Vol. 50.] ON THE PURBECK BEDS OF THE VALE OF WARDOUR. 07 



are evidently the equivalents of the ' Cockle-beds ' and especially 

 resemble those of the Upwey (Itidgeway) section, where there is 

 much more limestone than marl and where one of the limestones 

 is described as 'oolitic-looking.' The Upwey 'Cockle-beds' also 

 contain small forms of Cardium, Gorbula, Leda, and Serpula, which 

 are probably the same species as those found at liidge. There was 

 evidently a widespread irruption of the sea at this particular epoch, 

 converting the freshwater lakes of the Cypris-f rcG&tonea into 

 saltwater lagoons. 



The highest beds of the Lower Purbeck, with their Cypris pur- 

 beckensis, insect- and fish-remains, show that freshwater conditions 

 were once more predominant, the marls, marlstones, and oolites of 

 the Vale of Wardour being clearly the equivalents of the ' Marly 

 Freshwater Beds ' of Dorset. 



At the base of the Middle Purbeck group there is a set of beds 

 (between 10 and 11 feet thick) which are not unlike those of the 

 Cherty Freshwater group at Durleston Bay, except that they do 

 not contain chert. They have a similar black and brown clay at 

 the base, and they contain a flaggy shell-limestone of the same 

 character as the lowest beds worked in the Durleston quarries. At 

 Upwey this group is only from 5 to 6 feet thick, and does not 

 contain limestone. 



The 'Cinder-bed,' or oyster-limestone, occupies its usual position, 

 and is often crowded with Trigonice as well as Ostrece. The suc- 

 ceeding beds vary much in Dorset, and the same seems to be the 

 case in the Vale of Wardour, so that no special horizon above the 

 Cinder-bed is recognizable. We may, however, notice one point of 

 resemblance between the Upwey and Wiltshire sections, and that 

 is the prevalence of sand and sandy stone in the group which seems 

 to be the equivalent of the ' Got'bida-beds,' but which in Wiltshire 

 would be more suitably termed the ' Gyrena-heds.' 



As regards the Upper Purbeck group, there is no really clear section 

 of it, so that we cannot be quite sure that all its component beds 

 have come under our notice. We are, however, fairly certain 

 that there are no hard limestones like those of the ' Broken 

 Shell-limestones ' which form the base of the group in Dorset. 

 These appear to have thinned out northward, so that the repre- 

 sentatives of the Uuio-heds come to lie at the base of the group, 

 and these only contain very thin limestones. The thick bed of clay 

 w T hich we take as the basement-bed, and the soft yellow sand with 

 Endogenitcs which overlies it, seem to be quite peculiar to the Wilt- 

 shire succession. They appear to indicate an episode of what may be 

 termed Wealden conditions, the influx of rapid currents bringing 

 down mud and sand, and temporarily preventing the formation ot' 

 limestones or marls. This is only what might be expected to occur 

 on the margins of the Purbeck basin. 



The highest beds seen in the railway-siding and in the well at 

 Dinton bear much resemblance to the shales and clays of the ' Upper 

 Gypris-m&rh,' and here again the absence of Paludina-limestones 

 is worth noting. 



Taken as a whole, the Wiltshire series has a greater resem- 

 blance to the strata seen in the Upwey or Ridgeway section than 



' f 1' 



