Vol. 50.] ON THE PORBECK BEDS OF THE VALE OP WARDOUR. 61 



Under such circumstances it might be expected that the plane of 

 division between the Portland and Purbeck scries would be clearly 

 marked, if not accompanied by signs of erosion ; but this is not tho 

 case, and it is on the contrary difficult to decide where the divisional 

 plane should be taken. The flaggy and shelly portions of the 2 ft. 

 3 in. bed are firmly welded together, and would yield a slab like 

 that at the Museum of Practical Geology, in which Portland shells 

 are visible in the lower and Cyprids in the upper part, but these 

 Cyprids are not freshwater species, being in fact Candona angata and 

 C. bononiensis (which are estuarine forms). From the flaggy portion 

 two species of fish have been obtained (Ophiopsis breviceps and 

 0. penicillatus) and also a large species of Archceoniscus ; but none 

 of these fossils afford very good evidence for classing the flaggy 

 bed as Purbeck ; even the Arclwoniscus doubtless existed on the 

 land-surfaces of Portland times, and might therefore be washed into 

 the shallowing bay or estuary. 



The overlying marl contains Cyprids, but they are not recogniz- 

 able ; from the chalky limestone we obtained better specimens, 

 which Prof. T. Pupert Jones identified as Candona ansata, and a form 

 like Cypridea punctata, while in its highest cherty layer occurs 

 what appears to be Cypris purbeclcensis associated with a Cardinal 

 and Corbula alata. 



Thus both strata and included fossils show a gradual passage 

 from Portland to Purbeck conditions ; but, as the line must be 

 drawn somewhere, we prefer to take it at the base of the laminated 

 marl, and consequently to relegate the flaggy stone to the Portland 

 series. 



To account for the great difference between the Wockley and the 

 Chilmark sections, we can only suppose that Woekley lay in the 

 track of a tidal current which prevented rapid deposition and subse- 

 quently caused the erosion of the grey beds; while the Chilmark 

 area was either a low-lying sandbank or a backwater out of the 

 track of the main current, and close to the influx of springs or 

 streams which contained a large amount of carbonate of lime in 

 solution, the carbonate of lime being deposited partly as oolite 

 while the lagoon was still open to the sea, and partly as tufa after 

 this opening had been closed. 



If the white marl be taken as the basement-bed, the thickness of 

 Lower Purbeck exposed at Wocklcy is about 22 feet (see p. 49). 

 The Cyprids observed in the oolitic stone at the top of the quarry 

 proved to be Cypris purbeclcensis. 



Near the hamlet of Eidge, 1J mile north of AVockley, and about 

 1 mile west of the Chilmark section, is a quarry exposing beds 

 which we believe to be in the higher part of the Lower Purbeck 

 series, the floor of the quarry probably lying not far above the 

 horizon of the topmost bed in the Woekley section. The succession 

 observed at Ridge in 1SI»0 was as follows: — 



