has 

 printed some sections of the Wiltshire Purbecks which I 

 communicated to him for his paper. The middle and lower 

 Purbecks of Wiltshire are very similar to those of Ridgway- 

 hill, but the upper division is absent. 



