Vol. 50.] ON TEE rURUECK BEDS OF THE VALE OF WARDOUR. 63 



assigned to the Insect-Led, bnt it docs not appear that any insects 

 were found in it at that locality. 



When Mr. Brodie first visited the Vale of Wardour few exposures 

 of the strata were to be seen, and he bad no idea that the whole 

 Purbeck series was there represented. In his later paper he states 

 that when he collected from these beds he regarded "the Insect- 

 and Isopod-limestones as belonging to the lower part of tho Lower 

 Purbecks." It was in this way that he came to consider the 

 highest bed of ' lias ' in the Middle Purbeck at Teffont as the 

 equivalent of the Isopod- and Insect-limestones, though he notes 

 the litbological difference of the rock and the fact that it only 

 contains a few insects. 1 



It would appear, therefore, that the bed from which Mr. Brodie 

 obtained the greater number of his insect -remains occurs in the 

 upper part of the Middle Purbeck, just beneath the Isopod or 

 Archceonwcus-bedL ; but as we have failed to find any insectiferous 

 limestone at this horizon in the numerous sections which are now 

 open, we are compelled to conclude that it is a stratum of local 

 occurrence, confined to a very small area in the most easterly part 

 of the Vale of Wardour, a view in which Mr. Brodie now concurs. 



VI. General Conclusions, and Comparison of Dorset 

 and Wiltshire Purbecxs. 



Prom the sections given in the preceding pages it will be seen 

 that we have now better data than previously existed for calculating 

 the thickness of the several parts of the Purbeck series in the Vale 

 of Wardour. There is still some difficulty in fitting in all the ex- 

 posed sections with one another, and we have not found the actual 

 summit of the formation ; but we thiuk that there are not many of 

 its component beds which we have not seen, and that our estimate 

 of the total thickness will be found a near approximation to the 

 truth. 



Lower Purbeck. — If we take the Wockley section, which is re- 

 peated at Chicksgrove, as giving the more prevalent type of the 

 basement-beds, we have there from 22 to 24 feet of Lower Purbeck, 

 and in the Ilidge quarry there are over 21 feet (see pp. 49, 52). It is 

 clear that these two sections do not overlap, and how much comes in 

 between them we cannot say ; but in the lane from Chicksgrove to 

 Lady Down there is quite room for GO or 70 feet between the basi - 

 ment-bed and the outcrop of the oolitic beds. Finally, nearly 17 feet 

 of strata occur in the Teffont quarry which seem to be above any 

 exposed at Ilidge ; consequently we have actually seen over 60 feet 

 of Lower Purbeck beds, and, allowing for the gap between the 

 Ilidge and Wockley sections, we consider that 70 feet is a fair 

 estimate of their average thickness. The diagram on p. 05 (fig. 4) 

 shows the vertical succession which is assumed in this estimate : — 



1 See ' Fossil Insects,' pp. 18, 19. 



