MESOZOIC AND KAINOZOIC GEOLOGY IN EUROPE. 327 



by means of a graphic diagram. During the earlier part 

 of the period the area of maximum deposition moved from 

 the centre of the district towards the east ; while during 

 the later hemerae it moved towards the west. 



It is quite possible, as Buckman points out, that two 

 species which occur together in the same bed of rock may 

 really not have been contemporaneous and that proof of 

 the difference of age may be obtained elsewhere. At one 

 place we may have a great accumulation of deposit, with a 

 certain species at its base and another at its summit, while 

 some distance off the same period of deposition may be 

 represented by so thin a bed that the two species occur 

 together. Yet the one species is newer than the other. 

 In the western part of the Sherborne area, for example, the 

 deposits of the various hemerse are so thin that it is not 

 possible to distinguish them all ; but as the beds thicken 

 towards the east, it becomes easy to separate those of the 

 individual hemerae, and the characteristic ammonites appear, 

 not together but in their regular order. 



Comparison with the rocks of Normandy, the south of 

 France and Wurtemberg shows that the general succession 

 of the faunas is the same ; but it can scarcely be maintained 

 as yet that all of Buckman's hemerse have been distinguished 

 in those areas. 



A description of the uppermost Jurassic, or Purbeck beds, 

 as they occur in the Vale of Wardour in Wiltshire, has been 

 given by Andrews and Jukes- Browne (1). The whole series 

 is remarkably like that of the coast of Dorsetshire and has 

 been deposited under nearly the same conditions. Thus the 

 lower Purbeck consists of fresh-water beds towards the base 

 and towards the summit, while the presence of Cardmm, 

 Corbula, etc., in the middle portion shows that the waters 

 were then brackish: the middle Purbeck includes a "cin- 

 der-bed," which corresponds with that of Dorsetshire and 

 which is often crowded with Trigonia and Ostrea, showing 

 the incoming of marine conditions. The upper Purbeck is 

 more distinct in character and near its base includes a soft 

 yellow sand with Endogenites, which appears to be peculiar 

 to Wiltshire. Above, it consists mainly of clays and marls- 



